Suspension Rear Sway Bar just installed
#26
03-21-2019, 01:06 PM
I highly recommend this video titled "The Physics of Racing" https://www.youtube.com/watch?v=bYp2vvUgEqE, it explains all of this stuff. Very educational.
And, he confirmed a number of things I thought I was seeing/feeling with my car.
I also like how he talked about how he sets up his car to understeer going into a turn, and uses trail braking to create the oversteer necessary to make the turn quickly. This is a practice I use. As he said, a front wheel drive car will always understeer with gas on. With a FWD car and without trail braking, it is likely that gas will need to be applied before the apex and that will result in the car understeering around the corner.
I was, however, surprised as to how readily he glossed over the need for springs in combination with sway bars. He runs just stiff springs and leaves open the question as to the need for sway bars at all. For this he would have only needed to look at the primary purpose of each. That is, the primary purpose of the springs is to support the weight of the car. The primary purpose of the sway bar is to generate roll stiffness. A side benefit of springs is the generation of roll stiffness. But the sway bars are more effective at generating roll stiffness than springs. That is they need a lower spring rate to generate a unit of roll stiffness as compared to springs alone. This would lead to more easily creating roll stiffness (flatter cornering) and changes in understeer/oversteer, without compromising compliance of the car. At least that is my understanding.
Thanks for posting...
#27
03-21-2019, 10:17 PM
Moderator
Maybe having a degree in engineering which required a lot of college courses in physics and mechanics makes me overly pedantic. I will try to elaborate on what I said before, and also describe a couple of thought experiments.
First of all, what we call "weight" is really a force. F=mA so when you step on a scale it is measuring the force created by the acceleration of gravity on the mass of your body. I remember in high school physics that it took me a while to wrap my head around that. It is further confused by the fact that we use "Pounds" and "Kilos" interchangeably, when really a kilogram is a definition of mass, not force. (Force is Newtons in SI units.)
If we're talking about industrial machinery, yes on something like a forklift or an excavator it is possible to put enough of a load on the fork or in the excavator bucket so that the center of mass will shift outside of the "footprint" of the vehicle so it will tip. But we're talking about cars, where there's not any mass being added as the car goes around a corner, and the masses in the car stay in the same place unless you're driving a minivan with a bunch of kids not wearing their seatbelts.
Thought Experiment #1: Consider a brick. Say 6" long, 3" wide, 1.5" tall. Let's assume that it is of uniform density so its center of mass is at its geometric center. (the point 1/2 way along each of the dimensions. Look up the Wikipedia definition if you want more details.) Place it with one of the large 6" x 3" faces flat on a horizontal table. That's our car, minus wheels and suspension of course. Now tip the brick up on one of its long edges. This is the car starting to roll to one side as it goes around a corner. Keep tipping it up, the center of mass will will move up and toward the edge still on the table, until it moves to the other side of the edge and falls over onto its long, narrow 6" x 1.5" face. Before that critical point, it will fall back onto the 6"x3" face. The center of mass is still at the geometric center of the brick, it has moved relative to the table but it is still in the same place inside the brick. At no point did it move outside the brick..
Thought Experiment #2: Now the brick is on its 6"x1.5" edge, so it has a tall, narrow shape. Now it is more like a big passenger bus, the center of mass is up higher off the table, and as you repeat the experiment of tipping the brick, it will fall back over onto its larger face at a much smaller angle. Higher center of mass, narrower footprint, less stable vehicle. That's one reason why Lamborghinis corner better than minivans.
Thought Experiment #3: Replace the brick with a slab of lead that is 6"x3" and just thick enough to weigh almost the same as the brick, then add some styrofoam on top so you have a lead+foam sandwich that is exactly the same dimensions and mass as the brick. This is a different car, maybe a Tesla because they have the battery pack down in the floor, and the center of mass (with the lead side down) is now MUCH lower than in the brick. Repeat experiments 1 and 2. When the new lead+foam brick is laying flat on the table, you can tip it up to a far greater angle before it will tip on it's side. That's why lowering a car is beneficial - it lowers the center of mass. For the heck of it, turn the brick over so the foam side is down and the lead side is up, you know what will happen.
Thought Experiment #4: Let's go back to the brick of uniform density, again with one of the 6"x3" faces laying flat on the table. Put a scale under each end. Perfect 50/50 weight distribution! Now remove some of the brick from one end, changing only the thickness but not the length, and stack it on the other end to create sort of an L shape with the L laying on it's back. Adjust this until 2/3 of the weight is on one end and 1/3 on the other. Call the heavy end the front and the lighter end the rear, now we have a 67/33 front/rear weight distribution like a front wheel drive car, and the center of mass has moved forward, it is no longer at the geometric center. Put some wheels at each corner. Now take a paper clip, straighten it out, and place it across the brick from one rear wheel to the other. That's your rear rear sway bar. Did the center of mass move? No! Well, it moved a tiny amount because you added some mass, but you'd need really accurate scales to measure it. If I go from a 17mm RSB to a 22mm, I add maybe 3 pounds to a 2700 pound car - it's insignificant. It doesn't move the center of mass of the car. That's physics. Now if I add a 200lb driver to a 2700lb car, that will move the center of mass. If the 200lb driver moves from the driver side, to the passenger side, to the back seat, that moves the center of mass. It should be clear by now that to move the center of mass, you have to move mass! Unless your car is a fishbowl with water that sloshes side to side, the center of mass stays in the same place relative to the body of the car whether the car is at rest, or going around a corner. It doesn't magically jump up outside the car just because the car is going around a corner.
Thought Experiment #5: A sway bar couples the suspension between left and right sides of the car. There's a sway bar in front, and a sway bar in back. There is no coupling between the front wheels and rear wheels created by the sway bar. (The only suspension I am aware of that does couple front and rear is in really old Citroens with weird suspensions that had pressurized chambers of oil that coupled front and rear., but we're not talking about those.) So how can a sway bar move the center of mass or "balance the center of gravity"? It doesn't, except to the extent that it adds mass to the car.
What a sway bar does do, as I tried to explain in my earlier post above, is change the relative forces at each wheel as the car turns a corner. If you want to really understand that, watch the video I linked above, starting at about 50:00 if you're impatient. But seriously, watch the entire thing, I learned a lot from it.
First of all, what we call "weight" is really a force. F=mA so when you step on a scale it is measuring the force created by the acceleration of gravity on the mass of your body. I remember in high school physics that it took me a while to wrap my head around that. It is further confused by the fact that we use "Pounds" and "Kilos" interchangeably, when really a kilogram is a definition of mass, not force. (Force is Newtons in SI units.)
If we're talking about industrial machinery, yes on something like a forklift or an excavator it is possible to put enough of a load on the fork or in the excavator bucket so that the center of mass will shift outside of the "footprint" of the vehicle so it will tip. But we're talking about cars, where there's not any mass being added as the car goes around a corner, and the masses in the car stay in the same place unless you're driving a minivan with a bunch of kids not wearing their seatbelts.
Thought Experiment #1: Consider a brick. Say 6" long, 3" wide, 1.5" tall. Let's assume that it is of uniform density so its center of mass is at its geometric center. (the point 1/2 way along each of the dimensions. Look up the Wikipedia definition if you want more details.) Place it with one of the large 6" x 3" faces flat on a horizontal table. That's our car, minus wheels and suspension of course. Now tip the brick up on one of its long edges. This is the car starting to roll to one side as it goes around a corner. Keep tipping it up, the center of mass will will move up and toward the edge still on the table, until it moves to the other side of the edge and falls over onto its long, narrow 6" x 1.5" face. Before that critical point, it will fall back onto the 6"x3" face. The center of mass is still at the geometric center of the brick, it has moved relative to the table but it is still in the same place inside the brick. At no point did it move outside the brick..
Thought Experiment #2: Now the brick is on its 6"x1.5" edge, so it has a tall, narrow shape. Now it is more like a big passenger bus, the center of mass is up higher off the table, and as you repeat the experiment of tipping the brick, it will fall back over onto its larger face at a much smaller angle. Higher center of mass, narrower footprint, less stable vehicle. That's one reason why Lamborghinis corner better than minivans.
Thought Experiment #3: Replace the brick with a slab of lead that is 6"x3" and just thick enough to weigh almost the same as the brick, then add some styrofoam on top so you have a lead+foam sandwich that is exactly the same dimensions and mass as the brick. This is a different car, maybe a Tesla because they have the battery pack down in the floor, and the center of mass (with the lead side down) is now MUCH lower than in the brick. Repeat experiments 1 and 2. When the new lead+foam brick is laying flat on the table, you can tip it up to a far greater angle before it will tip on it's side. That's why lowering a car is beneficial - it lowers the center of mass. For the heck of it, turn the brick over so the foam side is down and the lead side is up, you know what will happen.
Thought Experiment #4: Let's go back to the brick of uniform density, again with one of the 6"x3" faces laying flat on the table. Put a scale under each end. Perfect 50/50 weight distribution! Now remove some of the brick from one end, changing only the thickness but not the length, and stack it on the other end to create sort of an L shape with the L laying on it's back. Adjust this until 2/3 of the weight is on one end and 1/3 on the other. Call the heavy end the front and the lighter end the rear, now we have a 67/33 front/rear weight distribution like a front wheel drive car, and the center of mass has moved forward, it is no longer at the geometric center. Put some wheels at each corner. Now take a paper clip, straighten it out, and place it across the brick from one rear wheel to the other. That's your rear rear sway bar. Did the center of mass move? No! Well, it moved a tiny amount because you added some mass, but you'd need really accurate scales to measure it. If I go from a 17mm RSB to a 22mm, I add maybe 3 pounds to a 2700 pound car - it's insignificant. It doesn't move the center of mass of the car. That's physics. Now if I add a 200lb driver to a 2700lb car, that will move the center of mass. If the 200lb driver moves from the driver side, to the passenger side, to the back seat, that moves the center of mass. It should be clear by now that to move the center of mass, you have to move mass! Unless your car is a fishbowl with water that sloshes side to side, the center of mass stays in the same place relative to the body of the car whether the car is at rest, or going around a corner. It doesn't magically jump up outside the car just because the car is going around a corner.
Thought Experiment #5: A sway bar couples the suspension between left and right sides of the car. There's a sway bar in front, and a sway bar in back. There is no coupling between the front wheels and rear wheels created by the sway bar. (The only suspension I am aware of that does couple front and rear is in really old Citroens with weird suspensions that had pressurized chambers of oil that coupled front and rear., but we're not talking about those.) So how can a sway bar move the center of mass or "balance the center of gravity"? It doesn't, except to the extent that it adds mass to the car.
What a sway bar does do, as I tried to explain in my earlier post above, is change the relative forces at each wheel as the car turns a corner. If you want to really understand that, watch the video I linked above, starting at about 50:00 if you're impatient. But seriously, watch the entire thing, I learned a lot from it.
Last edited by squawSkiBum; 03-21-2019 at 10:24 PM. Reason: punctuation
#28
03-21-2019, 11:07 PM
Moderator
Ok, just finished the video. Perfect. Right at my level of understanding.
And, he confirmed a number of things I thought I was seeing/feeling with my car.
I also like how he talked about how he sets up his car to understeer going into a turn, and uses trail braking to create the oversteer necessary to make the turn quickly. This is a practice I use. As he said, a front wheel drive car will always understeer with gas on. With a FWD car and without trail braking, it is likely that gas will need to be applied before the apex and that will result in the car understeering around the corner.
And, he confirmed a number of things I thought I was seeing/feeling with my car.
I also like how he talked about how he sets up his car to understeer going into a turn, and uses trail braking to create the oversteer necessary to make the turn quickly. This is a practice I use. As he said, a front wheel drive car will always understeer with gas on. With a FWD car and without trail braking, it is likely that gas will need to be applied before the apex and that will result in the car understeering around the corner.
Yeah, I learned this in the hairpin at Laguna Seca on my first track day. Plowed miserably through the corner with a huge slip angle and said to myself "let's not do that again."
I was, however, surprised as to how readily he glossed over the need for springs in combination with sway bars. He runs just stiff springs and leaves open the question as to the need for sway bars at all. For this he would have only needed to look at the primary purpose of each. That is, the primary purpose of the springs is to support the weight of the car. The primary purpose of the sway bar is to generate roll stiffness. A side benefit of springs is the generation of roll stiffness. But the sway bars are more effective at generating roll stiffness than springs. That is they need a lower spring rate to generate a unit of roll stiffness as compared to springs alone. This would lead to more easily creating roll stiffness (flatter cornering) and changes in understeer/oversteer, without compromising compliance of the car. At least that is my understanding.
Thanks for posting...
Thanks for posting...
#29
03-22-2019, 06:22 AM
4th Gear
cen·ter of grav·i·ty
/ˌsen(t)ər əv ˈɡravədē/
noun
The COG is fluid in a moving vehicle that is subject to braking, accelerating and going through bends or turns in the road. The COG is influenced by many different factors. Position of center of mass while at rest just being one of them. The lower the center of mass the better the starting point you have. I dont have enough time to give an online lesson on COG but I will give one more simple example to help visualize how fluid it is.
let's look at a dragster running a quarter mile. Yes I know this is a different animal but it will help to understand COG movements. When this car launches from the starting line its front wheels lift off the track. And all the weight of the car is on the back wheels. This weight transfer is done by moving the center of gravity behind the dragster. If the COG moves far enough back and up the car will keep going and flip over. This theoretical force or point cant be seen or touched but it's real.
back to track cars such as minis. There are again so many things going on with suspensions with so many different designs it would take a month of online courses to learn them all. Sway bars are the same. I'll try to give an example of how the center of gravity moves front to back by sway bar alone. When adding a larger rear bar we add load to the outside tire in a corner. This loading of that tire takes more weight of the car. This is what balances the steering out in a corner. Want more under steer? Smaller back bar. Less under steer? Bigger back bar. This is a very simplified example of what's happening but by adding or subtracting force from that rear tire (weight) you have moved the COG rearward or forward.
I am not trying to rewrite the book of suspensions here. I'm just trying to educate why just a rear bar has such an effect on your driving experience. You are transferring weight off of the front wheels to the rear with a simple rear bar change. This is called moving the center of gravity rearward.
/ˌsen(t)ər əv ˈɡravədē/
noun
- a point from which the weight of a body or system may be considered to act. In uniform gravity it is the same as the center of mass.
The COG is fluid in a moving vehicle that is subject to braking, accelerating and going through bends or turns in the road. The COG is influenced by many different factors. Position of center of mass while at rest just being one of them. The lower the center of mass the better the starting point you have. I dont have enough time to give an online lesson on COG but I will give one more simple example to help visualize how fluid it is.
let's look at a dragster running a quarter mile. Yes I know this is a different animal but it will help to understand COG movements. When this car launches from the starting line its front wheels lift off the track. And all the weight of the car is on the back wheels. This weight transfer is done by moving the center of gravity behind the dragster. If the COG moves far enough back and up the car will keep going and flip over. This theoretical force or point cant be seen or touched but it's real.
back to track cars such as minis. There are again so many things going on with suspensions with so many different designs it would take a month of online courses to learn them all. Sway bars are the same. I'll try to give an example of how the center of gravity moves front to back by sway bar alone. When adding a larger rear bar we add load to the outside tire in a corner. This loading of that tire takes more weight of the car. This is what balances the steering out in a corner. Want more under steer? Smaller back bar. Less under steer? Bigger back bar. This is a very simplified example of what's happening but by adding or subtracting force from that rear tire (weight) you have moved the COG rearward or forward.
I am not trying to rewrite the book of suspensions here. I'm just trying to educate why just a rear bar has such an effect on your driving experience. You are transferring weight off of the front wheels to the rear with a simple rear bar change. This is called moving the center of gravity rearward.
#30
03-22-2019, 06:36 AM
Alliance Member
#32
03-22-2019, 08:39 AM
Maybe having a degree in engineering which required a lot of college courses in physics and mechanics makes me overly pedantic. I will try to elaborate on what I said before, and also describe a couple of thought experiments.
First of all, what we call "weight" is really a force. F=mA so when you step on a scale it is measuring the force created by the acceleration of gravity on the mass of your body. I remember in high school physics that it took me a while to wrap my head around that. It is further confused by the fact that we use "Pounds" and "Kilos" interchangeably, when really a kilogram is a definition of mass, not force. (Force is Newtons in SI units.)
If we're talking about industrial machinery, yes on something like a forklift or an excavator it is possible to put enough of a load on the fork or in the excavator bucket so that the center of mass will shift outside of the "footprint" of the vehicle so it will tip. But we're talking about cars, where there's not any mass being added as the car goes around a corner, and the masses in the car stay in the same place unless you're driving a minivan with a bunch of kids not wearing their seatbelts.
Thought Experiment #1: Consider a brick. Say 6" long, 3" wide, 1.5" tall. Let's assume that it is of uniform density so its center of mass is at its geometric center. (the point 1/2 way along each of the dimensions. Look up the Wikipedia definition if you want more details.) Place it with one of the large 6" x 3" faces flat on a horizontal table. That's our car, minus wheels and suspension of course. Now tip the brick up on one of its long edges. This is the car starting to roll to one side as it goes around a corner. Keep tipping it up, the center of mass will will move up and toward the edge still on the table, until it moves to the other side of the edge and falls over onto its long, narrow 6" x 1.5" face. Before that critical point, it will fall back onto the 6"x3" face. The center of mass is still at the geometric center of the brick, it has moved relative to the table but it is still in the same place inside the brick. At no point did it move outside the brick..
Thought Experiment #2: Now the brick is on its 6"x1.5" edge, so it has a tall, narrow shape. Now it is more like a big passenger bus, the center of mass is up higher off the table, and as you repeat the experiment of tipping the brick, it will fall back over onto its larger face at a much smaller angle. Higher center of mass, narrower footprint, less stable vehicle. That's one reason why Lamborghinis corner better than minivans.
Thought Experiment #3: Replace the brick with a slab of lead that is 6"x3" and just thick enough to weigh almost the same as the brick, then add some styrofoam on top so you have a lead+foam sandwich that is exactly the same dimensions and mass as the brick. This is a different car, maybe a Tesla because they have the battery pack down in the floor, and the center of mass (with the lead side down) is now MUCH lower than in the brick. Repeat experiments 1 and 2. When the new lead+foam brick is laying flat on the table, you can tip it up to a far greater angle before it will tip on it's side. That's why lowering a car is beneficial - it lowers the center of mass. For the heck of it, turn the brick over so the foam side is down and the lead side is up, you know what will happen.
Thought Experiment #4: Let's go back to the brick of uniform density, again with one of the 6"x3" faces laying flat on the table. Put a scale under each end. Perfect 50/50 weight distribution! Now remove some of the brick from one end, changing only the thickness but not the length, and stack it on the other end to create sort of an L shape with the L laying on it's back. Adjust this until 2/3 of the weight is on one end and 1/3 on the other. Call the heavy end the front and the lighter end the rear, now we have a 67/33 front/rear weight distribution like a front wheel drive car, and the center of mass has moved forward, it is no longer at the geometric center. Put some wheels at each corner. Now take a paper clip, straighten it out, and place it across the brick from one rear wheel to the other. That's your rear rear sway bar. Did the center of mass move? No! Well, it moved a tiny amount because you added some mass, but you'd need really accurate scales to measure it. If I go from a 17mm RSB to a 22mm, I add maybe 3 pounds to a 2700 pound car - it's insignificant. It doesn't move the center of mass of the car. That's physics. Now if I add a 200lb driver to a 2700lb car, that will move the center of mass. If the 200lb driver moves from the driver side, to the passenger side, to the back seat, that moves the center of mass. It should be clear by now that to move the center of mass, you have to move mass! Unless your car is a fishbowl with water that sloshes side to side, the center of mass stays in the same place relative to the body of the car whether the car is at rest, or going around a corner. It doesn't magically jump up outside the car just because the car is going around a corner.
Thought Experiment #5: A sway bar couples the suspension between left and right sides of the car. There's a sway bar in front, and a sway bar in back. There is no coupling between the front wheels and rear wheels created by the sway bar. (The only suspension I am aware of that does couple front and rear is in really old Citroens with weird suspensions that had pressurized chambers of oil that coupled front and rear., but we're not talking about those.) So how can a sway bar move the center of mass or "balance the center of gravity"? It doesn't, except to the extent that it adds mass to the car.
What a sway bar does do, as I tried to explain in my earlier post above, is change the relative forces at each wheel as the car turns a corner. If you want to really understand that, watch the video I linked above, starting at about 50:00 if you're impatient. But seriously, watch the entire thing, I learned a lot from it.
First of all, what we call "weight" is really a force. F=mA so when you step on a scale it is measuring the force created by the acceleration of gravity on the mass of your body. I remember in high school physics that it took me a while to wrap my head around that. It is further confused by the fact that we use "Pounds" and "Kilos" interchangeably, when really a kilogram is a definition of mass, not force. (Force is Newtons in SI units.)
If we're talking about industrial machinery, yes on something like a forklift or an excavator it is possible to put enough of a load on the fork or in the excavator bucket so that the center of mass will shift outside of the "footprint" of the vehicle so it will tip. But we're talking about cars, where there's not any mass being added as the car goes around a corner, and the masses in the car stay in the same place unless you're driving a minivan with a bunch of kids not wearing their seatbelts.
Thought Experiment #1: Consider a brick. Say 6" long, 3" wide, 1.5" tall. Let's assume that it is of uniform density so its center of mass is at its geometric center. (the point 1/2 way along each of the dimensions. Look up the Wikipedia definition if you want more details.) Place it with one of the large 6" x 3" faces flat on a horizontal table. That's our car, minus wheels and suspension of course. Now tip the brick up on one of its long edges. This is the car starting to roll to one side as it goes around a corner. Keep tipping it up, the center of mass will will move up and toward the edge still on the table, until it moves to the other side of the edge and falls over onto its long, narrow 6" x 1.5" face. Before that critical point, it will fall back onto the 6"x3" face. The center of mass is still at the geometric center of the brick, it has moved relative to the table but it is still in the same place inside the brick. At no point did it move outside the brick..
Thought Experiment #2: Now the brick is on its 6"x1.5" edge, so it has a tall, narrow shape. Now it is more like a big passenger bus, the center of mass is up higher off the table, and as you repeat the experiment of tipping the brick, it will fall back over onto its larger face at a much smaller angle. Higher center of mass, narrower footprint, less stable vehicle. That's one reason why Lamborghinis corner better than minivans.
Thought Experiment #3: Replace the brick with a slab of lead that is 6"x3" and just thick enough to weigh almost the same as the brick, then add some styrofoam on top so you have a lead+foam sandwich that is exactly the same dimensions and mass as the brick. This is a different car, maybe a Tesla because they have the battery pack down in the floor, and the center of mass (with the lead side down) is now MUCH lower than in the brick. Repeat experiments 1 and 2. When the new lead+foam brick is laying flat on the table, you can tip it up to a far greater angle before it will tip on it's side. That's why lowering a car is beneficial - it lowers the center of mass. For the heck of it, turn the brick over so the foam side is down and the lead side is up, you know what will happen.
Thought Experiment #4: Let's go back to the brick of uniform density, again with one of the 6"x3" faces laying flat on the table. Put a scale under each end. Perfect 50/50 weight distribution! Now remove some of the brick from one end, changing only the thickness but not the length, and stack it on the other end to create sort of an L shape with the L laying on it's back. Adjust this until 2/3 of the weight is on one end and 1/3 on the other. Call the heavy end the front and the lighter end the rear, now we have a 67/33 front/rear weight distribution like a front wheel drive car, and the center of mass has moved forward, it is no longer at the geometric center. Put some wheels at each corner. Now take a paper clip, straighten it out, and place it across the brick from one rear wheel to the other. That's your rear rear sway bar. Did the center of mass move? No! Well, it moved a tiny amount because you added some mass, but you'd need really accurate scales to measure it. If I go from a 17mm RSB to a 22mm, I add maybe 3 pounds to a 2700 pound car - it's insignificant. It doesn't move the center of mass of the car. That's physics. Now if I add a 200lb driver to a 2700lb car, that will move the center of mass. If the 200lb driver moves from the driver side, to the passenger side, to the back seat, that moves the center of mass. It should be clear by now that to move the center of mass, you have to move mass! Unless your car is a fishbowl with water that sloshes side to side, the center of mass stays in the same place relative to the body of the car whether the car is at rest, or going around a corner. It doesn't magically jump up outside the car just because the car is going around a corner.
Thought Experiment #5: A sway bar couples the suspension between left and right sides of the car. There's a sway bar in front, and a sway bar in back. There is no coupling between the front wheels and rear wheels created by the sway bar. (The only suspension I am aware of that does couple front and rear is in really old Citroens with weird suspensions that had pressurized chambers of oil that coupled front and rear., but we're not talking about those.) So how can a sway bar move the center of mass or "balance the center of gravity"? It doesn't, except to the extent that it adds mass to the car.
What a sway bar does do, as I tried to explain in my earlier post above, is change the relative forces at each wheel as the car turns a corner. If you want to really understand that, watch the video I linked above, starting at about 50:00 if you're impatient. But seriously, watch the entire thing, I learned a lot from it.
Also, it seems that some people may be using the term “moving the CG”, maybe incorrectly or to just use terminology people are familiar with, to describe this change in loading on the tires when cornering and the sway bars have been changed. But, if you take your 4th brick example, you could set the brick on 3 thumbtacks, one in each of the front corners and one in one back corner to mimic a tripoding MINI. Then you could take a chunk of brick from the front corner diagonally opposite the unsupported rear corner and place that chunk on the diagonal a bit towards that unsupported corner. It seems that this would represent the change in the force on that support front location when the sway bar causes the inside rear wheel to come off the ground when cornering. If this is the case, then isn’t this the same as saying the sway bar causes the CG to change in a dynamic situation? Or did I mess up the example?
#33
03-22-2019, 08:48 AM
#34
03-22-2019, 08:57 AM
Moderator
LOL that (brick with thumbtacks) was going to be my thought experiment #6.
Agreed, after thinking about this, people are saying "moving the CoG" as a simple (and incorrect) analogy for the effect of different forces on the car, causing the forces on each wheel to change. The CoG/CoM stays in the same place, but as the car goes around a turn (for example) the lateral acceleration results in a force F=mA pushing to one side, that F acting on the CoG multiplied by the distance of the CoG away from the roll center creates a moment (a term used in physics and mechanics), think of it as a lever arm. That through the suspension geometry causes greater force on the outside wheels, and less force on the inside wheels.
I suppose if you took a snapshot in time of a car going around a corner and measured the forces on each corner, then you could calculate a "virtual" or effective position of the CoG/CoM, which would obviously be different from the CoG/CoM of the car at rest. But none of the stuff I have studied on suspensions does that, since it isn't useful in doing the calculations for what is going on. Again, to move the CoG/CoM you have to move mass, the positions of the masses inside the car are not changing, there are just varying forces being applied as the car accelerates, brakes, and corners. Calculate the moments and the suspension ratios to get the forces on each wheel.
I'll come up with more thought experiments later.
Agreed, after thinking about this, people are saying "moving the CoG" as a simple (and incorrect) analogy for the effect of different forces on the car, causing the forces on each wheel to change. The CoG/CoM stays in the same place, but as the car goes around a turn (for example) the lateral acceleration results in a force F=mA pushing to one side, that F acting on the CoG multiplied by the distance of the CoG away from the roll center creates a moment (a term used in physics and mechanics), think of it as a lever arm. That through the suspension geometry causes greater force on the outside wheels, and less force on the inside wheels.
I suppose if you took a snapshot in time of a car going around a corner and measured the forces on each corner, then you could calculate a "virtual" or effective position of the CoG/CoM, which would obviously be different from the CoG/CoM of the car at rest. But none of the stuff I have studied on suspensions does that, since it isn't useful in doing the calculations for what is going on. Again, to move the CoG/CoM you have to move mass, the positions of the masses inside the car are not changing, there are just varying forces being applied as the car accelerates, brakes, and corners. Calculate the moments and the suspension ratios to get the forces on each wheel.
I'll come up with more thought experiments later.
#36
03-23-2019, 05:23 PM
Boy, this thread takes the fun out of MINI's. Guys trying to convince each other that their viewpoint is wrong. Just enjoy your car and try and help others make choices based on our experiences.
I'm on my 3rd MINI that I've modded, all 3 have had larger RSB's. Every car I've tried different things, based on what I liked and didn't like about the previous car.
08 - Koni Yellows, TSW's, 1" H-sport bar, Powerflex front LCA bushings, WMW undercar brace. Fun car, suprised at the difference with the undercar brace and LCA bushings, very stable car.
13 - Bilsteins, Swifts, 1" H-sport bar, undercar brace. Swifts a little stiff for my taste, went with OE sport springs. Fun car, seemed less nimble than the 08.
13 (current), Bilsteins, OE sport springs, 22mm NM bar, undercar brace, 27 mm FSB, powerflex LCA. Recently put the FSB and LCA bushings, nice improvement in turn in feel. Just daily driving on winter tires the front seems more planted when hustling around the streets. I am running the RSB in the middle, so if there is more understeer, I can stiffen the rear to balance it.
The way that I always think of sway bars is that stiffer bars make both wheels less independant (someone stated this above). Lets not forget that the chassis flex under cornering, so the imaginary front and rear axles may not be parallel in a hard turn. If the inner rear wheel lifts off the ground, then it would be interesting to see if that wheel is at full extension, or if it is being held up by the RSB (because the outside wheel is being compressed due to the cornering loads). More food for thought?
Have fun,
Mike
I'm on my 3rd MINI that I've modded, all 3 have had larger RSB's. Every car I've tried different things, based on what I liked and didn't like about the previous car.
08 - Koni Yellows, TSW's, 1" H-sport bar, Powerflex front LCA bushings, WMW undercar brace. Fun car, suprised at the difference with the undercar brace and LCA bushings, very stable car.
13 - Bilsteins, Swifts, 1" H-sport bar, undercar brace. Swifts a little stiff for my taste, went with OE sport springs. Fun car, seemed less nimble than the 08.
13 (current), Bilsteins, OE sport springs, 22mm NM bar, undercar brace, 27 mm FSB, powerflex LCA. Recently put the FSB and LCA bushings, nice improvement in turn in feel. Just daily driving on winter tires the front seems more planted when hustling around the streets. I am running the RSB in the middle, so if there is more understeer, I can stiffen the rear to balance it.
The way that I always think of sway bars is that stiffer bars make both wheels less independant (someone stated this above). Lets not forget that the chassis flex under cornering, so the imaginary front and rear axles may not be parallel in a hard turn. If the inner rear wheel lifts off the ground, then it would be interesting to see if that wheel is at full extension, or if it is being held up by the RSB (because the outside wheel is being compressed due to the cornering loads). More food for thought?
Have fun,
Mike
#37
03-23-2019, 06:04 PM
4th Gear
13 (current), Bilsteins, OE sport springs, 22mm NM bar, undercar brace, 27 mm FSB, powerflex LCA. Recently put the FSB and LCA bushings, nice improvement in turn in feel. Just daily driving on winter tires the front seems more planted when hustling around the streets. I am running the RSB in the middle, so if there is more understeer, I can stiffen the rear to balance it.
I run my rear sway bar in the rear not the middle..... LMAO all in fun Mike. Sounds like you have a great set up. Are you running the stock rear sway bar links? They seem a little flimsy. I'm still in the build stage of my 08 clubby and my 09 clubby is stock and very nose heavy. This forum and its endless archive of knowledge has been of great help In my build. This is an addition to my three other play cars and 1970 Tahiti jet boat. Just love anything that goes vroom!
I run my rear sway bar in the rear not the middle..... LMAO all in fun Mike. Sounds like you have a great set up. Are you running the stock rear sway bar links? They seem a little flimsy. I'm still in the build stage of my 08 clubby and my 09 clubby is stock and very nose heavy. This forum and its endless archive of knowledge has been of great help In my build. This is an addition to my three other play cars and 1970 Tahiti jet boat. Just love anything that goes vroom!
#38
03-23-2019, 06:28 PM
I have the aftermarket endlinks sold by WMW. The studs are marginal in length where they go into the control arm. They are more beefy than the stock ones.
The MINI really responds to more front camber. I'm using the IE fixed camber plates, plus I slotted the strut tower mounts. I was able to get 2.0 negative, and turn in is much more crisp.
Mike
The MINI really responds to more front camber. I'm using the IE fixed camber plates, plus I slotted the strut tower mounts. I was able to get 2.0 negative, and turn in is much more crisp.
Mike
The following users liked this post:
MikeMJCW (03-23-2019)
#39
03-23-2019, 08:12 PM
Mike, you and I have very close to the same setups, except you have the brace and a touch more camber (I have the IE fixed plates too). How much did you have to slot the strut tower by to get to 2 deg?
I have the RSB on the softest setting and been very happy with it there. Part of the “happiness” is knowing I have a certain stability if I have to lift off the throttle in the middle of a turn and, even so, I also have a car that is responsive and very fun to drive.
As for your your question about lifting the inside wheel, from what I have see in watching the IMSA ST MINIs, it is the sway bar that is holding the wheel up off the ground. It is not at full extension.
And, yes, it is about having fun. Motor on!
I have the RSB on the softest setting and been very happy with it there. Part of the “happiness” is knowing I have a certain stability if I have to lift off the throttle in the middle of a turn and, even so, I also have a car that is responsive and very fun to drive.
As for your your question about lifting the inside wheel, from what I have see in watching the IMSA ST MINIs, it is the sway bar that is holding the wheel up off the ground. It is not at full extension.
And, yes, it is about having fun. Motor on!
#40
03-24-2019, 04:57 AM
Ok, just finished the video. Perfect. Right at my level of understanding.
And, he confirmed a number of things I thought I was seeing/feeling with my car.
I also like how he talked about how he sets up his car to understeer going into a turn, and uses trail braking to create the oversteer necessary to make the turn quickly. This is a practice I use. As he said, a front wheel drive car will always understeer with gas on. With a FWD car and without trail braking, it is likely that gas will need to be applied before the apex and that will result in the car understeering around the corner.
I was, however, surprised as to how readily he glossed over the need for springs in combination with sway bars. He runs just stiff springs and leaves open the question as to the need for sway bars at all. For this he would have only needed to look at the primary purpose of each. That is, the primary purpose of the springs is to support the weight of the car. The primary purpose of the sway bar is to generate roll stiffness. A side benefit of springs is the generation of roll stiffness. But the sway bars are more effective at generating roll stiffness than springs. That is they need a lower spring rate to generate a unit of roll stiffness as compared to springs alone. This would lead to more easily creating roll stiffness (flatter cornering) and changes in understeer/oversteer, without compromising compliance of the car. At least that is my understanding.
Thanks for posting...
I had one observation, and one disagreement with regard to the content.
The observation is simply that the discussion of camber, and the effect of body roll, apparently assumed a McPherson strut (no mention of unequal A arms - leaves out God's Chariot)
The disagreement is the good Professor notes that static negative camber can reduce the contact patch - not so I believe! I think the contact patch is determined solely by the vertical load divided by the tire pressure. In my car: LF: 800/40 = 20 sq in.. IMHO what changes is the shape, not the area, in response too mad camber.
Cheers,
Charlie
#41
03-24-2019, 06:19 AM
Charlie,
I remember years ago when car magazines like Car and Driver and Road & Track would do tire tests. They would do actual ink and paper prints of the contact patch of the various tires. These were definitely different for different widths of tires, which your equation would say, that could not be.. I also know that as a tire is tilted (camber), it loose traction under braking and acceleration. This was a particular problem with the ‘60s Z28 Camaro Trans Am race cars when they added enough camber to make it corner well and becomes a problem with MINIs with camber much above 3 deg. There is more going on in a tire in its ability to support the weight of the car than just a what is going on with a simple latex balloon support a weight, to which your equation would apply. That equation doesn’t account for the belts of steel, nylon, Kevlar and what not, and how they affect how a tire performs. I know this anecdotal and I wish I could put my hands on an exact proof, but I believe the video is correct. Someone might know of an experiment where this was measured.
I remember years ago when car magazines like Car and Driver and Road & Track would do tire tests. They would do actual ink and paper prints of the contact patch of the various tires. These were definitely different for different widths of tires, which your equation would say, that could not be.. I also know that as a tire is tilted (camber), it loose traction under braking and acceleration. This was a particular problem with the ‘60s Z28 Camaro Trans Am race cars when they added enough camber to make it corner well and becomes a problem with MINIs with camber much above 3 deg. There is more going on in a tire in its ability to support the weight of the car than just a what is going on with a simple latex balloon support a weight, to which your equation would apply. That equation doesn’t account for the belts of steel, nylon, Kevlar and what not, and how they affect how a tire performs. I know this anecdotal and I wish I could put my hands on an exact proof, but I believe the video is correct. Someone might know of an experiment where this was measured.
#42
03-24-2019, 10:12 AM
Moderator
Regarding camber and contact patch shape/area, you have to take sidewall stiffness into account. I think this also means that the distribution of force across the contact patch is uneven at anything other than 0 camber, so negative camber would mean greater force on the inside edge and less on the outside edge. This is supported by the inside edge wearing faster with high camber.
This is wandering far off topic but there's this: http://www.cambertire.com/
Maybe I'll start a thread in the Competition section on suspension setup and theory.
This is wandering far off topic but there's this: http://www.cambertire.com/
Maybe I'll start a thread in the Competition section on suspension setup and theory.
#43
03-24-2019, 10:38 AM
Regarding camber and contact patch shape/area, you have to take sidewall stiffness into account. I think this also means that the distribution of force across the contact patch is uneven at anything other than 0 camber, so negative camber would mean greater force on the inside edge and less on the outside edge. This is supported by the inside edge wearing faster with high camber.
This is wandering far off topic but there's this: http://www.cambertire.com/
Maybe I'll start a thread in the Competition section on suspension setup and theory.
This is wandering far off topic but there's this: http://www.cambertire.com/
Maybe I'll start a thread in the Competition section on suspension setup and theory.
One question that I have never seen addressed, is the shape of the contact patch and what interaction that has with respect to the traction circle. Any effects of this nature must arise from the squirming and dynamic nature of rubber. The devil, as always, lies in the details.
If my 17x225/45 rubber creates a contact patch 9" wide and 2.?" long for a 20 square inch area, does that provide better lateral force than longitudinal?
Cheers,
Charlie
#44
03-25-2019, 10:00 PM
Moderator
Check out this article: http://downloads.optimumg.com/Technical_Papers/RCE2.pdf (and there's more here: http://www.optimumg.com/technical/technical-papers/ ) Yes this is the sort of stuff I read.
The important things here for this discussion: The forces at the tires can be (for computational simplicity) summed to an equal and opposite force at the car's CoM/CoG. That's Newton's 3rd law. From the forces at the CoG, the rest is calculation of the moments. Prof. Marziali discusses all of this (with simpler math) starting at about 0:32:30 in the discussion of trail braking, and 0:50:00 for turning and the analysis of why swaybars change the understeer/oversteer balance.. These are (in my opinion) the best parts of the video.
Note that the CoG isn't "fluid" and doesn't move, just the loading on the wheels changes.
The important things here for this discussion: The forces at the tires can be (for computational simplicity) summed to an equal and opposite force at the car's CoM/CoG. That's Newton's 3rd law. From the forces at the CoG, the rest is calculation of the moments. Prof. Marziali discusses all of this (with simpler math) starting at about 0:32:30 in the discussion of trail braking, and 0:50:00 for turning and the analysis of why swaybars change the understeer/oversteer balance.. These are (in my opinion) the best parts of the video.
Note that the CoG isn't "fluid" and doesn't move, just the loading on the wheels changes.
Last edited by squawSkiBum; 03-25-2019 at 10:23 PM.
#45
03-25-2019, 10:16 PM
Moderator
I agree, that the distribution of pressure changes with camber, but a detailed map of the distribution thereof is something I have never seen. I also note that running -2 degrees of camber does not help acceleration and braking. Nonetheless basic physics compels me to adhere to the mathematics of opposing forces, according to Newton.
One question that I have never seen addressed, is the shape of the contact patch and what interaction that has with respect to the traction circle. Any effects of this nature must arise from the squirming and dynamic nature of rubber. The devil, as always, lies in the details.
If my 17x225/45 rubber creates a contact patch 9" wide and 2.?" long for a 20 square inch area, does that provide better lateral force than longitudinal?
Cheers,
Charlie
One question that I have never seen addressed, is the shape of the contact patch and what interaction that has with respect to the traction circle. Any effects of this nature must arise from the squirming and dynamic nature of rubber. The devil, as always, lies in the details.
If my 17x225/45 rubber creates a contact patch 9" wide and 2.?" long for a 20 square inch area, does that provide better lateral force than longitudinal?
Cheers,
Charlie
Thinking about it, my theory is that we think of the contact patch as having equal pressure across it's area, but that isn't correct since the tire is round and has inherent stiffness from its construction. So the normal force on the road is different across the contact patch. A narrow tire will have a long, skinny contact patch where the maximum force on the road is at the point directly under the center of the wheel, and decreases from that point towards the front and rear. A wider tire will have a wider and shorter (rear to front) contact patch, so more area at maximum pressure under the center of the wheel, and if you do the integral of F= u*N(x,y) you end up with a higher overall friction force (u is the coefficient of friction, N is the normal force at point x, y). That's a theory - don't know if it is correct or not.
#46
03-26-2019, 03:26 AM
This is a question that I have never seen a good answer to. Simply stated - why do wider tires provide better lateral acceleration than skinny tires, assuming the area of the contact patch is the same area (Force at the corner divided by tire psi.)
Thinking about it, my theory is that we think of the contact patch as having equal pressure across it's area, but that isn't correct since the tire is round and has inherent stiffness from its construction. So the normal force on the road is different across the contact patch. A narrow tire will have a long, skinny contact patch where the maximum force on the road is at the point directly under the center of the wheel, and decreases from that point towards the front and rear. A wider tire will have a wider and shorter (rear to front) contact patch, so more area at maximum pressure under the center of the wheel, and if you do the integral of F= u*N(x,y) you end up with a higher overall friction force (u is the coefficient of friction, N is the normal force at point x, y). That's a theory - don't know if it is correct or not.
Thinking about it, my theory is that we think of the contact patch as having equal pressure across it's area, but that isn't correct since the tire is round and has inherent stiffness from its construction. So the normal force on the road is different across the contact patch. A narrow tire will have a long, skinny contact patch where the maximum force on the road is at the point directly under the center of the wheel, and decreases from that point towards the front and rear. A wider tire will have a wider and shorter (rear to front) contact patch, so more area at maximum pressure under the center of the wheel, and if you do the integral of F= u*N(x,y) you end up with a higher overall friction force (u is the coefficient of friction, N is the normal force at point x, y). That's a theory - don't know if it is correct or not.
Cheers,
Charlie
#47
03-26-2019, 07:05 AM
I still need to read the papers you put links to, but I came across this post about contact patch size. The writer claims this is data that came from a tire manufacturer. He “talks” a lot, which you can skip over, but he does post the data he has:
http://www.enginebasics.com/Chassis%...t%20Patch.html
Food for thought...
http://www.enginebasics.com/Chassis%...t%20Patch.html
Food for thought...
#48
03-26-2019, 10:12 PM
Moderator
#49
03-26-2019, 10:13 PM
Moderator
Started a new thread here: https://www.northamericanmotoring.co...ml#post4459312 to carry on the discussion since this thread has wandered far off topic. Hopefully some Gen1 and Gen3 people will join in.
Thread
Thread Starter
Forum
Replies
Last Post