OK, please explain. Understeer happens when the front tires lose grip before the back tires do. So how does adding a RSB reduce understeer? I realize it does, just not the physics behind it.

Thanks!

 

It occurred to me that I also did not fully understand the topic, so I went here to find out: http://en.wikipedia.org/wiki/Understeer and was happy to see the MINI's place on the list in the entry. Our cars are in some pretty good company!

 

In short, the RSB causes the inside rear to loose traction a bit. You net less traction in the rear and some of the cornering force is transmitted to the outside front tire. Assuming that tire is still within its adhesion limit, it will force the nose into the corner better.

dan

 

I just pulled out my notebook from a course I took last semester, "Vehicle Dynamics". I remembered that stuff with roll stiffness didn't seem intuitive at the time (and still are a little fuzzy). As to the part talking about roll stiffness changes and how it changes handling:
To this day I'm not certain I understand this part of the course (everything else I understood but the part I wanted to understand - the handling part - gave me issues). Based on the notes, putting on a stiffer rear bar will cause an increase in weight shift in the rear and a decrease in the weight shift in the front. Again, my notes cause me to think this change in weight shift could change suspension geometries.

I just sketched a free body diagram (FBD - a simplified picture of a system showing forces acting on it) and I think that since a lower weight shift in the front would cause the front, inside tire to have a larger weight on it than before, it would also have a larger friction force pushing it toward the center of the turn than with the smaller rear bar. Because of the angles of the front wheels, this would cause a greater rotating torque on the car, causing it to ratate faster and yeild less understeer.

Edit: my sketch assumed 0 toe on the front and rear alignment. toe in either direction will mess with my very basic sketch, but without knowing weight distrubition of the car, measurements, and alignment settings, it's the best model I can make.

edit 2: as minibeel said, the inside rear would lose some traction due to the increased rear weight transfer. Depending on tire adhesion limits of the rear tires, it could be possible to overload the outside rear causing it to break loose and rotate more.

 

For suspension tuning see
http://www.rallylights.com/other/stuning.htm#sway

You can reduce understeer (very common in front wheel drive cars)
by increasing weight transfer at the rear by increasing rear roll stiffness (a stiffer rear sway bar- which can be adjustable with various holes).

And you can reduce understeer by softening the front sway bar which reduces weight transfer at the front.

Do one and you get some change. Do both and you get more change.

Also see
http://books.google.com/books?id=cr4...Xc7k#PPA108,M1

 

The way I like to simplify the whole rear swaybar function is like this:

In stock form, the front tires are asked to to the majority of the "work", which is why they give up first, causing understeer.

Add a big fat rear swaybar and it allows the rear tires to do more work, taking some of that work away from the fronts. You are not reducing traction at the rear at all, you are asking the rears to do more work, bringing them closer to their limit of adhesion. It's not just a "feel" thing, making the car more neutral- a big rear bar also increases the steady state cornering ability of the vehicle (you can carry more speed in the same corner). Of course, there are hundreds of variables involved, and this is a gross oversimplification.



On a bit more theoretical standpoint:

The most fundamental concept in vehicle dynamics that makes this all work, and the most counter-intuitive concept for me to understand when I was learning this stuff is the concept of tire load sensitivity.

Basically, the more you load a tire, the lower it's coeficient of friction.
This means that if your car understeers, you don't want to push down harder on the outside front tire, that will only make things worse.- you actually want to push softer on the front tire and push harder on the outside rear tire. It's all about balancing the work load of the tires.

Jason

 

Basically it helps keep the inside front tire planted in the turn.

 

not true.

this is the equation for friction:

• Fr is the resistive force of friction
• μ is the coefficient of friction
• N is the normal force pushing the two objects together.

as you can see, for higher values for N (weight, in this case), Fr also increases.

in other words, the more weight, or the harder you push down on a tire, the higher the friction. that's why lifting the throttle mid-turn will make the rear end come around; weight is transfered to the front, increasing traction in the front and decreasing the in rear.


i also had a hard time trying to visualize what is happening with the swaybar situation. here's how i made myself understand weight transfer and how it affects traction:

you ever see a car lift the inside rear wheel in a corner? there is no weight on that tire now. where did the weight go? to the other three tires. the majority goes to the outside front tire, increasing its friction (grip). that's how a stiff REAR bar increases traction in the FRONT.

 

I never said that Fr didn't increase with N, I stated that coefficient of friction decreased with increasing N.

Your equation is 100% correct, I will agree with you there. What you are incorrectly assuming is that μ remains constant with regards to normal force. Tire behavior is funny, as you increase N, μ will fall off. Yes, Fr does increase with N, but at a decreasing rate. In other words, two equally loaded tires can create more total frictional force than two tires unequally loaded.

I disagree with this one. A stiffer rear bar will DECREASE the load on the outside front tire, along with the inside rear tire. The payoff on this is the INCREASE loading on the outside rear, and inside front. Think diagonally here, for example, if you were to lift the right rear tire far enough off the ground with a floor jack, you could balance the car on two tires- right rear and left front. The other two tires would have zero load at that point.


Jason

 

There is a LOT going on and many actually move the wrong direction in tha battle against UNDERSTEER.
here is a good photo with a stiff suspension setting, and a H sport bar set on STIFF

The weight is on 3 tires (most of it on drivers front) and if I backed off the gas the rear would come around as the is no or very little weight on the rear tires.
This does open the question as to what the contacring tire can handle and the suspension settings are critical.
Many put on a stiffer front bar ... and this woud need an even STIFFER rear bar.
I have NOT done it but there are auto-x ers that disconnect the front bar...in essence this would relitivley make the rear bar very stiff...
but they are called ANTI ROLL bars for a reasion...the body roll would be very great

This photo indicated to me that the rear bar is doing its job.. if i was to make a change, I would stiffen the front shocks to help keep BOTH front tires more firmly planted

 

But stiffening the front shocks will make the car more prone to understear. Your saying you would give up a little front grip so you don't spin up the inside front on corner exit? No LSD?

Alan

 

One thing to remember- shocks are used for tuning transitions

springs/bars are used to tune steady state.

 

I also think that a lot of folks get a little lost when thinking about how a swaybar works because we sometimes think about weight transfer as a pushing or lifting. If we examine all the swaybar endlinks in existence, we might think otherwise; these tiny little things cannot possibly handle the compressive loads generated as a car corners. These are designed to work in tension and therefore pull the car down as weight is transfered. And this is how, as Jasonsmf wrote, the inside front wheel gains more potential cornering power due to additional weight transfer. Picture this another way; as a car corners there is a rotational speed about the car's axis - centroid axis if you like. As the bar is pulling the front end down it is also causing the car to spin about its centroid axis faster. If the bar transfered more weight to the outside front wheel, the car would fail to rotate.

And remember too that once the inside rear wheel leaves the ground, there is no more weight transfer. Fine tuning with a swaybar will help to achieve that fine point where the inside rear tire is just barely in contact with the ground.

I've been a dormant contributor here latley, but the level of expertise within NAM is definately on the rise. Nice info above!!!

 

Thanks Jason, that was very well-put and helped me to "see" what is going on more clearly. Makes total sense now that it has been explained.

In an issue of Car and Driver recently, they did some suspension mods to a Subaru Forrester, I think. They took a picture of it in full oversteer, and I found it interesting that the body was leaning towards the inside front tire. A very odd image, but it is clear that the suspension tuning was forcing the weight to transfer heavily over the inside front tire.

I get it now.

 

jasonsmf--> i get it. now i see how the rear swaybar loads the inside front tire. i just had to wrap my head around that one with a little thought experiment, but now it makes sense. i was just focusing too hard on those three wheeled turns (like the photo above) and seeing that outside tire squished down like that... threw me off..

however...

meb--> your swaybar theory is not making sense. link rods work in both tension and compression. one side pulls on the swaybar, the other side pushes. there is no other way. how else can you twist a swaybar if not buy pulling the ends in opposite directions?

succubus--> what you were seeing was a tail happy car being corrected by application of opposite lock. the car was actually sliding, and what looks like the inside front tire is actually the outside front tire. that was not caused by the suspension "tuning".

 

Right the photo is NO LSD ... so I could soften up the rear bar , but I was set at 15 (mid way ) on the shocks at that time.
I went to 25 (closser to stiff) ant the initial transition kept the wheel right at ground, and a bit more weight on the inside front.

still ..the wrong setting did make for a better photo

 

When the sway bar is loaded on one side it actually pulls up on the opposite side (pulling upwards on the tire), keeping the car flatter and reducing weight transfer to the outside.

 

right, it pulls up on one side, but the other side, the side that is "loaded up" as you say... that is being PUSHED by the link rod. that's the only point i'm trying to clear up. link rods have pulling and pushing action, not just pulling.

 

Exactly, when the swaybar is compressed on one side the link is pushing up on the sway bar, as this is happening the sway bar is pulling up on the opposite link, trying to mimick the same compression on the other side.Think of the sway bar as a lever.

 

However, I think, and I'm not an engineer, that the action is at the tension end...the pulling down on one side causes the twisting. I don't believe that there is a compressive force per se, but I could be wrong. The problem I have, and I am completely sympathetic to your reply sonichris, is that endlinks cannot absorb a lot of compression. I may not have a clear image of the total forces acting on these links, but I don't think these forces are small.

I understand that as a car turns, the outside spring and damper compress but the length of the endlink does not change.


Compounding things up front, the mini's endlinks (upper joint) must rotate with the strut as the steering wheel is turned - following the SAI.

 

the endlinks DO rotate with the strut, that's why they have ball joint ends.

as far as the forces on the links, one side pulls down, the other side HAS to have an equal but opposite force on it. that's just physics. you see, the only thing holding the swaybar in place (besides the bushings) is... the other endlink.

the links can withstand compresive forces becuause the force is transmitted directly through the middle, along it's axis. if the link was bent, even just a tiny bit, with enough load it would bend in two and snap. that's why aftermarket links are beefier, to withstand the increased load of stiffer swaybars.

 

The end links will see equal force butopposite loading (ignoring friction in the bushings...). One in tension, the other in compression. As long as the ball joint in the end link is not binding, the endlink body itself is a purely tension/compression member there is no bending involved. I have run some numbers on stock endlinks before I made my own adjustables using the stock ball joints- I can tell you that the failure mode of stock end links is that the compression side endlink will buckle LONG before the tension side snaps. Even if the stock endlinks hold up to a bigger sway bar, the compression side endlink is most likely flexing with the increased force of the bigger bar, reducing the effictiveness. Stiffer endlinks will allow the bigger swaybars to do their job more efficiently.

Jason

 

I'm still not so sure that these must work in a tension/compression relationship...again, with the caveate that I am not an engineer.

Why can't the awaybar on one side pull down while the damper or strut on the other side pulls up? These are opposite forces for sure, but still in tension.

One of the chief reasons I like the Powergrid endlinks is that they are robust, have ball joinsts and are very easy to adjust.

I left Chip Mink a message regarding endlinks - he is the owner engineer for Powergrid.

 

Here is my understanding on how this all works:
In it's free state, the swaybar is free to rotate in it's bushings.
If the endlink on one side of the swaybar is pulling down (endlink in tension), this creates a torque trying to rotate the swaybar in it's bushings.
If this is the only torque acting on the sway bar, it will start to spin around in the bushings (until it hits something that stops it.....)
In order to keep the swaybar in place, and not spinning around in circles, an opposite torque must be applied to the bar to cancel things out.
This is accomplished by the endlink on the opposite side pushing up with the same amount of force, but in the opposite direction (endlink in compression).

There are only two points where a force can be applied to the swaybar- the end link attachment points on either side. All of the forces are transmitted thru the endlinks.

Jason

 

Yes, very intuitive to me too Jasen. But I cannot help but wonder about the delicate design found in most endlinks...I also don't know the exact forces acting on these. By analogy, a lay person might expect a 4"x12" piece of lumber to support a load that can be sustained by a 2"x6". I may have a completely distorted view of endlink operation... And this wouldn't be the first time.

I guess another way of looking at this is if both ends work in tension, then perhaps there is no resistance to sway...