Suspension Mini Mania Positive Steering Amplifier (PSA)
Ummm...
I'm gonna have to agree with Rye on this, but the degree is what's open for debate....
What it comes down to is you trade some degree of bumpsteer for the changes in roll coupling that happens when you lower the car. Really, the biggest mistake I see in lots of street cars is lower the car > Much stiffer springs to supress roll and bottoming > leads to a car that's no fun to drive on most streets. While one can find that induced bump steer is too much for some, not that bad for others, it does fight the number one misconception out there in lowering cars.
Perfect? No, woth the $100+ Mini Mania is charging for it? That's up to the buyer.
The guy who sent it to Don found it helped a lot in AutoX, but that's pretty flat with hard turns. Maybe he found that the effect was more beneficial than detrimental in that use case. I liked it while it was on my car. But then, my car is a couple day a year track car, so I'm not pushing the envelope the way Greg or other track rats are.
Matt
What it comes down to is you trade some degree of bumpsteer for the changes in roll coupling that happens when you lower the car. Really, the biggest mistake I see in lots of street cars is lower the car > Much stiffer springs to supress roll and bottoming > leads to a car that's no fun to drive on most streets. While one can find that induced bump steer is too much for some, not that bad for others, it does fight the number one misconception out there in lowering cars.
Perfect? No, woth the $100+ Mini Mania is charging for it? That's up to the buyer.
The guy who sent it to Don found it helped a lot in AutoX, but that's pretty flat with hard turns. Maybe he found that the effect was more beneficial than detrimental in that use case. I liked it while it was on my car. But then, my car is a couple day a year track car, so I'm not pushing the envelope the way Greg or other track rats are.
Matt
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I didn't think of this Doc...so as we measure geometry, all things considered equal except the lower ball joint location, scrub radius/king pin changes.
Some of the improved feel others are posting about may come as a result of altering RC for the better - duh! But be careful folks as weight transfer can be an aid as well...as the rc gets closer to the center of gravity, it will begin to try to flip itself over the center of gravity having the exact opposite affect, the removal of weight transfer - not a scientific string of words, but written in a way that hopefully makes simplistic sense. I believe this is the reason Greg is so concerned about mis-applying this mod. In some scenerio, a wheel will actually lift off the ground...
Some of the improved feel others are posting about may come as a result of altering RC for the better - duh! But be careful folks as weight transfer can be an aid as well...as the rc gets closer to the center of gravity, it will begin to try to flip itself over the center of gravity having the exact opposite affect, the removal of weight transfer - not a scientific string of words, but written in a way that hopefully makes simplistic sense. I believe this is the reason Greg is so concerned about mis-applying this mod. In some scenerio, a wheel will actually lift off the ground...
the steering ratio is unchanged. The only thing that could change how much input vs directional change would be something having to do with scrub or understeer/oversteer or something like that. I'd have to think about it but I'd have to say that nothing jumps to mind right now, unless your toe is so whacked out that it's comeing into play. Geometrically, there is a very, very, very tiny change to steering ratio, but it's a second order effect and drops in the first order approximation.
Matt
ps, I liked em too! But steering feel is very subjective.
Matt
ps, I liked em too! But steering feel is very subjective.
Last edited by meb; Aug 12, 2008 at 06:14 AM.
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From: Orcutt, CA
Here is a summary of the July-August MC2 Magazine article. From the "The Hows and Whys of Suspensions Gone Bad" article:
"...we chose the spring or the coil-over setup we think is hot...but we have already made our first mistake...Because something happensto the way the car leans in turns that is due to the contact points on the chassis. The car's roll center gets lower. It goes down more than the center of mass does and this has an interesting effect. It increases the forces that lean the car in turns. An odd outcome of this is that if your spring rates are the same as stock, the lowered car leans more in the turns that (sic) an unlowered car!" ... "There are some parts that help with the roll center motion as a car is lowered. Mini mania has come out with something they call the Precision Steering Amplifier, a spacer that goes between the lower ball joint and the steering knuckle. For cars lowered about 1-inch, this restores the lower control arm to stock location, keeping the roll center unaltered. This means lowered cars won't increase the roll coupling as much as those without the part, so the need for stiffer front bars is reduced or eliminated." ... "Here's a decent plan of attack for improving real world handling without really creating too stiff and bumpy a ride. First off, don't severely lower the car if you want a compliant suspension. But if you have to do it for whatever reason, make sure to do something about the roll center motion." Item two was add a bit more front camber. Item three was drop unsprung weight. Item four was don't use too big a rear sway bar. Item five was if you change springs keep rates below about 260 ibs/in. From a picture caption "These are Mini Manias Precision Steering Amplifier. They restore lower control arm geometry on cars lowered about 1-inch. They take about 10 minutes per side to install."
"...we chose the spring or the coil-over setup we think is hot...but we have already made our first mistake...Because something happensto the way the car leans in turns that is due to the contact points on the chassis. The car's roll center gets lower. It goes down more than the center of mass does and this has an interesting effect. It increases the forces that lean the car in turns. An odd outcome of this is that if your spring rates are the same as stock, the lowered car leans more in the turns that (sic) an unlowered car!" ... "There are some parts that help with the roll center motion as a car is lowered. Mini mania has come out with something they call the Precision Steering Amplifier, a spacer that goes between the lower ball joint and the steering knuckle. For cars lowered about 1-inch, this restores the lower control arm to stock location, keeping the roll center unaltered. This means lowered cars won't increase the roll coupling as much as those without the part, so the need for stiffer front bars is reduced or eliminated." ... "Here's a decent plan of attack for improving real world handling without really creating too stiff and bumpy a ride. First off, don't severely lower the car if you want a compliant suspension. But if you have to do it for whatever reason, make sure to do something about the roll center motion." Item two was add a bit more front camber. Item three was drop unsprung weight. Item four was don't use too big a rear sway bar. Item five was if you change springs keep rates below about 260 ibs/in. From a picture caption "These are Mini Manias Precision Steering Amplifier. They restore lower control arm geometry on cars lowered about 1-inch. They take about 10 minutes per side to install."
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From: Orcutt, CA
I believe the key issue here is whether or not the PSA is a good part to add for an 1-inch lowered Mini. Occording to MC2 (Matt), it should be added to correct the geometry. Does it do a proper job of correcting the geometry after the car is lowered 1-inch, without causing an other ill affects?
Based on the suspension suggestions in the MC2 article the PSA would be used on a car with Adj. Camber Plates (let us assume -2 deg in front and -1 deg in rear), 19mm Adj Sway Bar, and slightly stiffer than stock springs (used to lower the car). Let us all assume that the track is based on using 38mm offset wheels or spacers to obtain an equivalent offset (15mm spacers with 52mm offset stock wheels).
Should the PSA be used on a car with the above suspension mods or is it just a waste of money with these other mods?
Based on the suspension suggestions in the MC2 article the PSA would be used on a car with Adj. Camber Plates (let us assume -2 deg in front and -1 deg in rear), 19mm Adj Sway Bar, and slightly stiffer than stock springs (used to lower the car). Let us all assume that the track is based on using 38mm offset wheels or spacers to obtain an equivalent offset (15mm spacers with 52mm offset stock wheels).
Should the PSA be used on a car with the above suspension mods or is it just a waste of money with these other mods?
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Roll center and center of gravity relationships are - outside of mass displacement - the most fundament of concerns. All else is secondary and depends upon RC and CofG evaluation.
...you could just try it...part of the fun at some level...and part of the educational expense
...you could just try it...part of the fun at some level...and part of the educational expense
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From: Orlando, FL
That could be an interesting topic. I'm not a suspension guy but being on a Formula SAE team for the past couple years makes me understand the importance of suspension geometry. I wonder how far you have to push the wheels out to counteract the roll center change of a mildly lowered MINI. It would be nice to get all the measurements for the suspension and do front/side view geometries in Solidworks or Pro-E to see the effects of these changes.
It's easier than that...
There are very nice suspension modelling programs out there for ~$500 that are all set up to do all the nice calculations for you already. If you've got SolidWorks or Pro-E already, then you still have to set it up to do all the calcs for you in the motion analysis modules. With the suspension programs, put in the geometry and get all the relevant calcs done pronto, no real set up needed.
Different ways to skin the same cat.....
Matt
Different ways to skin the same cat.....
Matt
We ALL need to go back to highschool
Motion ratio is the ratio of the spring comression (or expansion) to wheel motion. Nice little graphic in here.
http://www.worksevo.com/Spring_Rates_1.pdf
Ryan, I think you got it backward! Mac Strut track changes affect MR. Trailing Arm, not so much so.....
Matt
http://www.worksevo.com/Spring_Rates_1.pdf
Ryan, I think you got it backward! Mac Strut track changes affect MR. Trailing Arm, not so much so.....
Matt
No matter where the axle is, or where the tire is located in a MacPherson Strut, the motion ratio is always the same. The only thing that can change motion ratio on Mac struts is the angle of the strut. In Works document, their little drawing of D1/D2 is highly misrepresentative as they show a strut setup, which is n/a in that equation! Even in Works document they say that with their formula:
[1-[strut angle/360]]x100 = MR of strut
The reason the strut is the only variable is it's the only part of the suspension that acts on the spring. The control arm in a strut suspension only locates the knuckle in terms of camber and caster, it does not leverage any part of the spring load in a coilover strut like the MINI.
The motion ratio of the front axle is 0.99:1 because the front strut is on a very small angle in relation to the tire. The rear motion ratio is 0.875:1, just FYI.
[1-[strut angle/360]]x100 = MR of strut
The reason the strut is the only variable is it's the only part of the suspension that acts on the spring. The control arm in a strut suspension only locates the knuckle in terms of camber and caster, it does not leverage any part of the spring load in a coilover strut like the MINI.
The motion ratio of the front axle is 0.99:1 because the front strut is on a very small angle in relation to the tire. The rear motion ratio is 0.875:1, just FYI.
Been thinking about this more...
and what Ryan says is only true if you have solid bushing at the top of the struts. If you increase the track width you increase the lateral force on the strut guide bushing, and if it's rubber, it will move a bit. How much? Depends on the track change, the bushing stifness and the like, I'm guessing its a small order effect, so for street driving can be ignored. For race driving, there are probably solid bushings there, so it doesn't apply! But strictly speaking, it does have a small effect.
But whatever the magnitude of the effect, it's much less than just assuming that the difference in track length goes straight into the MR calculation, that is for sure!
Matt
But whatever the magnitude of the effect, it's much less than just assuming that the difference in track length goes straight into the MR calculation, that is for sure!
Matt
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Not so...when the tire's center line is moverd father away from the pivot points(s), the ground has more mechanical leverage on the tire - just like a see saw. Changing the angle of the strut via the upper mount may also equat into changing motion ratio, but this change will alter the strut's velocity curve and also alters RC.
Motion ratios must take into account wheel offset or back space...the tire's relationship is very important. I've seen a number of motion ratio formulas that do not consider the tire's geometric relationship the lever arm and I think this is misleading.
Motion ratios must take into account wheel offset or back space...the tire's relationship is very important. I've seen a number of motion ratio formulas that do not consider the tire's geometric relationship the lever arm and I think this is misleading.
No matter where the axle is, or where the tire is located in a MacPherson Strut, the motion ratio is always the same. The only thing that can change motion ratio on Mac struts is the angle of the strut. In Works document, their little drawing of D1/D2 is highly misrepresentative as they show a strut setup, which is n/a in that equation! Even in Works document they say that with their formula:
[1-[strut angle/360]]x100 = MR of strut
The reason the strut is the only variable is it's the only part of the suspension that acts on the spring. The control arm in a strut suspension only locates the knuckle in terms of camber and caster, it does not leverage any part of the spring load in a coilover strut like the MINI.
The motion ratio of the front axle is 0.99:1 because the front strut is on a very small angle in relation to the tire. The rear motion ratio is 0.875:1, just FYI.
[1-[strut angle/360]]x100 = MR of strut
The reason the strut is the only variable is it's the only part of the suspension that acts on the spring. The control arm in a strut suspension only locates the knuckle in terms of camber and caster, it does not leverage any part of the spring load in a coilover strut like the MINI.
The motion ratio of the front axle is 0.99:1 because the front strut is on a very small angle in relation to the tire. The rear motion ratio is 0.875:1, just FYI.
Think of it this way...
Picture an infinitely long axle coming out of the knuckle that is parallel to the ground. No matter where the wheel is on it, one inch of motion up or down equates to a one inch displacement of the knuckle, hence the MR stays the same.
In the case of our cars, there's a bit of negative camber, so as you move the wheel out the car gets a little lower. Independant of camber, you change the torques for steering, and you change the track for weight transfer.
The analogy with the see-saw is valid, but for torques about the lower ball joint that leads to lateral forces on the upper strut bushing.
Matt
In the case of our cars, there's a bit of negative camber, so as you move the wheel out the car gets a little lower. Independant of camber, you change the torques for steering, and you change the track for weight transfer.
The analogy with the see-saw is valid, but for torques about the lower ball joint that leads to lateral forces on the upper strut bushing.
Matt
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...there is a torque being applied, by the ground...I should have written wheel rate and not motion ratio. You and Ryan are correct, motion ratio relates to fixed points on the suspension. However, wheel offset or its distance from the ball joint will affect wheel rate, what the wheel feels.
Sorry for mixing terms...but they are related and I am old
And, what's up with NAM? I've been trying to get on all day to correct myself...NAM just doesn't load well at all lately.
Sorry for mixing terms...but they are related and I am old
And, what's up with NAM? I've been trying to get on all day to correct myself...NAM just doesn't load well at all lately.
Sent you a PM Michael.
Wheel Rate/Spring Rate = [motion ratio]^2
When motion ratio is inner pivot to spring pivot over inner pivot to outer pivot
Wheel offset on a strut suspension does not impact motion ratio, thus does not impact wheel rate. Wheel offset does however impact mechanical leverage, meaning wheel bearings, strut damper shafts, and ball joints are stressed more then the same wheel without spacers. Remember that the wheel, hub, knuckle, and strut move as one unit and there is no pulley action or mechanical advantage going on besides the angle of the strut versus the wheel plane of rotation.
Cheers,
Ryan
Wheel Rate/Spring Rate = [motion ratio]^2
When motion ratio is inner pivot to spring pivot over inner pivot to outer pivot
Wheel offset on a strut suspension does not impact motion ratio, thus does not impact wheel rate. Wheel offset does however impact mechanical leverage, meaning wheel bearings, strut damper shafts, and ball joints are stressed more then the same wheel without spacers. Remember that the wheel, hub, knuckle, and strut move as one unit and there is no pulley action or mechanical advantage going on besides the angle of the strut versus the wheel plane of rotation.
Cheers,
Ryan
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From: Magnolia, Texas
Very interesting thread...
I'm going to have to go back to my library and dust off some of the suspension design books I have. Do any of you have recommendations for suspension design & engineering books that would shed some light on the issues touched on here?
Things were much simpler when all you had to do was remember to keep the sliding pillars on your Morgan, or the sliding trunions on your MG lubricated.
I'm going to have to go back to my library and dust off some of the suspension design books I have. Do any of you have recommendations for suspension design & engineering books that would shed some light on the issues touched on here?
Things were much simpler when all you had to do was remember to keep the sliding pillars on your Morgan, or the sliding trunions on your MG lubricated.
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From: Orlando, FL
Carrol Smith has a lot of good info in his "to win" series. Engineer To Win and Tune To Win should have some good info. I don't have my copies in front of me so I can't check. Also, Race Car Vehicle Dynamics will have the info you need. It's virtually the bible in Formula SAE. $$$ though.
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From: Magnolia, Texas
Thanks for the suggestions. I'll check those books out. I have a well worn copy of Fred Puhn's book and also another book of the same vintage by Colin Campbell titled "New Directions in Suspension Design".
Kapps, I like your idea about putting all of the MINI suspension geometry into a CAD program. The first step would be to get all of the dimensions from a factory drawing. Although I don't have much spare time, I do have Pro/E...
Matt, I appreciate your insight and thoughtful comments throughout this thread. Thanks for giving your input after actually driving a car with the PSA. It seems to me that the only drawback to the PSA on a slightly lowered MINI is that the steering arm is no longer parallel to the lower control arm hence the bumpsteer. I think it would be interesting to calculate the actual bumpsteer effect for such a small angle of deviation. I suspect that it would back up your experience.
Thanks!
Kapps, I like your idea about putting all of the MINI suspension geometry into a CAD program. The first step would be to get all of the dimensions from a factory drawing. Although I don't have much spare time, I do have Pro/E...
Matt, I appreciate your insight and thoughtful comments throughout this thread. Thanks for giving your input after actually driving a car with the PSA. It seems to me that the only drawback to the PSA on a slightly lowered MINI is that the steering arm is no longer parallel to the lower control arm hence the bumpsteer. I think it would be interesting to calculate the actual bumpsteer effect for such a small angle of deviation. I suspect that it would back up your experience.
Thanks!
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From: Orlando, FL
I wish I had access to the drawings. I just did a similar thing in Solidworks for the rear suspension on my old Lotus Europa. It's pretty easy to set up once you have the correct data... just a 2D sketch with some defined dimensions and dependent dimensions.