Roll center location? :iono:

[d1ck]

Originally Posted by DEnd

I don't know the specifics for this car, but typically lowering the car results in an increase of the distance from the roll center to the center of gravity. This results in more body roll. This body roll results in a camber change at the wheels, this may or may not be beneficial. Whether or not it is depends on how the car behaves and which behavior you want at which ever track or corner you are trying to tune for.

Strut type suspensions typically only gain positive camber when the control arms are above horizontal. Therefore it is highly doubtful this would be beneficial.

[DEnd]

Originally Posted by d1ck

Strut type suspensions typically only gain positive camber when the control arms are above horizontal. Therefore it is highly doubtful this would be beneficial.

Lowering a strut type suspension also results in less negative camber change in bump until the control arms are horizontal. Depending on the corner you are trying to tune for it may be beneficial.

The point I was trying to make is just because it is an issue with STIs does not mean it is or will be an issue with the car, even though their suspensions are very similar. They could be exactly the same, and what benefits one car may harm the other due to differences in the height of the centers of gravity. Also the type of track/turns one is tuning for is a large difference as well.

[7thgear]

Originally Posted by brzranger

I don't think anyone's does. After dropping 1.5" i don't notice any negative or unwanted effects.

Reading about Roll center adapter (RCA) on the STI forums, some don't notice any real benefits. Some reports of broken whiteline RCA and some removing them after install.

if one took the time to calculate the amount of car enthusiasts that sit on the forums and cross reference them with how many show up at local autocross, then filter the ones that come in the top 15-20, then you'll realize that the vast majority of car enthusiasts are shitty drivers that wouldn't know what to do with a good handling car.

roll center adjustments are vital to a macstrut car, the closer it is to the center of gravity the less roll you have, less roll means you can keep your springs soft, soft springs mean mechanical grip,

more grip with less roll and soft springs = WIN

Originally Posted by DEnd

The point I was trying to make is just because it is an issue with STIs does not mean it is or will be an issue with the car, even though their suspensions are very similar. They could be exactly the same, and what benefits one car may harm the other due to differences in the height of the centers of gravity. Also the type of track/turns one is tuning for is a large difference as well.

struts are struts, it is an issue on the Sti, it is an issue on the FRS/BRZ, it is an issue on VWs, roll center balance and vehicle height is an issue on every single car out there that runs on MacPherson struts

some are better than others in stock form, but all have the same basic problems.

[wheelhaus]

IMO There's two reasons many drivers with cars lowered "too much" don't notice the effects of poor roll center or don't notice the benefit of an RCA kit.

For one, they are likely not sensitive enough and/or don't know exactly what to look for. You may not necessarily "feel" a difference in body roll (especially with stiffer springs/coilovers), but your suspension geometry will This could manifest itself as better/poorer grip in certain situations. Secondly, lowering a mac strut typically requires stiffer and stiffer springs to counteract the excessive roll forces of inverting the LCA's. Coilovers are a necessary step when you lower a car beyond a certain point, because the springs need to be stiff enough to prevent both excessive body roll and frequent bottoming, and the damping needs to be more aggressive to manage the shorter travel and higher spring forces.

The stiffer springs are improving weight transfer, so whatever negative roll center effects are present are likely masked by a different, stiffer, more responsive sensation during directional changes.

I personally don't like the design of the Whiteline RCA's, they're functional, sure, but they look like they're begging to fatigue and break. I'd at least like to see something with a wider "foot" to distribute the ridiculous lateral loads placed across those joints...

[jamal]

Originally Posted by 7thgear

roll center adjustments are vital to a macstrut car, the closer it is to the center of gravity the less roll you have, less roll means you can keep your springs soft, soft springs mean mechanical grip,

more grip with less roll and soft springs = WIN

It doesn't work that way. Using geometry to limit roll reduces grip because the damper and spring aren't actually doing any work. Lowering the CG is one thing because it reduces the amount of weight transfer. Raising the RC only creates more weight jacking and more side load on the tire which reduces grip.

Originally Posted by wheelhaus

IMO There's two reasons many drivers with cars lowered "too much" don't notice the effects of poor roll center or don't notice the benefit of an RCA kit.

The third is that most drivers aren't doing any sort of actual track testing where they take tire temperatures and measure lap times. Hence all the "OMG this suspension rocks and my car handles like a go kart!" posts. Go out and try to optimize lap times and you will very quickly find the limits of your cheap coilovers.

Also, the WL RCA doesn't provide enough correction to have to worry about the extra load breaking things. And I have seen what happens when you have too tall of a corrector first hand. When a car is lowered around an inch it is perfect though. You get some RC correction back toward stock while improving the camber curve, both which are very important on the stock strut suspension. At the same time you're not raising it way up high and reducing the effectiveness of the damper or changing how the tires are loaded.

[7thgear]

Originally Posted by jamal

It doesn't work that way. Using geometry to limit roll reduces grip because the damper and spring aren't actually doing any work. Lowering the CG is one thing because it reduces the amount of weight transfer. Raising the RC only creates more weight jacking and more side load on the tire which reduces grip.

i dunno about you but i don't live in a world of glass roads

there are plenty of bumps at even the best of race tracks to keep that suspension working at all times.

as for side load, well then it's an argument of whether you want your suspension absorbing the weight transfer or your tires, you're not in a better position in one place over another

having said that this only applies to normal cars, once aero gets into the equation it's time for super stiff

[GC86]

Originally Posted by d1ck

Strut type suspensions typically only gain positive camber when the control arms are above horizontal. Therefore it is highly doubtful this would be beneficial.

This is a common misconception, apparently because a similar statement was printed in the book "How to make your car handle" in 1978.

The camber gain on any MacPherson strut car, will change from negative to positive after the LCA becomes perpendicular to the strut inclination.

This angle will be waaay upward past horizontal.

The tire will hit the inner fender before the camber curve inflects to positive.

Ground Control

[d1ck]

Originally Posted by GC86

This is a common misconception, apparently because a similar statement was printed in the book "How to make your car handle" in 1978.

The camber gain on any MacPherson strut car, will change from negative to positive after the LCA becomes perpendicular to the strut inclination.

This angle will be waaay upward past horizontal.

The tire will hit the inner fender before the camber curve inflects to positive.

Ground Control

I've never thought of that before but it makes a lot of sense. I guess I've always visualized the strut being vertical which obviously isn't the case. It is worth noting however that the amount of camber gained upon compression will be less on a lowered vehicle when compared to the same car that isn't lowered.

[7thgear]

here is an animation a smart fella made a long time ago on the VW forums

this gives an example of a very bad macpherson setup and what would be a good macpherson setup... but not without it's own issues (such as having the LCA too long = flexible/brittle) or having the strut on too much of a angle and thus introducing weird wheel angles during rotation.

but you get the idea..

[DEnd]

That's an awesome animation and it clearly shows how a MacPherson strut suspension can be kept out of positive camber change with the correct geometry. Even with the LCA above horizontal. The question with this car is now is how flat does the geometry keep the tires in relation to the road during body roll, and what corrections actually need to be made when it is lowered. I'm almost tempted to set up string in my barn and convince an owner to come over... Almost.

[wheelhaus]

Wow that animation really does make it clear. Once the outer joint of the LCA goes above 90deg it still pulls inward, but the strut angle maintains increasing neg camber throughout travel. Now we need some geometry data on the front and rear of the 86!

[EarlQHan]

Your kinematic roll center does not matter (the roll center you are all referring to). It does not matter if it's located at the earth's core or on the moon. It is just a single point in space defined by the suspension at any particular given moment. In that respect, it is more like a thermometer, where it reads the temperature, but does not affect the temperature of the system. The force-based roll center is what counts. The body does not roll about the roll center. It reacts to forces and moments applied at the force application points, which is directly under the center of gravity.

If you believe the kinematic roll center to be true and the body to roll about it, then you must also believe the roll center migrates, meaning it moves vertically and laterally as the suspension moves. If this were true, and the roll center migrated outside the track of the vehicle, how could the vehicle roll about this point? All the corners would simultaneously be in either compression or tension, meaning there could be no reaction, which is an impossible physics problem.

[GC86]

Originally Posted by DEnd

That's an awesome animation and it clearly shows how a MacPherson strut suspension can be kept out of positive camber change with the correct geometry. Even with the LCA above horizontal. The question with this car is now is how flat does the geometry keep the tires in relation to the road during body roll, and what corrections actually need to be made when it is lowered. I'm almost tempted to set up string in my barn and convince an owner to come over... Almost.

It takes most of a day, two people, some special measuring tools, and a reasonably expensive program to get the numbers from an FT86 geometry into a computer.

We checked the hand written notes, and the front king pin inclination (kpi) is pretty much 14.5 degrees, and the strut is 0.546 meters long at static ride height and zero degrees.

So that will get you started.

That is a fairly low kpi, lower than previous Subarus.

Not sure why. It seems to have to do with cost and engine width more than any geometry "trick". I have been noticing that in a few places on this car suspension.

Ground Control

PS. In that cool illustration, the strut on the left is 7 degrees, and the strut on the right is 29 degrees at static ride height.

[jamal]

Probably to reduce camber loss with steering angle.