Handling and Ride Height

[belize1334]

I managed to answer my own initial question so I'm changing the direction of this thread...

When you look up optimal ride-height for performance driving on other platforms you usually get quotes about distance from hub-center to fender flare. This is a useful measure of ride-height because it is independent of wheel/tire size and reflects the underlying suspension geometry. To find this figure for our car I dug through some forums and found that Scott Yorga measured the stock suspension to have a 2-7/8" front fender gap and 2-3/4" rear gap with 195/60/15 tires. I used that information to determine that the stock ride height (assuming the accuracy of his measurements and the stated wheel tire size) is roughly...

F: 14.98"
R: 14.86"

Now, that suggests that ideal handling characteristics will be found for a hub-fender gap in the range of 13.5" to 15" front and back, below which the geometry will be an issue. What I'm hoping for now is the opinion of any seasoned racers (OneLap guys, the roadracing crew, etc) on what they've run and what were their impressions.

 

[Olson]

control arms level with the ground no lower. that will yeild the best gains with out messing with roll centers and what not.

 

[belize1334]

My understanding was that having the control arms level is already too low as it places the roll center directly between them, well below the CG. The should be angled down a bit so that the roll center is raised... but the question is how much. Without an exact model of the suspension we can't determine it empirically. We can take the stock ride height as a starting point but that ride height is also designed to accommodate stock suspension and the amount of wheel travel that can be expected. For stiffer performance we needn't allow so much room for travel. I'm leaning toward about 1" lower as that seems to be what alot of subie people run and they have a similar suspension layout. Still, the best would be the advice of seasoned racers who have put in the time to tune their suspension by trial and error...

 

[belize1334]

Further research suggests that you're right. On MacPhereson strut with firm suspension you want control arms level with the ground. On my car this came out to 14.0" from hub center to upper fender. Then I just put the distance from ground to jack-rail equal front and back. That put the back at 13.75".

 

What you're looking for is more of a roll center near but a little above ground at static ride height and mitigation during roll translating across the ground surface with little change in height and not changing from above to below or vise versa during chassis roll. The reason for it near ground level is that its the plane that dictates if you have a jacking on the outside, diving on the inside, or a balance of the two. Balance will keep the car rolling somewhat evenly so the roll center translates instead of climbing/diving.

Basically, the general consensus for mac-strut without doing a full suspension model is to get the lower arms close to but not greater than parallel with the ground plane and as little kingpin inclination as possible.

The reason to abide by this is generally its the best solution for most cars that don't have access to real suspension geometry changes and at that point you're time is better spend doing the math and research on proper spring rates, roll bar rates, quality dampers, and chassis stiffness where it matters. I say where it matters cause the center of the car can be the stiffest structure in the world and it wont mean a lick of different if the front strut area is a noodle.

Hope this helps.

 

[KiNgMaRtY]

Quoting belize1334:
I'm leaning toward about 1" lower as that seems to be what alot of subie people run and they have a similar suspension layout.


Correct, I had an STi and that was rule, don't go lower than about 1 inch

 

[bazeng]

I'm in the process of tweaking the AMG Galant (FWD traction issues) but do not want to sacrifice handling.

Would corner weighting be a good option?

 

[belize1334]

If you can adjust the spring heights then yes you should corner weight the vehicle. Remember to disconnect the swaybars when you're doing it. Then once it's right you can adjust the ride height of one side of the vehicle (front and back together so as not to spoil all your work) until the front swaybar attaches with no preload. Then attach the rear sway-bar and use washers to shim the end-links so that it too has no static preload.