Suspension setup - Springs (and
By Dennis Hale
|1. Road race setups are very different than autocross ones
for several reasons. The tracks are very smooth and not much travel is needed to absorb
the bumps ao ride heights are set very low to minimize the body lean. This in turn
requires stiffer springs to absorb the body weight over the reduced travel, kind of a
perpetual motion game 8), Further, oversteer is forbidden at 100+ mph, and the stiffer
springs especially in front help that be true. Finally, roadrace is a series of long turns
connected by distinct transitions and straights so the game is to maximize the cornering
force as soon as possible and hold that through the corner, accellerating as soon as
2. Autocross is different, there are rough lots and variations from lots to lots to deal with. Bumps require travel or you bottom the suspension and lose traction rather suddenly at the worst times. So we run a higher ride height for clearance and softer springs for compliance which in turn requires a higher ride height. If you always run on airports you can run a bit lower and stiffer than if you run asphalt or worse yet try the hillclimbs. Also autocross is all about transitions, not steady state stuff so we sort of encourage the instability of soft springs in the pursuit of fast transitions in opposite directions, I guess I want to call that improved turn in response at the expense of midcorner stability. You can toe the soft car better than the stiff one. We use at least as much shock though, we want the toss to only happen once....
For street spring selection, the game is more to balance the load than to outsmart the terrain. The weight of the car divided by the available travel is mostly the issue. I like to allow at least twice the corner weight in available spring. If you have a 500# corner and 250# wheel rate and 2" of travel you are pretty close to right. I also prefer the rear spring or rather wheel rate to exceed the front by about 10%. This is a preference from a guy who likes a rather loose car, 10% less will slow the roll rate down a bit and improve rear traction with your big motor thing, probably what you want. Most street setups fail to allow this suspension load thing and are driven on the bump stops alot. Bumpstops have very bad suspension characteristics and very high spring rates. Bending suspension parts are even worse and are what you have with no bump stops or way overloaded ones. Most complaints of rough ride are due to too soft springs that turn the bump stops into the suspension. I doubt you will find 250# rear wheel rate to be uncomfortable, at least for the front seat riders. Rear seat in a 510 is generally a penalty box no matter what your setup is 8) The only problem I forsee is the rear traction issue I brought up earlier and that may be improved with the reduced squat of the stiff springs more than the loss due to jacking up the unloaded side due to the stiff springs. Dunno.
You might like to read the Carroll Smith books..Tune to win, Engineer to win, etc. I beleive there are 4 of them, Plus Puhn's book, and we like Code's Twist of the wrist a lot... I beleive we included a bibliography in the little track book we gave you at Thunderhill.
Inexact summaries; 1. Springs will balance the car front to rear, swaybars from side to side; 2. Springs are primary tuning devices, swaybars are secondary; 3 Stop messing with the car and just drive it! 8). You do need to get the springs close before you can make any valid swaybar adjustments. Many street cars violate this rule and it never works out. As stated before, get the springs sized to the load and travel. The guess is what fraction of load you need to allow for. It is typical of a 510 to lift the inside front wheel in a full on corner, that means you will be loading the ouside tire by the entire weight of the front end plus some dynamic mass transfer effect. You will at least want to absorb twice the corner weight in the spring.
A very similar argument works for the rear. I think this means something like 150#/" wheel rates and 4" of travel beyond static ride height is about right for a 2300# car. The 4" of travel is the difficult part to sell, and 2" of travel would require 300#/" wheel rates. Swaybars are diabolical things that work by taking away from something that works well and giving to something that doesn't. Sounds like mechanical socialism... Anyway, we use them to give a firm feel to the transitions and they are easily over relied on as tuning aids. If you are running in stock rules classes they are helpful but mostly for this discussion I think a normal pair of street bars will get us a very long way. The VG car with its twisted weight and torque problems may well be best off with the stock tiny front and no rear, really. This will keep the inside rear wheel down the best and the car's strong suit has changed from graceful cornering- the dime dance- to a power play, that is why you made the change isn't it? (referring to my V-6 510 - ed) The car will need to be driven more like a Mustang, less in the corners and more on the straights and I think tuning it accordingly will be appropriately. Lots of springs, in front for the big weight bias change, in back to control squat. Lots of shock to keep the springs from oscillating, minimum swaybars and live with the slower transition feel. I think.
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