THSCC Forums

From your example if the strut sits at a 20 degree angle you'd have a motion ratio of .66, which is way low.

Or am I missing something?

--Donnie

Yeah,but what's the maximum negative camber that I can expect to get on the front of my wrx from an alignment shop?

the problem. car works good on asphault, plows like a pig on concrete. so i looked at the difference between the two surfaces and obviously the main difference is grip levels. ie, concrete is grippier. (duh) so, i spent a lot of the off season reading books on theories and such and spending way too much time testing different configurations using the Bosch LapSim program. the fruit of this labor is this.
the why:
lets assume a really simple car. corner weights are 800lbs on both fronts, 500lbs on both rears. we set the car up so that it unloads the inside rear and finish balacing the car with alignment/tire pressures and whatnot. now, we know it's balanced with this setup on a lower grip surface, but when we take it to topeka it pushes. why? well, we've already unloaded the inside rear, so the rear grip is higher then before. (verticle load on outside rear is still 1000lbs but the tire is on a stickier surface, thus more grip) since overall grip has increased though, we've increased our total lateral load transfer. since nothing can come off the inside rear, it all came off the inside front, reducing the amount of grip the front axle can generate. (well, it does generate a little more grip, but the front grip doesn't increase as much as the rear grip does) 
the theory:
start over. we take the same car, but set it up so that on a moderately grippy asphault surface the inside rear still has a _little_ load on it. we use alignment/tire pressures/whatnot to balance the car. now we take the car to topeka, and with the increase in grip we now have the inside rear completely unloaded, thus reducing rear grip hopefully at relatively the same rate as front grip. in theory, the car would handle the same.
supporting theorems and ideas:
one of the main ways we would get the car to rotate on the lower grip surfaces is with a reduction in rear camber. again, going to a higher grip surface, the car would roll more, the outside rear would de-camber more, the car would be even looser.
rear spring rates wouldn't have to be so high. the car would be easier to drive over the bumps in kansas.
in a way it sounds like we are throwing away grip. in reality, the only grip we lost was on the lower grip surfaces, not the high grip surfaces like topeka. as overall grip goes down front/rear balance is thrown to pot anyhow. i find in my car that it gets too loose as grip decreases, indicating that more rear grip is needed. also, as grip goes down, so does roll, decreasing the effects of a low rear camber setting.
less adjustment to the car for each type of surface and weather condition means more consistent car which means an easier job learning to drive it at 100%. that and it's a shitload of fun to walk through pre-grid watching everyone frantically adjust swaybars, tire pressures, and shocks while you calmly reflect on your last run.
final thoughts:
so far my car has handled the most consistently it ever has since i switched approaches. i'm down from 800lb rear springs to 550lb. the car has become easier to drive aggressively in slaloms, my rear shock adjustments make a much more noticeable change on the handling, mid-corner understeer has been mildly reduced (not from the spring change, i went from -1.75* rear camber to about -.6*, still with about 1/8" total rear toe-out) and i can actually keep it pointed were i want in the wet. (unless i'm hydro-planing) the best part is that i can more precisely place the car at corner entry and have found it easier to prevent driver induced understeer.
you guys may have ended up at roughly the same location i have as far as setup is concerned, so don't think i'm suggesting a change.

Tom,
In regard shock valving being too soft for the stiffer springs, the "traditional" approach to racing shocks has been to over damp the springs by introducing a lot of hydraulic "friction". However, at the top road racing levels the approach now is to minimize any source of "friction" and to run shocks softer and springs stiffer while maintaining appropriate spring and chassis movement control.

It is all way over my head at this point.

Best I can tell my shocks still don't result in under damping other than when I set them full soft (hence the wheel hop/oscillation over a series of bumps). Keep in mind that my car is extremely well balanced for autocrossing with no need to use shock tuning to change the balance or to compensate for soft springs, etc. like people have to do in Stock (due to rules limitations). You might check with Lee Grimes to see if the Tien shocks "give up" anything compared to what a racing Koni might be able to do . . . especially in regard to high shaft speed.

In regard to using stiffer springs and a softer bar to improve inside wheel traction: This has been a common tool for rear wheel drive cars for a long time and AFAIK with fwd. In many cases rwd cars don't even run a rear bar once we stiffen the springs. Think about how a bar works . . . it "wants" to unload the inside tire. Weight transfer doesn't change but the inside spring can extend more if the spring is stiffer or the bar is softer. The hard part is getting enough roll resistance with springs since bars can "easily" have very high wheel rates. However, if you significantly increase the spring rates even without changing the bar, the bar becomes a smaller percentage of the total.

I've skimmed Nate Whipple's comments and they make lots of sense. A couple of thoughts: What does the rear of his Honda weigh and what are the motion ratio's compared to your car? My guess is that his car is a lot lighter in the rear and that 550 springs may still be rather stiff.

I agree that balance is critical for autocrossing.

I finally got my almost Stock street prepared Lotus Europa to be consistently fast by raising the rear ride height with slightly stiffer stock free length springs and adding a rear sway bar. Of course it got even faster the next year when I went to lower and stiffer springs at both ends.

What you said makes sense to me. Since the strut/spring combo is sitting at a 20 degree angle, it is not seeing the same amount of compression as the wheel is.

On most Mcpherson strut that angle is so negligeble that the motion ratio is 1 or *very* close to it.

Tom-
Big bar generates more inside wheel spin, more rebound damping will generate more inside wheel spin.

My understanding is that the Teins change compression and rebound together, so when you "stiffen" them, you are doing more than change the rebound, you are changing the bump too (one equation, 2 unknows = unsolvable problem).

PS: I think we've deviated from the original question, "How do I maximize the negative camber on my WRX by taking advantage of stock tolerances?"

PS: I think we've deviated from the original question, "How do I maximize the negative camber on my WRX by taking advantage of stock tolerances?"

Okay, it makes sense to me that you use that angle to determine the motion ratio. I assume you would measure the angle at the "normal" ride height of the car since that's where you start most motions from (seeing as the motion ratio actually changes through the range due to the change in angle as the wheel moves up and down) and normalize from there.

I haven't looked at the STi, but the struts on the Spyder do sit a fair amount off from vertical. More than ten degrees, I think, which is very significant. You end up with a motion ratio of .83 which when squared drops all the way to .68. That drops the wheel rate to 2/3 the spring rate, instead of being equal to it.

Thats what I would think. That angle though, shouldn't change with wheel travel should it?

Sure it will. Think about a triangle. Change the length of *only* one side. Every angle inside the triangle changes. That's what you're doing there, since the other two sides of the triangle do not change length (the lower control arm and the distance from the top hat to the inboard end of the lower control arm).

Why do I keep saying "lower" control arm when there is only one damned control arm in the whole thing?

BTW, this is a great thread. Great info here and I've ordered several more books.

It's too bad most "real" race cars use double wishbone suspensions...those of us racing strut cars could definitely benefit from not having to adapt everything. Of course, it's also too bad that everything is written from a road race perspective and not a solo2 perspective

Thanks to everyone who has contributed!