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Equal Spring Rates

Old 01-14-13, 07:34 AM
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Equal Spring Rates

Anybody here tried square spring rates on an FC track car? I have been racing my car for over 10 years and have messed with soft rates, 400 front 200 rear to hard rates 800 front 600 rear. I have also messed with a number of bars from stock to stock front no rear, to Suspension Techniques front and rear to what I am running now which is Suspension Techniques front and stock rear.

In all that time I have never tried going to a square setup. At the present time I am running 600 lb fronts with 400 lb rears pretty constently. The car works very well and my lap times are good, but from the seat it always feels like the car is falling over on the rear outside tire and from shots taken steady state cornerings, I always have my front inside tire either off the ground or just on the verge of lifting. When I add more rear bar the car gets very tail happy.

So in brainstorming last night I thought about the fact the my car runs at 49% front and 51% rear as it comes off the track and why am I going firmer on the front than the rear. The attached shots are from Mosport end of September 2012. It is at the apex of Corner 3 at about 80 mph.

Comments and experiences would be interesting.

Eric
Attached Thumbnails Equal Spring Rates-rx7-celebration-2.jpg   Equal Spring Rates-rx7-celebration-3.jpg  
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Old 01-14-13, 08:48 AM
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Check your motion ratios front and rear. With those spring rates, 600/400, your wheel rates may not be too different.
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Old 01-14-13, 12:25 PM
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we use a spreadsheet, and with some of the data from the car to calibrate it, we can now make changes in the spreadsheet and it becomes really close to real life.

Autocross to Win (DGs Autocross Secrets) - Dynamics Calculator works too, although his understeer/oversteer thing is probably car specific.

having the same spring rate all the way around is unconventional, but that doesn't mean it won't work, i would try it if i was you
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Old 01-14-13, 10:07 PM
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I currently run 600/500 with no rear sway and a tire stagger and I wouldn't really want to go any further... but everyone likes different setups. try it and see.

I think the need for rear traction necessitates a softer rear end... if you're non-turbo without much power it might work, but even miata guys have pretty large spring staggers. (700/450)
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Old 01-14-13, 10:35 PM
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dont forget the rear roll center is quite a bit higher than the front. that means for equal roll stiffness front/rear, you will need more elastic roll resistance in the front. generally people achieve this with stiffer front springs + bar, and small/no rear bar.

sure, you could run equal rates all around, but you'll need a pretty big front sway bar to balance the car. and by big, i mean much bigger than anything off the shelf.

another thing to consider is that mac struts generally suck at everything, so preventing them from moving (with stiff springs) isn't always a bad thing. the rear has plenty of camber recovery, front doesn't.

FWIW, i run 600/430, solid 1.125" (Susp techniques) bar, no rear bar, and staggered tires, and it's a little loose steady state for my liking. ideally, I would switch to a bigass speedway bar and up the rear spring rate to help battle squat. Just haven't gotten around to it yet :-P

For a track car (higher speeds) with less power, I'd lean towards softer springs, since squat isn't such a problem. 350whp in 2nd gear really drives the *** into the ground.
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Old 01-15-13, 10:33 AM
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Interesting responses and everyone seems to be confirming what I am running now may be fine and directionally correct. I am running almost the identical setup to Josh18 2k, but with equal tire sizes all around. It handles great and I can really get up on the wheel when I need to tiger the car.

I think that maybe what I really need to do is replace the shocks and struts to control transient responses as the steady state is fine. This gives me more reason to replace my Bilstiens with the triple adjustable Morrisdampers that I have to fab up a way to mount them.

Thanks,

Eric
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Old 01-15-13, 12:43 PM
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Originally Posted by Josh18_2k View Post
sure, you could run equal rates all around, but you'll need a pretty big front sway bar to balance the car. and by big, i mean much bigger than anything off the shelf.

another thing to consider is that mac struts generally suck at everything, so preventing them from moving (with stiff springs) isn't always a bad thing. the rear has plenty of camber recovery, front doesn't.
we run a gigantic rear bar in the honda, which i mention because its the same as the FC, just backwards.

the Mac strut isn't horrible either, the Honda is double wishbone and we still run -3.5 camber, and like -5 in the rear (its an FC backwards).

the big difference is that we actually want it to pick up the inside rear wheel
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Old 01-15-13, 04:24 PM
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Originally Posted by j9fd3s View Post
we run a gigantic rear bar in the honda, which i mention because its the same as the FC, just backwards.

the Mac strut isn't horrible either, the Honda is double wishbone and we still run -3.5 camber, and like -5 in the rear (its an FC backwards).

the big difference is that we actually want it to pick up the inside rear wheel
Interesting that you say this as this is what I believed as well. My other racecar is an ex WC Cougar that had its suspension analyzed by Bill Mitchell, then designed by Dynamic Suspension and Ford Motor Company and totally fabbed from the unibody all the way to the ground. The car handles fantastically well and drives like a RWD car and you can balance it on the throttle in faster stuff. Anyways it has 600 front and 450 rear springs. A stock bar located with rod ends on the front and a freakin' huge Speedway bar on the rear. It used to run 17x8's on the front and 17x7's on the rear, but now I run it square all the way around with a larger splitter. I never see a shot of it lifting an inside rear, but it has little to no understeer. Also, the cambers are a lot less aggressive as well.

Take a look at the car at;

www.nummelinmotorsportgroup.com

Eric
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Old 01-15-13, 05:51 PM
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Actually, here are 2 shots that better show what I am talking about when I say that it seems to fall over on the outside wheel.

Just an FYI.

Eric
Attached Thumbnails Equal Spring Rates-great-shot-rx7-ii.jpg   Equal Spring Rates-rx7-3-wheel.jpg  
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Old 01-15-13, 11:34 PM
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that really doesn't look bad at all. I wouldn't even worry about it.
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Old 01-16-13, 10:38 AM
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how is the tire wear in the rear? even?
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Old 01-16-13, 12:21 PM
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^ This. What the car looks like doesn't matter. What are the tire temps and wear showing? How well are you actually using each corner of the car? Ultimately, tire temps and lap times are going to make the decision for you. That said, no harm in testing. Only way to learn what works. Even though it may not work, you may learn something about something else completely unrelated. But your description, good steady state, needs help in transition, certainly sounds more like a shock issue than anything else.
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Old 01-16-13, 01:25 PM
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Tire wear is very good, but if I push the car hard (so that it tricycles) my rear tire temps go up by about 20 degrees and the rears wear faster than the front. The wear pattern is even across the tire, but I do get blueing on the outside edges of the rears, again driving the car really hard. This leads me to believe that I am getting temp spikes, even though the temps are even across the tires.

I have tried changing alignments and pressures and camber and it works okay, just on the transients.

I think you guys are right and it just may be a shock setting issue to control transients. Maybe I should have a stern talking to with the driver to stop him from backing the car into the corners so hard.......

Eric
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