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Rear-end Instabilities

Old 06-30-08, 12:34 PM
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NC Rear-end Instabilities

Just got back from a great weekend at the SCCA PDX down in Rockingham, NC. They were also running a short enduro on Saturday and a Carolina Cup race on Sunday so we got to see a lot of ITA 7's running as well as an ITS 2nd gen. Fun weekend overall and darn cheap track time with only a half dozen drivers in my run group.

A friend and I were double-stinting the FC (he was instructing, I was running advance group). We have noticed that the back end seems to get a little unstable under heavy breaking. It seems to want to dance around a little. I'm trying to track down the cause of this and figure out what to look at first. I'm hoping some of you guys can help. Here is the current setup:

91 NA FC, 2400 lbs w/o driver. GC coilovers, 400f/275r. KYB AGX 5f/3r (need to double check this). RB front swaybar, no rear bar. Camber -2.5f/-1.7r, 0 toe f/r. Brakes: hawk blues in front, blacks in the rear, stock proportioning valve. DTSS eliminated. Tires are Toyo RA1 205/50/15 on stock rims.

So, given the above setup, does anyone have any ideas about what could be causing the rear end to get so happy? Any thoughts would be appreciated.

Thanks,

-b
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Old 06-30-08, 01:02 PM
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I just fixed a slightly more intense version of rear stability problems under braking myself.

Cliff notes: I had a dead rear shock, a dying rear shock, (also KYB AGX) and uneven rear toe, as well as a lot of front toe out. Now that those things are fixed, the car is stable under the brakes.

full thread:

https://www.rx7club.com/race-car-tech-103/spinout-under-brakes-fixed-766415/
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Old 06-30-08, 01:02 PM
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My first suggestion would be to remove the stock proportioning valve and put an adjustable on in so the driver can change it.

First, I would pay attention to the brake bias, sounds like it could have too much rear braking, even with the blacks. (When I had mine, I ran blues with an adj prop valve)

Second, Check corner weights, if they are off significantly that can cause this.

Third, Too much rear rebound (if that is adjustable, but this is a "if all else fails" guess, this would most likely be the case if all else was dead nuts and you were looking for those few last tenths)

Forth, Too little/too much rear pressure. Would have to be extreme in either case most likely

Fifth, Uneven rear anti-sway bar

I just noticed you are running zero toe in the rear, go 1/8" to in. I ran 0 toe on a formula car once and will never, ever do it again. I never tried it on my ITS Rx7 so I can not be certain in this case, but I would suggest never running zero toe. Change the front to 1/8" toe out...yes toe out. It will make it turn a bit better and with these heavy cars you need all the help you can get. The reduced tire scrub is not noticeable with 1/8", I have data testing to prove it in multiple cars or all types. It will be much more relaxing down the straights.

As for your competition....sounds like you have hoarded all the good rotary competition. Up here all we have is one, maybe two (and one of those is my old car) ITS cars. Most have transitioned to E-prod now, so that field is huge, haha.


Best of luck, let us know what works!!!

~Reid

Last edited by bean13; 06-30-08 at 01:06 PM. Reason: I need to learn to spell
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Old 06-30-08, 01:04 PM
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You probably mean 3f and 5r for the AGX's, but you're aware that the damping adjustment on the AGX's is literally nil everywhere besides the low speed range?

Why not run blues front and rear?
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Old 06-30-08, 01:15 PM
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I have no experience with these shocks, sorry.

Sometimes people run different pads front to rear because one is has too much/little bite. Blues are a bit more aggressive in initial bite than blacks, so often folks run blacks on the rear. I did and didn't like it, not enough grip for me, but I come from karts to take that for what it's worth.

Best-
~Reid
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Old 06-30-08, 01:22 PM
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Originally Posted by Roen View Post
You probably mean 3f and 5r for the AGX's, but you're aware that the damping adjustment on the AGX's is literally nil everywhere besides the low speed range?
Thanks, and yes that is what I meant - 3f/5r.

In terms of adjustability, I always went under the assumption that usually the highest and lowest settings were suspect and the middle ranges were more reasonable. If that's not the case with the AGX's and only the softer settings are usable, then I appreciate the heads up.

Why not run blues front and rear?
Since I don't have an adjustable proportioning valve, I was trying to get a little more front bias. I was worried about rear lockup if I ran blues all the way around. I've talked to some folks who have had better luck running a heavy street pad (eg. HPS+) in the back.

Last edited by wrankin; 06-30-08 at 01:38 PM.
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Old 06-30-08, 09:21 PM
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Bill,
I currently instruct DEs, primarily for NASA-MA, but have done other clubs as well. I have been battling crappy brake bias for a while, with the right rear wanting to lock up prematurely. Instead of downgrading the rear brakes from blacks to HP+, I upgraded the fronts from blues to HT-10s. I REALLY like the way the HT-10s bite, and they seem to behave better than the blues- I can apply LOTS more brake before the car starts dancing.

I am running 440/275 springs, tokico blues front, Koni Yellows rear; weight ~2450 caged, empty; RB front sway, no rear; -2.7* f, -1.7* r, 0 toe.

I will also try Reid's toe advice, but I think your problem is pad compound.

Talk to: Jay Andrew
http://www.Andrew-Racing.com

Jay has taken the time to talk to me personally to solve the bias issues (as much as can be done without a bias adjuster, anyway), so I buy pads from him (there is a NASA instructor discount).

Good Luck,
Pete
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Old 07-01-08, 08:33 AM
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Thanks for all the input guys - it's nice to know that I'm not alone in having to combat the "dancing butt" syndrome.

I think that the plan of attack right now (before the next event) is to pull and check the rear shocks, double check the bushings, and may be soften up the rear a bit (a friend at the track mentioned this after taking a ride around for a session). In hind site I should have dropped the rear tire pressure a pound or so while at the track and checked if that helped.

The second part will be planning the time and budget to put in a real proportioning valve.

Pete - thanks for the link. The prices seem pretty decent. I'll definitely be talking to Jay in the future.

-b
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