During my last on track session at VIR north I was having issues with my car becoming unstable in the braking zone on main straight. The car felt like the back end wanted to come around if I was anything but perfectly straight. I know braking should be in a straight line but this can't always be the case. Is there some trick to setting up your car to aid in braking stability? Any suggestions?

My setup is 99-00 SI brakes front and rear, Panther plus in front and Hawk HP plus in rear. 1/8 total toe out in front and 0 in rear, ride height is 1/2 inch higher in back, -1.9 camber front and -1.7 rear. 375 lb springs up front and 500 lb. spring in the rear.


[Modified by 6ghatch, 11:41 AM 12/14/2002]

 

Does the car have any Rear Toe out?

 

No

 

Nice setup. The rear will always be light under breaking. You will get used to it.

Only worry if the car is pulling the one side in the breaking zone.

 

That's close to my setup: EG hatch, GSR brakes, P+ front, HP+ rear, 500F/550R

I solved my instability problems when moving from stock rears to HP+. Now, however, I've got too much rear braking as I can lock the rears up under hard braking. I haven't changed anything though because the overall feel is better.

It's going to get light regardless, but I'm thinking that having more spring in the front might help the issue some. Maybe not... there's the whole spring rate vs body roll vs weight transfer thing that could negate that.

Reread your post... if the problem, as you see it, is braking while turning you may want to consider rear pads with less grip.

 

I also think that 375f/500r spring rates contribute to rear being a little light. I would increase the front spring rates to compensate for weight transfer under braking.

 

As others have noted, different pads may help, different front springs may help (however, they springs change many other handling traits, do you want those to change?)
you could also change the rear toe, add a little bit more toe in, that might help, but again, this may influence other handling properties.

Since I'm not a Honday guy, tell us more about the brake swap. They are parts off of an Si, did that include the master cylinder? If not, is the proportioning of the original master cylinder right for the new brakes? The engineers spend a few minutes on these things. If the brake bias is wrong (I know the line pressures for the '00 Celica GT rear drums and front disks are different than the line pressures for the '00 GTS 4 wheel disks), then how do you plan to make it right?

Scott

 

weight transfer is the same, soft spring or stiff spring, soft shock or stiff shock. first bet would be to put stock pads in the rear. the hp's like heat, and the solid rotors will hold heat longer then the vented fronts. they will also heat up at a different rate due to the amount of braking force they contribute. if you still have a problem, start playing with rear shock rebound. you may have it set too high, speeding up the rate at which weight is transferred.

 

Thanks for the replys.

Well I have a ITR M/C and brake booster and the porp valve is the same one on the 99-00 SI with 4 wheel disk brakes.

Being that I'm driving a hatchback I'm thinking the heavy front to rear weight dist. would be adding to my problem. Would some rear downforce help? Or lowering the rear of the car help? Also, I plan on upgrading my Koni/GC setup this spring to Tein RA's, 10k front 14k rear.


I've talked to Andy from Cobalt and he metioned the HP+ don't handle heat very well(max 750). He recommened some DS3000 up front and Hawk blacks in the back. The blacks have a good temperture range(900+) and medium torque.


I have the rears set close to full stiff (rebound), with the Koni's maxed out they can hardly control the spings.


[Modified by 6ghatch, 10:12 PM 12/14/2002]

 

right there is most likely the main contributor to your problem. if the koni's can't handle that rate, then they are most likely pretty tired. i was using 700lb rear springs on my car with ots koni's with no issues other then a crappy ride. if you can't control the spring and its only 500lbs..........

 

IMO, this is an alignment issue.

I've noticed a tendency for my SE-R to do the same thing (rear end get squirrley under very hard braking) when I get low on gas. Doesn't seem to matter much what pads I have in the back.

Try a touch of toe IN in the rear. If you have an adjustable rear bar, stiffen it up to take out any additional understeer you'll create from doing that. The sway bar setting should not affect straight line stability, but the rear alignment sure will.

Umm, err....Holy CRAP!!!!


[Modified by MaddMatt, 9:24 AM 12/15/2002]

 

quote:
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Also, I plan on upgrading my Koni/GC setup this spring to Tein RA's, 10k front 14k rear.

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Umm, err....Holy CRAP!!!!


Yeah, Holy Crap!!!! Umm...sorry to steal this thread, but what is the purpose of going with such stiff springs? As I have learned, the purpose of a suspension is to absorb impacts while controlling the body's movements and maintaining a maximimum tire patch without losing control everytime you hit a bump. In essence, those spring rates seem too stiff for me. Unless you're on a glass smooth race track at all times, those rates are going to upset the car in even the slightest bumps.

Could you explain your line of reasoning?

 

thats 10 kg/mm, not lbs/in.

 

Well, This car will NOT be driven daily next year. I do a lot of autocross events and track driving, I don't feel these rates are over the top. Also, the 45mm single piston is very capable of handling these rates.


[Modified by 6ghatch, 12:45 PM 12/15/2002]

 

Ohh. Thanks for the edumacation

 

Those rates are only ~ 550/770 lbs/in. Not a bad autoX/road race setup. Many IT/ECHC cars run higher rates. You will be shocked at the forces that are applied on the suspension on a race track. Even at those rates, the suspension will move a lot.

As for bumps upsetting the car, that is up to shock tuning. A good shock is a must for running high rates.

 

Eventhough your weight dist. is different with the hatch, I had some rear end squirrilness on my 00si when I had P+ up front and stock rears. I stepped up to a R4S rear and it really helped to plant the rear. I have had no problems with lockup with that setup. But, you have less weight back there so who knows. Did Andie recommend Ultimates for the rear? I run them on the street up front and they are a great pad. I would guess the P+ or DS3000 up front with the Ultimates out back would be a great setup.

Did you try stiffening up your front konis?

 

He recommened some DS3000 up front and Hawk blacks in the back. I think I'll give this a try and see if the brake bias is any better.

 

If you want to go really hard-core, you can install a brake-bias valve. This would allow you to take some of the clamping force out of the rear brakes and improve stability.

I would doubt that a new suspension setup would cure the problem. If you're straight-line braking (or approximately close), the chassis should be pretty much un-tweaked diagonally. Physics say that during hard braking the rear will unload: less down-force means less traction in the rear...reducing the braking capability should keep the tires further from the point of impending lock-up and keep things less "squirrely".

...Of course ABS would solve your problem too! Just kidding.

 

actually, stiffening the front koni's will do nothing, since you are only adjusting rebound, not compression. the rear shock rebound is where you would look. i would also make sure the pads are seated properly in the caliper and that there are no worn out suspension components.

nate

 

What's your theory about having the car ride 1/2" higher in the back? I noticed that when I lowered my car in the rear, it became more stable under decelleration, especially when trailbraking. It wasn't so quick to slide away from me and seemed to rotate a tad better (read: smoother).

 

There is no theory, except to get the desired camber setting in the rear,(I don't have a camber kit, just a couple of washers ) that is why I asked. I will get a rear camber kit and lower the rear a bit to see if that works. Thanks for the suggestion.