Rear of car unstable under long hard braking

[subsonic]

While braking in straight lines into turns 4, 8, 10 at Mont-Tremblant the back of my car (WRX STI) was wobbling left and right, trying to get around me (fun in 4...). This is even visible from onboard camera. Would this be due to having too much front torque brake pads compared to the rear? I was running with Hawk DTC-70 in front and Hawk HT-10 on rears. I really love the DTC-70 (handles the heat well), yet I would like to make sure the car is stable. So if somebody has experienced similar issue, would getting a more torque pad for rear help or just make things worst. I am starting to thing the pad is too agressive for my car's suspension.

I have 8k springs back and rear. The same springs that a member was looking for 2 weeks ago.

[Grippy]

It sounds like your rear brakes are not doing their share of the work. Either the temp range is wrong, or they are adjusted wrong, or they need a good bleed, but that should be felt at the pedal.

Spring rates should have nothing to do with it. If the car in neutral while cornering your rates are fine. Unless you are getting too much dive under braking.

[zip4zat]

Hey Paul,
Ive run the ht10's before and they're great pads for the rear. Sounds like you need to get more grip in the rear. Try lowering tire pressure a tad (easy way) or you need more rebound damping in your shocks (might be more difficult).

Also check your alignment, you might have some toe out which would also make your rear end dance a bit. I would assume your car is pretty straight in 8 and 10. Its hard to get the car dead straight in 4. Do you feel your ABS going off? It doesnt help things if its trying to compensate too.

I see if anything else comes to mind.

[subsonic]

It sounds like your rear brakes are not doing their share of the work. Either the temp range is wrong, or they are adjusted wrong, or they need a good bleed, but that should be felt at the pedal.

Spring rates should have nothing to do with it. If the car in neutral while cornering your rates are fine. Unless you are getting too much dive under braking.

The fluid was ok - pedal was fine. Which is usually an issue...not this time though. I felt brake heat/fluid was easily under control during that weekend (I was not driving for long - 5-6 laps/session.

[subsonic]

Hey Paul,
Ive run the ht10's before and they're great pads for the rear. Sounds like you need to get more grip in the rear. Try lowering tire pressure a tad (easy way) or you need more rebound damping in your shocks (might be more difficult).

Also check your alignment, you might have some toe out which would also make your rear end dance a bit. I would assume your car is pretty straight in 8 and 10. Its hard to get the car dead straight in 4. Do you feel your ABS going off? It doesnt help things if its trying to compensate too.

I see if anything else comes to mind.

I found out my gage was reading 5 psi too high...so my rears were probably around 42 PSI on NITTOS ... so you are saying this could be a contributor. I like a bit lower pressures on rear (38),

The alignement had just been done, but I am looking at the sheet and noticed that the rear camber is -2.0!!! Could this be the problem for lack of grip under braking in a straight line? I usually run it at -1.0..never more than -1.5...I have stock sway bars. The toe is set to zero all around.

I have single adjustable dampers and I didn't bother looking at them during the whole 2 days! I was working on something else...So I just checked and the settings are softest in the rear (for street driving)... and faster in front. I will bring rear up to about 1/2 next time.

My concern was that I simply had the wrong pads - sounds that the pads aren't the problem and that this has to do with rear traction... I will pay closer attention to your remarks.

Joe: What gear oil do you put in STi transmission/front diff?

Thanks for everyone help. I have things to watch out for next time (I need to move car closer to you guys in the paddocks) I put it close to class room last time.

I will be away for all of next week

[nhsilversti]

less rear camber and reset to 0 toe in the rear.

almost all unhappy rear ends in subys are from toe out in the rear. keep in mind, under load of braking the rubber bushings will flex and give a little toe out. you could try a very minimal amount of toe in to help combat the problem IF you are stock bushings.
04 had the stiffist of all sti's bushings
05 was a fair amount softer
06/07 they should have been labeled butter.

ted

[subsonic]

less rear camber and reset to 0 toe in the rear.

almost all unhappy rear ends in subys are from toe out in the rear. keep in mind, under load of braking the rubber bushings will flex and give a little toe out. you could try a very minimal amount of toe in to help combat the problem IF you are stock bushings.
04 had the stiffist of all sti's bushings
05 was a fair amount softer
06/07 they should have been labeled butter.

ted

Awesome information - this is great help.

I need to get within earshot to you guys next time.

[nhsilversti]

i forgot to mention the high tire pressure in the rear will cause a little dancing but, that would be minimal.

you are more than welcome to pit at my trailer. look for the trailer that is parked opposite from everyone else. i do this so we can get 4 cars pitted around the trailer and all have access to tools/air/shade/good times.

[brucesallen]

too much front brake power will make the car understeer- and be stable but go in a straight line. Too much rear braking will make the rear end loose and will spin the car if trail braking in a corner.

[offcamber09]

+1 on the rear alignment. Was the car trailered ? If so how was it tied down? That may have screwed up your alignment.
Contrary to the above opinion:
If you have static toe-out, under hard braking that should reduce the toe-out somewhat. As the weight transfers to the back, toe-out is increased. I have set up a lot of STIs with 1.5 deg. neg. camber in the back...2 is too much but not crazy bad.

[breakaway500]

I get the same sensation in my fwd car under heavy braking.The back end gets light,rear traction is minimal.All the more reason to be damn straight before hitting the binders hard.Otherwise,I will be chasing the back end. I believe it is a skill I must master rather than something that needs to be tuned out of the car.At least that is how it seems to me. I got to work on it a lot at Mont Tremblant.What a great track!

[JackFFR1846]

. I believe it is a skill I must master rather than something that needs to be tuned out of the car.:

UB great student with that attitude! 5 jack points

jack