Rear End Very Unsettled Under Brakes

[hrd-hr30]

personally, I reckon fitting a proprtioning valve is treating the symptom rather than the problem. I would go looking for the actual problem if it was me.

rule out alignment issues - slightly out alignment (that you can't pick by eye) will never cause brakes to lock.

it is either:

1) rear wheels loosing contact with the road - worn or just poorly valved shocks, worn subframe bushes, worn upper or lower arm bushes. it may pay to go to a pedders and have them check out the rear shocks on their "shock dyno".

how have the pineapples been fitted? I think its misleading for them to call pineapples a "lock & align kit". "all-round" will still allow some vertical movement under accelleration and decelleration - depending on the condition of the bushes. "traction" setup will stop vertical movement under accelleration, but do nothing under braking where it will rely totally on the condition of the bushes, and "drift" setting will do nothing to stop torsional movement of the subframe under accelleration, again relying on the bush to do the work. You still need bushes in good condition with pineapples. They certainly do not lock the subframe in place. And they do nothing (or very little at best) to locate the subframe laterally either. have the subframe bushes leaked?

2) brake system problem. bleed and flush fluid in the front circuit if the gtst has seperate resorviours, otherwise the whole system. start by syphoning or sucking the old fluid out of the resorviour with a syringe, top up with new fluid and bleed, bleed, bleed. starting with the left rear which is furtherest from the master cyl, then right rear, left front and finally fight front. doing each one until nothing but clean new fluid is coming out.

remove and inspect flexible brake lines to front calipers, checking for internal blockages/obstructions. replace if even remotely suspect - they're not very expensive. if there are steel lines downsteam of the rubber hoses to the caliper, remove them (both ends disconnected) and blow them out with compessed air. and visually check the condition of all the steel lines looking for damage (ie dented or partially crimped). That's all the cheap things covered...

finally if none of the above steps helped, its either master cyl or front calipers. I'd probably lean towards the master cyl out of these two - its possible that the front circuit piston seal isn't in great condition.

[AzzurrA]

1)

Shocks are only around 1 year old - if they're worn out, then whiteline/bilstein have something to answer for, one or the other of the upper or lower arm bushes have been changed (i have new bushes for camber adjustment).

Pineapples have been fitted in the 'handling' setup apparently

2) have done all that, have syphoned out all old fluid, added new fluid, flushed the whole system front and rear, all seems to be fine

as for alignment, this is what i'm running:

Front:

left castor - 4.96

right castor - 3.82

left camber - -2.08

right camber- -1.79

front toe - 0mm

Rear:

left camber - -1.44

right camber - -1.75

rear toe - 0mm

[Roy]

Ride heights, and is it an issue after a few laps when hot, or off the bat when cold, or doesnt temp influence it?

[AzzurrA]

quick/rough measurements:

Rear Left: 328mm

Front Left: 327mm

Front Right: 325mm

Rear Right: 325mm

p.s. Thanks King Springs you useless wankers

[r33_racer]

Not much left to try. If it hasnt been done already, i would go for reco on the master cylinder. Or get some -toe on rear to help drive it straight. Did you end up checking the rear tie rods from the HICAS lock bar. Im guessing your kit uses the std arms and rods also?

If its not geometry, then it sounds like the front brakes arent working as good as they should, but the backs are. Process of elimination. Your car will be in top shape by the time you sort out this problem

[Color_Of_Green]

When you brake hard and locks up does it drift to either side and if so is the initial direction of drift consistent? i.e. seems to drift to the left first?

Curious, what diff u running?

[AzzurrA]

it doesn't seem to drift to any particular side, no.... can go either way, dependant on which way the car is turning, or the camber of the road etc.

running standard LSD, that is fairly shagged (single-wheels alot of the time)

[Roy]

...did you sort the problem? Can you tell if its a temperature based thing, in that it does it when its cold, or hot, or both. If you really spend the first lap warming up the brakes, do you still have the issue?

[Color_Of_Green]

it doesn't seem to drift to any particular side, no.... can go either way, dependant on which way the car is turning, or the camber of the road etc.

running standard LSD, that is fairly shagged (single-wheels alot of the time)

hmmm I wonder if the diff is locking unevenly on deceleration and causing this issue.

U running std rotors on the back?

Do your rims allow air to flow through the brakes?

Have u tried running air ducting to to the rear brakes?

[AzzurrA]

...did you sort the problem? Can you tell if its a temperature based thing, in that it does it when its cold, or hot, or both. If you really spend the first lap warming up the brakes, do you still have the issue?

Hey mate.

I did a track day back on the 12th, and i ended up putting the Bendix Ultimates back on the rear a couple of days beforehand.

The problem still exists in street driving, though i did not notice it at all on the track, and it seemed very well behaved actually, so it could just be that the rears are working alot more efficiently on the street, and the fronts aren't getting the chance to come up to temp properly...

still doesn't explain why it was doing it with same pads front and rear though

[Roy]

Well in my R32 GTSt i have never been able to run the same pads front to rear. The car never stopped very well doign that. The with the bigger front rotors off a GTR, again i am not able to run the same pads front to rear.