Rear End Very Unsettled Under Brakes

[AzzurrA]

Hi All,

have a problem with my R32 GTS-t at the moment, in that when braking hard, the rear end seems to lock up VERY easily, and slides around all over the place, basically steering the car from the rear...

This problem was evident with my last brake setup (standard calipers/rotors, with RB74 pads on the front and generic pads (probably bendix ultimates) on the rear.

i thought it might go away with the new brake setup (standard calipers, DBA4000 slotted rotors up front, machined standard rotors rear, Ferodo DS2500 pads all around) but it hasn't... if anything, its worse...

Do these cars have any sort of factory adjustable brake bias?

other than that, i have another theory that maybe the front springs are a bit soft, causing too much weight transfer to the front, thereby unloading the rear suspension/nearly lifting the tyres off the road, however, i don't think this would be the case, due to the suspension being all pretty much new Whiteline stuff, which is designed to work together?

thoughts anyone?

[steve_gts4]

what are your rear subframe bushes like?

[AzzurrA]

fine... brand new whiteline lock & align kit

[Haysey]

i have the exact same problem, under heavy braking the back does an el doorriiiffttooo.. pretty annoying i think.....

any way of adjusting the bias?

[ferni]

Are you sure it is locking up?

Does the front of your car dive under brakes? Maybe your front spring rates aren't high enough so all the weight is being transfered to the front of your car, leaving your rear end light and wanting to come around.....

Just an idea, don't take my word for it......

[Sydneykid]

My first suggestion would be to get the rear wheel alignment checked, toe and camber. Make sure they check the tie rod ends for play.

If that is all OK, then repeat after me.........

HICAS is not my friend

cheers

Edited June 16, 2006 by Sydneykid

[djr81]

The other suspect is tyres. I (once) had some el cheapo rears (mis)matched to some good fronts. Made the old dear oversteer like a cat on lino & also lock the back brakes.

If you don't find anything untoward it might pay to chuck the old rear pads in. (Or atleast a set with a lower frction coefficient, DS2500 should be about 0.5, so try 0.45 or similar)

Are you confident you bedded in the DS2500's properly?

[AzzurrA]

Are you sure it is locking up?

Does the front of your car dive under brakes? Maybe your front spring rates aren't high enough so all the weight is being transfered to the front of your car, leaving your rear end light and wanting to come around.....

Just an idea, don't take my word for it......

Yep, definately locking up... sometimes it howls, sometimes it is silent though....

Front of the car dives, yes... which is why i made the same suggestion in my original post... though its not too bad...

My first suggestion would be to get the rear wheel alignment checked, toe and camber. Make sure they check the tie rod ends for play.

If that is all OK, then repeat after me.........

HICAS is not my friend

Rear wheel alignment should be fine, it wasn't done that long ago... can post up specs when i get home tomorrow. AFAIK there is no play in the tie rod ends, but i will have a look.

Car does not have HICAS any more either

The other suspect is tyres. I (once) had some el cheapo rears (mis)matched to some good fronts. Made the old dear oversteer like a cat on lino & also lock the back brakes.

If you don't find anything untoward it might pay to chuck the old rear pads in. (Or atleast a set with a lower frction coefficient, DS2500 should be about 0.5, so try 0.45 or similar)

Are you confident you bedded in the DS2500's properly?

Tyres are the same front and rear... they are Goodyear Eagle Revspec RS-02's... i've found them to be a fairly good street tyre actually.

As for if they're bedded in correctly.... well, i'm not sure... doesn't seem i can get enough heat into them on the street to bed them in....

[Chris Wilson]

On my R33GTS-t the master cylinder had what I believe to be a rear brake pressure limiting valve on the side of it. I converted to twin master cylinders, so never explored exactly what was inside this gizmo, but I suspect a spring and plunger to shut off the rear circuit above a specifice line pressure. If this is seized open it MAY allow enough rear bias to lock up the back tyres, especially if they are poor. As standard all production cars have heavy bias to the FRONT brakes, to stop rear lock up.

I suspect it may be rear alignment issue though, I doubt even without the bias valve working the setup would be designed to give so much rear bias, it'll be pretty much failsafe.

[gts4diehard]

How are the bushes in your rear hubs, they are designed to give some compliance to assist the 4WS, and after a while they can give you a little more steering than you would like. The rear steering rack can wear considerably at both the hub and rack so if the car has a lock kit using old arms there could be movement at the lock kit end - if your tierods are ok. A friends GTR had over 5mm play in rear steering due mostly to the rear steering arms & tierods and some wear in the bushes in the hubs.

Is the subframe lock kit bits sitting 100% correct?

If all the bushes are ok(& nothings bent) - then your geometry should be fine (no abnormal camber/toe-in happening) and the car should pull up straightish.

From what you say alot of suspension work has been done so hopefully everything has been done right i.e. suspension arms torqued up with the cars weight on them etc.

[Roy]

My money having been there done that, is ride height or front to rear compunds....lol, then again could be anything i suppose so i will get back in my hole

[AzzurrA]

well, i pulled the front pads out yesterday, and they seemed to be a little glazed, so i scuffed them up a bit and put them back in, will have to try and bed them in again when the roads dry up a bit :S

don't know if it will fix the problem or not though

[Cubes]

I had an issue with the rear locking up when I first bought my car.

I slapped on a set of new rotors, bendix ultimates and bled the brakes.

The brake fluid was well over due a flush, so I flushed and flushed and flushed.

Whoohooo brakes no longer locked up.

2 weeks later they began to lock up again, I noticed the fluid had already started going black again so I flushed again.

After ever really good flush (a 2-3bottle flush with a little drive around the block inbetween) the brakes would work really nice.

It took a good 6 months of flushing almost every second weekend and now FINALLY no more brake problems and the fluid stays clear for a good few months before it becomes a little dirty looking again. But the rear brake locking issue doesn't appear.

Since then its been around 3.5-4years and they are still fine, just give it a really good flush every pad change, which for me is every 1 to 1.5years.

The good old castrol red bottle stuff will work well until the problem is sorted. Thats IF its the fluid causing the lock.

[mad082]

are the front and back brake pads the same sort? if you have crappy pads on the front, and good ones on the back then that could be the problem.

[dangerous_daveo]

are the front and back brake pads the same sort? if you have crappy pads on the front, and good ones on the back then that could be the problem.

He did say same pads all round. And slotted front disks.

Jason, did you end up getting the calipers rebuilt at all? If so there MIGHT be a problem there.

[AzzurrA]

pads are ferodo DS2500's front and rear...

nah, didn't get the calipers rebuilt, however, we tested them when we had the pads out... all the pistons are moving freely and working correctly etc, dust boots seemed fine. so we didn't see any need to rebuild them.

HOWEVER

i now have another problem, that i am getting very stressed about, due to there being a track day on saturday that i REALLY want to make it to....

there is a noise coming from what seems to be the front left wheel.

at first i thought it might be a stone in the tyre - nope.

what it sounds like, is a 'plasticky-sounding' clicking/whirring/grinding noise that increases/decreases with wheel revolution, only seems to start once the car has warmed up (i.e. doesn't do it first thing in the morning).

I dropped into my suspension place on the way to work this morning, and he seemed to think everything is fine, and that its just a high spot in the pad or rotor that rubs at a certain point....

now, i've heard what he's describing before, and it sounds nothing like that, and the other thing that makes me think its not that, is that the noise is still there when you're using the brakes (i.e. if you left foot brake and still drive along, its still there etc)

now, this has me stumped, and fairly worried...

HELP!!!!

[mad082]

He did say same pads all round. And slotted front disks.

my bad

[mad082]

the noise could be the wheel bearing.

[AzzurrA]

Digging up a bit of an old topic here...

have tried just about everything i can think of to resolve the brake bias issue, but nothing seems to work...

have re-bled the lines, have taken front pads out and scuffed them up a bit and put them back in incase they were glazed/not bedded, have changed rear pads back to standardish ones (i have a feeling they're bendix ultimates), have put better tyres (semi-slicks) on the car incase it was tyres being too crap for the brakes... nothing seems to change it...

about the only thing i can think of left to try is to fit a proportioning valve.... gonna call around on monday morning and see if i can get somewhere to fit one on Monday (Track day on tuesday, which i could use to set it up)...

anyone know what a good one to get is? Spoke to a place that said they could fit an in-cabin adjustable one for me but that seems to be a bit hard-core for what i'm using it for, so probably just a simple one that controls the rears only would be fine... i've seen some Wilwood ones for sale on Ebay for around $80 (and know a place that can buy them in from their supplier too) would this be okay?

[T04GTR]

not enough toe or to much toe on the rear