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Car wriggling with big braking

Abo Bob
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Abo Bob Grand Master

Abo Bob

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When I'm on the track and brake really hard with a completely empty boot I find the back end of my car wiggles around a bit which can turn into a rear end slide out if I turn my front wheels too much more than straight.

It doesn't seem to to do it with full kit on though, perhaps the extra weight in the boot is stabilising or something.

Anyway, I've just had my front rotors replaced with slotted DBA4000's and the standard rears skimmed. Now on heavy braking I am getting the same wiggling but more pronounced. It's s little scary because I feel now I can't use the full force of the brakes in case I go sideways.

Oh and also I had the brakes bled at the same time.

Any ideas peeps?

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benm Grand Master

benm

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I was getting this towards the end of the last Wakefield trackday, it only happened when I really heated up the back tyres and they became a bit worn/skatey.

Does it happen out on the street or only at trackdays?

Abo Bob Grand Master

Abo Bob

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benm - it did only happen at the track but now on the street too.

Adrian - I do actually do that on the track. Got it nearly down at Wakefield but EC corners still a bit too big for my balls.

So there's no adjustment for front/rear bias that can be made?

Sydneykid Grand Master

Sydneykid

Posted
Posted
benm - it did only happen at the track but now on the street too.

 

Adrian - I do actually do that on the track.  Got it nearly down at Wakefield but EC corners still a bit too big for my balls.

 

So there's no adjustment for front/rear bias that can be made?

Still got the HICAS?

Tried adding a bit of rear toe in?

Neg camber on the rear?

I have a few others, but these are where I would start :P

GTR32 Rising Star

GTR32

Posted
Posted

At the risk of stating the blindingly obvious, it's probably happening because you are losing grip/traction at the rear. Might seem like a simplistic sounding statement but this can happen and end in a spin like you've described. The rears losing grip is a bastard of a thing to have happen but there are only so many reasons for it.

When you brake weight transfers onto the front and off the rear. The more the weight is transferred forward the more chance you have of losing traction as the rear has a tendency to rise; watch the Moto GP bikes brake for corners with their rear wheel in the air, same principle. A soft/stuffed rear/front shock combo will do it, opposite to the way drag cars are set up to squat in the rear with soft shocks. When the back end lifts enough it loses traction and under braking can lock easily. The fact that you've given the brakes some attention and are now probably generating more weight transfer, and that the lose happens more when there is nothing on the boot, supports this.

Ditto wheel alignment; if it's not set up right it will go birko when the weight is transferred to the front and lose traction again.

If you are running worn or crappy tyres on the back it will cause them to lose traction/grip quicker than if you are running tyres that are good quality.

Any of these seem familiar?

Duncan Grand Master

Duncan

Posted
Posted

I would start with the alignment....but it could just be the front brakes are working well.

I find that my gtst used to lighten up a fair bit under really heaving braking when using good tyres as well :P

It could be that you need to reconsider how you brake into a corner on the track, generally you would make around 90% of your braking in a straight line...when you do this it doesn't matter too much if the rear end is light. If you are braking too hard as you turn the rear can get too light on you and the car spins....

Abo Bob Grand Master

Abo Bob

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Hmm,

The phenomenon I'm describing I have also experienced in your car Duncan (now that I think about it).

Lately I have noticed a squeaking coming from my front end when the car is stone cold only going over biggish bumps. This goes away when warm and although I was thinking creaking springs, springs don't warm up, but shocks do. I have done 82000 k's and the rear left shock shows signs of having leaked at some stage (all original shocks) but doesn't anymore. Also I have hit the bump stops on the front with mega hard braking before the last turn at Wakefield several times. Perhaps my shocks are stuffed?

Sigh. Time for $2300 coilovers.

Thanks for the help guys.

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