Near Side Rear Wheel Locking Under Moderate - Heavy Braking

[SweetBadger 93]

I have a 1.6 gti brake system (single line to the rear, standard 1.6 compensator), 1.9 front calipers / discs, and a 1.9 rear disc beam.

The last race we did, the car was constantly locking the near side rear wheel. I put this down to the fact that the ride height on the beam was uneven so it was lifting the n/s rear wheel more.

I've reset the ride height, but the wheel is still locking!

I'm guessing that the issue is the rear brake compensation, but if that's the case then why is only the n/s wheel locking, as the pressure should be the same at both rear calipers?

I've ordered a Tilton Brake Bias Adjuster Valve with M10 x 1 ports, so I should just be able to bolt this in place of the 1.6 compensator on the chassis leg and adjust it so that the rear doesn't lock, but I'd like to understand how/why it's only the n/s rear wheel locking.

 

Any ideas?

 

Thanks,

[Tom Fenton 1,536]

What are your corner weights like across the diagonals? NSR will be the most unloaded wheel with the driver sat in a RHD car.

 

I would also plumb the adjustable valve somewhere within reach. You can use a lot more rear bias in the dry compared to the wet, for example.

[SweetBadger 93]

When they were last checked the corner weights were quite even across the wheels, but that was with the rear ride height uneven so I'd need to get it rechecked.

 

I guess adjusting the offside coil over at the front to raise the offside front a little will transfer more weight to the n/s rear, so could try that?

 

Good advice on the location of the valve - I plan to plumb the valve within reach, but need it in place before this Saturday, so it'll have to live on the chassis leg short term in place of the standard compensator until I get chance to run new copper lines.

[petert 578]

I run a single line with a Tilton valve and still suffer lock up if the car isn't set up correctly. In a RH drive car that wheel has the least mass on it and locks up easily. Corner weighting definitely helps, as does droop and having sticky tyres. Adjust the valve so it has almost zero bias to the rear.

[SweetBadger 93]

I managed to get the Tilton valve fitted in place of the standard regulator yesterday, and with the valve set to minimum rear pressure the n/s wheel wasn't locking - that's with road tyres in the wet so seems to have done the job.

 

Will probably increase the pressure to the rears slightly tomorrow if it's dry.

[petert 578]

Leave the rear pressure at an absolute minimum for circuit work. Some racers even block off the rears completely with a ball bearing in the line, so it appears legal.

[welshpug 1,646]

check you actually have a pair of 30mm pistons, not a 30 and 32.

[Anthony 1,002]

Leave the rear pressure at an absolute minimum for circuit work. Some racers even block off the rears completely with a ball bearing in the line, so it appears legal.

Out of curiosity, why is that Peter?

 

I would have thought from a logical point of view you'd want as much rear brake effort as you could without locking the wheels, so as to maximise braking force and to take some of the load (heat) off the front brakes by sharing the load.... so clearly I'm missing something

 

Certainly driving a road car with a failed rear compensator leading to low rear effort didn't feel nice and was dramatically better once the compensator was replaced and rear effort restored to normal.

[Tom Fenton 1,536]

I also don't get the logic behind that. I have always ran as much rear effort as I can get away with, without locking rear wheels on hard braking. Its markedly different what you can use in dry compared to wet conditions. The standard compensators are of course set up so the rears won't lock in the worst of wet conditions to be safe for Joe Public. I have found on a number of different cars that you can make them stop a fair bit better in the dry by allowing more rear brake bias.

[petert 578]

Every corner is a full, emergency braking situation. In a short wheel base, fwd car, there is a huge weight transfer to the front axle, leaving minimum weight on the contact patch. Thus the tyres just lock up. Sure, try and limit it, but in a 205 I don't think it really matters if the rear brakes work or not.

[Toddy 7]

A brake test at an MOT station will inform you of the percentage difference being applied on the rear axle, most likely be the compensator valve (I can't explain why it applies more force to one wheel, when they are knackered, but they sometimes do!) which you are going to change anyway.