Brake imbalance

ejmaztec

On an nct re-test, the rolling-road came up with a 43% imbalance on the rear brakes. How much difference do any of you hi-tech mechanical intellectuals think that it would have made had the offside rear tyres not had 7lb more pressure than the other one? The offside rear brake wasn't as effective as the nearside according to the test.

BrianD3

I don't think it would make much difference for the brake roller test. Tyre pressure would make more difference for the suspension test.

The UK MOT testers manual mentions that damage may occur if underinflated tyres are subjected to the brake roller test but it mentions nothing about under/over inflated tyres causing a brake imbalance.

The NCT testers check/adjust (or are supposed to) tyre pressures before the test, did you check your tyres after the test and find the imbalance? Was the offside tyre at the correct pressure or was it overinflated? Also, seeing as this was a retest presumably you already failed on brake imbalance?

ejmaztec

"Somebody" was supposed to check the tyre pressures before the re-test, in the same way that "somebody" was supposed to check the car over prior to the first test. The nct testers couldn't check all of the braking system at the first test as one of the rear brake-hoses burst during the test:eek:

I checked the tyre pressures after the re-test and found that the offside rear was 7lbs over-inflated. I read somewhere that altering tyre-pressures can adjust the brake imbalance.

BrianD3

The reason I think tyre pressure wouldn't really affect the roller test is because it is surely a test of how well the brakes stop the wheels rather than how well the tyres stop the car. If its the latter then cold tyres, hot tyres, wet tyres, worn tyres will affect it as well. If the friction on the brake roller surface is high (which I think it is) then these factors should be minimised.

Steve

I had this a few years back - turned out to be a small crack in one of the calipers.

It's not something you should dismiss as being caused by tyre pressure.

crosstownk

Sounds like a possible leak/malfunction from a rear wheel cylinder. I doubt tyre pressure is the culprit.

Avns1s

Came across a similar problem and changing the rear shoes sorted it, even though the one's already there weren't work.

They could have become contaminated or one of them might hvae become hard.

Slidey

I have often seen totally different readings off the brake tester because the wheels have been wet.
I dont think an over inflation will give an imbalance of 43% though..
The max allowed is 30%.
Just a point to note, the side with the lower brake readings is not always the side you should concentrate on. Sometimes it is better to tweak the high side as this may be the side that is causing the imbalance..
Post your readings and rear axle weight if you want...

ejmaztec

Thanks for the input lads.

Mislaid the NCT report but found it again after searching for a couple of days:rolleyes:.

Rear Nearside 1.78kN

Rear Offside 1.02kN

Vehicle weight 1312kg.

Took off the rear drums to find that the offside shoes are worn more than the other side. I'm assuming that, as there was a problem with the handbrake not self-adjusting, coupled with the fact that the offside handbrake cable was 2 inches longer than it should have been (stretched to buggery), this screwed the offside shoes.

Pity LIDL don't do rolling-roads for €30 so that I can experiment for free with what difference different adjustments to different things make.

The shoes having been replaced both sides and it's off to the test-centre for the third time:eek:

Slidey

Well best of luck, if you were near sligo i'd pop it on the rollin road for ya!
Give the new shoes a good bedding in by pulling the handbrake as you are driving for a bit.. not enough to lock the wheels or set them on fire though!
At 1312kg you need a total of 55%
Ya should be getting well over 200 on the front a side which is 400
and 150 a side on the back would then give you a pass..