NCT brake ovality query

Irish Steve

A query, based on suspicion that it's the test procedure that's not doing what's expected, or something along those lines.

2011 Grand Picasso C4 was in for test today, which it passed, but if the figures for the rear brakes are to be believed, by the skin of it's teeth, due to brake ovality on the rear discs, one side was on the limit of 90%, the other was 88%.

Brutal honesty, I don't believe those figures, if they are valid, and if the discs were that much out of true, I would have expected to feel that back through the pedal when driving, especially when braking gently rather than hard. The day to day driving is very much the opposite, the brakes work well and there's no hint of any problems with them, but if there is an issue developing, I'd rather get on top of it before it becomes a problem, even though it won't need another test for just over 2 years.

And in passing, this is a test after 2 years, all the discs and pads were replaced when the vehicle was imported and it's only done 24K since that time, the front discs are showing 15 and 11% ovality, and there's plenty of life left in the pads, as the vehicle doesn't get driven hard as such.

The performance figures are fine, service brake 79%, and parking brake 30%, with the brake effort from the parking brake being higher than the service brake.

What was pleasing was the smoke test, 0.010, which is actually better than the figure it got 2 years ago, probably helped by a good long motorway run at the weekend.

So, am I just being paranoid, or is there an issue that will need to be looked at before too long?

mickdw

Irish Steve wrote: »

A query, based on suspicion that it's the test procedure that's not doing what's expected, or something along those lines.

2011 Grand Picasso C4 was in for test today, which it passed, but if the figures for the rear brakes are to be believed, by the skin of it's teeth, due to brake ovality on the rear discs, one side was on the limit of 90%, the other was 88%.

Brutal honesty, I don't believe those figures, if they are valid, and if the discs were that much out of true, I would have expected to feel that back through the pedal when driving, especially when braking gently rather than hard. The day to day driving is very much the opposite, the brakes work well and there's no hint of any problems with them, but if there is an issue developing, I'd rather get on top of it before it becomes a problem, even though it won't need another test for just over 2 years.

And in passing, this is a test after 2 years, all the discs and pads were replaced when the vehicle was imported and it's only done 24K since that time, the front discs are showing 15 and 11% ovality, and there's plenty of life left in the pads, as the vehicle doesn't get driven hard as such.

The performance figures are fine, service brake 79%, and parking brake 30%, with the brake effort from the parking brake being higher than the service brake.

What was pleasing was the smoke test, 0.010, which is actually better than the figure it got 2 years ago, probably helped by a good long motorway run at the weekend.

So, am I just being paranoid, or is there an issue that will need to be looked at before too long?

How do they test this at nct?

Irish Steve

Not sure exactly how the system arrives at the figures, I am assuming it's part of the overall brake test, which happens on a rolling road system of rollers that test each axle in turn, and from what I have seen, the front axle of the vehicle is driven into the rollers, which then start turning, and the test continues as the tester applies the brakes until the vehicle is effectively spat out of the rollers by the strength of the brake force applied, they then do the same for the rear axle, and then repeat the rear axle for the parking brake. I am assuming that there are sensors on the rollers that detect the brake force being applied by the vehicle, and the computers do the rest based on the weight etc. I don't know which phase of the test does the ovality part of it.

I do know that this system can't be used on vehicles that have permanent 4 wheel drive, but there's very few of them that go through the NCT system.

That's my understanding of how it works, but there may be others here that have a more in depth understanding of exactly how it works.

greasepalm

Look closely at the discs any black marks showing high spots,wheel bearing can also throw your disc out of alignment.Warped discs are usually from heavy braking.

Rolling road is the only thing that check brakes.

jetfiremuck

Most of the braking is done on the fromts. If you look at the rear disc and it looks pitted you may have sticking pistons. I put a set of pads in my car (had same type issue) after a week the discs were clean and shiny.