Brake discs

They warp.

Apart from the car failing the MOT they are likely to crack and distort. They can actually wear quite a bit before they are unacceptable. However I have just had to replace a set on a Mercedes Vito van due to cracks after only 40,000 miles. This is a record as far as I'm concerned and barely acceptable, especially as it needed a new engine at the same time for unrelated reasons.

Huw

Or if vented types could break up with disastrous consequences.

Both are entirely related - White Van Man has been driving it ;-)

If I didn't know the driver and his history with other vans and the fact that he is a very skilled mechanic [NOT fitter] then I would agree. My own ML 270 did show very quick disc wear also. It only did 22000 miles in my [wife's mainly] care and showed nearly as much disc wear as my Land Cruiser automatic does at 100,000.

Checking disk thickness isn't part of an MOT test.

-- Dave Baker

I was given an advisory regarding these at my last MOT.

( incidentally re. mileage, like much on my Volvo 740, these discs are original, at 230k, is there any scope for a claim against the manufacturer?)

:-)

Really? I'm sure I had a car fail a few years back for this very reason. Could have been the garage trying it on, co-incidentally they also did repairs, but I did it DIY cheaper anyway. And the disks were thin, even if it hadn't failed and I'd got an advise I would have changed them.

In message , Steptoe writes

Perhaps the tester was just trying to help with some friendly advice. As Dave said, they can't fail on disc thickness because the don't know what the minimum thickness should be.

That is rather good.

Dr Wu ( snipped-for-privacy@tiscali.co.uk) gurgled happily, sounding much like they were saying :

Others have said that they warp and can crack. They can also cause the brakes to fade more quickly, by not being able to dissipate the heat as they should.

Not directly, but a vehicle can fail if a brake disc is "...excessively scored, pitted or worn".

Chris

Yes indeed and also for being cracked excessively. The tester has a discretion and he should steer towards the safe side because if a disc failed catastrophically within a reasonable time after the test he would be in shit up to his neck. The disc is after all part of the MOT test as shown above.

Huw

However it's a completely subjective thing. Tyres have a legal wear limit, pads fail the MOT if they're worn to the backing plate but disks have no legal wear limit and would only fail on examination if they were down to the ventilation holes. If the car passes the brake roller test there's not much an examiner can say just because the disks are thin. Brake fade clearly isn't an issue on a 4mph brake roller test although the car might not be so happy during a 100 mph emergency stop.

The disks on my Focus when I bought it were down the the makers recommended wear limit of 2mm under spec after only 30k miles. I skimmed them in the lathe and put new pads on and to anyone's casual eye they look brand new. You'd only know if you had a disk thickness chart to hand and happily that isn't part of the MOT test. I'm sure they'll be fine for another 30k miles, mainly because I use engine braking rather than the brakes in most situations. Maybe the previous owner was one of these cretins you see dabbing the brakes before every corner even when they're going so slow you don't need to.

-- Dave Baker

They shatter with catastrophic results.

sponix

Tut, tut.

Learners are taught not to use engine braking nowadays but the brake pedal.

This is because brakes rarely 'fade' nowadays and normal braking is considered safer..

sponix

The message from snipped-for-privacy@email.com (sPoNiX) contains these words:

Neither. The preferred method is to teach proper anticipation so you don't need either /but/ they should also be taught to brake lightly even if they've predicted and correctly implemented a nice gentle slow-down so that the brake lights come on.

I know this 'cos I was picked up on it by the SE[1] during a check-test[2] where I'd spent twenty minutes getting a learner to read far enough ahead to not need the brakes. I got a Grade 5 (max 6) 'cos I should have remembered to eplain the need to show the brake lights.

[1] Supervising Examiner - sits in on driving lessons sometimes to ensure quality of instruction. One one occasion I also had the regional bloke watching the SE watching me watching the learner. Just a /little/ pressure, you understand. [2] Occasional tests of instructors - like having OFSTED visit a school.

Not being a learner for some 25 years now I'm not too fussed about how they might be taught to drive nowadays. How is being forced to use the brakes safer than driving with such forward anticipation that engine braking alone is enough to reduce your speed sufficiently for each corner and still having the full force of your braking system available to use if you should need to?

Fuel injected cars use no fuel at all on the overrun so using engine braking not only saves brake system wear but also fuel and emissions.

Everytime you lift off the accelerator you're using engine braking. Is that illegal now? Should we all be keeping the throttle flat to the floor at all times and fighting the acceleration produced by having the brakes on to compensate? Bollocks we should.

-- Dave Baker

Show them to whom? When I'm cracking on, using brakes or otherwise, there won't be anyone keeping up to show any brake lights to unless they're in something fairly sporty and trying fairly hard.

Whatever happened to "if you run into the back of anyone else it's your fault pretty much regardless of the situation"? WTF should anyone need to dab their brakes on if they're going slowly enough not to need to just to show brake lights to some f****it behind them? This sounds like utter numptiness to me.

-- Dave Baker

Gears are for going, brakes are for stopping. Engine braking is only used for going down steep hills to prevent runaway.