Brake disk thickness

jon1981

lads,

how do you know if your brake discs are worn and need replacing, i was feeling the discs the other day and could feel a minute lip at the edge... but how do i know if they are still within limit? (its a bmw e46 if that matters)

cheers

frank gooding

A minute lip is not a worry.

Generally you will see a noticable amount of difference.

Discs near finished can show tiny hairline cracks or will tend to warp.

You could go into a motor factors and ask them to give you a look at new ones to get an idea.

johndaman66

Are the thickness of the brake discs measured in the NCT. Mine seem pretty thin at this stage but on an examination of the eye do not appear warped or no hairline cracks.

Also my front brake pads most certainly need replacing. Would anybody be able to give me a ballpoint figure of how much I'd be looking at to replace these...labour only as have the pads and brake fluid bought. Car in question is a 1.0L 1996 VW Polo.

OP apologies for half hijacking your thread.

sundodger5

if you speak to the motor factors most of the catalogues they use will have the disc spec including minimum thickness.
armed with this info measure them. most discs go below minimum spec after 2/4mm of wear.
waiting for hairline cracks etc is just letting them wear down way too much.

JHMEG

Minimum thickness is usually written on the disc. Take off the wheel to see it. Measure with a caliper. Lip will be very pronounced before discs need replacing.

NCT do not measure thickness. Labour on a 96 Polo prob about 50 quid, or less if you're down the country.

Captain Slow IRL

You will need a special vernier to measure the disc - the min. allowance should be stamped on your disc, as JHMEG said - probably 18.4mm min (I think)

As a general rule with BMW's, the discs are usually on or very near the limit when the pads have to be replaced. You might get 2 sets of pads to a set of discs, if you're lucky but never 3. If you're a hard driver, it'll be pads and discs each time.

johndaman66

JHMEG wrote: »

Minimum thickness is usually written on the disc. Take off the wheel to see it. Measure with a caliper. Lip will be very pronounced before discs need replacing.

NCT do not measure thickness. Labour on a 96 Polo prob about 50 quid, or less if you're down the country.

Thanks JHMEG. Thats not as bad a price as I would have imagined and am down the country as in County Limerick. Was actually going to attack the job myself with the help of a knowledgeable friend but he had to work over weekend and will be next weekend also and NCT is Friday week. May get on the blower tomorrow and ring some garages for bestr price. Thanks again

Avns1s

johndaman66 wrote: »

Thanks JHMEG. Thats not as bad a price as I would have imagined and am down the country as in County Limerick. Was actually going to attack the job myself with the help of a knowledgeable friend but he had to work over weekend and will be next weekend also and NCT is Friday week. May get on the blower tomorrow and ring some garages for bestr price. Thanks again

As a matter of interest, why do you say they need replacing? Are they emitting a scraping / squealing sound during braking or is it by visually inspecting them?

Unless they are really worn or making a noise during braking, I'd leave them until after the NCT, as new pads take a while to bed in and this could affect the braking performance test. Just my 2c!

johndaman66

Avns1s wrote: »

As a matter of interest, why do you say they need replacing? Are they emitting a scraping / squealing sound during braking or is it by visually inspecting them?

Unless they are really worn or making a noise during braking, I'd leave them until after the NCT, as new pads take a while to bed in and this could affect the braking performance test. Just my 2c!

Brake pads have worn very very thin at this stage. There is practically nothing left of them. Brake performance seems to be very poor too. Hell even the back shoes may need replacing also but as I know the front ones do the very most of the work I will take a chance in replacing them for the moment. I think with the amount of driving I do they should be bedded in before test....thats assuming weather picks up also;)