How fast do discs wear?

[silentbrown]

Obviously something of an "it depends", but what are ball park figures for mm per 10K miles with modern brake pads?

Prompted by garage saying these rear discs need changing in the next 1000 miles or so. Difficult to measure properly as there's an annoying guard behind.

They're only lightly lipped and no serious scoring. I'm tempted just to renew the pads (which do need doing!)

[Super Sonic]

It depends. There are too many variables to give a figure. Do you constantly accelerate and brake in stop start traffic, or trundle along slowly so by the time you've caught up to the vehicle in front it has started moving? Do you come off the gas early approaching junctions and roll up to them, or tear up to them and brake hard? The more you can anticipate traffic flow, the longer your brakes last.

[silentbrown]

Super Sonic said:

It depends. There are too many variables to give a figure. Do you constantly accelerate and brake in stop start traffic, or trundle along slowly so by the time you've caught up to the vehicle in front it has started moving? Do you come off the gas early approaching junctions and roll up to them, or tear up to them and brake hard? The more you can anticipate traffic flow, the longer your brakes last.

Fair enough. But a garage isn't going to know that when saying "your discs need changing". There must be some "typical" figures.

[Super Sonic]

That is true, they may last 1,000 miles or 10,000. Given the price of discs, and the fact they are quite important, I would err on the side of caution rather than trying to wear them down to the limit. The garage could have said they will need replacing 'soon', but that would have been even more meaningless. I would change them when you do the pads. I had advisories on all pads and rear discs on my MOT, so replaced them all at once within a month. I was told the front discs were getting worn, and they would probably be due replacement next time I changed my front pads.

[cerb4.5lee]

Looking at that disc I'd be just replacing the pads I think. In terms of how long they last/wear, there are too many variables I think. Our Merc is on 70k miles now, and it is still on the original discs and pads from the factory for example.

[silentbrown]

Super Sonic said:

That is true, they may last 1,000 miles or 10,000. Given the price of discs, and the fact they are quite important, I would err on the side of caution rather than trying to wear them down to the limit. The garage could have said they will need replacing 'soon', but that would have been even more meaningless. I would change them when you do the pads. I had advisories on all pads and rear discs on my MOT, so replaced them all at once within a month. I was told the front discs were getting worn, and they would probably be due replacement next time I changed my front pads.

Thanks. Price of a pair of discs is about 4 times the price of a pair of pads... Also, it's the first time I've seen the disc wear indicator 'cutout' (see pic) and my assumption was that when the wear reached that, you should change them. Probably 2mm each side before they reach that?

[Super Sonic]

The rear discs on my car were about twice the price of the pads. At the end of the day, only you can judge how heavy you are on brakes,so you have to use your judgement. If you have had the car for more than a year, you may have some idea how much they have worn between mots, so this may help you decide.

[wyson]

I always ignore them when they say brake pads or discs need to be changed in x thousand miles.

I wait until they say these need changing now.

I went 3 years and 15k miles with them saying my rear brake disks needed changing soon!

My only regret was applying that strategy with bushings. I should have changed them out earlier because it quietened the ride. Live and learn I guess.

[Maxdecel]

silentbrown said:

Thanks. Price of a pair of discs is about 4 times the price of a pair of pads... Also, it's the first time I've seen the disc wear indicator 'cutout' (see pic) and my assumption was that when the wear reached that, you should change them. Probably 2mm each side before they reach that?

Min thickness spec quoted in here, just dial in your car; the spec may be engraved on the disc periphery or wheel face.
https://www.brakebook.com/bb/textar/en_GB/93087900...
The cut out on the disc is for balance.

[E-bmw]

Maxdecel said:

silentbrown said:

Thanks. Price of a pair of discs is about 4 times the price of a pair of pads... Also, it's the first time I've seen the disc wear indicator 'cutout' (see pic) and my assumption was that when the wear reached that, you should change them. Probably 2mm each side before they reach that?

Min thickness spec quoted in here, just dial in your car; the spec may be engraved on the disc periphery or wheel face.
https://www.brakebook.com/bb/textar/en_GB/93087900...
The cut out on the disc is for balance.

^^^^ Wot 'e said.

Correct.

Also, you have shown 1 pic of one side of one disk, what about the inside face & what about the other disc/discs they say need replacing.

[Sheepshanks]

What car is it, and was it the franchise dealer who said they needed doing?

With a few brands in the family ‘fleet’ I’ve always found VW related dealers keen to say the pads and discs are wearing out. Apart from the SEAT dealer we use, who said they were all 50 % at 60,000 miles then the rear pads wore out between services.

[silentbrown]

Maxdecel said:

The cut out on the disc is for balance.

D'oh. I'd read it was a wear indicator, which is clearly wrong. So only way to really measure is with suitable Vernier/micrometer which may be awkward.

Thanks!

[Simon_GH]

It used to be common to keep the same discs and just replace the pads but as pad material became harder (banning asbestos is one reason) then the pads tend to wear out the discs so they are commonly replaced together.

If the discs look fine then I’d trust your common sense. Rear brakes are important but don’t provide much of the braking force. I’m not sure how many drivers would even notice if their rear brakes stopped working.

[wyson]

When their car rolled down a hill parked with the handbrake on?

[cerb4.5lee]

Simon_GH said:

If the discs look fine then I’d trust your common sense. Rear brakes are important but don’t provide much of the braking force. I’m not sure how many drivers would even notice if their rear brakes stopped working.

Rear discs play a bigger role than they used to I'd imagine with traction control etc though. I remember being confused at first as to why the rear pads in the E90 330d we had years ago wore out before the fronts for example. It was because the traction control was always nipping away at the rear discs though.

The rear pads are wearing much better than the fronts though in the F82 M4 I have now, because if I decide to push on I make sure that the traction control is loosened off now, and it is prolonging the life of the pads. I also switch the traction control off in the 370Z, because if I don't it just cuts in far too enthusiastically for my taste for example.

[Maxdecel]

cerb4.5lee said:

Rear discs play a bigger role than they used to I'd imagine with traction control etc though. I remember being confused at first as to why the rear pads in the E90 330d we had years ago wore out before the fronts for example. It was because the traction control was always nipping away at the rear discs though.

The rear pads are wearing much better than the fronts though in the F82 M4 I have now, because if I decide to push on I make sure that the traction control is loosened off now, and it is prolonging the life of the pads. I also switch the traction control off in the 370Z, because if I don't it just cuts in far too enthusiastically for my taste for example.

Spoke to a Beema owner with the same earlier in the week, warning came up 1350 miles remaining on rear pads. Damn clever these computer things.

[bimsb6]

Our lexus nx300 has worn out the discs before the pads ! 54k milesfrom new and the discs were down to the minimum limit when the pads were only half worn , the rear discs were on the limit which is only 1.5 mm below new thickness.

[alabbasi]

When I lived in Atlanta which is hilly, It would be often. Now I'm in Texas where it's flat. Hardly ever.

[MustangGT]

alabbasi said:

When I lived in Atlanta which is hilly, It would be often. Now I'm in Texas where it's flat. Hardly ever.

Exactly, it is totally dependent on driving conditions. Most of my miles are motorway, rarely touch the brakes in 100 miles, around town it is every few metres.

[Smint]

The only car i've known wear the discs as rapidly as the pads was the ex MiL's Golf 2, the pads were hard as hell and needed a heft shove on the pedal to stop the car.
I swapped out the front pads for standard Ferodo pads, a much softer pad which required probably half the pedal pressure to stop the car, only trouble was it caused an imbalance of the braking and prone to front wheel lock up in the wet (no abs), so had to fit Ferodo brake shoes to the back too.
Must be said it was a much nicer car to drive after this.

I've usually got the wear of two sets of pads from a set of discs, doubtless the move to regenerative braking on hybrids has changed things so maybe dis corrosion is more of a problem than it once was.