Do these brake discs need replacing

[Welshbeef]

On my car they are shiny from edge to edge always - and that’s both cars.


My personal car I had the front discs changed end of last year and new pads all round. Car had done 90+k on the front discs. The rear discs were fine and remain on - they look like new and based on my usage should last to 150M+.

Re those dics in Original post I’d change them it’s moving into winter they are 85% worn you want the best possible stopping distance. On your usage they will last another 5-6years so very reasonable pence per mile and so crucial to your safety.
Out of interest when did you last change the brake fluid?

[Boosted LS1]

The pads aren't bedding in fully with the discs. Just replace the lot, it can't be that expensive surely.

[38911]

stevieturbo said:

If the visible side is like that, you can be sure the other side is a lot worse.

How can “you be sure” of that? Please explain?

If it’s a sticky calliper then the chances are the inboard side of the disc will be much cleaner - assuming it’s a single piston calliper driven from the inboard side.

It’s just passed an MOT which means the brakes meet the required effectiveness. End of story.

[Welshbeef]

38911 said:

How can “you be sure” of that? Please explain?

If it’s a sticky calliper then the chances are the inboard side of the disc will be much cleaner - assuming it’s a single piston calliper driven from the inboard side.

It’s just passed an MOT which means the brakes meet the required effectiveness. End of story.

They “pass” the test.

Remember for the handbrake to pass an MOT they only needs to meet 15% of the original specification....

Fact is his stopping distance WILL be longer /nearly the longest possible from a discs and pad perspective currently v brand new plus of course going into the winter season it’s the last thing you need.

Do you run your tyres down to 1.6mm too? It’s legal but under 3mm they have used up 90% of the wear and in wet conditions are vastly longer stopping distances.

To me stopping distance and lighting is the most important part of vehicle safety period - and for a few quid your be pretty daft not to change it

[CrutyRammers]

Welshbeef said:

Fact is his stopping distance WILL be longer /nearly the longest possible from a discs and pad perspective currently v brand new plus of course going into the winter season it’s the last thing you need.

Nope. As long as disks and pads are within operational limits, stopping distance should be the same as new ones.

[Welshbeef]

CrutyRammers said:

Welshbeef said:

Fact is his stopping distance WILL be longer /nearly the longest possible from a discs and pad perspective currently v brand new plus of course going into the winter season it’s the last thing you need.

Nope. As long as disks and pads are within operational limits, stopping distance should be the same as new ones.

I call custard on that one. Where are the tests to prove it mirrors the same as brand new pads and discs.

Also 60k mikes without question will have reduced its thickness so stopping distance increases.

[CrutyRammers]

Welshbeef said:

I call custard on that one. Where are the tests to prove it mirrors the same as brand new pads and discs.

Also 60k mikes without question will have reduced its thickness so stopping distance increases.

You think that stopping distance is related to the thickness of the disk?

[steveo3002]

things people have to argue about on here lol

yes it will limp on for a bit more , if you want it right then its time to think about replacements

all the ones ive ever changed are always worse on the inside

[Welshbeef]

CrutyRammers said:

Welshbeef said:

I call custard on that one. Where are the tests to prove it mirrors the same as brand new pads and discs.

Also 60k mikes without question will have reduced its thickness so stopping distance increases.

You think that stopping distance is related to the thickness of the disk?

Yes - disc will get hotter quicker and become less efficient meaning longer stopping distance.

It’s related to a huge number of different things but braking efficiency is certainly one

[227bhp]

38911 said:

stevieturbo said:

If the visible side is like that, you can be sure the other side is a lot worse.

How can “you be sure” of that? Please explain?

If it’s a sticky calliper then the chances are the inboard side of the disc will be much cleaner - assuming it’s a single piston calliper driven from the inboard side.

It’s just passed an MOT which means the brakes meet the required effectiveness. End of story.

It's normal for the inner face to rust far more than the outer.

[CrutyRammers]

Welshbeef said:

Yes - disc will get hotter quicker and become less efficient meaning longer stopping distance.

It’s related to a huge number of different things but braking efficiency is certainly one

And do you think that heat build up is a significant factor in the braking performance on the rear disks of a road car in normal use?

[t400ble]

In a word, yes

[Welshbeef]

CrutyRammers said:

Welshbeef said:

Yes - disc will get hotter quicker and become less efficient meaning longer stopping distance.

It’s related to a huge number of different things but braking efficiency is certainly one

And do you think that heat build up is a significant factor in the braking performance on the rear disks of a road car in normal use?

It’s efficiency is less - handbrake brake test only has to achieve 15% of a brand new part....


Anyway looking at those discs they are probably £20 items so it’s crazy not to get them done along with a brake fluid change.

[CrgT16]

Discs are fine... they are not even close to manufacturers minimum spec. I bet if measured they probably not even half way so... why change?

On track duty/racing the heat Issue with the disc is more of a problem on normal road, providing is whitin spec makes no real world difference. If the OP can noticeably feel the difference with more spirited driving he should also be more aware of the need or not to replace anyway. Put it this way, if it’s within spec it will be just as safe for 95% of the drivers where more worrying about brake system performance is the poor reaction times and driving without proper attention.

As for tyres, I just changed my rear ones close to the legal limit and got another 10k out of them because of that. It was dry and they where fine. I knew what I had and drive accordingly. If it was winter time I would probably would have changed them earlier. My car is RWD and fairly rapid. I just drive to the car and conditions. My wife’s car... different story.

Horses for courses and all that....

[38911]

Welshbeef said:

Fact is his stopping distance WILL be longer /nearly the longest possible from a discs and pad perspective

Just because you start a statement with “fact is...”, doesn’t make it a fact. Especially when you’re talking twoddle.

Welshbeef said:

Do you run your tyres down to 1.6mm too? It’s legal but under 3mm they have used up 90% of the wear and in wet conditions are vastly longer stopping distances.

No I don’t let my tyres wear down to 1.6mm. Also I don’t stick my fingers in live electric sockets either, a fact that is about as relevant to this discussion as tyre tread depth.

Let me try and break it down into nice simple bite-size chunks for you. Tyres have treads to enable them to displace water. The depth of the tread affects their ability to do that. Therefore the performance of a tyre in the wet diminishes the more the tread wears as there is less space for it to displace water.

Brake discs work on friction between the pad surface and the face of the disc. Their effectiveness is not affected by the WIDTH of the disc. Just think hard and I’m sure you’ll figure it out. As the disc wears thinner, braking performance will remain linear until the disc wears so thin that it becomes structurally weakened. The disc in the photo is nowhere near that point - it’s probably tens of thousands of miles away from that.

Ironically, the best performing tyre in the dry is a bald one - but I fear If I try and explain that one to you, you’ll start dribbling.

Edited by 38911 on Monday 8th October 17:08

[TooMany2cvs]

38911 said:

Brake discs work on friction between the pad surface and the face of the disc. Their effectiveness is not affected by the WIDTH of the disc. Just think hard and I’m sure you’ll figure it out. As the disc wears thinner, braking performance will remain linear until the disc wears so thin that it becomes structurally weakened.

...although the heat dissipation of a thin disc will be much worse than a thick one. Which is one reason why there's a minimum thickness...

So, to use the tyre tread analogy, by the time a disc has got thin enough to fail to dissipate heat effectively in normal driving, it's "illegally bald", probably by quite a decent chunk.

[stevieturbo]

38911 said:

How can “you be sure” of that? Please explain?

If it’s a sticky calliper then the chances are the inboard side of the disc will be much cleaner - assuming it’s a single piston calliper driven from the inboard side.

It’s just passed an MOT which means the brakes meet the required effectiveness. End of story.

Clearly you have zero experience working at cars, nor any clue how calipers etc can seize.

The discs are st, end of story. The inner face will be worse, end of story. Stop being a tight arse and replace them.

[38911]

TooMany2cvs said:

...although the heat dissipation of a thin disc will be much worse than a thick one. Which is one reason why there's a minimum thickness...

So, to use the tyre tread analogy, by the time a disc has got thin enough to fail to dissipate heat effectively in normal driving, it's "illegally bald", probably by quite a decent chunk.

Nope, still wouldn’t affect the friction or braking power under normal use. The amount of heat generated even from a high speed emergency stop, is nowhere near enough to overwhelm the heat absorption abilities of a worn brake disc.

You might - after repeated deliberate high speed emergency stops - overheat the disc, but I’ve driven a car on a track so hard that I had the brake discs glowing red - literally - and have the photos to prove it. The brakes were still every bit as effective as when they were cold.

[TooMany2cvs]

38911 said:

TooMany2cvs said:

...although the heat dissipation of a thin disc will be much worse than a thick one. Which is one reason why there's a minimum thickness...

So, to use the tyre tread analogy, by the time a disc has got thin enough to fail to dissipate heat effectively in normal driving, it's "illegally bald", probably by quite a decent chunk.

Nope, still wouldn’t affect the friction or braking power under normal use. The amount of heat generated even from a high speed emergency stop, is nowhere near enough to overwhelm the heat absorption abilities of a worn brake disc.

You might - after repeated deliberate high speed emergency stops - overheat the disc, but I’ve driven a car on a track so hard that I had the brake discs glowing red - literally - and have the photos to prove it. The brakes were still every bit as effective as when they were cold.

If data is the plural of anecdote, then I'd like to offer up my first CX. It'd fade noticeably by the end of a single high-speed stop. The old ones were well below maximum wear limits, and a new set of discs and pads resolved it absolutely.

[38911]

TooMany2cvs said:

a new set of discs and pads resolved it absolutely.

I’d suggest any perceived improvement was either psychological (very common) - or your old discs had defective friction surfaces - or your new discs had a superior friction surface. But it had nothing to do with the thickness of the disc.

A car is something that people have an emotional connection with, so people often get a psychological affirmation when a part is replaced, even when it makes diddly squat difference.

If you’re happy then great, but please stop scaremongering based on a theory that - when you really think about it rationally - just isn’t very credible.