Does braking efficiency decrease as pads wear?

[TallTim]

My Audi is in for a service. Audi just rang me to say the front pads & discs are 80% worn and they recommend changing them now. Since I've got the best part of 40K miles out of them so far, I figured that the remaining 20% should give me another 10K miles which is probably another 5 months, so I said no thank you. They then proceeded to tell me that even though they were not completely worn out, my braking efficiency would be reduced so I really should change them now. I'm pretty sure this is BS & still said not to change them - am I right? My thinking is that the same surface area of pad is in contact with the disc at the same pressure so braking effect should be the same whether worn or not. In extreme conditions maybe more heat is being transferred to the caliper because there's less pad to absorb it, and maybe a more worn/hotter disc is more prone to warping but I can't see this being a major factor, especially in day-to-day road driving. Anyone actually have any data on this?

[Watchman]

I agree with you in principle because the pad material is the same throughout the pad thickness, but in practice I find the initial bite better with newer pads. Perhaps the heatcycles change the compound slightly? Perhaps they're glazed a little? Dunno, but there is a little difference. Possibly nothing to worry about as you're clearly used to the performance you have now out of your brakes. In fact I have just turned down my own garage's attempts to have mine changed at this point based on the same argument however I do look forward to new pads next month (or maybe Feb).

[Lowtimer]

Your brakes with their thin pads will be more prone to fade than on new pads, and you will have more pedal travel.

There's not a lot of merit in trying to take the last millimetre off a brake pad. If you run them down to the backing plates you will definitely screw the discs, and you will have no braking at that point. If you are the sort of person who is happy taking the wheels off and changing his own pads, you could run them down a bit further by giving them weekly inspections, but if you like to have other people do the work what's the point having to take the car back to the garage?

Mind you, on modern-ish cars with big asbestos-free pads like these, the discs are often also pretty much worn out by the time the pads have had it. What do the discs themselves look like, or haven't you inspected them recently?

Edited by Lowtimer on Thursday 29th December 15:04

[WeirdNeville]

Their estimate of 80% worn is probably a bit over stated anyway, to make you spend the money.

If you've got big gappy alloys just poke through ,it's normally possible to identify the pad and check its thickness visually. If not you might have to pull a wheel off. But I'd be tempted to get another 5-10k's miles out of your current pads.

Pads work fine until they're very thin - when I changed my BMW's pads there was just a smear of material left, but they worked fine. Sure ,I wouldn't have done a trackday on them ,but they stopped the car fine.

The main issue is disc thickness - the disc acts as a heat sink and is the primary means of dumping heat from braking. If the disc gets too thin it will overheat, causing all sorts of problems.

Just keep an eye on it an change it when you actually need to.

[mrloudly]

Lowtimer said:

Your brakes with their thin pads will be more prone to fade than on new pads, and you will have more pedal travel.

There's not a lot of merit in trying to take the last millimetre off a brake pad. If you run them down to the backing plates you will definitely screw the discs, and you will have no braking at that point. If you are the sort of person who is happy taking the wheels off and changing his own pads, you could run them down a bit further by giving them weekly inspections, but if you like to have other people do the work what's the point having to take the car back to the garage?

Mind you, on modern-ish cars with big asbestos-free pads like these, the discs are often also pretty much worn out by the time the pads have had it. What do the discs themselves look like, or haven't you inspected them recently?

Edited by Lowtimer on Thursday 29th December 15:04

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

[jimxms]

mrloudly said:

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

^^ this is how I understand it.

[E30M3SE]

Do Audi not fit brake pad warning sensors??

[PhillipM]

As others have said, you don't take brake pads down to the last mm or two - you're risking overheating the material, transfering more heat through to the fluid (so the brakes fade earlier), and even the material peeling off the backing plate as the remaining thickness isn't stiff enough to keep the bond in shearing rather than peeling modes.
They're like tyres - there's a minimum thickness rating.

However, doesn't always mean the dealer is being completely truthful..

[thinfourth2]

TallTim said:

My Audi is in for a service. Audi just rang me to say the front pads & discs are 80% worn and they recommend changing them now.

Thats your problem right there

I would trust a main dealer as far as i could throw them

If you can take a wheel off without adult supervision then check them your self when you get your car back

[zcacogp]

jimxms said:

mrloudly said:

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

^^ this is how I understand it.

This is my logical response as well, but in practice it seems that there is less pedal travel with new pads. And I don't understand why.

In terms of braking efficiency, the same applies; my logic says that new pads should work as well and new ones, but new ones always seem a bit better. Maybe it's psychosomatic.

Good thread BTW. Reading with interest - thanks OP.


Oli.

[The Black Flash]

jimxms said:

mrloudly said:

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

^^ this is how I understand it.

Absolutely correct, for callipers.
For older drums the pedal travel does increase as the shoes wear, and you have to wind the adjusters on the drums in to compensate. I think that modern drums do this automatically though.

[DuckDuck]

Its worth mentioning that a lot of cars have tapered pads with the contact area actually increasing as the pad wears. Is this to help with heat dissipation as they wear?

[kambites]

Have you tried re-bedding in the old pads? You're meant to do it regularly because the pads lose density over time and need to be re-compressed... or something like that.

[mrloudly]

DuckDuck said:

Its worth mentioning that a lot of cars have tapered pads with the contact area actually increasing as the pad wears. Is this to help with heat dissipation as they wear?

Think you'll find it stops squealing

[mrloudly]

The Black Flash said:

jimxms said:

mrloudly said:

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

^^ this is how I understand it.

Absolutely correct, for callipers.
For older drums the pedal travel does increase as the shoes wear, and you have to wind the adjusters on the drums in to compensate. I think that modern drums do this automatically though.

They do

[thinfourth2]

mrloudly said:

The Black Flash said:

Absolutely correct, for callipers.
For older drums the pedal travel does increase as the shoes wear, and you have to wind the adjusters on the drums in to compensate. I think that modern drums do this automatically though.

They do

No

They are meant to but i've yet to buy a car where they do

But i save most of my cars from a scrapyard

[davepoth]

mrloudly said:

The Black Flash said:

jimxms said:

mrloudly said:

Why will the travel be more? Surely the calliper pistons move out to take up pad wear, thus the pedal travel remains the same (although the fluid level will fall in the reservoir)

^^ this is how I understand it.

Absolutely correct, for callipers.
For older drums the pedal travel does increase as the shoes wear, and you have to wind the adjusters on the drums in to compensate. I think that modern drums do this automatically though.

They do

Not exactly "modern" either - My car from 1973 had auto adjusters.

[smack]

TallTim said:

Audi just rang me to say the front pads & discs are 80% worn and they recommend changing them now.

What is the min thickness of the discs (have they actually measured it???), and what are they new? I remember someone on here with an A8 from memory getting told from a dealer that he had only (from memory something like) 3mm left on his disc before the minimum thickness. He checked what they are new, and found they had only worn 2mm, so 60% left. And told them to shove it.

Changing discs is more money for them, and the industry is keen as ever it seems to get owners to think they have to replace both at the same time.

[Nedzilla]

I recently had my front pads and discs done one my E46 M3.I probably could've got a few thousand miles out of the but decided to get them done while it was in for a service.The pads and discs were very worn but to be honest i didn't notice any difference in braking performance as far as regular driving is concerned with the new stuff fitted.
Maybe under continuous heavy braking the old stuff may have suffered from fade quicker but unless your driving on a track this isn't really going to be an issue.

I do remember taking it for a service at a main dealer soon after i bought it and they told me the rear pads needed changing.I said i will leave them for now untill the brake wear light comes on.
I ended up having them done at the next service some 18 months and 15k miles later and the light still hadn't come on!!

[sparkythecat]

I noticed when I put my winter wheels on a month ago that the front pads on my Audi A6 were well down. As if to confirm my observations the dashboard warning light came on about 2 weeks later.
I changed the pads yesterday, they had around 4mm of friction material remaining, which is really about as thin as you want to go.