Warped brake discs - is it me??!!

[PositronicRay]

I don't know what the catch is Kwik Fit do a life time guarantee

http://www.kwik-fit.com/brakes-at-kwikfit.asp

[kambites]

Dodsy said:

If doesnt case a problem at the rear as most of the braking force comes from the front, so the rear brakes dont get as hot. This is why you see cheaper cars with rear drum brakes and front disks. As most braking force is at the front rear drums are fine for smaller cars and are cheaper.

I was under the impression that drums were typically more expensive than discs?

[DAVEVO9]

Studio117 said:

Handbrake doesn't use the pads..

• not in all cases** though

EVO 8/9?

Rear discs won't get anywhere near as hot..

[Dodsy]

kambites said:

I was under the impression that drums were typically more expensive than discs?

I thought that the costs were actually slightly cheaper for drums as they arent made to the same high tolerance that disks are. And as the drum can be used as the handbrake as well it means there is no secondary braking assembly required therefore its cheaper. Disk brakes do not make good handbrakes.

If disks were cheaper then thats what the manufacturers would be fitting - even a saving of £1 on a component adds up to lots of £££ across a production run so they would do it.

[lostkiwi]

mini me said:

Next time it does it, or if it is doing it now. Go for a drive and when you find yourself able to WRT safety/ people around/ behind etc give it some uber hard braking a couple of times. Don't actually stop though just very hard on the pedal down from a good speed and then carry on trundling along so the discs cool again. This will often clear any deposits on the disc. Works well on track where this is a common issue with road cars. Don't do it too much though just a couple of good stamps on the brakes should clear it up if its not too bad.

That will help with pad transfer but pad transfer is only part of the issue with hot brakes.

When the brakes get hot and they are locked in position with the pad (at traffic lights for example) the disc material under the pad can undergo a molecular change where it form cementite. This is caused by the heat in the disc altering the molecular structure of the steel. Cementite is very hard and resistant to wear and has less coefficient of friction than the surrounding material. The hard area formed eventually becomes a high spot on the disc and juddering starts.

There are steps you can take to avoid it.
1. More gradual braking
2. Don't sit with your foot on the brake after heavy braking
3. In an auto leave more space in front of you when stopped to allow the car to creep slowly forward whilst held on the brakes


Some brands of brakes seem more prone than others. I've never been happy with EBC for example but have had no issues with Brembos or Black Diamond.
Nitrac cryogenically treat their discs to reduce this tendency but I've never tried them.

[kambites]

I suspect discs are cheaper if you use the main calliper for the hand brake; drums are cheaper if you don't.

Setups with discs where the calliper is used for the hand brake are an arse because the handbrake comes off as the brakes cool down. The hand brake on my Lotus is almost completely useless because even after relatively gentle driving, the brakes are hot enough that it comes off as they cool down. I think discs are fitted to the rear of modern family cars as a fashion statement; I'd actually rather have drums. Even on a sports car it's more the reduction in weight that would steer me towards discs than the improved braking consistency.

[james_gt3rs]

I'd be quite surprised if normal road driving could generate enough heat in the brakes to warp the discs??

[DocJock]

eltax91 said:

Hi guys. Thanks for the speedy replies!

The 5 series and the accord are box slush 'boxes, so yeh, after any heavy braking if I'm stopped for a short period of time I'll hold it on the foot brake.

The octavia were both manual though. Although all my driving in these cars is pure motorway, I live 2 miles from the m1 and it feels like I spend my life on M roads.

I guess at all points though I'm braking down a slip road and sitting at lights often. Although I wouldn't say I braked very early on slips, I'm not last of the late brakers either.

You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.

[kambites]

DocJock said:

You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.

Ignoring aerodynamic drag and engine braking you put the same amount of heat through the system - the car has a finite amount of kinetic energy all of which needs to be shed through the brakes as heat. However, the earlier you brake, the longer it has to dissipate while the brake is still turning rather than sitting on a single piece of the disc.

[SuperHangOn]

I've heard about warped discs being a myth but I recently changed my fronts due to severe judder - one disc in particular was visibly not true when spun and rubbed on the pad at each rotation.

I can't say what might have happened to the brakes as they were juddering when I bought the car.

New discs and pads fixed the judder.

[willmagrath]

I got new discs as my old ones were warped. For the first few days I tried my best to avoid heavy breaking of any form to bed them in, but 3 days in, a lorry pulled out on me causing me to stand on the brakes......they were then warped......

[Pickled Piper]

TA14 said:

I doubt that they are warped discs: http://www.stoptech.com/technical-support/technica...

All you need to know is here. Follow the brake break-in procedure whenever you are getting symptoms of "warped discs". Works 99% of the time.

pp

[Freddy88FM]

DocJock said:

You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.

I suspect this is true. Of course in each instance the exact same amount of heat energy is generated however the time for the discs to absorb the heat is longer. I suppose it's like a three second burst of 2000 watts vs a min of a 100 watt candle on a pan of water. Same energy, but one will absorb more than the other.

[Ranger 6]

Last time this happened to me I took the wheels off to check and sure enough there was an outline of the pad stuck onto the disc. It appeared to be the resin or whatever binds it together that had melted and left a high point on the disc which gave the impression of warped discs.

Followed the bedding in procedure again and all good.

[lostkiwi]

Freddy88FM said:

I suspect this is true. Of course in each instance the exact same amount of heat energy is generated however the time for the discs to absorb the heat is longer. I suppose it's like a three second burst of 2000 watts vs a min of a 100 watt candle on a pan of water. Same energy, but one will absorb more than the other.

Total heat generated will be the same either way. Total heat dissipated will also be the same either way.
The difference is the rate at which the heat enters the system to be dissipated. A sudden hard stop will cause much greater temperatures to be generated as there is less time for it to be dissipated through the metallic mass and airflow.

Think about holding matches one at a time to a 1kg mass of metal. Hold it on for a month and there will be little change in the temperature of the metallic mass.
Now put an oxy torch on for 10 minutes. The metal will heat up significantly even though the total heat input is probably about the same.

Ranger 6 said:

Last time this happened to me I took the wheels off to check and sure enough there was an outline of the pad stuck onto the disc. It appeared to be the resin or whatever binds it together that had melted and left a high point on the disc which gave the impression of warped discs.

Followed the bedding in procedure again and all good.

Yep - pad transfer. Thats not permanent. Cementite 'warping' has only one cure and thats disc replacement.

[Rangeroverover]

I've been wondering about "warping" I often drive through a ford/stream on the way home, if the discs are hoy when I go into the cold water what effect it has on the discs, would they warp or are they used to extreme heat cycles

[blueg33]

DocJock said:

[
You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.

Not convinced.

Brakes dissipate kinetic energy as heat (that's how they make a car stop). Whether you brake for a protracted period or a short period of harder braking, you are still converting the same amount of kinetic energy into heat energy. The only difference is that in hard braking you are converting kinetic energy to heat in less time, ie a higher temperature.

So I think the converse of the statement above is true, unless I am missing something?

[p1esk]

Dodsy said:

Handbrake varies by car - Some cars the handbrake acts on the rear brakes pulling them on, others have a separate brake just for the handbrake.

If doesnt case a problem at the rear as most of the braking force comes from the front, so the rear brakes dont get as hot. This is why you see cheaper cars with rear drum brakes and front disks. As most braking force is at the front rear drums are fine for smaller cars and are cheaper.

Rear brakes do still get hot but not hot enough to cause pad transfer. But it can cause handbrake issues. My TVR after a run is parked up with the handbrake full on and it holds the car. Give it half an hour for the rear disk to cool down and it contracts slightly which slackens of the handbrake and car will slowly creep down my sloping drive until it hits the house. I was always told to leave a parked car in gear and still do it today.

That was a problem covered by the BBC 'Watchdog' programme some years ago, the model concerned being the Citroen Xantia. Handbrake slowly lost grip as the discs/drums cooled and contracted, IIRC.

[Peanut Gallery]

How close to the sea are you?
Photos of the face of one of "warped" disks?

I have never warped a disk, but have been harsh on the brakes on some runs. Also only have drums on the back, and these do get some rust in spots on them - pulling the hand brake for long downhills is noisy but wears the rust off and I get a nice clean sharp handbrake for 6 months or so.

[Mave]

blueg33 said:

DocJock said:

[
You sink more heat into your braking system by braking earlier and more gradually, sitting on the pedal for longer.

Not convinced.

Brakes dissipate kinetic energy as heat (that's how they make a car stop). Whether you brake for a protracted period or a short period of harder braking, you are still converting the same amount of kinetic energy into heat energy. The only difference is that in hard braking you are converting kinetic energy to heat in less time, ie a higher temperature.

So I think the converse of the statement above is true, unless I am missing something?

The subtle difference is where the heat goes. The conductivity of the pads is lower than the discs- in a short hard stop more of the heat goes into the discs. In a longer more gradual stop there's more time for the heat to conduct through the pads and pistons to the fluid, so although the total energy is the same, you're more likely to boil the fluid.