Brake disc warping patterns
 

Just booked my car in for service to have the brake discs assessed and brake pads changed. Unsurprisingly this is the third time we have to send the discs for skimming - which is obviously not possible hence requiring replacement. Am I the only one experiencing disc brake issues? Car has 42k on the clock and is my daily driver. No track use.

why are you skimming the disks each time? did you have vibration or anything of that sort?

Lots of vibration and humming on moderate brake pedal pressure at highway speeds. Dealer mentioned that it was hot spots but it's progressively gotten worse requiring skimming apparently. Explanation from dealer was that it was caused by rapid contraction of disc typically in rainy conditions.

Do you use your brakes very gently?
That can cause uneven disc wear and cause brake judder.
I assume that brake judder is the symptom?

It's a fairly common Subaru problem.
Always braking gently = brake judder = have to skim discs.
The solution is to occasionally hammer the brakes really hard.

My wife's Impreza has this problem, my Forester and my 86 don't - go figure :)

I call BS.
Brake discs aren't made from glass and rapid contraction and/or heat/cooling issues cause small surface cracks not warping.

This is what a disc looks like after being on a racetrack in the rain. Much more extreme temp variations than you'd ever get on the road.
No judder, just little cracks which aren't a problem.

Also had the discs skimmed about 10 000km.

No vibrations but had like a wheel bearing noise.

Brake usage is difficult to attest to but if you are in stop start driving all the time, you do brake hard every now and again to avoid taxis, pedestrians and the odd 1 tonner insistent on occupying the same space and time as you.


The only observation that offers any correlation is that this hot spot behaviour only develops after a significant downpour.


Are our car discs sensitive to rapid temperature fluctuations or do I have a one off problem?

Wow...I probably have a factory fault with the discs fitted to my car.

If you're still under warranty, then let your dealer replace your discs and pads.
If not, then talk to Francois at Powerbrake.

Then comes the part which might seem strange if you haven't done it before...

You need to give your brakes some serious horns to bed them in properly.
Like - ride gently for 100-200 kms, then do 5-8 crash stops from 100 to 30 kph as hard as you can.
I mean - accelerate to 100, and hit the brakes so that the ABS kicks in down to 30 or 40, then immediately accelerate back up to 100 and do it again, and again.
Repeat at least 5x, but could be as many as 7 or 8 times.

The intention is to create massive heat and burn in the new pads and rotors.

You need to get your brakes hot enough that they smoke and smell.
If you don't smell them burning, then do it again and harder.
When you smell the burning strongly, and you can see smoke, then that's enough.

Trust me - it works.

I hope that some of my mates who know about this will chime in and support me so that you can believe that it's not crazy and that it's the right thing to do...

Here's a pic from a week or two ago of a modded Lumina SS with new pads.
This was lap 2, and he was making some brake pad smoke. Equates to 6-7 hard brakings.

When we got back to the pits, I spoke to him and told him he was making smoke. He knew, and was intentionally burning in new pads on track.

Didn't help him much though. Despite V8 horsepower, he was a mobile chicane, and the STi in front of him was even worse.
Amazing how many big hp cars are actually quite slow on a racetrack.

I'll guarantee you that @JouMaSeHoes with an almost stock NA 86 would easily put both of those cars to bed on Zwartkops.
.

Notably I have the same vibration, but I'm pushing 56000km and my brakes are warped no doubt. Have experienced some pad fade under Lowveld conditions @Fish Eagle :lol::lol::lol:

Dealer replaced discs under warranty, pads for my account (they were worn already). Car braking is back to normal. While not smoking, they are bedding in as evidenced by the burning smell.


On an adjacent note: Toyota dealership service and approach was well below par. This whole process was painful. I don't understand why a warranty claim is so hotly contested by the dealer especially if there is a history of evidence supporting the claim. Not going back to this particular dealer.

I also read a while back about proper bedding in procedure for new brakes - scrapping off old brake pad layer and putting the new layer for best performance - slight differences depending where you read. I do sometimes notice oxidation on the discs surface itself if the cars been parked for several days under the car port. Maybe I don't have enough transfer layer on the disc?

The powerbrake website is a great resource for all things brake related: http://www.powerbrake.co.za/tech_info/tech.htm

Here's their advice for bedding in

Bed-in Cycle:

Drive slowly for a few kilometres, using the brakes gently to bring them up to a moderate operating temperature.
Perform 6 consecutive brake applications of increasing intensity from 100 km/h down to 30 km/h. If wheel lock-up (or ABS activation) is referred to as a 100% brake effort then you should be aiming to build up to a 60 - 70% brake effort for the last 2 applications. The applications should be performed consecutively, accelerating normally back up to 100 km/h after each application, before braking again.
If possible, do NOT come to a complete stop throughout the 6 brake applications described above. Doing so can result in ‘pad etching’ during which friction material is unevenly deposited onto the brake disc causing DTV and brake judder. The will be visible as the outline of a brake pad on the disc surface. (To learn more about pad etching and DTV click here)
It is normal to experience a strong smell and even see a small amount of smoke rising from the brakes from the 4th application onwards. This is not a problem. Be prepared to experience “green fade” from the 4th application onwards.
Drive on, using the brake as little as possible, for a number of kilometres allowing the brakes to cool substantially in the air-stream before parking the vehicle and allowing the brakes to cool completely. This cooling cycle is important.
Gradually build up your driving style over the next 500 - 700 km's while keeping an eye on the MTR temperature measuring paints on the discs. If the red paint turns white, you are running your discs over 610 deg C and disc life will be reduced as a result.