Brake disc warping

[Tim Falce]

I am having problems with warping my front discs. They only seem to last half a dozen or so races before I need to have them skimmed or replace them.
I try not to leave my foot on the brakes when they are hot when the car is stationary so unless the discs I am buying are crap (Maybe a good possibility)I can't think what would cause this.
They were slightly juddery when I started the Birkett yesterday but by the time I had finished after a 40 minute stint my fillings were ready to fall out.
Has anyone got any suggestions whats going on.
I am now on my second set of discs this season although my first set although slightly warped were replaced by the supplier as after a couple of race I found they had machined the one of the faces the wrong size and the pads were wearing a step on the outside.

[GORDON STREETER]

Tim if its any consolation I have the same problem but it only affects my car at lower speed with gentle braking (ie green flag/finishing lap) I use genuine AP discs with carbon metallic pads and renew the discs every 2 seasons.

[Tim Falce]

I'm just wondering if it's of a case of "you pay for what you get", I can buy 3 of my discs for one AP disc.

[AU N EGL]

have you tried taking spray brake cleaner on the rotors and clean them off? Some times that warp feeling is un even brake pad material deposited on the rotor.

[GORDON STREETER]

As for "you get what you pay for" it doesn't always ring true. I have fitted aftermarket discs on hot hatches for boy racers road cars that cost a fortune, only to fit the original ones back on within a couple of weeks because they have warped. On the other hand I have fitted disks that I have brought for 7 quid each (probably made in India or China) that have happily done 40 000 miles. The bedding in period for race brakes is very important and should be done to the instructions supplied with the parts. This is obviously more important on something that is going to be abused . As I said I do have problems myself but then my brakes have to fit inside 13 in wheels.

[johnny yuma]

Originally Posted by AU N EGL

have you tried taking spray brake cleaner on the rotors and clean them off? Some times that warp feeling is un even brake pad material deposited on the rotor.

Agree its only pad material which gases out and solidifies on the rotor in invisible irregular layer.You can remove it with cordless drill and GARNET PAPER disc.

[trikesrule]

Got plenty of ducting to the discs and calipers? Just where is the air pointed at. It's less likely to warp discs if it's aimed at the centre of the disc. Also you may have a case of too much cooling down before the next application. Brake discs I've noticed prefer to stay in the envelope they were designed for. Your's may be over cooled or over heated?.......trikes

[turbophil]

If this is a frequent problem, then you're probably not running a well suited pad for the application or need to work on your bedding in process....or both

It's not often that rotors warp. Most of the time it's pad build up. The heat on the rotor is highest where the pad build up is since it's a high spot, which will self perpetuate into a larger build up... If one continues to drive with the problem, cementite can occur from the heat. Then you truly have a hardened spot on the rotor which may be impossible to turn out...

Another thing to check is to be sure that you're not holding your foot on the brakes while stopped without proper cool down... That can cause pad build up as well...

A couple pennies for the jar...

Best,

Phil

[johnny yuma]

the spinning of the rotor will "pull" any air ducted onto it's surface into the pad and caliper if the duct aims it at the "upstream" side of the caliper.To introduce air to the vent area between the rotor halves,aim the duct into the bottom of the vents and The(centrifugal) Force will be with you and the air should exit the vents with a nice swag of B.T.U.s
Snooping in the Pits .

[888_Fan]

I recently ran my new sports car at a race meeting which was hot, 35 degrees on the Saturday and absolutely bucketing down with rain on Sunday for 2 races. I put new front rotors on for Saturday.

Then, 2 weeks later, in a dry practice session, one of my front wheels was locking consistently. I noticed there are hot spots all over this rotor. Upon asking my brake rotor supplier why this was so, and he told me that the wet really takes its toll on brake rotors, and hot spots wasn't uncommon after running in the wet.

Anybody else heard of this theory?

[graham bahr]

although it doesnt address the intial warping issue, unless the discs are well worn, its better to reface them than replace, the reason being once warped they have worked out there internal stresses and are far less likely to warp again than a new set.

[Raceng]

Just a thought , have you checked the disc mounting surface is true , we had no end of problems with them . Theory is when disc is new they are not work hardened , so any small amount of runout can be absorbed by the relative soft disc , as the disc goes through repeated heat cycles it will no longer flex and so more vibration is felt , also try phasing the disc , ie turning it on flange until the least amount of runout is measured using a dti gauge

[dtype38]

Originally Posted by graham bahr

although it doesnt address the intial warping issue, unless the discs are well worn, its better to reface them than replace, the reason being once warped they have worked out there internal stresses and are far less likely to warp again than a new set.

Now, intuitively, I would have thought that using a warped disc would cause uneven heat build up, and 'increase' the internal stress. You are suggesting that new discs have internal stresses from the manufacturing process, and using them gets them hot enough to 'relieve' the stess. If that's indeed the case, would it be useful to stick new discs in the oven for a while before putting them on the car? Anyone idea how hot and for how long a cast iron disc needs to be heated to stress relieve it?

[GORDON STREETER]

Gas mk 9 for 30mins turn over and season with a decent chili powder, mmmm lovely

[SidewaysFeltham]

The normal reason for cast iron disks developing "grabbing", part from all of the obvious ones like they machining tolerances were out, is free carbon.

What happens under adverse conditions, is that the disk overheats and the free carbon which tends to be scattered in "Clumps" throughout the material hardens, rather like any carbon steel would. It is these hard areas that cause brake grabbing, pedal oscillation etc.

We always found that re-surfacing disks only worked for a very short time: even on road cars, particularly Jags.

The problem with racing is that you are stressing the material far beyond normal usage and running typically "hard" competition pads simply exacerbates the problem.

The solution was to change to steel disks.

Obviously, the more expensive the price with cast iron, then the better the quality, in theory. There are all sorts of "Cast Iron": some is different composition to others and probably more suited to brake disks. Cheaper grades tend to be old ships, bits of cars, trucks and telephone boxes etc which have been re-smelted and turned into "Pigs" re re-casting.

Off topic, is that a real Strat, Gordon: and why do you keep changing your avatar?

[GORDON STREETER]

No its a Squire copy. I gave a guy £100 for it complete with a Fender amp !
As for the avatar, its because I'm a nonsense man. Anybody that "really" knows me would know that !

[SidewaysFeltham]

So, with my Real Sunburst Strat Japanese re-issue circa 1976, I can feel marginally superior.................

Humph! Wish I'd never sold my 1964 US original with rosewood neck!

£100 including the Fender amp? You wos robbed, Mate! Not!

It's rather strange: the more I discover about real race drivers, so many are musicians, too!

[GORDON STREETER]

Its because we don't want to grow up. I was talking to somebody the other day at a dinner/dance and he told me that he had sold his old mans genuine 60s Strat for peanuts a couple of years back. Hindsight is wonderful.
Anyway we digress, maybe I'll start a thread on favorite guitar players as with the age range on 10/10ths it would be interesting to some. The only thing I liked about Tony Blair was he played a guitar !
Anyway back to brake discs !!!!!

[MGDavid]

Originally Posted by GORDON STREETER

....... The only thing I liked about Tony Blair was he played a guitar ! ......!!!!!

... I gather he didn't do even that terribly well....

[1200Datto27]

Originally Posted by dtype38

Now, intuitively, I would have thought that using a warped disc would cause uneven heat build up, and 'increase' the internal stress. You are suggesting that new discs have internal stresses from the manufacturing process, and using them gets them hot enough to 'relieve' the stess. If that's indeed the case, would it be useful to stick new discs in the oven for a while before putting them on the car? Anyone idea how hot and for how long a cast iron disc needs to be heated to stress relieve it?

I have access to a computer controlled pottery kiln that lets me setup curves for heating/cooling of metals (I use it for heat relieving of welded components) and have used the kiln to process rotors. I slowly ramp the temp up to 500 degrees C (takes about an hour), soak the rotors at that temp for a little while, then quickly ramp them up to a little over the transition temp for the materials used, Once there, and having soaked for about 5 minutes, I pull them out and quench them in oil, down to about 200 degrees, after this I put them back in the kiln and reheat them to just below the transition temp, soak them for a short while, and them pull the rotors out and place them in heat blankets to slowly cool them back to ambient temp.

I have done this to 4 sets of rotors so far, and have gone from cracking rotors and having issues with build up, wear and fade, to not cracking a rotors since, whilst being able to run more aggressive pads for less wear and build up.

I also prebed the pads using the kiln, by heating them to about 75% of there operating range, soaking them for for about 5 minutes, then getting them out and cooling them down in the heat blankets.

[SidewaysFeltham]

Some useful data on this site.

http://www.racingbrake.com/v/main/ro..._important.asp

I also did a quick surf on heat treatment of cast iron and as I suggested earlier, the actual composition of the cast iron (which varies enormously), has a direct bearing on rotor performance, durability and longevity.

Back in the late 60s, Ford led the pack with nodular cast iron cranks, which with various forms of hardening and heat treatment were far superior to the standard issue.

For ultimate performance, as always, steel has inevitably been the way to go.

Unless you can afford Carbon-Fibre.........................................

[GORDON STREETER]

As for refacing them it isn't easy to do unless you specialize in them. Most motor factors used to do them (even on the car ) but that type of grinding was OK for normal road cars. The problem comes with vane'd discs as the machine follows the opposite side and tries to put a ghost one on the other.
Even my mate who can make just about anything hates working with cast iron as he loves to keep his workshop clean !!

[SidewaysFeltham]

I used to re-face road disks on the lathe, Gordon.

It was gap bed and I could throw pretty large diameter disks and flywheels.

Made a set of special carbon-tungsten tools and on a good speed and very slow feed the final finish was excellent.

However, as I said earlier, after a few weeks the grabbing was back, 'cos it's impossible to turn out or grind out the clumps of free graphite which have turned!

In the end we invariably fitted new disks.................

[GORDON STREETER]

At least scrap is worth a bob or two now:

[SidewaysFeltham]

Yeah.

To make new Chinese brake disks!



In fact just fitted two new rear disks to Mrs Sideway's Volvo estate: couldn't believe how cheap they were: Swedish patterns from Euro Carparts.

Complex vented disk; with a brake drum integral for the handbrake.

Cheap as chips.