Can warped discs be repaired?

[Davie_GLA]

As above really, the bike i've recently bought has a very slight warp in them. I can feel it through the lever.

The discs are in good condition and nowhere near being worn out so i'm wondering if an engineering firm would have any success straightening them?

Anyone done it?

[MrOrange]

Are you sure they are warped? Lots of other reasons that discs become "grabby".

[Davie_GLA]

i think so, they don't feel grabby but rather a pulsing through the lever.

[wildone63]

Is it an old or high mileage bike?,Ive had discs which felt 'pulsey' when they were getting near to their wear limit,or they may have a high spot which may need skimming.

[Davie_GLA]

It's a 2003 bike with 5700 miles on the clock.

[moanthebairns]

how much are we talking to replace discs and pads?

Driving with a car with warped discs is bad enough a bike no chance.

also even if you could bend discs back into place this would weaken them further would it not? are they not more likely to fail!

[John D.]

Warped discs = myth, no?

Uneven pad material build up. Remove/smooth that. No need to bend discs!

[gavgavgav]

you say straightening them, are you convinced that they are bent? The only way of bending them is to collide with something. The pulsing you get could be simply a section of pad transferred to the disc, have a look for discolouration, and try giving that area a little sanding and a clean. Done this on my car a few times, have had pad transfer after thrashing the brakes and then left standing.

[Davie_GLA]

Aye, fair point.

Discs are a couple of hundred quid if you go middle of the road with pads around the £40 so although not break the bank i've just bought the bike and would rather not have a fairly large expense so soon.

However i guess it's not worth taking any chances.

D.

[SystemParanoia]

time to buy a lathe

[HeatonNorris]

moanthebairns said:

how much are we talking to replace discs and pads?

Driving with a car with warped discs is bad enough a bike no chance.

also even if you could bend discs back into place this would weaken them further would it not? are they not more likely to fail!

Well, you don't bend them, you skim them.

[Chipchap]

It could be as simple as the floating bobbins are a bit clogged and the disc is now sitting skew.

[Davie_GLA]

Maybe, the bike has sat for a year and not turned a wheel. Will have a look at that.

[moanthebairns]

HeatonNorris said:

Well, you don't bend them, you skim them.

really thought once they were gone that's it.

[HeatonNorris]

moanthebairns said:

HeatonNorris said:

Well, you don't bend them, you skim them.

really thought once they were gone that's it.

If there's enough 'meat' on them and the warping isn't too bad, they can be skimmed so they run straight and true again.

[Davie_GLA]

gavgavgav said:

you say straightening them, are you convinced that they are bent? The only way of bending them is to collide with something. The pulsing you get could be simply a section of pad transferred to the disc, have a look for discolouration, and try giving that area a little sanding and a clean. Done this on my car a few times, have had pad transfer after thrashing the brakes and then left standing.

I reckon a pic or two will say a thousand words. I'll organise that and get them posted. I did notice that where the pad contacts the disc, the part where the centre of the pad is making a darker shade on the disc.

TO clarify the discs are lighter towards the outside than the inside.

[Chipchap]

It could also be "Pad deposit" where the high spots are actually a build up of friction material on the surface of the disc. Get brake cleaner from Halfrauds and give it all a good going over, strip the bobbins and reassemble after cleaning, use a high heat/copper grease to ensure they slide.

Then perhaps emery or wire wool the disc surface both sides to remove the deposits.

Should all be fine after this.

A

[Davie_GLA]

I don't understand the bobbins. I know what they do but thought they were fixed to allow the disc to float.

Are they serviceable?

[Rubin215]

gavgavgav said:

you say straightening them, are you convinced that they are bent? The only way of bending them is to collide with something. The pulsing you get could be simply a section of pad transferred to the disc, have a look for discolouration, and try giving that area a little sanding and a clean. Done this on my car a few times, have had pad transfer after thrashing the brakes and then left standing.

Brake discs on bike don't need to be struck by something to become warped; the disc is much thinner than on a car so will not stand the same abuse.

A common way of warping them is for a ham-fisted spanner monkey to lever against the disc to get the pistons back into the caliper prior to replacing the pads. Once one side of the caliper is done, the disc is left unsupported so pressure against the other side bends it.

Also, if the brakes are used very hard again and again (practising emergency stops perhaps ) they can overheat and this is enough to warp the metal
As an example, try holding a butter knife over a gas flame and see what happens.
Again, if they are already very hot and are suddenly cooled (deep mpuddle, washing with a hose etc) it can warp them due to the localised cooling.


Regarding skimming, I haven't met an engineer yet who will skim a floating disc when it is still on the carrier; the disc will move around and "ring" and there is no way you can produce an even surface.
Some shops can re-face the disc using a bench grinder, but fewer and fewer people have one these days.
By the time you add up taking out the bobbins to get the disc off the carrier, skimming and refitting you are probably as well to get either decent pattern discs like EBC or buy a good second hand pair from a breaker.

[smegmore]

The only way is to take the discs off the carrier and have them surface ground. I've done this but not with floating discs, I've also had steel clutch plates surface ground, not cheap.