Why do disks warp?

[The Stranger]

Ok based on another thread I thought it may be of interest to find out.

Now there is the obvious reason, impact damage of some kind, but what else would make a disk warp and can it reasonably be prevented?

[Motu]

Cut a 250mm disc out of sheet metal - now run a gas flame across it and see what happens.A better question would be why don't they warp?

[Ixion]

This question generated a lot of discussion a while back on an Alfa Romeo forum I subscribe to. Some models suffer from this. Some of the members are professional automotive engineers (ie designers)

The general conclusion was that warping is caused by differential heat buildup on the disk.A brake disc is a large flat thin plate, in effect. A very unstable shape from a heat distortion point of view.

If one bit gets hotter than the rest the only way the extra expansion can be accomodated is to buckle the disk.

The reasons for heat difference include variations in the material and thickness (ie manufacturing quality), and an "oven" effect. If you brake hard from high speed to a standstill, the disks get VERY hot (try touching them sometime - carefully. Have burn gel ready).Once you are stopped the disk starts radiating that heat to the air and cooling down. But the cooling down will be much slower in the bit of disk that is under the caliper, and much slower again in the bit of disk under the pads. And much much slower if you sit stationary with the brakes on.

So you get part of the disk cool(er) and part still hot. The only way the disk can accomodate that if it exceeds the plastic limit of the metal, is to warp. It MAY or may not "unwarp" when it cools. But if you do the cycle repeatedly, and by chance the same bit happens to be the "under caliper" bit several times running, you are likely to end up with a permanent distortion.

One way to minimise this is to move slowly forward (or back) after stopping, thus moving the hot bit out into the air.

[sAsLEX]

Old callipers that aint retracting from the disc, rubbing only on one side could do it.

[The Stranger]

Thank you sir, that is what I was looking for.

Was always taught that after a caning, run the vehicle easily for a while to allow the brakes to cool and or don't allow the vehicle to sit stationary whilst the brakes are cooling for just this reason.

Wonder how many people considered this at the track day.

[F5 Dave]

Worse in cars with drum brakes. Go down a windy downhill & get them red. Stop & yank on the handbrake. Go for coffee. Opps.

Back to discs; The distance between the inside & the outside of the swept area in speed, which is one reason for 'many-piston' callipers being smaler can use thinner swept area. Sadly this hasn't been a complete cure as they have made discs thinner thickness for weight reduction, less mass means less stability.

Some people cut slots in discs so the expansion has somewhere to go, sort of like bridge expansion joints. But the potential of one section bending & the sudden total lockup that would ensue would defer most thinking people.

Some materials like cast plate have different density across them so if you cut a disc from it some areas will heat up & cool at different rates. Decent discs use better material. Some cast in large tube like shapes & laser cut the discs from that.

When you skim a disc on a magnetic table the disc may pull flat but spring back a little after, this may also make the disc vary in thickness once both sides are done.

[Hitcher]

Let's do the disc warp again.

But those pelvic thrusts really drive me insane...

[UrbanMyth]

i hate warped disc they feel so horrible

[kickingzebra]

Poor mans ABS!!!

[N4CR]

To basically sum all that up (Ixion was right) when you brake hard and end up sitting at the lights, the pad is rather toasty and causes expansion due to heating blah blah, disc cools quicker than pad and it makes it bend. Magically.

So when you brake real hard for a while, move the bike a bit if you have to sit still, don't keep brakes clamped on and sit for a few minutes, not good with localised heat spots.

[sugilite]

1998 ZX9R discs are known for dishing/warping under racing conditions. At two tracks especially...puke and teratonga. Both tracks you really only brake hard once or twice, then the disc cools down and the cycle goes on causing the warp. The disks were to thin. I've got 2002 rotors on at present I think...I sometimes call it the bike that ebay built hehehe

[Sketchy_Racer]

As explained by others, you know how a disk warps...

After i go for a good thrash on my bike and get the front really hot,

(over Piecock hill for example) then at the kapiti side of the hill you are sitting at a giveway facing down hill. Have a few friends that warped their disks there. So because I don't use my back brakes during any 'sporty' riding i just use them at the bottom of the hill and let it roll slowly to stop any 'hot spots' on the disk.

I got the disk on my FXR red hot coming down teh hill one night..... that was fun!!

[dickytoo]

1998 ZX9R discs are known for dishing/warping under racing conditions. At two tracks especially...puke and teratonga. Both tracks you really only brake hard once or twice, then the disc cools down and the cycle goes on causing the warp. The disks were to thin. I've got 2002 rotors on at present I think...I sometimes call it the bike that ebay built hehehe

That's funny, I didn't see either ebay or sugilite there when i had to fabricate new brackets for the calipers to accomodate the larger discs. It was really nothing, just sustained hard labour with a hacksaw, a hand held drill, a bastard file and the good ol' mark 1 eyeball.:gob:

The discs were actually 2000 ones. the 2002 are even bigger, up to 320mm diameter.

[sugilite]

Hehehehe, bait taken on queue!

[Marmoot]

Why do disks warp?

To go boldly where no disks have gone before....



.....is this a trick question?

[Pixie]

Perhaps they weren't breastfed long enough