Why do discs warp?

19/07/2017 12:45pm

If you read the previous pages, you may remember that we said a brake was in fact an energy converter and it converts that energy into heat. The problem is that we are always trying to manage that heat. Too little can cause problems and too much can be even worse. Impact or mechanical interference such as a disc lock can cause a disc to bend but if it is warped it will be caused by heat.

First we need to look briefly how discs work. Through friction, we convert forward motion into heat. The heat transfers mainly into the brake discs and pads and they are effectively ‘air cooled’ by the speed. As the disc starts to get hotter it expands. Then as it cools it shrinks back to exactly the same size and shape it was before the process started. The trouble is it can only do this within certain parameters and if the disc gets hotter than its maximum temperature then it will no longer return to the same size and shape that it started. It will cool as a totally different shape. In other words it is warped. You can generally feel a warped disc by feeling for a ‘pulsing’ at the master cylinder lever.

The poorer the material the more easily the disc will warp, and I swear some well known manufacturers discs are warped before you even get them out of the box! 
In order to prevent this warping, first of all you need a top class material then the more heat you have, the more material you need to cope with that heat. In other words, you make the disc thicker. Brembo manufacture front discs for superbikes between 4.5mm and 6mm thick. 4.5mm for road bikes, 5mm for track days and club bikes, 5.5mm for Supersport and Superstock racing bikes and 6mm for Racing Superbikes. Even up to 7mm for certain riders and disciplines.

It is partly to do with how the bike is ridden and partly to do with who is doing the riding. For almost all road riding 4.5mm is just fine because road brakes never really get that hot. In World Superbike though you can sometimes see the discs glowing red hot! If you gave Troy Bayliss 4.5mm discs he would warp them fairly easily. Not because the disc is poor but because the product wasn’t designed for WSB racing. 

In general, a quality disc shouldn’t warp. If it has done then either you are applying pressure to the pads by holding onto the lever or pedal or something in the system is sticking like the caliper pistons. In Moto GP Brembo noticed that the onboard electronics recorded that the front brake was being ‘applied’ at high speed. This was eventually traced to air pressure on the brake lever. Moto GP levers now have cut outs in the lever to prevent this happening. Well worth considering at 200mph!


Tags: brakes