19/07/2017 1:08pm
If your disc is warped or bent you should be able to feel a pulsing back through the brake lever as the brake is applied. If you can, you will need a dial gauge and mounting stand to measure it and find out by how much.
To measure it, leave the wheel in the bike and raise it off the ground with a suitable stand and follow all relevant safety procedures to ensure both the bike and you are safe!
Now set up the dial gauge and zero it whilst just touching the disc. Then move the wheel round slowly while watching the gauge and note how far the disc is out of true. On a fixed disc if it is more than about 10 thousands of an inch or 0.25mm out then it is either warped or bent. With a floating disc you need to allow up to twice this to allow for the disc float. Less than this isn’t enough and is sometimes just production tolerance.
Whenever they are measured out of tolerance most people assume the disc is warped. This is because the term ‘warped’ is known to relate to discs so is used to mean ‘not flat’ rather than truly warped. Warping is, and can only be, caused by heat! To find out if your disc is warped first of all you need to look for discolouration. When a disc gets hot enough to warp it will change colour from its normal silvery colour to a combination of blue, yellow and brown. Not just in one spot but all over the disc. The disc face can do this as well as the disc edge. This can happen because the disc is used in an extreme situation such as racing, track day riding or stunting. It can also be caused by rider error if the rider is putting the brake on without realising it. By leaving their foot on the brake pedal for instance.
Then lastly and the street riders favourite, is sticking calipers caused by lack of maintenance. If the caliper sticks and the pads are constantly touching the disc then the constant friction will send the temperature sky high and this can happen very quickly. With a quality disc these are really the only ways you can get enough heat into a disc to warp it.Cheap low quality discs will warp at much lower temperatures than high quality discs!
If your disc isn’t warped and measures out of tolerance then you have a bent disc. A bent disc is a completely different problem. The disc remains the same silvery colour but is no longer straight or flat. This is usually caused by some sort of impact. The infamous ‘disc lock’ is a very common cause and has probably doubled the number of discs sold! Far more common though are bent discs caused by tyre fitters. Applying pressure to the side of the disc, whilst fitting a new tyre is by far the most common cause. It even happens in World Super Bikes believe it or not. The only cure is replacement. Better still, if you remove the disc whilst the new tyre is fitted and refit it afterwards it won’t happen!
With a high quality disc, general road riding will never produce enough heat to warp the disc. Perhaps with a fully loaded Harley Electra Glide, with ‘generous’ rider and passenger being ridden enthusiastically down the Alps with fixed discs you might manage it. Short of that it just isn’t going to happen and if the disc truly is warped it is almost certainly being caused by a sticking caliper.
If a disc is warping because of extreme use, such as racing or track riding, then the only thing you can do is replace it with a stronger one. Nobody will guarantee a disc against warping in racing but without a doubt the strongest discs out there are Brembo. Winners of practically every major race series practically every year!