Contrary to what you’d think, the most common cause of rotor damage isn’t the result of a crash, but when loading your bike into a car or, if you have a bigger bike (and bigger rotor), into the back of a truck or trailer on an uplift day. With often a dozen bikes rattling around in a confined space some damage is inevitable. There’s also a risk of someone just bunging a 40lb rig on top of yours and a sharp pedal dinging the rotor.
The first thing to do when attempting to straighten a bent rotor is to check that the bolts holding the rotor or caliper on haven’t come loose and that the rotor is centred in-between the brake pads. If a rotor is badly aligned it can rub on one side more than the other and this can give the impression it’s bent.
Just tapping the surface with a soft-faced mallet can correct small flaws in the rotor — handy if it’s just nicking one of the pads and making a tinkling noise. You can even take out slight twists easily and quickly with an adjustable wrench or a dedicated tool like the Park DT-2 Truing Fork. However, more severe bends or warps require a bit more time and more sophisticated tools. Both Park and American firm Morningstar make dedicated Rotor Truing Gauges. The Morningstar R2 O.C. Tech wheel survey tool is the better of the two because it can be fitted to a bike with the wheels intact, but unfortunately it’s not available in the UK. For this reason we’ll be focusing exclusively on the Park DT-3 Truing Gauge.
A Dial Indicator is available for the Park and this allows you to detect small (0.1mm) defects in the rotor surface. This is useful if you have a tight-fitting brake with a small gap between the pads, but most truing can easily be done by eye.
When straightening a bent rotor it’s worth starting with the rotor arms — these are the thinnest and weakest part of the rotor and usually bend first. Going straight onto the braking surface can often mean chasing a kink round the entire circumference. Lastly, most rotors will also get worse before they get better, so expect to spend at least 15-20 minutes, maybe longer, on this job.
TOOLS FOR THE JOB:
Adjustable wrench / Allen keys / Disc brake cleaner / Park DT-2 Rotor Truing Fork £14.95 / Park DT-3 Rotor Truing Gauge £39.95
Contact: Park Tool www.ultimatepursuits.com
1 A lot of alignment problems are caused by the rotor being loose on the hubs. Check the bolts are tightened to the correct torque.
2 Next check the caliper bolts are tight.
3 Finally make sure the rotor is centred between the pads. Remove the wheel, push the pads (and pistons) back into the caliper with a screwdriver, then reinstall the wheel and measure the gap either side of the rotor. Adjust if necessary.
4 A sticky piston can cause the rotor to rub on one side. Remove the pads then pump out both pistons 3-4mm. Apply some silicone lube, which you can buy from a plumbing merchant, to each piston then reset them and wipe off the excess lube.
5 If the rotor only rubs in one place, lightly tap the braking surface at this point with a soft-faced mallet.
6 If that doesn’t work try a truing fork like this one from Park.
7 At a push you can use an adjustable wrench for this job. Set it to the thickness of the rotor.
8 Park’s DT-3 Rotor Truing Gauge works in tandem with a Park TS-2 Wheel Truing Stand and mounts as shown. Secure the gauge and align the silver stop with an area that has the least amount of side-to-side movement — this acts as a reference point.
9 Spin the rotor and mark the start and end of a warp or bend with a semi-permanent felt-tip pen.
10 Position the truing fork in the centre of the Xs. Steel rotors have a tendency to spring back so you may have to bend it past the centre line or do it a number
11 On most rotors it’s usually the rotor arms that bend first. The stubby end of the Park truing fork is ideal for getting in to straighten them.
12 Once you’ve finished truing give the rotor a clean with methylated spirits or a specific rotor cleaning product such as Finesse Stop Disc Brake Cleaner, £4.99.
With most brakes you can afford to have some run-out between the pads and rotor, usually about 1mm. Meaning if you get the rotor pretty straight leave it there.
If the rotor is this badly bent then bin it.
If you’re checking the rotor position by eye, place a piece of white paper on the floor underneath it