One of the benefits of running disc brakes is that there's no wear to your wheel rims when you brake, but the disc brake rotors will gradually wear with use. Thankfully, they tend to have a long lifetime and are relatively cheap and easy to swap.
Worn out disc rotors
Disc brake rotors usually last so long that many people treat them as 'fit and forget' components. However, manufacturers provide minimum thicknesses for their rotors. Shimano recommends that its rotors, which start out 1.8mm thick, should be replaced when the braking surface has been reduced to 1.5mm. This information is given on the rotor; it says "Min.TH=1.5".
SRAM rotors are usually 1.85mm thick to start with, although some of its 140mm rotors are 1.9mm, and they should be retired once they get down to 1.55mm.
Different brands recommend different minimum thicknesses so check the details for the rotors you use. If you could slice ham with yours you've left it waaay too long!
If a small step has developed between the braking surface and the rest of the rotor, that's an indication that it could need changing, but we'd recommend measuring the thickness properly. The easiest way is with a micrometer, vernier calliper or a digital calliper.
Something like the Park Tool DC-1 digital calliper (below, £49.99) is perfect for the job.
It isn't possible to give an estimated mileage for a disc rotor because wear will vary according to the type of rotor used, the pads applied to it, rider weight and braking habits, the terrain ridden, typical conditions, the cleaning regime, and many more factors.
Don't be tempted to continue using rotors beyond their minimum thickness. These are safety items, after all, and it's not worth the risk.
Bent disc rotors
Another reason to replace a disc brake rotor is if it is badly bent.
Disc rotors can bend if they get too hot, are damaged in a crash or if too much pressure is applied to the side, frequently during travel. Putting a bike in the back of your car and resting something on the rotor is quite a common cause.
Sometimes you'll clearly see that a rotor is bent, and other times you'll realise when one part rubs on the brake pads as the wheel turns.
If a rotor is badly bent, you should replace it, especially if it was caused by crashing. In other circumstances, you might as well try to straighten the rotor. It's not rocket science and you have nothing to lose. Use a rotor truing tool like the Park Tool DT-2 (£19.99) or the jaws of an adjustable spanner to bend the rotor back into shape.
The easiest way to do this is with the wheel still on your bike. You simply find the section of the rotor that's bent – either by eye or by passing it through the brake calliper and listening for rubbing – and push it back in the opposite direction, being careful not to overdo it.
Upgrading disc brake rotors
You might simply want to replace your disc brake rotors because you fancy an upgrade.
Cheaper rotors are one piece, cut from steel, while floating rotors are more expensive. This is where the central carrier is aluminium with the braking surface made from steel. The idea is that you get consistent performance even if your braking system heats up during use. Ones from Hope (above) are priced £50.
Shimano incorporates what it calls Ice-Tech in many of its rotors, using a three-layer sandwich structure of stainless steel and aluminium, the aluminium included because it transfers heat better than steel. Reduced heat in the braking system reduces brake fade (a fall-off in performance on long descents). It also increases pad life and can lead to quieter braking.
Shimano's SM-RT99 rotors have 'fins' inboard of the brake track designed to maximise surface area for improved cooling. Shimano says that the increased surface area leads to a reduction in heat and a consequent increase in braking force.
Changing rotor size
You'll usually want to replace a disc brake rotor with one of the same size, although a different size can make sense in some circumstances (assuming your bike can support this and you have the correct adaptors). All other things being equal, a large rotor will slow you down faster than a small rotor.
Shimano's road disc brake system has been designed for use with 140mm or 160mm rotors, the idea being that users can choose the size to suit their weight and intended use.
SRAM’s Paul Kantor said, “We recommend 160mm rotors front and rear for road use and 140mm is fine for cyclocross. We have tested 140s extensively but we like the margin of safety that 160s offer for the road.”
Focus has told us that in testing it found 160mm rotors preferable to the more common 140mm, handling the buildup of heat more effectively. This goes against the trend for smaller rotors on the road.
The choice is yours but if you’re a larger rider you might want to start with 160s and see how you go.
Buying new rotors
There are two different disc rotor and hub standards: 6-bolt (see the Hope rotor pictured higher up the page) and Centerlock (see the Shimano rotors pictured above). The difference is self-explanatory: a 6-bolt disc rotor is fixed to the hub with – you guessed it – six bolts while the Centerlock system uses a lockring.
You can get adaptors, but make things simple for yourself by just swapping like for like.
Mat has worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.