Home » Bike Maintenance » Disc brakes : introduction to rotors

Disc brakes : introduction to rotors

The rotor is the proper name for the disc that gives the disc brake its essential raison d’être. As with calliper mounts, it has taken time for the industry to decide on a standard – four- and five-bolt hubs have been and gone (and in the case of Hope Technology, four and three bolt hubs have come around again) but the sixbolt hub is now the most commonly used.

Disc brakes - introduction to rotors

Tighten rotor bolts alternately across the centre to hold tightening plates (A) in position

Shimano’s alternative to the six-bolt mount, the Centrelock, has been adopted by other manufacturers including DT Swiss. It uses a rotor with a splined ring which mates with a splined ring on the hub shell and is locked into place with a modified cassette lockring. It’s quicker and easier to swap discs with this system, as you only need to loosen and tighten one ’bolt’, but that bolt can only be tightened with the dedicated lockring tool and a large spanner, rather than the easy-to-carry Torx/Allen key that a six- bolt mount requires. If you have Centrelock hubs and want to run six-bolt rotors, you can buy adapters that allow you to switch between the two.

As well as the choice of size and mount, you will also have to decide whether you would like regular, one-piece rotors or floating and vented versions. The former are lighter and slightly easier to coax back into shape should you manage to bend them, whilst the latter have appeared as a crossover from the motorbike industry. Floating rotors consist of an inner spider and outer braking surface which are only loosely joined, whilst vented rotors are two very thin braking surfaces with a gap in the middle that allows air flow. Both allow the faster dissipation of heat than a standard rotor, but both types usually carry a weight penalty. If you’re interested in shaving grammes from your bike, you can even upgrade your rotors to featherweight versions.

Your disc brake rotors can get very hot, particularly on extended descents. Don’t touch them they’ve it’s had long enough to cool down or you really will burn yourself and end up with a nice scar. Rotors are also razor-sharp and quite capable of slicing right through a finger when spinning – blood will contaminate your rotors and pads just as effectively as oil, so keep your hands well clear of the brakes when the wheel is turning.

Each model of calliper is designed to work with a specific rotor – the diameter and thickness are crucial. If you are attempting to mix and match, make sure that the rotors aren’t rubbing on the calliper body and that the full face of the pad is hitting the rotor rather than overhanging the inner edge. It really is best to use the dedicated rotor for your brakes, though.

Rotors do wear out eventually, though it takes a while and they last much, much longer than rims do under V-brakes. To check the wear, you can either measure the rotor with a pair of vernier callipers or simply take a close look at the surface. If there’s a noticeable difference in thickness between the rotor face and arms, then it needs to be replaced.

The braking surface also needs to be smooth and shiny – torn, pitted or corroded surfaces mean inconsistent braking and accelerated pad wear. This can be caused by the pad compound wearing out, leaving you braking on the metal backing plate – so if you’re out riding and the scraping noise of a muddy disc suddenly turns into an excruciatingly loud banshee wail, it’s well worth checking whether or not you’ve worn your pads out and, if so, replacing them with the spares you’ve (of course) got in your toolkit to avoid damaging the rotor irreparably.

It’s vital that the bolts securing the rotor are fitted securely, otherwise they will rattle loose. Many manufacturers supply their rotors with Torx bolts, which require a tool that’s a little like a starshaped Allen key. The Torx bolt is no stronger than a standard Allen key and is actually easier to damage if tackled without care. Don’t attempt to bodge the tool, as a standard Allen key or screwdriver simply will not work. Park make several excellent Torx key sets and many multitools now include them as standard, too.

If you ride frequently in muddy conditions, you may find that a wavy or sawtooth-edged rotor like those manufactured by Hope and Avid will stop the calliper from clogging. Curved cutouts on the rim of rotors allows hot rotors to expand uniformly, reducing cracking.

Braking efficiency

Your rotors are your braking surface. Your brake system’s efficiency will depend as much on their condition as on the condition of your brake pads. Cleaning the rotors and replacing your pads should be the first priority if your brakes aren’t behaving as they should be. The majority of problems here are due to contaminated pads, or worn or dirty rotors, rather than more glamorous bleeding issues.

Cleaning rotors

Disc brake pads willingly absorb grease or oil from any nearby source, with an immediate effect on your braking power. Keep the rotor clean; it’s best to avoid even touching the rotor with your bare hands if you can as they leave grease behind. Some people dab grease or copper slip onto the back of the pads to try to stop any squealing. This is a bad idea, as the copper slip will soften under the heat of sustained braking and inevitably find its way onto the rotor.

Clean your rotors with either dedicated mountain bike disc cleaning spray or isopropyl alcohol, which doesn’t leave an oily residue behind. You can get the former from bike shops and the latter from chemists and electrical wholesalers. Don’t be tempted to use a car disc brake spray. Car brakes run much, much hotter than a mountain bike disc brake and can burn off the residue the spray leaves behind, whereas you’ll find yourself needing to start anew with the cleaning and a fresh set of brake pads.

Bikes that get a lot of use on the road will need their rotors cleaning more often, too. The roads are covered with an oily residue that contains the remnants of fumes as well as leaks; this finds its way onto rotors very quickly and can cause reduced braking power. You may also find it helpful to change or sand down your brake pads more often. The absence of abrasive mud means that the pads become glazed (shiny to look at) and noisy; you can minimize this problem by using softer resin brake pads rather than sintered.

Replacing a rotor

Some rotors – primarily Shimano – are supplied with tightening plates. These re thin metal plates which sit behind the bolts, which you bend up and around the head of each bolt once it’s tight. If you wish to fit them then a flat-blade screwdriver will help you form them around the bolt but they serve no real purpose once the bike is out of the factory.

To replace your rotor, first arm yourself with the correct tool and some thread-locking compound. Never try to bodge the tool – an Allen key or screwdriver will not substitute effectively for a Torx key, and vice versa. If you do find yourself in the sticky position of rounding off the bolts, enlist the help of either a Dremel with a fine cutting disc or your friendly bike shop of choice, depending on how confident you’re feeling, because once you’ve destroyed the heads on rotor bolts they are extremely difficult to remove.

Rotor bolts tend to be fitted tight and abandoned to the ravages of salt and grit, so they will often let go with a sharp bang. Wrap a rag around the tool to protect your hand if you find the percussive crack uncomfortable, and if you’ve managed to undo five of the six bolts but the other is sticking, try tightening the rest again to relieve the pressure on the stuck one – chances are it will come free with ease.

Once all six bolts are free, remove them, clean up the hub and threads, then work out which way your new rotor needs to be fitted. Most will have direction arrows etched into them but if not, the offset of each arm of the rotor usually needs to point forwards at the top.

If you’re reusing the old bolts, clean them and apply a small dab of thread-locking compound. New bolts will have this already applied. Fit each bolt through the rotor and thread them loosely into the holes in the hub. Once the bolts are snug but not tight and you can still waggle the rotor slightly, rotate the rotor clockwise as far as you can, then tighten the bolts in the correct order, shown below.

Once you’ve followed the star-shaped sequence below and the bolts are tight, go round a final time – you’ll probably find that the first bolts in the sequence are not actually tight and need further work. Use a torque wrench if you’re worried about over-tightening. The correct torque is 2–4 Nm.

Once you’ve been out on your first ride, check the bolts are still tight, and then check them again every 500 miles.

See also bike maintenance tips, tricks and techniques “Disc brakes : fitting Post Mount callipers to your bike”