Earlier this month we showed you how to stop the squealing from your rim brakes. But what if you use discs, whether cable-operated or hydraulic? By following the 10-step guide below, you should be able to banish the three S's: shrieking, scraping and squashiness.
Before you start, here are some useful tips:
- Lube your chain sparingly – too much can cause contamination.
- Avoid storing bikes with hydraulic discs upside down. Trapped air can get lodged in the callipers, causing squashiness.
- Use threadlock or the anti-loosening devices provided with calliper retaining bolts.
- Wear hand protection against disc rotors and brake fluids, as well as eye protection when bleeding brakes.
- As always, if you have any doubts about your results, take your bike to a pro.
- Adjustable spanner
- Needle-nose pliers
- Disc brake cleaner, eg. Muc-Off
- Medium emery cloth or sandpaper
- Allen key set
- Thin shim washers
- Brake fluid
- Plastic bottle
- Fuel or vacuum hose
- Protective wear
- 6, 7, 8mm box/open-end spanners
10-step guide to sorting out your disc brakes
1 Straighten disc
A wobbly rotor is easy to spot. Give the wheel a spin and look at it from above. Note the section you want to straighten. A commercial disc truing tool is nothing more than a slab of steel with a slot cut into it, designed to grab the disc at the correct spot. An old adjustable spanner does just as well. Position the jaws in the middle of the area to be straightened and gently pry in the right direction. Spin and repeat as necessary. For truing a smaller portion of the disc, place a short piece of wooden dowel against the spot and use a hammer to strike the dowel with a measured blow (easy does it!).
2 Check for worn pads
Deciding whether or not to replace the pads is simple. Some manufacturers use a gauge of sorts, such as the little bump on the Magura pad on the right of the picture. On the left is a more typical pad, which should be replaced just before it reaches the thickness of the return spring. Inspect the pads’ contact surfaces. Partially worn pads can sometimes be improved with cleaning (see step 4). Glazed pads have a shiny hard surface with curved wear ridges in the shape of the rotor. Revitalise them by roughing up the surface with coarse sandpaper or a file. Check pads every month to avoid damage to the rotors.
3 Install new pads if necessary
Brake pads are usually accessed from the bottom, having removed the wheel in order to get the disc out of the way. For hydraulics, insert a screwdriver or tyre lever between the old pads, then pry them apart until the backing plate contacts the calliper. Cable disc pads are often magnetically held. The Shimano design pictured enables installation from the top. Remove the split pin with a pair of needle-nose pliers, or if it’s a threaded pin, use a 3 or 4mm Allen key. The pads should pull out with the return spring. Clean away caked-in dirt before inserting new pads and, ideally, a new return spring.
4 Spray away squeals
Brake fluid and cleaners are pretty vicious, so you’ll want to protect your hands and avoid getting the fluids on paint. We find Muc-Off Disc Brake Cleaner is by far the most effective at reducing noise and improving performance. Badly glazed discs can be roughed up first with some medium emery cloth or sandpaper, improving bite. Spray generously (as pictured) and allow to dry. Now get the brakes hot and re-apply as necessary. Earth friends can try a strong solution of eco-friendly washing up liquid and water, followed by a rinse. Slam on the binders. Then repeat and rinse with water.
5 Get things straight
A frequent cause of squashy brakes is a misaligned calliper, causing the pads to strike at an angle. The springiness of the steel disc as it is straightened between the pads is what you then feel through the lever. It’s crucial that the pads are perfectly parallel as they clamp the disc. Some brake mounts have enlarged holes that offer some lateral adjustment – loosen the bolts just enough to nudge the calliper over; you might have to try a few times, as it can move when bolts are re-tightened. Otherwise, use thin shim washers that are widely available. Back out the bolts and slip in the washers.
6 Optimise leverage and cable pull
On cable discs, check brake lever and calliper lever arm return: if they’re not snapping back nicely and you feel resistance when you squeeze, replace the cable and/or housing. Lube the cable with oil rather than grease before inserting it into a sound outer, and anchor the cable (usually with a 5mm Allen key) with the calliper lever arm at its most open position, as shown above. This is the optimal pull angle, with plenty of throw available. As the lever runs out of travel and the angle of cable pull changes, you get a drop in braking power. As in the image, the old pinch point is evident by the kink in the cable; this inadequate setting limits lever travel and power.
7 Adjust pad position
Most systems enable pad bite point adjustment. On cable types, one or both pads can be dialled inwards, using either a knurled knob or an Allen bolt. Bring them in as close to the disc as possible. If needed, turn the barrel adjusters counter-clockwise to take up some cable slack and move the pads closer to the disc. On single-dial callipers, bring the lever side up against the disc by moving the whole calliper, then move the dial-adjustable pad in close (see step 5). On hydraulics, turn the grub screw, usually inside the lever at the fulcrum, a few turns inward. There might also be an adjustment dial on the lever – actuate it with your fingers. For further adjustment, see step 10 below.
8 Top up fluid
Two types of fluid are used in hydraulic brakes, either mineral or Dot. This is usually marked on the reservoir cap. Do not introduce the wrong fluid – the seals will be damaged and leak. Shimano and Magura use mineral oil; Hayes, Hope and Avid use Dot 4 or 5.1. Immobilise the bars with a strap around the wheel and down tube. Position the levers with the reservoirs level, remove the caps and check the fluid level. If the level is really low, this could be part of the problem, as air may have worked its way into the line. Top up before bleeding. Make a cheap fluid capture system with a suitable bottle (an empty oil bottle of the same material) and some clear fuel or vacuum hose, available at auto parts stores.
9 Bleed brakes
When it comes to hydraulic brakes, air in the lines will make a squashy lever pull back to the bar. This step applies mainly to Shimano, Hope and Tektro, with a regular reservoir and standard bleed nipple as pictured. Wear protective gloves. Push a hose onto the bleed valve and secure with a zip-tie. Attach the bottle below the level of the valve. Back off any bite point adjustments, then pump the brake lever four or five times and hold it, squeezing. Open the valve with a small spanner (6 or 7mm) and watch for bubbles as fluid is forced into the tube. Close the valve, release the lever. Repeat five to 10 times, keeping the reservoir topped up, until no more bubbles appear and the lever is firm.
10 Equalise pads to get rid of the rub
With the wheel removed, make sure the pads are fully retracted (see step 3). Re-install the wheel, and refer to step 5 to centre the calliper over the disc. Squeeze the lever a couple of times. The pistons should move in, then return about equally, relative to the rotor. If they don’t, try this: pull the disc firmly over, holding it against the pad that was too close, blocking movement. With your free hand, squeeze the lever a couple of times, forcing the pad that was furthest away to extend towards the disc beyond its original travel point. Release the disc and give the lever a couple more tugs. This will establish new piston protrusion points. Repeat as necessary.