Replacing the brake discs

Really good braking is absolutely essential in today's road traffic. Which is why it's vital that every biker regularly checks their brakes – and that means more often than the just the vehicle safety inspection required by law every year or two. Proper maintenance of your braking system includes changing old brake fluid, replacing worn brake pads and checking the brake discs. Every brake disc has a minimum thickness specified by the manufacturer, and when this service limit is reached, the disc must be replaced. You should check this with a micrometer, not a caliper gauge, because wear and tear causes the brake disc to form a small ridge at its outer edge which would falsify the measurement if you use a caliper gauge. 

However, exceeding the thickness service limit is not the only reason for changing a brake disc. During heavy braking, the discs reach temperatures of up to 600°C.

Temperature differences between the outer ring and inner carrier of the brake disc in particular produce uneven thermal expansion, which can cause the disc to distort. Extreme temperatures also arise during everyday use. Travelling downhill on a mountain pass, where you're using your brakes constantly (often with heavy luggage and a pillion passenger), can push temperatures to mind-boggling heights. Sticking brake caliper pistons often generate high temperatures, and the constant contact with the brake pad can cause discs to wear extremely quickly. This can also cause discs to distort – a problem that particularly applies to discs with a large diameter and those that are rigidly mounted. 

On modern motorcycles, the low-cost rigid discs tend to be used where the loads on the brake are relatively low. Today, the state of the art is floating brake discs on the front axle. These offer the following advantages: 

• Reduced rotating mass enhances handling
• Less unsprung mass
• Better adaptation of the materials to the demands made on them
• More spontaneous brake response 
• Less tendency for the brake disc to warp

Floating discs have an inner ring which is bolted to the wheel hub and connected by movable floaters (a.k.a. bobbins) to the outer ring, which the brake pads press against during braking. If the axial play of this connection exceeds 1 mm, the brake disc will rattle, which is when it needs replacing. Any type of radial play generates a type of "run-on" when braking and will also cause your bike to fail the vehicle safety inspection.

First jack up your motorcycle securely in order to unload the wheel you are working on. If your bike is not fitted with a centre stand, you will need to use a paddock stand. Release the brake caliper(s) from the mount and replace the brake pads as described in the DIY tip Brake pads. Secure the brake caliper, e.g. with an insulated wire, to a part of the bike where it won't get in the way while you're removing the wheel – don't let it dangle on the brake hose.

Loosen the wheel axle and remove the wheel from the front fork/rear wheel swing arm. If the wheel axle does not detach easily, check first whether it is still secured, e.g. with additional clamping screws. If you still cannot undo the screws, see DIY tip Releasing screws for assistance.

Lay the wheel on a suitable surface and undo the fastening bolts of the brake disc in diagonally opposite sequence. Always use quality tools that fit well, particularly if the hexagon socket screws are proving stubborn, and make sure that you insert the tool as far into the socket as possible to ensure a secure grip. If you strip the screw head so that no tool grips, removing the screw is going to be a real headache. If the screws prove very hard to shift, warming them with a hot air blower and giving the tool a knock with a hammer can often help to loosen them. If the hexagon socket of the screw head is stripped, you may be able to undo it by using a bit that is one size bigger and hammering it in so that it grips. 

Take the old brake disc(s) off the hub and clean the contact surface. You need to remove any unevenness (residual paint or Loctite, etc.), and this is also a good opportunity to clean the rim and axle, as access is easier. If the axle has rusted, the rust can be removed, e.g. using sandpaper.

Fit the new brake disc(s). Tighten the fastening bolts to the torque specified by the vehicle manufacturer, in diagonally opposite sequence. Always replace badly corroded or damaged fastening bolts with new ones. 

Before actuating the handbrake cylinder, make sure that there is enough space in the brake fluid reservoir for the fluid level to rise. New pads and discs push the fluid upwards in the system, and you must make sure it doesn't exceed the "Maximum" mark. Actuate the main brake cylinder in order to bring the brake pads into contact with the disc. Check the pressure point of the brake. Check that the wheel turns freely once you have released the brake. If the brake drags, you may have fitted it incorrectly or the brake pistons may be sticking in the brake caliper.