How to check & replace brake discs

By MCN Technical Staff -

Parts & accessories

 29 November 2006 16:51

My discs look OK, so why check them?

Looks can be deceiving. It’s not only high mileage bikes that will suffer from worn or damaged discs – today’s sports bikes can suffer just as badly if regularly used to the limits of their design. Trackdays, for instance, are killers of discs as constant hard braking exaggerates wear and tear, leads to excessive heat build-up and can cause discs to warp.

Stuff you’ll need

Front and rear paddock stands to allow the wheels to be removed. Sets of appropriate Allen keys, spanners and sockets for wheel and disc removal. You’ll also need a torque wrench to tighten the disc bolts to exactly the correct tightness. To measure for warpage a dial gauge is a must, as is a vernier caliper to measure the thickness of the discs.

Do I have to replace like for like?

If the bike is still within its warranty period, yes. If not, then cheaper aftermarket discs are available and often make a bettter value buy. If the discs are worn or warped and the bike is still under warranty you’ll only get them replaced free if the dealer and maker believe the damage is due to a manufacturing defect. If they believe the discs are damaged from your regular trackday outings, for instance, or an accident, or are just worn due to heavy use, then you’ll have to fork out yourself.

Can I really do it?

If you haven’t got access to a dial gauge or vernier caliper and you think there’s a problem with the discs, you can pay a dealer to do the measuring part. As far as changing the discs goes, if you can remove a rear wheel from a 15-speed pushbike then you should be up to the demands of this job.

1. Sight and feel are just as important in deciding whether a disc is buggered. If you can feel a pulsing sensation through the front brake lever (or rear) under gentle braking, or the front end violently judders at high speed with the first stroke of the lever then it’s a sign the discs are warped. But get the steering head bearings checked, too.

2. A good look at the discs will also show up problems. Check for cracks, severe pitting and uneven wear on the disc’s face (rotor). If the discs are of the semi-floating (as shown here) or full floating type also pay attention to the condition of the rotor buttons that connect the disc rotor to the inner rotor (the bit that actually bolts to the wheel).

3. If the rotors have thin ridges or dips running around their circumference rub a finger across the radius of the rotor. If the ridges feel deep or abnormally high, then the discs need to be measured properly for run-out and wear. If there’s an obvious difference in thickness between the rotor’s outer lip and the face, check for wear.

4. A dial gauge mounted solidly on the fork leg or stand and placed against the rotor’s face at two different points (one-third and two-thirds distance across the radius) will highlight the rotor’s run-out – the amount a disc is warped. On the CBR600 shown the maximum amount of run-out on the front discs is 0.2mm either side of the rotor.

5. Vernier calipers measure how thick the discs are. On the CBR600 seen here, the minimum permissible disc thickness (dealer figure) is 3.5mm (4.4-4.6mm standard) on the deepest ridge. Make sure the rotor’s outer lip is not included in this measurement as it isn’t touched by the brake pad.

6. If the discs need to be replaced the wheel needs to be removed. Label the wheel’s direction of rotation before removal for reference when fitting new discs (they are handed). Prop the wheel (eg on an old tyre) so the discs aren’t damaged by the floor surface – not an issue with the old discs but you don’t want to bugger the new ones.

7. Disc bolts are notorious for being hard to undo – they’re Loctited (non-setting glue) in place. Shock the bolts by hitting them full on with a hard plastic mallet to help crack the glue, or even corrosion, before undoing them. If you can, use a new, perfect fit Allen key to cut the chance of rounding off the bolt’s key-way.

8. All bike manufacturers recommend disc bolts should never be reused. That may be so, but when you’re talking £1.60 per Honda disc bolt and there are 12 of them on the CBR600’s front end, the expense (£19.20) soon adds up. As long as the key-ways are in good nick and the bolts are torqued up tight using them a second time isn’t a problem.

9. With the discs removed check the mounting face on the hub is clear of corrosion (similarly if you take the discs off to have the wheels painted then there should be no paint where the hub and disc join). Gently rub clean with wet & dry grit paper wrapped around a flat block, or a flat-bladed screwdriver.

10. When fitting new discs, (EBC Pro-lite replacements used here), ensure the rotation of any new disc is the same as the wheel and lay it on the hub. Some replacement discs have more than one set of holes to suit other models, so pay particular attention to align the mounting holes to match those in the wheel hub.

11. Apply a drop of medium/hard Loctite to the first few threads on the disc bolts before screwing all bolts in finger tight. Now torque the bolts tight (get the right figure from your manual or dealer) but in a criss-cross pattern to ensure the disc is tightened down flush. Repeat the same procedure for the opposing disc. Refit the front wheel.

12. New discs should be mated to new pads – the old ones will have worn to match the old rotors, will take for ever to ‘bed in’ and can even score the new disc. Some replacement discs come with a zinc coating so the unused area remains corrosion resistant. This gets scrubbed off the main part with use.
* Thanks to EBC Brakes (01604-583344)