Now we’ve happily got you swapping and swilling oil around inside your forks, we reckon it’s time to take our basic maintenance series on to cover that other often fluid filled frightener – disc brakes.
We’ve abandoned our normal walk through format as where each bit goes is pretty self explanatory. Instead we’re going tell you what to watch for before you part with cash, and then discuss a few tricks to make the whole process easier and swifter. Then we’ll move onto servicing, cleaning and bleeding your brakes in following weeks.What fits where?
The first essential is making sure that your chosen brake actually fits your bike. Most brakes are designed to fit the International Standard mounting (ISO) which uses two bolt holes on a plate aligned parallel to the brake rotor.
With this system correct alignment relies mostly on the alignment of the actual frame mounting. Correct spacing to prevent the pads rubbing on the rotor is then achieved by adding or taking off thin washers on the mounting bolt in the gap between frame and brake. As you can guess this is a damned irritating and time consuming procedure, but it’s light and you only have to do it once.
Some cable brakes (Avid, Grimeca, Diatech, Formula) get round this problem by mounting the caliper itself on sliding posts fixed to the frame mount. The brake is then moved sideways by a small thumbwheel or just allowed to ‘float’ from side to side. This makes setting up very easy, but adds weight and potential for squeak under braking.
Hayes get round the problem of alignment by using a different mounting standard altogether. They use a pair of bolt mounts that are perpendicular to the rotor face, with a slotted mount on the caliper to allow sideways movement and ensure accurate alignment. Rear Hayes brake mounts are becoming rare now, but Manitou forks still use them as standard up front. Shimano Deore and Avid cable brakes add a curious twist by using an international mount that attaches to the actual brake caliper with a Hayes style two bolt plate. Again very easy to set up, but not for fans of minimalist mass and clean design.
Although this sounds like an absolute minefield, the good news is that there are adaptors available to fit practically any brake to any mounting as long as there is space. Hope even do replacement caliper segments to allow you to swap just the mounting portion of the brake rather than adding an extra bracket.
Another fitting consideration is the size of disc rotor used. Smaller, lighter disc rotors are favoured by the XC race set, but if you want more stopping-power larger rotors are the way to go. As these put the braking surface of the disc further away from the hub (and brake mounting) then you’ll need at least an adaptor and possibly a new caliper to bridge the gap, so check before you change.Pot luck
While shopping for disc brakes you’ve probably bumped into two different types. Twin pot and Four pot. Twin pots are the most common design using two opposed cylinders to press the pads against the disc. As the pads are square and relatively narrow aligning them to stop rub is quite easy. Four pot brakes with twin pistons on each side are a lot trickier though as pads are much longer, leaving you less room for error in alignment.No mount misery?
If you’ve got an older bike with no disc mount on the rear end but are desperate to use discs then all is not lost. Hope have an adjustable disc mount that attaches to the axle of their rear hub in place of a spacer, with an arm that bolts onto the V brake mount on the seatstay to transfer braking stress to the correct place on the frame. Give them a ring on 01282 851200, or head for Hopetech.com.Top fitting tips
The best tip we can give anyone fitting disc brakes is to take your time. Faffing with spacers in between brake and frame and tightening then loosening the mounting bolts until you get the alignment right would try the patience of a saint, but getting it right is essential.
The second piece of advice is to make sure you don’t undo any bolts you don’t have to. We know several people who’ve pulled a caliper in half thinking it was something to do with the mounting. Not only will you lose all your brake fluid and let air in but you’ll probably douse the pads in fluid and need to replace them too.
When fitting the brakes, dab a small bit of high-performance grease between the back of the pad and the cylinder head. This helps stop small vibrations that can cause squeal. Make sure that none of the grease touches the pad face or rotor though.
If you’re setting up a cable disc brake that uses a straight arm rather than a cam, ensure that the arm is at right angles to the cable mount on the caliper when the pads touch the rotor. This gives maximum braking leverage, but needs to be checked regularly as pads wear.
If you’re trimming your hose, make sure you leave enough length to cope with a complete bar wrap round without pulling the hose out of the lever or kinking it. Brakes operate at big pressures and plastic hoses will soon split where they are kinked.
If you haven’t got dedicated hose guides on your bike, hoses can be attached with zip ties or purpose built cable clips which plug into your old cable guides. If you’re using a metal hose then be very careful to protect the bike anywhere it might rub, as the braided line will soon rub through soft alloy.
Our final piece of advice is to leave removing the old V brake set up till the last minute possible. That way if something does go wrong, you’ve less to bolt back on to ride to the bike shop ;-).