Dirt Bikes 101- Brakes

#2 in a series

Nov. 01, 2005 By Mike ""OldFart"" Hetrick
This month we'll be talking about brakes. I mean, what the heck. We talked about wheels last month and brakes usually attach to the wheels, so why not. Let's begin with drum brakes. There are still some of those beasts out there and we don't want to leave anyone out.

Drum brakes work very much like the drum brakes on a car except for the method they are actuated. Instead of a hydraulic wheel cylinder, bikes have a "cam" that turns and spreads the ends of the shoes apart and against the drum. There is either a rod or cable that pulls on the arm, which turns the cam. Some earlier (much earlier) street bikes had a "double" leading shoe brake. This means both shoes have their own cam that pushes the shoe against the drum. A "single" leading shoe brake has only one cam to operate both brake shoes. Double shoes were better but only when everything (the linkage between shoes) was adjusted properly. More parts meant more to go wrong, more to service and more weight.

Disc brakes. Again work like a car (how many ways can you make brakes?). Virtually all disc brakes use a hydraulic system with a master cylinder, which produces the pressure that is transferred down the brake hose to the caliper where the pressure pushes the piston (or pistons- some street bikes have up to six pistons per caliper) against the backs of the pads. This is what squeezes the disc and gives us the friction necessary to slow down.

Most brake hoses the manufacturers put on a bike is of a rubber or plastic variety. These can (and do) flex or swell when pressure is applied. This can give the brakes a slightly vague or mushy feel. Steel braided hoses can be bought or made and can help brake feel substantially. The steel braided hose doesn't expand under pressure like some of the plastic or rubber hoses can.

Brake pads come in a variety of compounds. There is a definite science in determining which type pads work for what application. This can be almost along the lines of choosing tires. If you're not satisfied with the stock pads, there are a few alternatives available. I won't go into them here but basically the metallic pads last longer (especially in muddy conditions) sometimes with a minor trade-off in braking performance. And again, just like tires, you get what you pay for. At a hare scrambles a few years ago, I was in a pinch for rear pads. I tried a set of "bargain" pads. They were 100% worn out before the end of the practice lap (about 4 miles).

Some brake pads have a metal shim between them and the caliper piston. This can be important. You may have a relatively minor symptom like brake squeal or you could have something more important like brake fluid boiling. This can (and normally does) result on a partial to complete loss of braking force at that wheel (normally the rear).

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The above picture shows an old brake pad (right) and a new pad with the old heat shield installed.

Rotors are pretty much maintenance free. They wear and you replace them or they bend and, if they can't be straightened, you replace them. Bike rotors are not machineable.

There are larger rotor kits available. These include a caliper carrier that moves the caliper out to the right place to contact the swept area of the rotor. This can give an increase in braking power for those who can actually use it.

The rotors that come on dirt bikes are either slotted or drilled. This can create problems with rapid brake pad wear in muddy conditions. If you ride in areas where dry dirt isn't very common, it may be beneficial to install solid rotors on your bike.

Some rotors are mounted on pins or collars that allow the rotor to move sideways just a little. These are known as floating rotors. Some of the Kawasaki KXs have floating rotors. When the pins/collars wear, they can make a heck of a lot of noise. Sorta like your front wheel is getting ready to fall off.

One accessory that I've had good results with are brake retaining pins. Some of the manufacturers use pins that have an allen head holding the brake pads in place. These can become seized in place even if anti-seize compound is used on the threads. The aftermarket pins have a regular hex head (normally an 8mm) that can eliminate them getting stuck since you can use a wrench or socket that can apply more torque to the pins to remove them.

A disc guard can be a good idea. They can prevent mud from accumulating on the caliper/rotor and protect the rotor from impacts with rocks, ruts or even other riders.

Oh, yes. One other thing about rear brakes. A design feature normally only found on drum brakes was the full floating backing plate. This had the backing plate stay arm attached to the frame of the bike instead of the swing arm. This required the center hole of the backing plate to pivot as the rear suspension moved through its travel. This made a fairly big difference in braking in situations where there were sharp bumps. The rear wheel would follow the contour of the terrain much better than without the full floating feature. There were only a couple models with a rear full floating disc brake. The early to mid ?80s KTMs come to mind right off hand. Nowadays they reduce the kicking with shock valving instead of reducing the braking forces acting on the rear suspension.

Maintenance

Brakes can be fairly reliable and trouble free. They can also be a nightmare. Let's examine the possibilities (some of the more frequent ones anyway).

Drum brakes are pretty much maintenance free except for an occasional adjustment. Go through one deep creek and you'll see one of the reasons they started using disc brakes on dirt bikes. You normally lose virtually all of your stopping power. Dragging the brakes usually restores the brakes within ten or fifteen miles. Some people have had a little success grooving the shoes. This requires removing the shoes from the backing plate and cutting slots in them to allow the water a channel to follow. These grooves should be made so as the wheel turns it pushes the water towards the outside edge of the drum. The grooves should be about one or two millimeters wide.

Drum brakes can be made to work fairly well in dry conditions. One thing that can make them work poorly is the lack of adequate contact area. This means only a small portion of the shoe is touching the drum. If you pull the shoes out and they are only shiny on half the shoe, then this is a prime indicator the shoes aren't working as well as they could. More contact area means better stopping. Sanding/filing the center of the shoe can help increase the surface area. Placing the shoe inside the drum (loose- not installed on the backing plate) will usually give you a fairly clear indication of how well it is touching the drum during use.

Another method of increasing the contact area is to apply the brake while tightening the axle nut. This insures the backing plate is centered if there is any play in the center hole where the axle goes through it. This can make a significant difference in how well the brakes work.

The only other thing drum brakes require is some lubricant on the moving parts. Waterproof grease works well on the brake cam sliding surfaces. This includes where it turns in the backing plate.

Disc brakes can be adequate for your average trail rider as is from the factory. More aggressive riders will want to replace the brake fluid with some better quality (higher boiling temp) fluid. There are even silicone base fluids but these can be miserable to live with due to the fact they don't mix with any moisture that may get into the system. This means if it gets hot enough to boil, the moisture turns to steam which is, more or less, mostly air. Air is not good in a hydraulic brake system. Stick with DOT 3 or DOT 4 approved fluids.

More aggressive riders change their fluid relatively frequently. This can help the feel of the lever/pedal. This can depend on the particular system. Seems like you can forget about Honda CR brakes but some KXs need to be bled fairly frequently to maintain a good feel.

Flushing a brake system that is full of fluid is pretty easy. Bleeding a system that is full of air can be pure hell. Here are a couple methods that work for me. There are about 20 other methods also.

Get a clear length of hose about a foot or two long. Tie a loose knot in it about halfway down. Remove the bleeder screw cap and loosen and retighten the bleeder screw. Now put the hose on the bleeder screw.

Open the bleeder and apply the brakes. When the fluid stops moving, close the bleeder and release the brakes. If you're flushing the fluid (not trying to eliminate an air bubble) keep the reservoir full and repeat this until you think you've got most all the old fluid flushed out.

If you have had the system opened and have an air bubble, a hand operated vacuum pump can be a big help. Hook the pump to the bleeder bottle (don't run brake fluid through the pump- it could damage it), and hook the other hose to the bleeder nipple, open the bleeder and suck the air bubble down through the system and hopefully, out the caliper. You'll see a huge amount of air moving through the hose. This is mostly coming from around the threads of the bleeder screw. A blob of grease around the bleeder screw can help reduce/eliminate this. The vacuum pump can suck a bunch of fluid fast, so keep a close eye on the fluid level in the reservoir. If it goes low enough to suck air in, you'll have to start from scratch.

An air bubble can be very elusive in the front brake hose. You're trying to force it down and it keeps floating to a high spot in the system. Okay, let's get that to work for us instead of against us. Unbolt the caliper. Pull it off the disc. Now take a flat blade screwdriver and gently push the pads apart. This will force fluid backwards through the system and hopefully force the air bubble through the master cylinder and out the reservoir. Reinstall the caliper and bleed again to make sure all the air is out. You may have to go through some small ritual to get all the trapped air out. Just use any combination of methods to insure you have no air remaining. Once you get most of it out, it should bleed out fairly normally.

Some people have had to resort to removing the entire brake system intact and bleeding it with the thing laying flat on the floor. Raising the caliper a little higher than the master cylinder can help. This lets the air bubble get pushed sideways and up instead of trying to raise up to a high point in the system in the installed position.

If the clearance between the master cylinder push rod and the master cylinder piston is not sufficient, it can give a few different problems. I was at a hare scrambles relaxing after the practice lap and saw some guy trying to push the pads apart on his rear brake with a BIG prybar. Being the curious and sometimes too helpful type, I asked him what was going on. Turns out his rear brake was dragging bad enough that the wheel wouldn't turn. He had adjusted the pushrod the night before trying to get his rear brake to feel tighter. Boy was it ever tight. Readjusting let the fluid back up into the reservoir the way it was intended and the brakes started working normally. There are holes in the master cylinder bore that allow the fluid to flow backwards through it. If the pushrod is adjusted too far, these holes won't be uncovered and won't allow the fluid to "back up" normally. Then you get dragging brakes type symptoms. On the same token this can also prevent the system from bleeding properly since it won't allow fresh fluid into the system.

A bent disc can give the same symptoms as air in the system (mushy pedal or lever). As you ride the bend pushes the pads back into the caliper. Then the next time you apply the brakes, the initial lever or pedal movement is used pushing the pads back up against the rotor instead of squeezing the rotor. I had a Yamaha that I couldn't get to work right. No matter how many times I bled it, it always had a soft lever feel to it. I t would feel okay sitting still and then I would ride it and the lever would again feel like shit. Then it dawned on me. The week before, I had been in a first turn pile up. That's when my disc had gotten bent.

Bending a disc back is no big deal. Just use a pair of needle nose pliers or an adjustable wrench. Don't use pliers with sharp jaws. You don't want to gouge the rotor. With the wheel in the air, spin it so you can pinpoint the deflection. If it isn't too big of a deviation, you can usually get it bent back by grabbing right in the center of the bend and flexing the disc back into position. It'll have to go past straight since it has some "memory" to it, but it isn't difficult to get it straight with the least bit of common sense.

If it is a big bend, you may have to start near each end of the bend and work your way towards the center of it. If you were to start in the middle, you could end up with a very wavy disc. Look at it this way. What's the alternative? Right. Replacing the disc anyway. Just work slowly and carefully. You can usually get a disc virtually perfect. They're pretty "flexy".

The master cylinder is a "no maintenance" item unless it quits making pressure properly. In this case, a rebuild kit can restore normal operation if the bore is okay. To remove the master cylinder guts, pull the rubber boot off the end. Some boots have a metal ring that holds them in place. Try not to tear the boot unless you're sure the new rebuild kit comes with one. Removing the boot will expose a snap ring. Chances are this snap ring will be corroded in its groove. It may take a little "persuading" but it has to come out. Once the snap ring is removed, the guts will almost jump out at you (since they are spring loaded. The rebuild kit may come with a new piston with all the cups in their correct position. If the rebuild kit only has the new rubber cups, take careful note of their position and direction. They have to go in 100% correctly or it just won't work right. While you have the piston removed, clean the master cylinder with some brake or contact cleaner. If there are any significant pits (small imperfections in the surface the rubber parts slide over), these may reduce the effectiveness of the new parts right away or they could show up as a short life span of the rebuild parts. If so, it may be time for a new master cylinder. Lubricate the new piston assembly and reinstall into the master cylinder. Put the brake line back on and bleed.

Hydraulic clutch master cylinders are the exact same thing as a brake master cylinder except a mirror image. It works the same and can be rebuilt the same. A word of caution. Be sure to use the correct fluid. For instance, the KTMs use a mineral fluid that isn't intended to be mixed with brake fluid. Using the wrong fluid could cause permanent damage.

Calipers can be a problem if neglected. The piston can stick. If the caliper ha sonly one piston, it has to have pins that it slides on so it will squeeze the disc from both sides. These pins can also stick. Taking everything apart and using a little of your favorite waterproof grease will do nicely here.

If a piston is stuck or the seal is leaking brake fluid, the caliper can be rebuilt without much trouble. Usually the hardest part is getting it apart. There is a small lip at the outer edge of the piston that can be used for grabbing with a pair of channel locks. Just don't scar the sides of the piston. Air pressure can be used in the hose fitting (once disconnected) to force the piston out unless it is stuck. Be careful because the piston can come out with enough force to damage fingers. Remove the sliding pins and clean everything. Some emery paper or steel wool can be used on the sides of the piston. It needs to be pretty smooth since this is the sealing surface for the square cut o-ring. Clean the bore of the caliper also. Make sure everything is spotless when getting ready to reassemble. Lubricate the new square cut o-ring with clean brake fluid before installing it. Make sure the inside of the caliper bore and the sides of the piston are coated with fluid and push the piston back into the caliper. It needs to be virtually perfectly lined up with the bore before it will go in. If it is the least bit crooked, it won't go and you may damage it if you try to force it. Reassemble the sliding pins and you're ready to go into the bleeding nightmare. Have fun. Send me a card- something like "wish you were here" might be appropriate.

While you have the pads out, inspect the pins that hold the pads in place. If they get a notch worn into them, the pads can be reluctant to move correctly. New pins are fairly cheap. Some bikes have a plug that screws into the caliper to prevent dirt from collecting in the head of the pin. The picture below shows where this plug lives.

 

The pin is directly inside the plug. Here I'm getting ready to remove the pin.

Rear pedal height can make a difference in how the brakes act, not so much in function but the way you apply them. This can normally be adjusted and is one of those "personal preference" things. I like mine really low so I can keep my foot on the pedal while I'm sitting down without dragging it. This makes it a bit farther away when I'm standing up but, for me, it is the lesser of two evils. The adjustment is usually either a stop screw or bolt near the pivot for the pedal. Some models (Yamaha YZs come to mind) only have a provision for adjusting the clevis at the point the pedal and the master cylinder push rod join. Normally the locknuts are loosened and the threaded pushrod has flats for a wrench to turn it on this type adjustment. In the picture below, the locknut is at the top of the "U" shaped clevis that connects with the brake pedal. The height adjustment is barely visible under the black rubber boot at the bottom of the master cylinder.

I hope you got something out of this months column. I had fun playing with the digital camera. Wanna see some pictures of my dog?

New Info

This section will be dedicated to any additional information that has "surfaced" since last month. This may be anything from an important point I neglected to include in a past column to any good lies I have thought up. I will try to include clarifications from past "unclear points" or anything else that might merit some follow-up.

From the Wheels column:

When mounting/dismounting a tire, it is important to push the tire bead (the inside edge of the tire) into the center of the rim (right above the spokes) to allow the tire adequate room to be pulled over the rim at the other side. In other words, if you're trying to lever the tire off the rim at the 12:00 position, be sure to push the tire into the center of the rim at the 6:00 position. Otherwise, it ain't comin' off!

Rim straps. These are important. These help protect the tube from possible damage from spokes. If yours isn't in good condition, either replace it or use a layer or two of duct tape.

There is an alternative to the inner tube this is called a foam insert. This is basically a stiff loop of foam that is in the shape of a fully inflated and installed inner tube. These are mostly for desert or woods racers who don't want to chance getting a flat during a race. These have a very limited lifespan. They are also extremely difficult to install for even the most experienced. Not something your average weekend warrior should consider.

Slime is another alternative to flat tires. This is a gooey "liquid" (for lack of a better term) that you squeeze into your tube. This stuff will repair a small puncture in a tube. It doesn't work for pinch flats where a small slit is formed from getting pinched between a rock (or other hard object) and the rim. Slime can be pretty miserable to live with too. It can make the valve core not seal correctly and cause a slow leak unless you have a tight sealing valve cap.

Tire pressures. Some guys run up to 18 psi in the desert to help prevent pinch flats (it can be hard to see the rocks at 80 mph let alone trying to avoid them). I've run as low as 6 psi in a really soupy motocross. I even got away with it for almost 4 motos before the tire spun enough to tear the valve core out of the tube. Normally I use 12 psi unless something dictates different. More in rocky conditions, less in muddy or sandy conditions. Generally speaking, the more pressure you run, the more traction you give up.

Thanks to all who helped point out some of the things that I've neglected to bring up.

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