Most bikes converted to racing use have pretty good basic brake components installed from the factory. Some of the latest bikes with radial calipers and/or radial master cylinders come with track-spec brake pad compounds and hardly need any performance improvements before taking to the track.
Many manufactures still ship supersport bikes with street-compound brake pads, rubber brake lines and lackluster brake fluid, all of which compromise brake performance on the racetrack. We always replace the brake lines with braided steel lines, replace the brake fluid with a high temperature fluid and replace the brake pads with high friction racing pads.
It’s a good idea to pay close attention to brake components. This is a brand new brake line which had been improperly assembled by the manufacturer and leaked upon installation.
We change the stock rubber front brake lines to braided stainless steel (actually, braided-stainless-steel-covered plastic) lines to eliminate the expansion inherent in rubber lines. These lines deliver a firmer feel at the lever and, we feel, improve safety in the event that the brake line should end up rubbing on the front tire or someone else’s front tire. We usually don’t bother replacing the rear brake line.
We have used Kevlar lines, braided steel lines made by a variety of manufacturers and have tried braided steel lines of two different internal diameters. Although we have rarely had problems with any material we have shied away from the Kevlar lines because the care warnings which accompany these lines are a little more strident (i.e., never let the calipers hang from the lines) than the warnings which accompany the steel lines. The care warnings seem like a little too much for us to worry about when trying to replace the whole brake system after a crash in under two minutes. That means we now always use the braided steel lines.
Cable tie on the brake lever holds the master cylinder open to allow gravity to fill the calipers after the stainless lines have been installed.
In the old days braided steel lines were shipped without any exterior plastic coating, which meant the braided steel would quickly wear away at fenders, fairings, radiators or anything else it touched. We try to buy the ones that have a clear plastic exterior coating, to avoid tearing up the aforementioned parts. We have also tried both the normal internal diameter brake lines and the fashionable smaller diameter lines. None of our riders could tell a difference between the two. Your mileage may vary.
Having nice brake lines won’t help you one bit if your brake fluid has bubbles in it. Fluids don’t compress much (this is for the real geeks who know that fluids will compress in some situations although not in a brake system) while gases compress quite easily. When you pull on the brake lever you are applying pressure at the master cylinder which is transferred through the fluid to the pistons in the calipers. If there are bubbles in the fluid the gas will collapse before any usable pressure is exerted on the caliper pistons. Bubbles appear in the fluid for a variety of reasons including leftovers from disassembling the hydraulic system (i.e., to replace the brake lines), from water boiling out of the brake fluid (those calipers get hot) or from the fluid itself boiling (when the calipers get really hot).
This gets us into one of those racing dichotomies where we need to trade longevity for performance. Virtually all brake fluid is hydrophilic (and the stuff that is not hydrophilic is not a great idea because the water tends to separate out and sit around in various spots in your brake system) meaning that the brake fluid actively soaks up moisture from the air. The absorbed water is mixed into the brake fluid, lowering the boiling point of the brake fluid. When the brake fluid in the calipers gets really hot (as in, slowing for turn one at VIR or Summit) some of the liquid turns into a gas (little bubbles) which then reduce the subsequent braking power of the system as the lever comes back further.
We look for a brake fluid that has the highest boiling point that we can find (currently we have the top-of-the-line EBC and Motul in our tool box) but there seems to be an axiom that the higher the boiling point on the brake fluid, the more hydrophilic it is and, therefore, the more often the fluid must be changed. On our endurance bikes, this is every weekend. Due to the whole "boiling creates bubbles" phenomenon we like to bleed the brakes at the track directly after a hard practice session when the calipers, fluid, and, unfortunately, the rotors are all hot. We will also change the fluid entirely before each race.
Bubbles in the brake fluid are bad. Here a number of them are being purged from the brake system. Eventually each bubble expunged from the system becomes a personal and moral victory.
Now ultimately we run into tracks and brake systems for which any brake fluid is just not going to be able to withstand the abuse. That is when those remote adjustable brake levers (on trick racebikes) or actual pumps to circulate the brake fluid for cooling purposes (on trick racecars) come into play.
All of these heat-related problems (and remember, the brakes are really just dispersing the potential energy of the racebike at speed into heat and sometimes, light) will be compounded by the use of the sintered metal HH-friction- rated pads that we love. Not too many years ago it was tough to get brake pads that were really aggressive for stainless steel rotors. At the time builders installed ductile iron rotors for the increased friction but they tended to be very fragile and had a bad tendency to explode on the track. Now that there are many really great HH (a friction rating for brake pads) available for stainless rotors it is often possible to just change the pads, lines and fluid and leave the rotors alone. We have used EBC HH kit pads to good effect and have been using the Vesrah RJL pads for the past few years. These pads are so powerful, and thus create so much heat, that any weakness in the brake fluid will be revealed after a few hard laps.
The brake pads themselves have a hard life. The high heat can glaze the friction material (which can sometimes be brought back to life by scrubbing the pad lightly on a concrete floor in a figure-eight pattern) and warp the backing plates. Long six-piston pads are particularly prone to warping. Backing plates in general are being made thicker to prevent warping. If the lever feels spongy and the fluid is good check the pad for wear or warping. To check for pad warping, take the pads out and place the pads together back-to-back and see if you can rock them against each other with your fingers. Also, we have had, on three different occasions on two different brands of bikes a year apart, Dunlopad brake pads actually delaminate and lose part of their friction surface so that sort of failure can happen as well.
The unsung heroes of your typical brake system are the caliper piston seals. These poor seals have to resist the brake fluid pressure and the heat, and they are responsible for ever-so-slightly retracting the brake pistons so the pads don’t drag on the rotor. They have a tough job so don’t make their lives any worse (or shorter) by pushing the pistons back into the calipers without carefully scrubbing off all the baked-on brake dust with a soft toothbrush. When we re-install the calipers we pump the pistons back towards the rotor with lots of quarter-to-half-stroke pulls on the brake lever to move the pistons slowly. This helps prevent over taxing the seals’ "pull back the pistons" function and helps reduce brake drag.
Brake fade is bad, brake failure is a disaster. We have seen complete brake system failure on everything from GP bikes down to club racers. Usually it is due to poor routing of brake lines or improper installation of calipers but often it is as silly as not having the brake pad pins and/or caliper bolts secured with safety wire or other redundant system. For instance, on many GSX-R calipers we will drill and tap a small hole next to the brake pad pin which allows us to install a small Allen bolt which is then safety wired. This allows the stock clip to fall out without any chance of the brake pad pin coming out. This is, granted, a serious commitment to keeping the brake pad pin (and, therefore, the pads) in place but, on the other hand, we’ve never had one go missing, either.
Brake Bleeding 101
To get the best out of new lines and pads you need to eliminate all air from the brake fluid.
For bleeding brakes, you will need a handful of paper towels or shop towels, some clear plastic tubing, some new brake fluid, and an 8mm combination wrench.
Brake fluid is extremely corrosive to paint and plastics. We’ve seen those plastic GSX-R ram-air scoops shatter where brake fluid was allowed to drip on them, and brake fluid spilled on the gas tank will bubble and peel paint just like gasket remover. So before you start bleeding the brakes, make sure that you have covered any painted or fragile surfaces that may be in the splatter area.
In order to avoid unnecessarily pushing all of the old fluid all the way through the brake lines, remove the reservoir cap and suck out all of the fluid from the reservoir. You can do this with a Mitey Vac, shop towel, or paper towel (if you’re using a paper towel or other lint-carrying device, beware of the lint). Lightly place the cap back on the reservoir.
Then, keeping a sharp eye out for anyone who may be tempted to squeeze the brake lever, remove the calipers and push the pistons all the way back into the calipers (after cleaning them to prevent messing up the seals) to force more brake fluid back up into the reservoir. Suck that fluid out, and then fill the reservoir with fluid from a newly or recently opened bottle of brake fluid. Gently squeeze the lever repeatedly until the pads contact the rotors, and then top off the reservoir again and replace the cap.
Place a paper towel on the calipers right next to the bleeder valve to catch any stray drips that might otherwise run down onto the brake pads. Attach a bleeder hose to one of the bleeder valves on one of the calipers and feed the other end into some kind of reservoir—an empty plastic bottle, for instance. Squeeze the brake lever and crack open the bleeder valve using the open end of an 8mm combination wrench until the brake lever hits the handlebar. Close the bleeder valve and pump the brake lever a few times until you feel pressure return at the lever, and repeat until you can see the fresh fluid coming out of the bleeder or no more air comes out. Top off the fluid reservoir as necessary. When you are finished with one caliper, repeat the steps for the other caliper, and don’t forget to top off the reservoir when you’re done. Sometimes rotating the wheel (with the bike on a front stand) and then applying the brakes will help the pressure come up quickly.
Two styles of master cylinders. The top one is a conventional Nissin master cylinder, the bottom one is a Brembo radial master cylinder. Hydraulic brakes work off of leverage so the smaller the master cylinder the greater the leverage (and lower effort) but the greater the lever travel. Radial master cylinders can get the best of both world because their pivot arrangement gives better leverage allowing the use of a bigger piston which decreases lever travel. Basically you get 20% more pressure at the pads for the same effort at the lever. Whether your pads and rotors and calipers can handle that is a different question. Also, if it is not done at the factory, score your brake lever with a hack saw about two inches from the end to avoid losing the whole lever in the event of a mild low side or banging handlebars.
If you’ve successfully bled both lines, you should have a firm feel at the brake lever. If the lever feels spongy or pulls back farther than it should before engaging the brakes and you’ve already tried adjusting it, there still may be some air in the lines. Air can get trapped at the banjo bolt on the master cylinder itself, or in bends in the lines or in the caliper bodies.
For more aggressive bleeding, wrap some paper towels around the lines coming out of the master cylinder and, while squeezing the brake lever, loosen the banjo bolt at the master cylinder. When the lever comes back to the handlebar, tighten the banjo bolt and pump the lever a few times. If you noticed any bubbles coming out from around the washers or the banjo fittings, repeat until no more bubbles are seen.
When bleeding brakes you can often get a bit of air out of the system by loosening the top banjo bolt ever so slightly (ie, just enough to let air and a couple drops of fluid) while squeezing the brake lever. This makes a bit of a mess so wrap a rag around it before you crack it slightly.
You can tap or manipulate the brake lines while you’re bleeding at the calipers to help dislodge any trapped bubbles. You can unbolt the calipers and turn them over and tap on them. And you can loosen the master cylinder from the handlebar and move it around and tap on it to help dislodge any trapped air.
Some other commonly missed tricks for brake systems include ensuring that the pivot of the brake lever is well lubricated and, more importantly, the contact area between the master cylinder and the brake lever is well lubricated.
If, after combining some or all of these methods, you are still dissatisfied with the feel at the lever, you may try looking for other causes such as warped brake pad backing plates, insufficient brake pad material (worn brake pads), herniated piston seals, or warped rotors. To check for warped rotors, first look to see if the rotor is "coned" or distorted to one side. Coning can be quite dramatic, but usually happens only to rotors with offset carriers or the rotors on a YZF600. One day at Summit point we coned over three sets of YZF600 rotors because we got them so hot that the crystals in the steel reformed to be more compact and the rotors shrank onto the floating buttons and coned over to the side. A dial indicator with a magnetic base mounted on a front stand is useful for checking for minute changes in runout at the track.
Caveat #1: Do not allow the fluid to completely drain out of the master cylinder reservoir or you will pump air into the lines and make your life a living hell as this is most likely to occur at the least convenient time, such as when the 5-minute board goes up. Monitor the fluid level closely in the reservoir as you are bleeding and top off frequently.
Caveat #2: Do not use the closed end of the combination wrench to open and close the bleeder valve. It’s too easy to forget to remove the wrench from the bleeder if you get sidetracked or distracted and the potential for disaster on the track is much greater if your bike has a wrench hanging onto the front caliper waiting to get thrown through a radiator or under a rear wheel, for instance. If you use the open end there’s little danger of accidentally taking an unwanted hitchhiker out onto the track.
Caveat #3: Always squeeze the brake lever first before loosening a banjo bolt or opening a bleeder valve. This will help prevent air from entering the brake system at the bleed point. Always close the bleeder valve or tighten the banjo bolt before letting go of the lever, or you surely will pull air into the system, past the threads of the valve, or past the washers of the bolt.
Brake Bleeding 201
Bleeding brakes after completely draining them (line replacement, caliper swap, etc.) can be easy or incredibly frustrating.
Refilling from empty:
After reassembling the system, bolt the calipers onto the fork legs.
Crack the bleeders open and gently squirt in a little compressed air to extend the pistons and pads out to the rotors.
Hook up bleeder hoses to the calipers and leave the bleeder valves opened.
Put a cable tie on the brake lever to hold it at half pull and pour fluid into the reservoir.
The fluid should flow through the lines and into the caliper.
When only small bubbles are coming out of the clear bleeding hoses shut the bleeder valves and proceed to bleed normally, remembering to bleed the top banjo bolt towards the end of the process as well.
Once all that is done, depress the brake pads and pistons back into the calipers and then extend them again to the rotor using only small 1/4-pull strokes on the brake lever. This will prevent the seals in the calipers from getting over-stretched and reduces brake drag from the pads dragging on the rotors.
Wire everything on the calipers: Banjo bolts, brake pins, caliper mounting bolts. Also, periodically check your brake pad pins for wear where the pads rub on them.