Brakes

By: Ron Milam

 

   Brakes are probably one of the most troublesome systems on a trials motorcycle. I bet if you asked 10 of your friends if they were satisfied with their brakes, 9 would say no. This is despite the fact that they are riding a fairly new $5500 motorcycle. What is it that makes brakes such a source of grief?

     I have a few things that came to mind. First, trials bike brakes are shrimpy. They are only a small notch above bicycle brakes. This is partly a price we pay for light weight. Second, the actuation systems (brake pedals for true trials bike dummies) of many bikes are poorly laid out. The main purpose of the rear brake pedal on a Beta Techno is to minimize any mechanical advantage between your right foot and the rear caliper. If you are a GasGas or other owner, don’t snicker. Yours is probably not that much better. Third, the brakes are generally full of mud and other unhealthy stuff. Fourth, we demand an awful lot of those tiny units. You expect them to lock up easily when you want to hop or nose wheelie, but you don’t want them to lock up when you get sloppy with their application. Generally, the brakes on a new bike work pretty well. It is only after a few days of use in bad conditions that you notice the loss of stopping power. I am going to list a few things that I have learned about brakes. Maybe you already know all of these. If so, you may skip over to the classified ads. If not, some of the things may improve your brakes. Those of you who know me, know that I ride a much fiddled with ’96 Beta. Soon after I got it, I discovered that most owners of  Betas don’t like their brakes. I now have a 4 piston caliper on the front and a rear rotor which is 38mm larger than stock. I now love my brakes whether wet or dry. You can make yours work well too, with a little work.

    You should make sure that your rotors are not bent or warped and that your calipers are in alignment with the rotors. If you see a rotor flexing as the brake is applied, that is a sign of possible misalignment. Uneven pad wear can also be a sign. This is normally caused by bent caliper mounting bracket or by the wheel being out of alignment ( especially on the rear). If you are new to trials, you may wander why brake rotors are mounted loosely to the hubs. This serves 2 purposes. It allows the system to compensate slightly for misalignment. It also keeps you from getting killed. This is always a nice thing. If you hit something with your front wheel and flex your forks, the caliper is momentarily knocked out of alignment with the front rotor. This happens because the caliper is mounted to the fork leg and the rotor is mounted to the  wheel hub. If the rotor did not float a little bit, it would push the pistons back into the caliper. You would not know anything was amiss until you grabbed for some front brake and the lever pulled all the way to the grip. If you were to think quickly and release the lever and pull it again, you would find everything normal again. Some people find the looseness of a floating rotor disconcerting, but it is definitely worth the aggravation. Side to side play is all that is needed. If your rotor rocks front to back, you can tighten it up by replacing the bushings and or mounting screws.

     Running pads that are excessively worn can also cause problems. Some calipers are prone to piston sticking if the pads are too thin. This is because the pistons must extend so far that they become cocked in their bores. Also, rear brakes can experience special problems if the pads are too thin. Rear brake pedals often do not have sufficient  travel to both actuate the caliper and also to allow the piston to uncover the charge port of the master cylinder. Ever notice the reservoir and hose connected to the middle of your rear master cylinder? This allows enough additional fluid into the high pressure side of the system to make up for the fact that your pistons are hanging out farther because of pad wear. When you release the pedal it must allow the master cylinder piston to return far enough to uncover the little hole in the cylinder where the hose is connected. If you have your pedal adjusted too low, the piston can’t retract far enough and no additional fluid can get in. You won’t notice the problem until one of those muddy, brake ruining events that cause rapid pad wear. You will find the pedal going lower and lower until finally it bottoms out on the skid plate just when you need it most. The only advice I can give you here is to pay attention to this possibility. Bikes vary in this regard. You may never see the problem or it may plague you. If you have ever thought about putting shims in behind worn pads, I can say from experience that this works fine and can save a lot of money.

        I guess the little running joke about the no name column is over. This happened as I was looking at my bookshelf. I just hope no one reports me to that computer book company that prints books on all subjects    ( except trials bikes ) by the same name. Next month we will finish up on brakes so you have 30 days to order parts and buy fluid.

 

   Last month as you may recall we began a discussion on brakes. Since that article was published,  several people have mentioned brake problems that they are having. I think that next to clutches, brakes are the system that cause riders the most headaches.I believe we left off on the subject of sticking pistons.

  If your pads are in good shape and your pistons are sticking, you may need to clean the seals. What I have found is that debris can build up in the seal grooves of the calipers and cause the seal to grip the piston too tightly. The first time I took a caliper apart I was scared to death the seals would get damaged and have to be replaced. Replacing them would be easy. Waiting a week for the parts was the worry. Happily, I found it to be no big deal. In fact I think everyone should disassemble their calipers yearly and clean them and change fluid. You will find the caliper to be made in 2 pieces, with a small o-ring sealing the 2 halves. The pistons will likely be very difficult to remove. The answer here is to blow them out with compressed air. Did I say CAREFULLY blow them out with compressed air? This is another one of those areas when I must say that this is a fairly dangerous thing to do if caution is not used. It is entirely possible to inadvertently launch a projectile at a velocity that could kill,maim or really get you in a lot of trouble. Just be sure to point the piston away from yourself or anyone else and into a rag or other soft item to avoid damage to the aluminum piston.

   You can use a small screwdriver or a packing puller to remove the seals. If you are careful, the seals can be reused. If you have had the foresight to order new seals, replacement would probably be better. After removing the seals you can clean out the grooves in the caliper housings. You should next reinstall the seals. My next step is somewhat unusual and really hard to do unless you have Houdini hands. I fill both

caliper halves with fluid, put the pistons barely in and lay the o-ring in one half. Then, at a high rate of speed, bring the two halves together without making a huge mess and letting air into the system.This will require great manual dexterity to keep the ports covered with your fingers and to prevent the o-ring from falling out of position. You can then bolt the parts up tight. The reason you left the pistons hanging out is that you can now push them in some to push out any air that may have been trapped inside.

   I have two theorems concerning hydraulic brake systems. Theorem 1- The less air you trap in a system, the less you will have to bleed out later. This means it is a good idea to fill all of your hoses and other parts. As you connect the hose to the cylinder, push the pistons in some more to expell any air in the port and fill the end of the hose. You’re going to trap some air. Your goal should be to keep it at a minimum. The next theorem ( lets call it Theorem 2 ) states that bubbles go up easier than they go down. If you don’t believe this just try pushing some bubbles to the bottom of a swimming pool. This sounds like the kind of thing only nut would try. Well then, why do most people bleed their brakes from the top down? The best way to bleed brakes ( by the way this whole column is simply my humble opinion ) is from the lowest point, which is normally the calipers to the highest. I have always used a large medical syringe to push fluid into the caliper. You should first remove the pads (if they are still in) to avoid contaminating them. If you get fluid on your pads they will probably never work right again. You should push the pistons all the way in and insert some object to hold them there. The less internal volume there is , the less place there will be for those pesky evil bubbles to hide. On some bikes the bleed fitting screws down onto a steel ball sitting over the passage way. The function of this ball is to drive you nuts if you don’t know it is there. You can take it out to bleed the system and put it back in afterwards. Since this open port is lower than the master cylinder, it will be under a positive pressure, allowing you to insert the ball and reinstall the bleed fitting without allowing any air to enter. If you have a large syringe , say 5 ounces, one shot should be sufficient to completely evacuate the system. You obviously have to put something under the master cylinder and have its top off, since 5 ounces of fluid will overflow.

    There are a couple of tricks  to using the syringe. First it is transparent. You should take advantage of that fact by making sure all air is out. Don’t get bubbles in while you are filling it. If you shake or bump the fluid container, you may entrain air. Any bubbles you put in the syringe must be expelled.You should also use transparent tubing to connect the bleed fitting to the syringe. Again, this allows you to check for bubbles.

   Next month, we will cover master cylinders and fluid and other interesting stuff like that. If anyone has good ideas or techniques they would like to share about brakes or anything else, please tell me about it and I will get it in next month.

 

   Last month as you may recall we continued the discussion on brakes. As you may have gathered by now, brevity is not one of my strong points. So I’m gonna talk about brakes some more. We thoroughly discussed calipers last time , so lets move to the other end of the hose. There are a few things to know about master cylinders. First of all, they are so incredibly simple that is hardly seems possible that they could give people the fits that they sometimes do. One piston, one cylinder, one or two seals, one spring. That’s it.

   Seals are the parts that are most likely to cause problems. If you ever look at a worn master cylinder closely ( that means a magnifier if you are my age ), you will probably see that it is worn more on one side than the other. That is because the actuating mechanism is putting side loads on the piston. In the short term, say 2 or three years, the seals will have to be replaced. Over a longer time however, the cylinder bore can become scored or out of round. It is possible to sleeve a worn master cylinder if it is valuable ( rare ). But in general an new one is the easy way out.

    If you look at the seals closely and think about how they work, you will see that the seals are self pressurizing. That is to say, that as more force is applied to push the piston in, the resulting pressure forces the seal lips outward onto the cylinder wall harder and harder. A worn seal would probably seal as well as a new seal if you could get the pressure to build up enough to get that initial sealing. Sometimes you can get a situation were you can slowly pull the lever all the way to the grip. But if you pull it quickly enough, the seal will pressurize and the brake will work normally. Worn seals and out of round bores can cause havoc when you try to bleed a system. Especially when using the master cylinder as the bleed source. They tend to let air enter around the seal and into the system on the return stroke

     A topic subject to much debate and speculation is brake fluid. There are two types relevant to trials bikes. DOT 4 is a conventional type fluid that is currently used in most passenger cars. DOT 5 is a silicone based fluid that is used in high performance applications. Some people will tell you that you need DOT 5 fluid in your trials bike because …. ). The only believable thing I have ever heard is that it helps seal the piston during that critical stage were pressure is just starting to build. I personally have not seen this, but it was passed on to me by the walking trials enclopedia with the initials C.R. so it must be true.However, the stuff costs 4 or 5 times what DOT 4 fluid costs and DOT 4 works just fine if your system is in good shape. I buy Valvoline synthetic DOT 4 at Auto Zone for about $5.00 a quart. At that price you can afford to fully flush your system every time you bleed it or any time you are just feeling masochistic. It is not quite as benign as the silicone stuff, but it doesn’t seem to eat paint or plastic as readily as the old DOT 3 stuff we all hate.

   Another item that can give trouble is the master cylinder piston return spring. They can break from having the piston pushed too far into the cylinder. This happens more on clutches, but be aware that it is an important part and should be checked closely when changing seals. If it is not returning the piston far enough to uncover the charge port, you will have brakes until your pads wear sufficiently to require all of the fluid in the cylinder ( not the reservoir ). Then you will be out of luck.

   I only have one comment about hoses. They are the source of a lot of the sponge you feel remaining in a well bled system. A little sponge is not necessarily a bad thing. Some folks want their lever rock hard and some want a little give. I personaly like it hard. I got a chance a couple of years ago to closely inspect Geoff Aaron’s Beta and hear him talk about brakes. His bike had after market hoses from Braking. They had a very small inside diameter. The hoses seemed to bulge very little when applying the brakes. The result was a very nice feel at the lever. I still have my stockers though, and they work fine.

   The last thing to cover is the subject of brake pads. There are several good brands and types. Basically the two types of pads are metallic and semi-metallic. They are differentiated by the composition of the friction material. Metallic pads are faced with a sintered ( using high heat and pressure ) mixture of metal powders. They seem to work better when wet than semi-metallic pads which are faced with conventional friction material. The semi-metallics are said to work better dry. I just use the red Galfers ( metallic ) and they work fine. There is a trick that you can use to make your new pads stop better. This is something that some people seem to know, but don’t talk about. It is a highly technical and secret process called  “seasoning “. I am just kidding. It is so easy to do that you would be downright derelict to not run right out and do it right now. All you do is ride with your brake on ( do it one end at a time ) until the pads are really hot. It is hard to say how hot really hot is. They should not be so hot as to turn the paint black. You can experiment a little. After getting them hot, you should use a garden hose to quench them as quickly as possible. You should already have the hose turned on in advance, etc. You should see a lot of steam and hear a lot of hissing. If not, you didn’t get ‘em hot enough. Go do it again. Then carefully test it out. If the process was done correctly, the brake will work better than it ever has, including the day it was new. If you are testing the front, you can quickly end up with your face in direct contact with your lawn. There is that much difference. They will work great for a while, but the effect will gradually fade as you feed the pads their usual diet of mud and debris.

     That was all I was going to say about brakes, but I feel in a pot stirring mood so I’m going to add a little bit on brake theory. Some of you may disagree with what I am going to say. If so, I welcome your arguments / rebuttals. First a little hydraulic theory. The amount of force that your caliper applies to the pads is a ratio of the master cylinder piston area to the slave cylinder ( caliper  ) piston area, accounting for the leverage at the lever or pedal. If you made the master cylinder piston smaller you would increase the force on the pads. Or if you made the slave cylinder piston area larger ( as in the 4 piston calipers ) you will also increase the force on the pads. The problem is that since the smaller piston displaces less fluid, more lever travel is necessary to actuate the brake. The same thing is true of the 4 piston calipers. On the front this is not normally a problem, but many bikes have minimal rear brake pedal travel. You can modify the pedal or lever to increase the leverage, but this will also reduce the amount of travel of the master cylinder piston, and again you may run out of travel at the pedal.  

       Now for the controversial part. Everybody thinks that larger pads stop better. This may be true on a road racer where heat and therefore fade is an issue. Trials bike brakes don’t get that hot. In engineering school I learned one or two things that really amazed me and fly in the face of conventional wisdom. One is the relationship between friction and surface area. The amount of friction between 2 surfaces depends only upon the force applied to the surfaces and their relative coefficient of friction. If you double the size of your brake pads but keep the clamping force the same, you will not stop any faster. ( Neglecting heat induced fade ). If the 4 piston calipers stop better it is because of increased piston area and not the larger pads. This is because the amount of friction is independent of surface area.  The reason I am saying all of this is to say that we have all of these problems because the factories keep putting on those little bitty brake rotors. Anyone can see that if the rotor is larger and the caliper is located farther away from the axle, the caliper will have more leverage on the wheel and will have more effect. The problems of a larger rotor are minor. More weight ( ounces ). More vulnerability to damage ( I have a larger rear rotor on my Beta and have not dinged it in the 2 years of use ). More rapid pad wear ( may be true ). The benefit would be that we wouldn’t have to try to get that last little bit of brake performance. The normal grungy operating condition would still work well enough. But instead, the factories go smaller and smaller. Look at a ’99 GasGas. If you had 2 quarters to go with it you could use that rotor in a soft drink machine. This is one area where most people would gladly accept a little more weight. If the factories used  this  ( larger rotor ) approach it would hamper marketing strategies tremendously. Every year of a bike’s production life  

“ improvements ” are made. One year they go from a 4 piston caliper to 2 pistons. After a few years they bring about a “ revolutionary “ 4 piston caliper. When they go to the stronger caliper they seem to counter the benefit by reducing the rotor size. But be of good cheer. Maybe someday the “ revolution “ will be larger rotors and we will not have to complain about brakes anymore.

    Now you know enough about brakes to keep yours in tip top ( I just love those technical words ) shape.

Next month I think I’m going to talk about that other really aggravating system: The clutch. If you have anything good to say about clutches (hopefully your successes), please tell me about it and I can pass your experience on.