By: Ron Milam
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.
Ron