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Most recreational vehicles have more than one brake. Motorcycles have at least one on each wheel, and on higher powered bikes there usually are two discs with calipers on the front wheel.
ATV’s usually have one on the rear axle and one on each front wheel.
Snowmobiles have only one brake, mounted either on the secondary shaft or directly on the driveshaft.
This simplifies the picture quite a bit for sleds; first because fewer parts are needed and costs are less, and secondly because you do not have the additional problem of balancing the brake force between the front and rear.
The disadvantage is that all the brake action goes into only one disc and one set of calipers. The demand on this system therefore becomes greater, and in case of a failure, there is no backup. An exception may be Yamaha which has a mechanical parking brake that could be used in an emergency, but many people would not think to flip over the small lever on the mechanical brake.
Disc brakes rule
Disc brakes have been the accepted application, since snowmobiling evolved through the 1970s and ’80s. By then discs were already on cars and motorcycles and the components were readily available.
The early mechanical calipers activated one brake pad through a helical screw center plunger and the other pad was stationary. To make both pads apply even pressure, the caliper would float in its mount and move the inside pad over to grip around the disc. In most cases the disc was mounted on a key or spline, allowing it also to move and seek the best position for effective braking.
On newer hydraulic brakes there is an actuation piston on each side pressing on the brake pads, and they clamp around the disc with the same force, as they are moved by the same hydraulic pressure. The discs still are usually mounted on splines, which make them float from side to side. Although this would not strictly be necessary, they have to be mounted somehow, and a little float makes the brake action quicker and more effective also when the brakes are released.
A master cylinder on the handlebar’s left side activates the brakes by pushing on a plunger that builds pressure behind the activating pistons in the caliper. The amount of pressure or leverage is proportional to the diameter of the plunger on the handle and the diameter of the brake caliper pistons on one side of the disc.
If the area of the pistons is 6 times larger than the area of the plunger we have a 6 to 1 leverage ratio. The master cylinder usually has a reservoir built into it that contains extra brake fluid, and every time the plunger is retracted it opens a small passage from the reservoir that makes sure the hydraulic circuits are always full. During brake activation, the inertia energy built up in the snowmobile, due to its weight and speed, has to be scrubbed off by friction between the brake pads and the brake disc. That energy is then turned into heat, which is dissipated into the disc and brake pads.
Dissipating all this energy into only one brake system puts a large demand on both the disc and the pads, resulting in heat becoming the limiting factor in a snowmobile brake system. As the pads heat up, some of this heat is transferred into the steel backing plate and into the hydraulic pistons. This in turn heats up the hydraulic fluid, which may cause it to boil and form steam pockets, causing brake fade.
Fluids play vital role
Brake fluid is rated for its ability to withstand temperature and moisture content. Over time moisture gets into your brake fluid through numerous microscopic passages in lines, and around piston seals, etc. Brake fluid moisture even in a fairly new sled can be near 3%, but after many years of service may reach a level as high as 7% or 8%.
As more moisture invades the fluid, its boiling point drops, which is not a good thing. At 3% water content, the boiling point of a DOT 3 fluid may drop by 25%. DOT 4 fluid, soaks up moisture at a slower rate, lowers its boiling point by as much as a staggering 50% when it reaches 3% water saturation. However, DOT 4 has a higher starting point, and takes longer to saturate.
Nevertheless at least one manufacturer recommends that only DOT 3 be used in snowmobile applications. DOT 3-rated fluid must have at least a wet saturated boiling point of 284° F, and a new boiling point of 401° F.
DOT 4 brake fluid is required to have a wet boiling point of 311° F and a dry boiling point of 446°, DOT 3 and 4 are glycol-based fluids that absorb moisture and are nasty stuff that will peel paint off your machine. So if you spill it on painted surfaces do not try to wipe it off; this makes it worse. Just wash it off with water.
DOT 5 brake fluids are another creature altogether. They are silicone-based, do not absorb moisture and have a minimum boiling point of 500° F. This makes it popular for racing applications, and there are considerably more expensive racing brake fluids too, some having a boiling point of 600°.
DOT 5 and other special racing fluids are much more expensive; up to 5 to 10 times the price of glycol-based fluids, and have other drawbacks, especially for your average weekend snowmobiler. One drawback is that they do not mix with the glycol-based DOT 3 & 4 fluids.
It’s very difficult to completely clean a system of glycol fluid before filling it with a silicone-based fluid. Leftover glycol will form sludge in the system that may boil and cause a loss of brake pressure. Generally DOT 5 fluids are used only by racers and only if installed in the system from the beginning.
Do NOT attempt to add DOT 5 silicone fluids to a system already containing DOT 3 or 4 fluid unless you completely dismantle and clean each part and passage.
Since snowmobiles operate in a high-moisture environment it’s advisable to replace the glycol-based fluid by performing a thorough brake bleeding at least every two years. After a bleed operation, use the rest of the brake fluid in your car or motorcycle since half a bottle sitting around for two years until the next bleed will just become saturated with moisture.
The pads
Brake pads are now usually metallic, and present few environmental problems.
However, with the increasing interest in vintage machines, more old sleds are being put back into use and sleds from the ’70s often had brake pads containing asbestos.
Asbestos was a very good brake pad material with a number of desirable features. It was relatively inexpensive, gave good service life, and was easy on rotors. The fibers had high tensile strength and could operate well in temperatures up to several hundred degrees.
The undesirable side effect is asbestos dust, which does not degenerate and if breathed in settles in the lungs and can cause cancer. When restoring older sleds that can be covered with old asbestos brake dust it’s important that the sled be washed off with cleaning fluid and not blown off with an air hose. When blown, the dust will mix in the air and can be ingested by anyone in the room.
While it may take a little more time to thoroughly wash it off (using rubber gloves), getting cancer is not a good alternative.
Heavy discs are better
Many racers feel that lighter brake discs are desirable, but for brakes to work well over an extended period, it’s important that there is enough mass to absorb the energy without overheating the disc, causing warping or glazed pads. When it comes to discs it is better to err on the heavy side.
Many racers try to cool the brake system by ducting cool air from outside. The cooling duct usually terminates on top of the caliper, which is good for cooling the pads or pistons, but does little for the disc. That’s because the disc acts as a centrifugal fan and hot air is flung out from the outside diameter, preventing cold air from hitting the disc.
The most effective way to cool a disc is to duct air around the disc’s sides or onto the center so the cool air is flung out by the centrifugal action.
Yamaha has a center fan hub, but the most effective is a rotor with internal passages in the disc, like Arctic’s larger drive shaft mounted units.
With inertia increasing by the square of the speed, and speed runners now reaching more than 180 mph on asphalt runs, braking becomes an increasingly large problem.
Nitro cars use parachutes, and they were made mandatory on speed sleds run on ice by NSSR for sleds going faster than 150 mph. At AmSnow’s 2009 Super Sled Shootout, run on asphalt each fall, the record-breaking sled had 4-wheel brakes on its ski wheels, although we do not know exactly how efficient they were.
A more practical solution on future speed sleds may be to have one brake setup on the secondary shaft and another complete setup on the driveshaft. This would double the brake capacity and if operated by separate levers, one on each side of the handlebar, it would be a better failsafe system. Brake development is an active part of AmSnow’s asphalt shootout, and the next step forward in brake technology may just come from racers or their aftermarket company sponsors.