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Early snowmobiles had very crude mechanical brakes, and that was really all that was needed. These simple machines were very light, low powered and slow, and the built-in drag from the drive system many times was enough to slow them all by itself.
Add the fact groomed trails were nonexistent, and the additional drag deep snow would exert on the skis, pan and track, and you almost had the same condition as a boat going through water.
That’s one of the major reasons you needed a CVT belt transmission to make a successful snow vehicle. With all that drag you couldn’t use a gearbox because you could not shift fast enough from one gear to the next without the sled slowing so much that you had to downshift to the lower gear again.
Constant power delivery through a belt drive transmission was needed for the sled to move successfully through such a high drag environment with small 20-horse industrial engines. As a result, the brakes usually consisted of a cable operated brake shoe rubbing the rim of the stationary secondary sheave. As horsepower increased, and speed with it, separate brake discs mounted on the secondaries and floating calipers with double brake pucks appeared.
Still mechanically operated with a brake handle often made of plastic, and not larger than the throttle handle, was the norm in the 1970s, although slowly metal handles with more leverage started to appear. When secondary cross shafts appeared, the brake disc moved to the chain case on the sled’s right side, but was still operated by a mechanically activated floating caliper.
Polaris was the first to start using hydraulically activated brakes, but others were not that eager to follow. Concerns about fluid overheating and brakes fading, and the increased cost kept others away, as they kept improving their mechanical brakes. Polaris countered by introducing water-cooled brakes.
There also was concern over the parking function of hydraulic brakes, some thinking that fluid pressure would drop over time and consequently lose grip on the brake disc. The thought was that sleds would fall off trailers etc.
Yamaha was the last to switch to hydraulic brakes, and when it did, the engineers insisted on keeping an additional mechanical parking brake. This double system is still used on Yamahas even today.
Power + Speed = Better brakes
As power and speed on groomed trails increased, larger and more efficient brakes were needed. Not only were sleds getting faster, they also were getting heavier, resulting in a lot more built-up inertia to slow a sled down to a safe stop.
Inertia is tricky because it does not increase at the same rate as the speed, it actually increases at the square of the speed. That means that a sled traveling 75 mph actually has twice the inertia to stop, compared to a sled going 50 mph. Increase the speed to 100 mph, and the inertia is extremely large. A sled’s brakes have to absorb four times as much energy to stop a sled from 100 mph as from 50 mph.
The energy is absorbed as friction into the brake disc. As a result the discs and calipers must be cooled to function properly.
During normal trail riding speeds usually are not so high, and the brakes are not used so frequently as to overheat. As soon as you start pushing the speed and braking frequency, as in cross country, snocross or ice oval racing, cooling and brake friction becomes a problem. In a typical 25-lap World Championship at the Eagle River Derby Track, racers resorted to large cooling ducts and sometimes double calipers to survive the race with functioning brakes.
When the inertia is scrubbed off in the form of friction into the brake’s disc, the disc can quickly become red hot, and then a number of unpleasant things may happen.
First the disc may warp and reduce the pad’s grip surface. Second, and more dangerous, is the buildup of a hot layer of gas between the disc and pad, which makes the pad lose grip, just like a tire hydroplaning when hitting a puddle on a wet road.
To get rid of the gas layer, racers would put grooves in the pads. Then they noticed that if you drilled holes in the brake disc, grip improved because the holes would evacuate the gas. Next step was to machine slots in the discs to wipe away gases.
Finally technology evolved into today’s saw-tooth shaped discs, these have notches to totally remove the gas. Although it looks like all the holes and slots and saw-tooth shapes are there to save weight, they all have to do with preventing that dreaded gas fade. As a matter of fact, you do not necessarily want the disc to be lighter, because that would prevent it from absorbing heat, making it heat up faster.
To absorb more heat you need a thicker brake disc and a wider disc with internal vent slots for cooling because it becomes necessary as weight and speed increase. Take a look at the monster vented cast iron discs on a short-track NASCAR racer, and then look at them glow red as they enter the turns on a short-track night race.
A good example of a well dimensioned vented cast iron brake disc is those used on larger Arctic Cat sleds with diamond drives, where the disc is mounted on the drive axle. Mounting the disc on the drive axle has been popular on race sleds for ages, but now both Arctic and Ski-Doo mount their brakes directly on the drive axle.
Although this seems like a no brainer it actually requires twice the brake torque to stop the sled, compared to a secondary shaft mounted disc which turns at twice the speed. As a result the discs and calipers have to be dimensioned up, which is clearly evident on the Arctic design.
To have efficient braking you not only have to have good brakes, braking is also greatly affected by the track and suspension design. More aggressive tracks will stop you faster, and of course the more studs you have in the track, the shorter the stopping distance, particularly on hard ice or hard pack. Suspension design also plays a part. On a conventional sled, the braking force through the track means that the track is under tension on the bottom, thus pulling up the front of the slide rail. As it pulls up the front of the rail, less track is left on the ground and the effective traction surface moved back on the slide rail. This puts more weight on the skis and less on the track, which again can result in a very squirmy track action, especially with no studs.
We experienced a good cure for this when we tested the rear-axle drive suspension from RAD Technologies. Because the RAD system tensions the back of the track and pushes the front of the suspension down, it not only puts more track on the ground, it also increases pressure on the front of the track. The result is a drastic 30% reduction in braking distance with the RAD design when compared to the same conventional sled.
Clearly there is still a lot to be gained in braking performance, and with more powerful and heavier sleds becoming more common, braking is an increasingly important part of the complete performance envelope. We haven’t seen the end of development in this area, as the manufacturers and aftermarket companies constantly are developing new and more efficient brake systems.
Watch for Olav’s Brake Tech 2.0 coming next issue.