Track Length Vs. Clutching

When Team Industries developed the first Team clutch (the TSS-98), we needed a place to test clutches on snow­mobiles that would load them hard.

Team bought the biggest stock Polaris machines you could get in 1999--the RMK 700 with 136-inch tracks--and headed for the mountains. Our Team test riders were about to get a serious reality check. When they came back, one of the test guys told me, “The guys who live out there had more radical sleds for their wives to ride than we had.”

OEM machines have come a long way since then, and so have we. The mountain segment is the biggest seller for Team aftermarket clutches and we put a lot of time into testing and study­ing clutching for this application.

In this article, I will try to elaborate on the challenges of clutching and gearing for sleds with large tracks, big horsepower, varying altitudes and vary­ing terrain.

Tremendous Loads

Today’s big tracks put tremendous loads through the drivetrain. These big loads cause a lot of friction in the system and this makes proper venting, clutching and gearing vital. Com­pounding this is the very tight packag­ing of modern sleds. There is not a lot of extra air under the hood and the manufacturers are forced to seal them up tight for noise restrictions. Add a cherry hot turbo to the equation and you have a lot of belt-killing heat.

Getting airflow to expel the heat is a good place to start. Venting near the clutches can help bring temperatures down to levels that your belt can tolerate. If you have belt temps higher than 200 degrees F consistently, you will not have satisfac­tory belt life.

Gearing and clutch calibration are also vital to making belts live and mak­ing your machine perform. You want to gear your sled down so that once the drive clutch is fully engaged, track speed is not too high. If you are con­stantly “clutching” (not fully engaged), the drive clutch creates tremendous heat. The OEMs have started gearing the mountain sleds down the last few years and belt life has improved because of this change.

Clutching can be kind of a black art and we have customers who clutch in completely different ways than we do, sometimes with good results. So I do not claim to know everything about it and I am no longer surprised when we get requests for custom helixes that are way different than anything we have done before. There are some things that we have learned about calibrations that seem to always ring true.

Helix Angle, Progression

Driven clutch helix angle and length of progression is a big thing to consider when clutching for the mountains. Team helixes use the following number­ing system: Start angle, end angle and length of progression.

For example, in a 73-57-.56 helix, the angles transition from 73 degrees to 57 degrees over the first 0.560 inch of roller travel. The helix is 57 degrees the rest of the way.

Too often, people get caught up on the finish angle of a helix, but at low to medium speeds, you may not even get to the finish angle in the shift curve. Attached is a chart that shows the angle of two helixes on the “y” axis charted against track speed on the “x.”

Bear in mind that the clutch is in “low” gear all the way up to 22.9 mph. Notice that the 73-57-.56 helix is at 64.4 degrees at 38.4 mph, while the 71-57.36 helix is at 59.3 degrees.

The driven clutch spring is also an important thing to get right for a mountain snowmobile. Once again, the very large loads of a large, high lug mountain track cause us to use higher rate springs than low load applications. You need to prevent up-shift in on/off throttle situations so that the motor can pull max rpm when it is needed.

Drive clutch flyweights determine shift curve as well in clutching. We spend a lot of time testing different profiles and we find that fine tuning through adjustable-gram flyweights is the easiest way to optimize this curve. There seems to be a new flyweight out every year that claims to radically change clutching, but at the end of the day, getting a flyweight to work with your driven setup is the goal.

Horsepower Changes

With the changes in altitude inher­ent with mountain riding, motor horsepower changes as well. To have a setup that works for a greater range of altitude, you need to have an efficient driven clutch that will not pull down engine rpm at high altitudes. This is one of the biggest differences we see in testing between the TSS-04 (non-tied) clutch and the Team Tied clutch.

Whereas we might see a 200 rpm change from 6000 to 9000 feet in the TSS-04, we see less than a 100 rpm change with the Tied Clutch. This is because all of the rotational torque is transferred through the helix in a Tied design, while only half is in the TSS-04. This makes the clutch “smarter.”

As a general rule, you will be happier with a mountain sled that is under-clutched (at or slightly above peak rpm) than one that is over-clutched (below peak rpm). In situations where you need on/off throttle response to get out of trouble, you will be glad you have quick response instead of bogging.