Clutches and Chains

Clutches

A clutch is what takes the power our engine makes and connects it to the ground. The clutch that came on your sled in most cases is a very good unit made by Max Torque. It’s very easy to maintain and will last through many years of hard use. The design is simple, centrifugal force, pushes the shows outward against a drum that has the drive gear on it. A simple garter sprint wraps around the shoes to control the engagement.

When it comes to adding performance, the demands on the clutch change, both in terms of the added power it must now handle and because that power is now usually made further up in the RPM range, often a higher engagement is preferred. We know from reading the engine section here, that as we improve and add power, we generally loose some on the low end. Stock if your clutch engaged at 2500 RPM and had maybe 2 ½ HP there, now that we have made some improvements, we might have lost a little at 2500, but at 4000 we now might have 6 HP. Clearly, it would be much better to now engage the clutch at 4000 rather then at 2500 and having to wait on till it pulls itself to the 6 HP level.

While your stock clutch is a strong unit and will takes years of hard use, it is limited in ability to adjust the engagement point. Help for this comes from the kart racing sport. Lets look at several of these.

The clutch I recommend for most HiPo trail sleds and racers in anything but the highest classes is the Max Torque Dragin Skin. This is a clutch Jim makes just for Kart racers. The biggest advantage to you is because it is made by the same company that makes your OEM clutch, and uses the same drum size, your stock band brake system bolts right on, a very simple swap. The inside of the clutch is completely different. Instead of simple shoes and a garter spring, the D/S has 3 much lighter shoes made with a special compound lining. These are held to the center hub by 9 springs, 3 on each shoe. Here are 9 different sets of springs with different force ratings. By using different sprint sets you can tailor your engagement to any setting you want. A big plus on this set up is Jim also sells weight sets that simply fit around these small springs. You can use the weights to dial in how hard you want the clutch to “hit” You can have a very soft engagement for trail riding or small kids, or, by adding more weight have it lock up instantly for a very hard launch for racing. This is the exact clutch both my boys have used in karting and have multiple World Championships with them in the Animal classes.

These clutches also have interchangeable sprockets so you can change gearing without having to buy a new clutch drum each time, they just snap in place.

From here the next step up again comes from karting. When we get to the power levels of strong Champs and Outlaws we often need a multiple disc style clutch to control the higher HP level. This becomes more of an issue when geared higher (numerically) for things like Snow X. These clutches are much like the clutches used in most automotive racing and has multiple friction disc that mate to floaters. At this level, for those that have not switched to a jackshaft and hydraulic brake set up, we do have a custom made drum that accepts the stock band brake set up.

There are several other shoe or puck clutch designs being marketed to the 120 world. To be honest, even though the parent company that makes them is a mile from my door, these are mostly variations of Karting clutches that were designed decades ago. To be honest, the technology has progressed so far that I don’t recommend using these clutches.

From this point on we get into the very serious stuff. CVT Belt drive clutches, same thing as the big sleds.

These CVT units work much like the automatic transmission on your car. At idle there is no tension on the belt and your sled will sit and idle. As RPM’s increase, the sheaves of the primary clutch move together and grab the belt to start the sled moving. As we gain speed the arms, weights, or pucks inside start to move the sheaves closer together forcing the belt out to ride in a larger diameter, at the same time this pulls the belt down in a smaller diameter on the driven clutch. This is what gives us the continuously variable gear ratio. We have about a 3/1 ratio with these depending on brand. Clearly 3 times the ratio off the line will be a tremendous help. If you have ever ridden a 10/15 speed bike and watch what happens as you shift this is the same thing. When you start out in 1st gear on a bike, it takes very little force, but soon you are pedaling very fast but your speed is relatively slow. To go any faster in first over revs the engine (your legs). As you start to shift up the bikes speed increases and your legs slow down, eventfully reaching the highest gear. At this point you are simply limited in speed by the amount of power your engine (your legs) can put out. If you ever tried to leave from a stop on a bike in high gear, you know how hard it is and how much force it takes. Starting out in low gear you will accelerate much faster and cover much more ground quicker. This is exactly what a CVT clutch set up does for your sled.

Before we get into the inner workings of these clutches, two things we should understand. First, these are not a bolt in swap, they require that a cross shaft be installed on your chassis and that a alternative brake system be used. All parts for this are readily available, but you do have to install this on your chassis. Second, this is not cheap; top of the line stuff will set you back about $1000.

Ok, on to the mechanics. There are several styles of CVT units, for the trail or lower level racers Jim at Max Torques makes a nice set up. There is a link to his site in the ling section, he does a very good job of explaining his product.

There also are some clutches made by Comet that were used on mini bike and yard karts around. With Comet now out of business, you have to hunt for used ones. Sorry but I really can’t be of much help with those, never tried them.

This takes us to the Jr. Race Car Shockwave. This is the clutch used by 90% of the serious 120 race sleds. It is a CNC Billet clutch very similar in design to the Comet 102/108 design most people will know from big sleds. You tune this clutch just like a Comet, there are different arms or ramps which vary in both weight and profile contour and work against a spring. The secondary is again just like most big sled designs where a roller helix works against a spring. There are different arm/ramp choices and springs for the primary clutch, and we have different degree helix’s and springs for the secondary clutch.

We have spent many years’ drag racing big sleds. We would bolt in the engine and get to the race. The day was spent tuning the clutches, that’s the difference between 1st place and the rest. If you seriously racing your 120, this is where you will find the speed, setting it up for the conditions of the day.

Chains

The last link in putting power to the ground is the chain. OEM, your sled came with a #40 industrial chain. I can tell you it’s strong, but that's about it. The gear, in its spacing of the teeth, must match the chain sized used. If your only going to change one or two gear sizes, maybe its going to be easier for you to stick with the #40 size. The advantage, and reason racers switch to #35 chain is the chain itself. The kart racing world again supplies the product. While there also is industrial #35 chain, the advantage racers want is the good quality chains the karters use. This chain is a double roller, o-ring sealed chain. It’s every bit as strong and much lighter and free running than industrial chain.

If you're going to buy different rear gears and front drivers for the clutch so you're set for any track or conditions, it's much better to switch to #35 right away.

Spring Engagement Chart

Regular Shoes Wedge Shoes*
Part
Number
Wire Diameter Color Ticking (RPM) Engagement (RPM) Ticking (RPM) Engagement (RPM)
GS-HOSTAR 0.025 Plain 270 400 170 300
GS-XLIGHT 0.049 Plain 1450-1500 1150-1650 1250-1350 1450-1550
GS-LIGHT 0.054 Blue 1650-1800 1850-1950 1550-1600 1650-1750
GS-SS 0.056 Plain 1900-2000 2200-2400 1800-1900 2100-2200
GS-SSW 0.057 Green 2400-2500 2500-2550 2100-2150 2250-2300
GS-820 0.059 Black 2800-2900 3000-3100 2600-2700 2700-2800
Engagement based on 5 spring sample
SS Shoe = 46 grams
SSW Shoe = 62 grams (35% heavier)
* Used by the OEMs of both go-karts and snowmobiles.
Shoes can be mixed as long as it is done consistently. The clutch MUST be evenly balanced.

Thanks for Jim Donovan of Max Torque, LTD, for this spring engagement chart.