You know the drill – everyone needs to set up a snowmobile to match the weight of the rider. Specifically, the rear suspension spring preload needs to be adjusted to match the rider weight. When the rider hops on the sled, the rear of the sled should squat a little, but not too much.
Most of us have seen a sled go by where a great big guy (or gal) is on the sled and the snow flap is dragging in the snow. This is a clear indicator the spring preload is not adequate for the rider weight. You know full well that most of the shock travel has already been used up, so when this rider hits a bump there is very little shock stroke remaining to absorb the bump. For sure this suspension is bottoming often, likely providing a very unpleasant ride but also greatly increasing the chance of component damage due to the hard metal-to-metal contact occurring. Something has to give! Chances are the ski pressure is also very light and the carbides are not getting much of a bite, so the cornering is going to be precarious as well.
Or, on the other extreme, you see a small rider on a sled and the rear end is jacked all the way up with zero squat at all. The sled will have heavy steering but will also have little weight on the track, so the track will spin and break traction with little throttle applied. This can also be a dangerous situation as the sled is out of balance, for sure.
So we all know we want the rear of the sled to squat some when we hop on it. But many times we don’t really know how much is enough, or how much is too much. We have learned over the years to just eyeball it and don’t even bother with precise measurements anymore. On our trail sleds we’ve always been trying to get something over 2” with right about 4” being as much as we’d like to see. Anything more than that and we’re using up more of the available travel than what we’d like. But some people like to be specific about things. OK then. By the book you want to target 20% of the total travel. On a short track sled try to get at least 2-3” inches of sag (50-75 mm) measured at the end of the tunnel where the aluminum meets the snow flap with the rider seated. Even this number is somewhat dependent on the tunnel length as a longer tunnel will sag more out at the end of the tunnel. Adjust the torsion spring adjustment cams to obtain the proper ride sag; it is acceptable to have the cams at different settings to obtain the desired sag, like one at #1 and the other at #2.
More importantly, worry more about how the sled behaves and less about the exact measurement. It’s OK to fudge this some as needed. It is quick and easy to adjust. If a sled is otherwise set up properly (traction, skis, runners, ski springs, center shock spring) then using the rear torsion springs is a quick and easy way to adjust the sled for riders of all sizes and weights.