Adam made some serious suspension modifications to his sled. Take a look. There's a write-up from Adam below.
I did the polaris track suspension swap with polaris drivers. The thing that prevents these suspensions from having huge travel is the spring adjusters on the side. You could use a coil over spring on this setup and it would be a lot easier. I did it this waybecause I had some 76 chassis parts and kawi era suspensions, they use a different spring tensioner bracket. The kawi ones have the mounting hole in the side of the adjuster closer to one side of the bracket. I took the kawi adjusters and fliped them to opposite sides so now when you put the bolt that secures them through the chassis plate in the adjsuters are swung down out of the way of the track. I also had to cut off and reposition and weld the tab that the adjuster eye bolt goes through.
So with the adjusters out off the way, I took another front suspension arm off of a spare skid frame and seen as the fronts are longer it gives you more vertical travel with out having to have the arm swing through a large angle. I ordered a new hyfax and made some longer sliders for the tops of the rail seen as the back wheels are pushed back from the longer track conversion. The long sliders allow the longer suspension arm to have full travel and not derail from the slider as it would if they were stock.
I then removed the top track idler shaft and wheels and ground out the 4 rivets on the little hole suport plates. I moved the hole for the idler shaft back and up until the mounting hole for the shaft lined up where one of the 4 corner rivet holes are. This moves the track up and back and keeps it from hitting the adjusters at full travel. The shock you see is off of a 80's yamaha, i just happened to have it and it fit perfectly. I simply cut the shock tabs off a rear suspension arm and welded them on the longer front arm thats in the back. I used a stock shock cross shaft and some existing hole in the back of the rails to mount the bottom of the shock.
I have not done anything to the front portion of the suspension, but plan to raise the point where the front suspension arm connects to the skid that will let the skid travel downward more with out loosing track tension through the suspension movement. The front of the sled has yamaha ski wideners with boa ski aluminum skis that i got on ebay and the stock springs. The only probelm with the sled is that i think the track drivers are too large and cause the track to hit the tunnel sometimes. If you wanted to do this mod i would go right for the coil over shock and that would eliminate most of the crazy mods i did. The only problem i can see is that under heavy suspension compression the shock angle decreases alot and almost flattens out, this causes it to apply less force on the suspension and you could have trouble bottoming out. The shock I have is just an absorber and works well. There are more updates to come that will improve the suspension so stay tuned.