Vintage Snowmobile Suspension Stories
Your set-up can benefit from using some theory and lots of practical application that will contribute to quicker times for your sled.
Ever wonder why some guys are strong and straight out of the hole, always? While running our SRX's we noticed some interesting physical factors at work. This is a simple guide to stuff I figured really worked.
1) You must understand that all the original traction force is applied directly to your rear wheels. The sharper the pic, the deeper the lug, the more the traction force the wheel gives to the axel via bearings and onto the skid frame.
2) Force travels down the slide rail to the skid arm then up the arm and into the chassis. All the force, less the friction, goes directly onto the front top mount bolts and into the tunnel.
3) The angle of the arm is critical. As force is fed up the arm the energy that, moments ago, was straight ahead is now pointed upwards. This means some lift is now involved.
4) To channel energy in a lifting requires the equivilent of down force. This causes the sled to wheelie to an extent, as the front suspension bolt divides the forces, some up, some down and some straight forward.
5) Movement in any direction costs energy. When you have a limited amount available, how you divide the 3 way split is important. You want all the forward motion possible and as much down force to keep the pics planted the entire length of the track. That will give you lift, (which is wasteful after traction is established).
6) Wheelies cost forward motion. You should set your arm by moving the front moounting holes up or down. Play around until you get enough traction and most of the energy pointed ahead. Snow is a bit different than ice. You need more angle for snow. Sharp pics allow flatter arms on ice, snow has less traction.
7) Check your skid frame for true along its length. The rails must be parallel and exactly the same side to side. Rear wheels must be the same size and allow nice easy rolling of the track off the track and onto the wheels. Too big gives a step that costs down force and lessens the traction by trying to hold the track free of the sliders for a few inches. Smooth is good, size really isn't critical for vintage drags. Good bearings will serve you well, if cleaned and greased annually.
8) Please don't assume that the guy who slapped your sled together did so with precision. Always measure and measure again. Make all your holes line up side to side and get your shafts nice and square to the tunnel and skid. This is called blueprinting the chassis. In the hayday of Yamaha racing (1974-77), the race director would say "don't phone us about handling issues until you've blueprinted the chassis". It is extremely important and can not be over done. It was a well kept factory racing secret.
9) Blueprinting a chassis will help give you a straight run, however let's not forget the other stuff. The engine must be true in both directions to the secondary drives. These must be true to the driveshaft and gears. Finally the drive must be true to the skid.
10) I will bet anyone...(except and old snow pro) that your sled is way off. In fact you will probably find you need lots of spacers and a rat tail file before all is said and "true".
11) Don't forget to turn the front wheels and driveshaft on a lathe. Gently take just enough off to true up the circle. Straighten the shaft if required.
12) A sled that twists coming out of the hole and runs off to one side is misdirecting some of our neat handy work. make sure you have enough spring tension side to side. Use 2 limiters to hold things true. Torque straps from the engine to the jackshaft will help. Two would be better if you have the room.
13) In closing let me say this about rear springs. They really only support your big bum. Set up the rear for just a bit of squat. If you need to drop your tail down to get a hole shot you need more angel arm, not softer springs. Stiffer springs give you traction quicker as the sled rolls back over centre. Quicker is faster.
See you there!!