ski pressure/chassis balance

[eric82930]

I wanted to explain/discuss ski pressure. I have had quit a few of my dirt bike buddies tell me they slid there forks DOWN in the clamps to "remove some ski pressure" This is bassakwards. It only works that way on a dirt bike with tires because it raises the front of the bike and slightly rolls the center of gravity (and rider weight) to the rear tire.

Snow bikes and sleds are completely different. You need to really think of it as a dirt bike with 3 wheels in a row. One wheel is the ski,one wheel is on the front of the track and one on the rear. The pressure on the chassis is basically a see-saw around the center wheel. If you slide the forks down you are just steeling pressure from the center wheel and adding it to the front and back. It is the same effect as lengthening the strut rod.

Having more pressure front and back makes the ski dig in and the rear of the track bites harder too the the chassis feels more like trying to turn a bus.

Going to the other extreme, the center "wheel" will carry all the load and the ski won't dig in enough to really carve aggressively. Also the track will float the best if the pressure is equal across the rails. I believe the bike/sled will somewhat auto equalize the rail pressure by rocking back under power but in bottomless snow with zero traction I don't think it can fully rock back to equalibrium so you loose floatation and start trenching.

I have tested more/less ski pressure many times and the less there is it always feels more fun like a dirt bike but always seems to suffer slightly in outright climbing performance.

Keep in mind snow density will always change which setup works best. It makes us really think we are stupid sometimes when really the conditions just changed from the previous day. Even back to back test days same weather, same temps, the snow is always more dense the second day because it settles a bit each day after the last storm.

Anyone else that can feel/explain the difference?

 

[Norma Stitz]

Thank you for posting this. My first ride on my bike yesterday left me feeling like I wanted to reduce the ski pressure. During the setup of the bike I ran the forks all the way down in the clamps, so I definitely have room to raise them and I'll try that.

Then I also thought about the other two ways to reduce ski pressure being shortening my strut rod and reducing spring preload on the rear shock, but your comment about the center wheel carrying too much weight was also my concern there.

To be smart we should only make one adjustment at a time but it's tempting to try multiple things when tinkering in the shop because most of us probably can't test out of the back door and when we go riding we are usually too busy riding to take time for adjusting.

 

[eric82930]

One adjustment at a time is smart for sure. Also look for lesser dense layers under the snow. If the lower layers are real dense then you can carry more weight on the center without trenching as much.