TY Comes Through Yet Again!

[Jaybaar]

After doing standard maintenance (oil, filter, grease, slides, chain case etc.) on my 16 Viper I noticed my rear shock was dead, or so I thought. After scouring the forums for days, I came across a sort of related post. One of the comments asked "is the sled on dollies?", I thought, that's an odd question and didn't give it much thought at the time. Well this has been living in my head since I read it given the fact my skis were on dollies, I just took the skis off of them and SOB the rear shock is fine! Can anyone explain this strange phenomenon?

 

[biffdotorg]

I've seen it as well shuffling sleds around the shop all summer. I can't explain it, but it happens often.

 

[Jaybaar]

I've seen it as well shuffling sleds around the shop all summer. I can't explain it, but it happens often.

It's really baffling. The shock acted like it was completely smoked, zero compression or rebound.

 

[STAIN]

It the angle of the dangle.

 

[GR8BBQ]

I bet the dollies allow the skis to move in any direction (including outward).... I've not experienced this, but my dollies only allows skis to roll forward or backward.

 

[ksiesel]

Many incorrectly believe that the shock is what provides the force to extend the suspension. Depending on the type of shock, this can be either a very small truth or no truth at all. The primary purpose of any shock is to restrict motion.

Non gas charged shock - these are not charged with nitrogen or air so when a loose shock is compressed by hand, it does not extend on it's own. This means that for any suspension using this type of shock, the shock itself provides exactly ZERO force to help lift the vehicle after the suspension is compressed.

HPG shocks (This would include any reservoirs, clickers, IFP's and some emulsion) - These shocks when loose will self extend after being compressed by hand. The reason they extend is there is more surface area for the gas pressure to act on the side of the piston that does NOT have the shock rod attached. This creates a force that is equal to the shock rod cross sectional area multiplied by the gas pressure. It's not a LOT - maybe 75 lb.

So, understanding the above, how does the suspension extend after you get off the machine? It's almost ALL because of the SPRINGS, not the shocks. In fact, if the suspension does extend, it's because the springs are too weak, the weight is too high, or something is binding it (note this could be a frozen shock if water got inside).

Back to the question about why having skis on dollies and track on level surface results in the rear being collapsed. This is perfectly normal as what you have done is increase (drastically) the weight carried by the front ski springs and the rear arm springs. Where did this weight come from? The center spring is now fully extended and likely not even touching the ground any longer - it is carrying no weight.

In essence, you've taken the 3 balance points of contact (rear of track, front of track, skis) from the normal machine on level ground and you've altered the geometry so only 2 of those 3 points are carrying all of the weight. More weight being carried by rear springs means there may not be enough force to fully extend the rear suspension arm.

 

[earthling]

Unlike the wheel on a car, the skid on a snowmobile doesn't move linearly up and down. On a car you can push down from the fender or up from the wheel and the force is translated either way - equally. To extend on what @ksiesel wrote, when you apply force from the skid (track) up or the bumper down (or from the seat/running boards), the forces do not translate equally. The skid is arranged as a trapezoid so its natural articulation is backwards as well as upwards. It cannot move straight up (like a wheel does). When you push down on the sled from the bumper or from standing on it, you may not be activating the skid in its natural direction of articulation, typically because of where the dolly is placed relative to the bottom of the skid. To far forward and it activates the front part and not the rear, too far backwards and it does the opposite. When the skid is on the ground the force is translated equally, and naturally through the entire mechanism of the skid (the trapezoid).

 

[Jaybaar]

Many incorrectly believe that the shock is what provides the force to extend the suspension. Depending on the type of shock, this can be either a very small truth or no truth at all. The primary purpose of any shock is to restrict motion.

Non gas charged shock - these are not charged with nitrogen or air so when a loose shock is compressed by hand, it does not extend on it's own. This means that for any suspension using this type of shock, the shock itself provides exactly ZERO force to help lift the vehicle after the suspension is compressed.

HPG shocks (This would include any reservoirs, clickers, IFP's and some emulsion) - These shocks when loose will self extend after being compressed by hand. The reason they extend is there is more surface area for the gas pressure to act on the side of the piston that does NOT have the shock rod attached. This creates a force that is equal to the shock rod cross sectional area multiplied by the gas pressure. It's not a LOT - maybe 75 lb.

So, understanding the above, how does the suspension extend after you get off the machine? It's almost ALL because of the SPRINGS, not the shocks. In fact, if the suspension does extend, it's because the springs are too weak, the weight is too high, or something is binding it (note this could be a frozen shock if water got inside).

Back to the question about why having skis on dollies and track on level surface results in the rear being collapsed. This is perfectly normal as what you have done is increase (drastically) the weight carried by the front ski springs and the rear arm springs. Where did this weight come from? The center spring is now fully extended and likely not even touching the ground any longer - it is carrying no weight.

In essence, you've taken the 3 balance points of contact (rear of track, front of track, skis) from the normal machine on level ground and you've altered the geometry so only 2 of those 3 points are carrying all of the weight. More weight being carried by rear springs means there may not be enough force to fully extend the rear suspension arm.

Thank you for knowledge bomb ksiesel! Your explanation makes perfect sense, greatly appreciated!

 

[Jaybaar]

Unlike the wheel on a car, the skid on a snowmobile doesn't move linearly up and down. On a car you can push down from the fender or up from the wheel and the force is translated either way - equally. To extend on what @ksiesel wrote, when you apply force from the skid (track) up or the bumper down (or from the seat/running boards), the forces do not translate equally. The skid is arranged as a trapezoid so its natural articulation is backwards as well as upwards. It cannot move straight up (like a wheel does). When you push down on the sled from the bumper or from standing on it, you may not be activating the skid in its natural direction of articulation, typically because of where the dolly is placed relative to the bottom of the skid. To far forward and it activates the front part and not the rear, too far backwards and it does the opposite. When the skid is on the ground the force is translated equally, and naturally through the entire mechanism of the skid (the trapezoid).

Thank you earthling! The trapezoid reference allowed me to picture what is actually taking place in the skid as it moves. Great stuff!