The physics behind suspension tuning.

[Cat-man-Doo]

Looking for someone that really understands the physics behind shock settings that can explain it to me. Specifically, I have Fox QS3's in my skid. The rear shock has 3 rebound settings and 3 stiffness settings. I understand that the refund settings change the speed at which the shocks will rebound. What I don't know is how these changes affect the sled in what conditions. When would I want the rebound slower and when would I want it faster and why?. The other adjustment, as far as I know is "the harder the snow, the sifter the setting, the softer the snow the stiffer the setting. I get the soft shock for hard snow but not sure I understand the stiff shocks for soft snow. Bottom line, what conditions require what settings and, more importantly, why?.

 

[AdrenalineAddict]

Your question is really difficult to answer because every rider has a different desired result. Ex. Hillclimb Racer may want stiff compression so as not to bottom out and stress A-arms and other suspension components every time he hammers a mogul, but not so stiff that the hit sends him vertical like a pogostick. Another Ex. Aggressive middle aged rider is not so worried about bottoming out but more concerned with absorbing the bump (mid compression) without hammering his spine, but wants a fast rebound to keep his track on the snow. A good example of something I didn't care for was a sled I purchased that had been revalved and resprung for a seriously aggressive rider. I turned every shock on this thing to it's lowest compression setting and every preload to where the spring would nearly fall out and in my opinion it was still the crappiest riding machine I had ever rode in decades of snowmobiling. I didn't notice it much in the powder but when the mountain became hardpack it would literally run up a chopped up slope like a pogostick, you couldn't keep the track on the surface no matter how you rode it. I ended up removing the supposedly superior revalved shocks and found some properly calibrated (to my liking) units in the shed and swapped them out. This made a world of difference in how the sled reacted to hard springtime snow and I quit cussing at the sled all day and began enjoying it, this change also made it much quicker up a slope because the track actually made contact with the snow. Some will tell you want a soft compression and slow rebound shock on your front track shock as it climbs onto the snow better and trenches less. I feel this is fine in the powder with no obstacles but what if you want to carry your ski's 6" off the snow to avoid rocks and stumps? In this case you want high compression and fast rebound to keep your front end up. I don't think there's a one size fits all rule for suspension tuning, more of a tune to your personal liking and riding style approach.

 

[Wheel House Motorsports]

As mentioned above the suspension style and application varies MASIVELY by age/fitness/riding style.

Personally I find that I prefer soft snow/soft shocks. Hard snow hard shocks. This is due to the forces exerted by the rider/sled into the snow. When the snow is soft you push into it and it acts like suspension so there is much less impact on the shocks so less demand on the valving. When its hard they need to work harder to keep you from blowing through the stroke and bottoming.

This theory only goes so far in firm snow as a weaker/less aggresive rider can only push on the suspension so much before there body becomes the weak link when hitting harsh bumps.

My personal examples on the Float 3 QS3s on my axys that I have run. Im 5" 11" 165#. Athletic and fairly aggresive. on compresion I almost never run position 1 on anything but FTS to help climb on really deep days. I find that they are just too soft for bopping around and I catch myself bottoming when crossing ruts or hitting edges. Position 2 is my go to for almost every day riding. I bump everything up to position 3 for big jumps in the spring in hard snow. I can run 3 on rough trails if I want to ride VERY hard but I need to ride super aggresive and its rather exhausting. I often dial back and ride 1 if im absolutely exhausted and just rolling whoops on my way home from a hard day.

As for rebound. I dont play with it a ton. Reducing the rebound damping on the rear skid especially makes for a very light and "bouncy" feel. I will sometimes do this in really deep snow to help get the sled to pop up when unweighting the skid and trying to float over obstacles as I sort of "bunny hop" up things. But generally I stick with 2 and call it a day. Rarely do I ever run the rebound on 3. reserved for very specific hard snow jump days with choppy outruns that need to minimize any sort of bounce.

 

[butthead]

good instructions on shock set up
i found this to be a good source

 

[Teth-Air]

As mentioned above the suspension style and application varies MASIVELY by age/fitness/riding style.

Personally I find that I prefer soft snow/soft shocks. Hard snow hard shocks. This is due to the forces exerted by the rider/sled into the snow. When the snow is soft you push into it and it acts like suspension so there is much less impact on the shocks so less demand on the valving. When its hard they need to work harder to keep you from blowing through the stroke and bottoming.

This theory only goes so far in firm snow as a weaker/less aggresive rider can only push on the suspension so much before there body becomes the weak link when hitting harsh bumps.

My personal examples on the Float 3 QS3s on my axys that I have run. Im 5" 11" 165#. Athletic and fairly aggresive. on compresion I almost never run position 1 on anything but FTS to help climb on really deep days. I find that they are just too soft for bopping around and I catch myself bottoming when crossing ruts or hitting edges. Position 2 is my go to for almost every day riding. I bump everything up to position 3 for big jumps in the spring in hard snow. I can run 3 on rough trails if I want to ride VERY hard but I need to ride super aggresive and its rather exhausting. I often dial back and ride 1 if im absolutely exhausted and just rolling whoops on my way home from a hard day.

As for rebound. I dont play with it a ton. Reducing the rebound damping on the rear skid especially makes for a very light and "bouncy" feel. I will sometimes do this in really deep snow to help get the sled to pop up when unweighting the skid and trying to float over obstacles as I sort of "bunny hop" up things. But generally I stick with 2 and call it a day. Rarely do I ever run the rebound on 3. reserved for very specific hard snow jump days with choppy outruns that need to minimize any sort of bounce.

I must say when the snow is soft the sled already tends to dive more into the snow so in this case I like firming up the suspension which is opposite to what you are saying. What I find is low speed dampening is to control ride height over gentle humps while high speed dampening is to stop bottoming on big jarring hits. Small chatter bumps on the trail need your suspension to move fast or it will all be fed back to the rider. Here you can't crank up the dampening. Trails with big hits and chatter are challenging to get dialed.