rear suspension theory

[eric82930]

I am going to try to build a home made kit and was wondering what you though of some wild ideas.

One Idea i want to try is to use a rising rate single shock scissor and put it near the front of the skid instead of the center or rear like normal. Then the solid torque arm (swingarm) will go from behind the scissor mount to near the back of the rails with no static spring force on the rear of the rails at all.

Having the torque arm so far back will put all the dynamic force to the rear of the rails to keep the front down under power but all the static spring force from the scissor should be forward enough to keep the ski pressure light off throttle and jump landings.

Track tension could be tricky. But if my side plates are solid and rails don't have too many holes, I could move the scissor and the torque arm pretty easy to acheave a balance.

 

[swedenturbo]

Nice to see more snow bike home builders!
Why not post a simple sketch of your planned design? Much easier to understand your thoughts in how your suspension is supposed to work...

 

[eric82930]

I'll try to attach a picture of a scetch.

 

[eric82930]

Sorry for the horrific art work. Hope it at least conveys the general idea.
The solid swingarm would be on the left and angle and position will be hard to get right I think. Too much angle will make the track tighten under compression. With no spring force behind the halfway point of the rail should make the frint feel pretty light in neutral power conditions.

I got this idea from having my rear shock "fox float" on my timbersled blow out and ran it just on the front shock with extra presure. The front shock even maxed out on pressure would bottom easily because of the falling rate mechanical geometry and a stiffer shock would probably eventually bend the cross bar.

 

[olymon]

If the line to the rear wheel and rail is a connection between the two I don't think the suspension will move much with the solid arm you have on the forward mount like that. You have a trapezoid that swings back and forth on the two bolt mount locations. Do a little wood mock up and you'll see.

Sent from my PC36100 using Tapatalk 2

 

[eric82930]

Sorry the front arm is not solid. It is just a crappy drawing. It will be a standard scissor like off a polaris pro. If you remove the solid link at the back the rails can move freely forward and back a few inches with the scissor still attached. The scissors always have to have a swing link on both ends where they attach to the rails or it will couple or bind and not allow the trapazoid to change.

I will definately do a mock up before I finalize anything, my main concern is the idea of no spring in the back and the torque input being in the back instead of the front. The torque of the motor pushing the rails forward creates an imaginary spring force between the rails and the tunnel but if this imaginary spring is too far back behind the center of gravity the rear of the bike will actually lift under throttle. Too far forward it will wheelie too much.

Drag racers change the torque arm angle to get the car to launch with the most traction. I have seen some drag setups so perfectly neutral that when they launch, the height of the rear bumper never changes but the rear axle drops two inches and smashes the rubber tires into the road.

This neutral balance point would be what I would like to acheive if possible by changing the position and angle of that rear arm. The angle can't change too much or track tension will suffer. I can mock the angles for track tension but won't know how it rides till I try it. Worst case I can just flip them back to a conventional design and have rails that look like swiss cheese.

The mototrax has no spring in the back but it seems to be coupled as far as I can tell. The rear arm on the mototrax is telescoping so it doesn't provide any change until it reaches the end of its stroke and couples the rear to the main arm.

 

[Sheetmetalfab]

Sorry the front arm is not solid. It is just a crappy drawing. It will be a standard scissor like off a polaris pro. If you remove the solid link at the back the rails can move freely forward and back a few inches with the scissor still attached. The scissors always have to have a swing link on both ends where they attach to the rails or it will couple or bind and not allow the trapazoid to change.

I will definately do a mock up before I finalize anything, my main concern is the idea of no spring in the back and the torque input being in the back instead of the front. The torque of the motor pushing the rails forward creates an imaginary spring force between the rails and the tunnel but if this imaginary spring is too far back behind the center of gravity the rear of the bike will actually lift under throttle. Too far forward it will wheelie too much.

Drag racers change the torque arm angle to get the car to launch with the most traction. I have seen some drag setups so perfectly neutral that when they launch, the height of the rear bumper never changes but the rear axle drops two inches and smashes the rubber tires into the road.

This neutral balance point would be what I would like to acheive if possible by changing the position and angle of that rear arm. The angle can't change too much or track tension will suffer. I can mock the angles for track tension but won't know how it rides till I try it. Worst case I can just flip them back to a conventional design and have rails that look like swiss cheese.

The mototrax has no spring in the back but it seems to be coupled as far as I can tell. The rear arm on the mototrax is telescoping so it doesn't provide any change until it reaches the end of its stroke and couples the rear to the main arm.

A point to consider is that when you are climbing a hill the neutral angle of the rear arm changes (vs flat drag racing)

Therin lies the rub........

 

[swedenturbo]

Just make a simple wodden mockup.
That will hopefully show if your suspension theory will work as you think..
At least, you will be confident that you keep track tension in most conditions....