Rear Shock positioning

Do you see what you want, or get your questions answered, at Sons of Thunder?

Interesting reading. Yeah, if the angle between the mounting points increases off of perpendicular then the amount of side load potential also increases when dealing with a shock moving in an arc. This can result in a binding moment, which usually results in the seals prematurely failing. The way the shock industry gets around this is to increase the spring minimum preload force and increase the length of the pistons bearing surface on quality built shocks. I haven't taken an old dirt bike shock apart that had been designed for use with those extreme angles but I can guess they also increased the minimum distance of the piston from the head bushing where the rod comes out of the body when the shock is fully extended to prevent jacking from having the bearing surfaces too close together.

A shock may feel tight when mounted but it should still move by hand pretty easy or else you have it too tight and you are artificially introducing binding at the pivot. Lines of force travel in straight lines through a material with any reduced diameter resulting in a stress riser. Too tight and possibly the shocks mounting eyelet hole would wear in an oblong shape or the pivot bolt will shear. This would be the only way you would get torsion from the mount. Torsion requires the binding of motion. As long as the shock can move freely in and out then torsion cannot occur to the shock. The internal piston binds, the spring is too stiff, the travel of the shock is exceeded or the mounts are binding and then you'll have torsion.

Be aware that as you lean the shock more you need to consider that the swing arm was designed to have a motion through an arc with a certain expected distance of travel. If the swing arm has the ability to travel further than when the shock is fully compressed (like in a bottoming out condition) then the shock can no longer move and torsion occurs unless it is perpendicular to the force. The end result is that something has to dissipate the energy. If the shock is fully retracted you could get a hydraulic ram effect inside that could extrude the seals if the force is still traveling through he length of the shock body. That doesn't mean the energy is spent, if the force is great enough to the shock in torsion something somewhere will bend or break. The weakest link fails.

It’s hard to see side loading in motion but you can see its calling card. Look at your shocks shaft, if it is dull on one side then that indicates side load. Scratches or gouges indicate severe side load with the probability the shock no longer has any oil or gas charge left in it and you’re just riding on the springs. If the shaft is bent then you get what is known as artificial or induced side load. This has happened from either an accident, torsion due to improper mount/application angles or if worn out they have bound up and bent the rod due to too much axial play.

This post has been great!!! I've been getting just the information (and education) that I was looking for.

Basically, If I were to copy the first mono shock designs, I would just need to build a basket on the swing arm: Thus changing nor only the posistion of the shock, but the direction of the forces being applied to the shock.

I think I've got a plan now. Thanks all!!!

So, I borrowed a pic from another site that I frequent of 75 honda 550. The frames are bascially the same as far as basic design go, so it works for what I want to do:
After this breif bit of research and education in shock design, structer and frame design, I have come up with an idea how to make what I want work.
I would like to thank cafe75-550 from the Honda Single Overhead Cam Four site for the use of the picture of his bare frame. It was perfect for "mocking it up" within photo impressions.
Basically, The whole rear section is removed. I have added a basket on the swing arm to hold the bottom of the shocks in a different loaction, and added a small perch for a seat. (for the seat, I am planning on having just the bare frame with a wicker-like seat woven in place - different, huh?)
The top pic is the bare frame. The second pic is with my modifactions. What do you all think? Keep in mind that I am planning on using both rear shocks. This will not be a mono shock setup.\

Post edited by: KB02, at: 2006/11/26 10:31

Post edited by: KB02, at: 2006/11/26 10:32

And the second pic:

KBO2, nice photochop job. I'm sure you will get input from people here with more experience in frame mods or engineering, but I can spot a couple of potential problems in your plan. You will be removing some of the triangulation from your frame that seems like it contributes to the overall strength- like the triangles in a bicycle frame or a roof truss. (Yeah, I'm a builder) You would have to brace the crap out of those "red" arms to hold them stiff, in terms of side to side movement. Also, you will change the "fulcrum" length- distance from swingarm pivot point to the shock mount is now considerably shorter- I think that will affect the performance of the spring and shock. Like I said, someone else could explain this a bit better.

One last thing, watch out that you don't get your inner thighs "bit" by those shocks! I do want to see that seat, that will be unique to say the least.

Contact member "Lorcan". I think he did one but it might have been with a swingarm assembly off of a newer bike with the mono setup. Sometimes when the mass of the bike gets to a certain point the shock would do better if it were a short stroke design mounted in a trapeze arraignment so the the shock is working on leveraged motion versus direct line movement.

Nevco48 wrote:
One last thing, watch out that you don't get your inner thighs "bit" by those shocks! I do want to see that seat, that will be unique to say the least.[/quote]
Yeah, tell me about it. :lol: