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Chemical X
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88 Posts
Discussion Starter · #1 ·
Just a quick question while im welding into place my shock.

If a shock is angled straight down would it take the shock less force to bottom out? rather than if it's angled say 45 degrees.

I'm getting a shock made and i need to know if the one im basing it off with bottom out at the angle i put it. Ive got the full range to pick from.


Im using a CBR shock the older model with the oil resi. It's nice and small and fits into the swingarm perfectly. It doesn't use a linkage just bolts straight into the arm.
 

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escape from prison planet
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2,296 Posts
shift would be the guy to ask, but he doesnt come around much anymore. if he does, he's normally in the 2stroke section.
 

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Old, bold rider
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179 Posts
Just a quick question while im welding into place my shock.

If a shock is angled straight down would it take the shock less force to bottom out? rather than if it's angled say 45 degrees.

I'm getting a shock made and i need to know if the one im basing it off with bottom out at the angle i put it. Ive got the full range to pick from.


Im using a CBR shock the older model with the oil resi. It's nice and small and fits into the swingarm perfectly. It doesn't use a linkage just bolts straight into the arm.
While studying suspension dynamics I came upon an article published by Graham Byrnes PhD for ProMechA.com.au which did a good job of clearing the high tech fog that has surrounded rear suspension since the advent of single shock linkage setups: http://www.promecha.com.au/leverage_linkages.htm
It served to confirm that which I had found by using the "change something and see what it does" method, a much more time consuming way of going about things.

In your case it's relatively simple: on a non link setup a progressive rate (soft on small bumps, and harder on larger bumps) without using a progressive spring is almost inevitable. In a nutshell: if the angle of a line drawn from the swingarm pivot to the shock mount and a line drawn from the lower shock mount to the upper mount increases as the suspension compresses you have a progressive rate. On the other hand if the angle decreases the rate is digressive (harder on small bumps and softer on larger bumps, obviously not a good setup for the street.) Basically, if the distance from the swingarm pivot to the lower shock mount is greater than the distance from the pivot to the upper shock mount the rate is digressive, and progressive if the distance is less. What you need to do is draft a representation of the swingarm you are using from as precise measurements as possible, draw arcs of the rear axle movement over the suspension travel you desire, usually five inches or so, and shock mount movement generated by the motion. An easy way to do that without a CAD program is to take the full scale draft and cut out a thin wooden model of the swingarm, nail it to a piece of plywood at the swingarm pivot, and directly measure the travel. From that you can determine what geometry will produce the results you are looking for.

If you want to use a shortcut you can use this freeware linkage simulator program: http://blog.rectorsquid.com/linkage-mechanism-designer-and-simulator/
All control points can be entered parametrically, and it will generate animations that can be quite educational.
Here is a screenshot of a generalized drawing of the setup you have: Non link illustration.png
And a video of a GSXR link simulation in action.

Have fun!

Rob
 

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lɐʇuǝɯᴉɹǝdx&#4
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3,516 Posts
Fukin A, wish I'd found that linkage designer last year. Will have to plug in my mono-lever derived design, see how it pans out. My direct measurements showed it as strongly progressive (especially near end of travel), but measurements can be mistakes, and it would be nice to know how I can tweak it (top shock mount is very easy to re-build).
 
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