Rear suspension design 2

[jeromek89]

With regards to mountain bike designs, can anyone tell me how the actual forces due to rider weight and pedaling are isolated from the actual forces on the rear shock? What are the advantages of the rear suspension designed like this?

 

[MikeHalloran]

The advantages of a rear suspension with a shock absorber are twofold:

1. The shock absorber absorbs energy to limit the force transferred to the serious rider when the wheel impacts terrain. It might be said that the shock absorber isolates the rider from the ground.

2. The fancy suspension isolates the dilettante rider from some of his money.

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Seriously, it's mostly a planar problem, so if you drag out your kinematics textbook, you can get an idea of how arbitray forces are transferred around the structure, and how linkages trade force for range of motion. You will have to go on to kinetics to get an appreciation of how the physical dynamics of riding translate into non- arbitrary forces. Then you can go on to strength of materials to see how dealing with the forces affects the weight of the bicycle... which affects the dynamics, so you have to recurse a little.

I.e., in a couple days with a spreadsheet and a tape measure, you can do a little comparative anatomy of different suspension designs and answer the question for yourself.

Or buy some more magazines and trust the bilge tripe produced by writers who couldn't hack the math.




Mike Halloran
Pembroke Pines, FL, USA

 

[Surestick]

The two most common ways are through valving, specifically low speed compression damping and through linkage and pivot placement to isolate rear suspension movement from chain tension.
The following page might be of interest to you:

http://www.rdrop.com/~/twest/mtb/pathAnalysis/

As far as valving goes a google search for 5th element, Fox Pro-Pedal, Curnut, Romic, Specialized Brain, and Rock Shox Motion Control Damping should get you started.
As far as linkage design goes look for VPP, DW link, GT iDrive, etc., etc.

 

[Heckler]

This is how suspension designers isolate pedal forces, chain growth, and other noncritical inputs.

"Propedal is a marketing term created by FOX to describe a digressive compression damping force curve. Digressive force curves have been around long before bicycles had suspension. Digressive compression tunes use preloaded valving to build compression quickly in order to give a firmer feel at low shaft speeds. It does effect the total compression curve and will have an effect on full travel or bottoming, but it puts more emphasis on the low speed characteristic of the shock.

Propedal has nothing to do with the air system of the shock. With adjustable systems such as the RP3 or RP23 you can activate a bypass hole in the shock which allows oil to free bleed past the valving. Propedal "off", and the bypass hole is open allowing fluid to move freely. Propedal "on" and the bypass port is closed off forcing the oil through the preloaded stack." - Darren from PUSH Industries



Heckler
Sr. Mechanical Engineer
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This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.