|   Paging D.F.L. Continuing Horst link thread from below | Steve from JH
Jan 10, 2002 6:26 PM | | This is the thread under discussion:
http://forums13.consumerreview.com/crforum?14@@.eea299a
Your last reply below says:
]You are performing the calculations as if the rear cogset (a lever arm) were rigidly attached to the suspension member (either the CSs or the upright link.)
The way I see it,because the suspension members are somewhat isolated from the suspension members by axle bearings, the lever arm should actually extend from the chain (where it contacts the top of the cogset), through the rear axle, down to the ground. This is missing from your diagrams. (Or maybe you left it out to keep it understandable to dufus' like me:)) This would leave the rear suspension affected by chain torque attempting to shorten the chainline, and by the forward force created by accelleration. Do I have this right?
Although I don't think you meant to repeat "suspension members" I think I know what you mean and you have it right.
Think of the top of the cog where the chain makes contact as a point on the wheel and the wheel as being anchored to the ground by traction. The point then can be thought of as anchored to the ground. The chain tension acts between that point on the wheel and the point at the top of the chain ring, which can be thought of as a point on the frame or front link of the linkage. If the chain line does not pass through the common center of rotation for the front part of the frame and the suspension member where the wheel is mounted, a torque moment will be created.
If the wheel is mounted on the swingarm, the swingarm pivot is the common center of rotation. If the wheel is mounted on the rear link of a linkage, the IC is the common center.
So the exact location of the IC is quite important. A chain line that passes above it when it is close to the BB might pass below it if it were moved farther forward, even out ahead of the front axle. So the torque would be reversed. And it could work the other way around. Thus the angle of the upper arm of the linkage can make a big difference.
Below is an illustration of a contraption I made last winter to demonstrate all this. The picture looks better than the real thing did. It was quite rickety and jury-rigged--made of wood with washers and bits of wire and held together by QR skewers. It fell apart after every few pulls on the chain. But it proved what I thought it would.
The exact position of the IC also matters as far as acceleration effects are concerned, both from pedalling and braking. I don't have time to get into that right now, but I will get back to you. |
| re: Paging D.F.L. Continuing Horst link thread from below | Peter E
Jan 10, 2002 11:59 PM | | Steve, In your diagram you look at the rear wheel and rear link as one unit. Are you sure that is right? I dont think you can do that. If you would redo you experiment (good experiment btw) and use a much larger cog I dont think you would get the same result. Should in not be like this?
Peter |
| Or like this | Peter E
Jan 11, 2002 7:36 AM | | Or like this on a mono shock.
Peter E |
| Parallel chainline? | Steve from JH
Jan 12, 2002 9:32 AM | | If you mean that a chainline parallel to the swingarm, or theoretical swingarm in the case of a linkage, would be neutral, well I used to believe that a couple of years ago.
After studying some engineering I would say that the top two red arrows in each of your diagrams look like textbook illustrations of a force couple. They would produce a torque moment on the swingarm with respect to the wheel and front triangle that would move the pivot down, if unblocked by anything else.
On an actual bike the chainstay lengthening effect would in fact block (isolate) the torque. In my experiment there is no chainstay lengthening effect because there is no crank and the chain is free to lengthen under tension.
So I could measure an actual upward movement (using cable ties on the shocks) whenever the chain passed even slightly above the IC. |
| Very cool... | D.F.L.
Jan 14, 2002 1:37 PM | | I only had time to print them off; no chance to evaluate.
Came within a wisker of being fired (or attacked, maybe) by my boss. Not a great time to be online.
I will review, thanks! |
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