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Re: [ST] New Member



Phil,

You are correct in that the shape of a a member in tension or 
compression does not change the reactions at the end.  A force applied 
at one end is resisted by opposite forces at the other end, if the 
member is stable and not moving. 

The shape of a member in torsion does effect the reactions, though.  An 
offset between the load and any part of the member creates a moment that 
must also be resisted by the other end.  Using your paper clip, in the 
shape of a square question mark, a force applied to the top of the "?" 
will make the tail twist in your fingers. 

Obviously Honda, Ducati, BMW, and Triumph have the capability of 
building bikes capable of withstanding the forces.  I prefer not to have 
a SSSA.

John Albright
'02 RS

Philip Las Gourgues wrote:

>Colt45 wrote:
>  
>
>>If the rear wheel applies a moment to the SSSA, what resists that
>>moment?  Specifically, what keeps the wheel's axle from rotating CCW
>>looking from the rear?
>>
>>If the rear wheel load is 400 pounds and the contact patch centerline
>>is 4" to the right of the center of the SSSA (essentially at the chain
>>line), what keeps the 1600 in-lbs from rotating the whole swing arm
>>CCW?
>>    
>>
>
>I don't think the wheel is applying a moment to the SSSA. The spindle/rear
>axle is rigid (as is the swinging arm itself) and the centre of the point of
>support is directly in front of the contact patch of the tyre. As long as
>the member transferring the forces is rigid it shouldn't matter what it does
>(in terms of shape) between the point of application of the force and the
>point of support (other than internal loads/stresses).
>
>You can demonstrate this to yourself by bending a paper clip into a shape
>with a U sort of thing 1/2 way along (basically so it looks like a highly
>simplified SSSA diagram) and apply a force at one end and hold the other.
>There is no twisting. But as soon as you press on the outside edge of the U
>(the force is now off center to the point of support and there is an overall
>torque) it twists in your fingers. I suck at ascii art but I've attempted a
>diagram of what I'm talking about with the paper clip below (viewed from on
>top).
>
>
>      |
>      |
>      |____
>               |
>       ____|
>      |
>      |
>      |
>
>
>Phil
>'02 S3
>
>
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