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Torsion Bar Swingarm Pivot
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Posted by: shed---------------------
I've been thinking and discussing with my mate about the rear shock installation on bikes. If PDS can be made to work as well as a rising rate linkage (which I see no insurmountable obstacles to), then this means that the spring can be re-packaged so that its not wound aroung the shock body and causing side-loads and the associated friction.
I would like to build a torsion spring into the swingarm pivot bolt, and react the wheel load through that. This would improve the packaging at the rear of the bike, remove the sideload associated friction on the shock. Potentially reduce the weight of the rear shock as it would only be a damper not a spring platform.
Rising rate could be handled by a trick torsion bar, by using the gas pressure in the shock, or bump-rubbers.
Shed
Posted by: ScottS---------------------
good ideas, one disadvantage is that there will be a lot more mechanical advantage to work with, could be a problem or maybe create opportunities.
I recall Suzuki made a street bike with a rotary damper similar to what you are describing, don't know if they still use it or not
I don't see what is wrong with dual shocks though- PDS style damping can create the progressivity and mounted at the end of the swingarm the forces are not multiplied so much.
Posted by: marcusgunby---------------------
suzuki dropped the rotary.
Posted by: YZ165---------------------
I don't think the "torsion bar" could be long enough. Given the frame width as a limit. Unless you used it as footpegs too. 
Anything is possible. Not sure of the practicality though. It would benifit the intake tract to loose the spring, and maby some weight. But you can buy a titanium spring for $600. So there goes the weight thing.
I just can't see the benifits outweighing the cost to make it happen. But then, I'm a brain dead GOOF, so who know's! 
Posted by: SoCal Steve---------------------
Shed,
Continue your thought process...... this sounds promising. It seems that the weight and packaging advantages could be real. The frame width being as narrow as it is would be the largest obstacle. Thing how much wire is in a current shock spring versus the width of the swingarm pivot.
How about a leaf spring type inside or underneath the swingarm..... possibly with an adjustable length, attachment point to pivot to adjust rate? The other issue here is not the total weight savings, but where and how high is the weight. Thats why the YZs feel heavier when leaning in.
Think outside the box.
Steve
Posted by: SoCal Steve---------------------
Shed,
Torsion bar under the motor attatched to the swingarm with a link?
Steve
Posted by: rick#3---------------------
It's not a torsion bar, but I recall Yamaha was using a leaf spring with a rotary damper a few years ago both connected to the swing arm by a linkage. The preload was sent by a slide you moved forward or backward under the leaf spring. I haven't heard anything on it for sometime now.
Posted by: shed---------------------
Torsion bar could be Ti and gundrilled so mega-light. Hadn't thought of a rotary damper, that might work really well for packging and free-ing up space in the airbox area. Bolt on a structural carbon airbox, and you have the lightest 'crosser in the world!
Leaf spring / flexure type rear end would have the disadvantes of friction and lack of travel respectively.
Posted by: ScottS---------------------
it;s all about part consolidation- get the existing parts to do multiple jobs and you save weight- swingarm pivot becomes a torsion bar, and the shock spring is gone- weight saved. Airbox becomes the rear subframe- two parts = 1 part , weight saved. Plus fasteners etc
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