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Hossack Motorcycle Front End
sebwiers:
I'm looking at maybe building one of these for my bike. I'm figuring to use a control arms design based off an oval track racer, so I'm not much worried about strength there (should be over done by a large margin).
However, the fork has me a bit worried. I was planning to use 6061 t6 aluminum tube for the legs - maybe not as efficient as a true girder, but better able to resist twisting, and easy to build with (I want adjustable mounting). I don't really know what numbers I should look at to figure out how big / thick my tubes should be, though.
My guess (based on some online beam deflection calculators) is that a 2.5 tube with .25 inch walls and 12" length (about as long as I can see needing to go from the ball joint to the axle) would (if loaded with 1000 lbs, at least double what I'd expect to ever see) deflect .05 inch and have a bending stress of 13,250 psi.
The formulas used for this are:
MI for Solid Round Beams = (pi * (OD^4 - ID^4)) / 64
Deflection = (length^3 * force) / (3 * E * MI)
Bending Stress = (force * length) / (MI / (0.5 * height))
I don't know what those mean, so want to be sure they are the right ones for situation at hand.
The deflection doesn't bother me much (that actually seems pretty stiff) but the stress seems pretty high, close to the yield point. On the other hand, I would have TWO of these tubes, and so the total load would need to be 2000 lbs (roughly 3g acceleration, which is MUCH harder than I could stop) to cause this strain. I figure the load won't always be exactly even, so want some safety margin even with the whole load on one leg. Bump loading doesn't worry me as much, as it would pretty much be entirely compression.
Would that tubing be sufficient, or do I need to step up the diameter (thicker walls are hard to find)? Can I get away with thinner tubing (.125 inch) in a similar diameter, while still having a decent margin before yield?
Brass_Machine:
What kind of bike are you building this for?
Eric
sebwiers:
It'll be going on my Yamaha XJ750 Seca (think its an '83). Bike is a bit over 500lbs, I weigh about 200. Target weight balance is 50/50. Design of the system will allow fairly easy removal for use on a different bike / return to using the stock telescopic Seca fork, although the bike will end up with some extra mounting tabs, etc.
No really compelling reason to do it, by all accounts the Seca fork is a decent piece for its age, with a fairly competent anti dive setup. Just having some fun, looking to make a rat bike / cafe racer mashup with an 80's influence. The design will also let me experiment with various rake / trail combos pretty easily; I've seen some info indicating that bikes with MUCH steeper head angles, but still running 3-5" trail, are actually a lot nicer to ride on rough streets than the normal setup.
Brass_Machine:
When I was considering building a hossack style front end, I had taken some serious thought of adapting a BMW duo lever system. Then also the thoughts went to doing machined style girders instead.
Have you looked at Chris Cossentino's setup?
LINK
Eric
bp:
As it happens over 20 years ago my final design project at college was entitled "Motorcycle Suspension". The system that I ended up with was based on a Hossack. From memory I made the decision to use steel for all structural bits subject to fluctuating loads. The fork legs were 50mm x 25mm x (I think) 3mm RHS steel. The design used a Honda 500cc XBR engine, because I had one. So no powerhouse!!
Best of luck
cheers
Bill
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