I've been thinking about this, and intuitively, I think wishbones should be more rigid up and down (I.e., about and axis through the bb), and side to side (axis normal to the ground), but less rigid in twisting.
I could probably work out how to prove it, but my structures fu is not strong.
If you can prove that then you should move on to prove that up is down and black is white. One big one will always beat two small ones...
G.
First and foremost, It's been a long time since I've done any work like this, so I would certainly defer to your expertise.
However, what I was thinking is that trying to twist the rear wheel, (like imagine putting a bar through the dropouts and standing on one side), the lower wishbone would be in pure torsion, and there would be a bending moment at the top wishbone, with a moment arm length of something like standover height - BB rise.
On a standard rear triangle, the single bending moment would be replaced by two bending moments, one at each seat stay, and the moment arm would be marginally longer (the hypotenuse of a triangle with vertices at the center of the axle, the dropout and the center of each seat stay). Additionally you would replace the torsion about the lower wishbone with two bending moments, but I can't remember if that's a good thing or not, especially considering that lower wishbones are rarely circular.
So that’s where I stopped, because even if this line of though was accurate (I’m not sure it is), I’m pretty sure that if you held mass constant, you could plug in all the material properties and over some range of section dimensions, one would be better than the other, and over another range of section dimensions, the other might be better, and there is absolutely no guarantee that you could make a tire fit on any of the optimized solutions.
And ultimately, that’s not really a loading I think we care a whole lot about. I don’t think a little twisting on a peg stall, or landing with some sort of yaw is going to be noticeable. Obviously, that’s something I think I would be willing to compromise to make the structure stiffer when you apply a downward force to the bottom bracket.
One of these days I’ll dig my books out and prove myself wrong, in the meantime, hopefully I’m not sounding like too much of a muppet.
Topic: Frame Features /Strength (Read 40401 times)
