RE: ?'s on frame material not for Muni
> Someone mentioned frame flex. What is causing that? I want to
> avoid that.
I can’t resist. You again asked a too-open question:
Frame flex is caused by applying force to it. Normal forces are what we call
End of joke part.
Other forces are gravity (you sitting on the seat) and drops/packing in the
car/falling on it. The gravity force you can pretty much ignore, as it will
be covered by nearly any design. You can try to build it to be proof against
drops, falling on top of it, being packed in the car, etc. but to do this
thoroughly would probably mean adding more weight than you want to. I would
just design to cope with the main forces of riding.
The harder you pedal the more force there is trying to bring your tire in
contact with one side of the fork or the other. Which way your frame flexes
depends on the type of frame. A Schwinn frame is obvious enough; as a piece
of flat steel it’s extremely strong in one direction, but extremely weak in
the other. So you can grab a Schwinn’s seat, with the wheel held in place,
twist it from side to side, and watch the frame flex.
My first “MUni” was a 26" wheel crammed into a 24" Schwinn frame. Bad idea.
Sure the wheel fit, but as soon as you tried to pedal hard it would rub the
sides. So whenever I was going up (or down) anything steep, I would get a
“brrrrip-brrrrip-brrrrip” with each crank of the pedals.
Even more rigid tubular frames have some flex in them. Greg Harper’s Uni.5
is built in a Torker or SemXL frame, but the tire is pretty wide, with
minimal clearance. When I pedaled hard it also rubbed.
Another example is my carbon frame. Carbon is super-strong and super-light,
but flexes more than steel. My frame (Roger Davies Mk 5) is built of carbon
tubing with aluminum lugs. The force of pedaling, especially when you hold
or pull up on the front of the seat, causes a lot of twisting force through
the frame. This puts a lot of stress on the joints where the tubing is glued
into the lugs, because the twisting force is in the same direction you would
twist them to take them apart. Mine has been reinforced with aluminum pins
to keep the parts from coming apart, but it’s easy to see the amount of
force going through that frame, as you generate work with the strongest
muscles in your body.
Any frame will flex. Your goal is to build it in such a way that the flex is
minimized, or allow more tire clearance for a more flexi frame.
Stay on top,
John Foss, the Uni-Cyclone
“We were discussing Big Mac Meals. I think that has little if anything to do
with cow parts. There are probably more UNICYCLE parts in a Big Mac than cow
parts.” - Greg Harper on cuisine