Re: Number of Spokes
In article <000001c06725$60767240$d52f6ad8@k4cybertech.mocty.com>, yoda
<yoda@socket.net> wrote: )Thanks for replying Chris, I knew there were people
out there )that know about these things. I have noticed for a long time )that
the spokes generally come from the hub at a tangent, that )is they don’t go
directly to the rim but lay over almost 90 deg. )from that line. I never did
know why, or how they came up with )a certain number of spokes to do the job.
And I agree that more )can hold more weight, that makes sense.
Bicycle/unicycle wheels get their strength from tension; all the spokes pulling
on the hub keep it in the middle. When you hit a bump, the wheel is compressed,
reducing tension at on the spokes at the bottom (and the top, to an extent). As
long as there’s enough tension left, that’s not a problem, although when the
wheel is significantly loaded (a big drop, for example), a relatively small
sideways force can potato-chip the rim.
Crossed spoke patterns spread the tension out over more of the wheel and thus
are stronger. Fully tangential (90 degrees at the hub) crossed spoke patterns
are marginally better than partially tangential crossed spoke patterns at this;
both are significantly better than radial spoke patterns (0 degrees at the hub).
Radial spoke patterns on drive wheels also suffer from wind-up; because all the
forces on the hub are pulling directly away from the axle, there’s nothing to
stop the hub from rotating more than the rim when you put a rotating force on
the axle. Really, radial spoking is just silly; it’s done, when it’s done, for
aerodynamics, but the aerodynamic profile of a radially spoked wheel is almost
identical to a crossed spoke wheel. It looks cool, admittedly. It’s also useful
on wheelchair wheels so fingers don’t get caught in the crosses.
I’ve found that it’s hard to get the spokes to lie flat at the elbows on a
fully-tangential spoking pattern; if they don’t lie flat, the everyday loading
and unloading of the spoke as the wheel turns puts strain on the elbow and
eventually pops the spoke. So, I usually use two-cross lacing on 32-spoke rims.
More spokes do not always make a stronger wheel! The rim has a maximal aggregate
tension that it can support; the more spokes you have, the lower the tension on
each spoke will be. The tradeoff is that the load each spoke has to carry is
also lower, so more spokes can make for an overall stronger wheel, but you can
reach a point of diminishing returns.
Far more information than you want about the physics of wheels can be found in
Jobst Brandt’s book, “The Bicycle Wheel”. -Tom