On Saturday, March 16, 2019 at 11:37:49 PM UTC-4, John B. Slocomb wrote:
On Sat, 16 Mar 2019 23:04:02 -0400, Frank Krygowski
wrote:

On 3/16/2019 5:50 PM, Ralph Barone wrote:
James wrote:
On 17/3/19 2:56 am, Ralph Barone wrote:

I missed commenting to John earlier. Your visualization of the problem
isn't quite right. With rim brakes, the spokes do not see any braking
forces, since the brakes sit between the spokes and the tire.

The spokes may not see any torque between the hub and rim, but are
certainly forces due to braking that the spokes see.


Now that I think about it some more, yes, there should be increased tension
on the rear facing spokes and decreased tension on the forward facing
spokes under rim braking.

I suspect it would instead be very much like the situation of a bike
wheel supporting a vertical load. In that case, there's negligible
increase in tension as a reaction to the load. Instead, there's a
decrease in tension in the spokes between the hub and the ground.

For a wheel with applied vertical load plus rim braking, I think the
spokes pointed forward and downward from the hub would see a decrease in
tension. The others would see no significant increase. Check out a free
body diagram of the wheel, and a FBD of the hub.

Certainly true for crossed spokes but what about radial spokes, which
are now common in front wheels where, likely, the greatest braking
force is imposed?

It might be noted that disc brakes impose enough force against the
fork to, possibly, cause the axle to move downward in the drop-outs,
thus solid axles in some modern bicycles.


However, under disc braking, there should be an
additional torque which should increase tension on all spokes where the
head leads the nipple and decreased tension on all spokes where the nipple
leads the head (I'm sure there's a technical term for it, but it currently
escapes my mind).

I imagine that these "torsionally derived tensions" in disc applications
are greater than the spoke tension changes with rim braking.

Perhaps- depending on how hard the brakes are applied. I think a lot of
folks visualize extreme applications of brakes. But I know almost all my
brake applications are very gentle. It just makes no sense to be braking
hard very often, and wasting all that energy. I'm pretty sure most
people plan ahead enough to minimize their braking.

--
Cheers,
John B.

Radial Spokes on disc equipped front wheels? Sounds like a recipe for broken spokes. Then again, if the disc brake is concentrating the applied braking force close to the hub perhaps a radial spoke front wheel isn't as prone to spoke breakage as it would be if the braking force was at the rim? I'm thinking about the torque at both locations.

It also seems to me that with through axles (iiuc) that a bicyclist LOSES the advantage of a quick release wheel. I know that those damn lawyer lips are a pain in the butt most times and largely negate the reason to have a quick release in the first place.

Cheers