Torsion Loads

Torsion Loads

Wheels, as spinning objects, experience torsion loads. Torsion involves a turning force. For example, when you pedal the bike, the chain going through the gears on your rear wheel tries to make the hub turn clockwise when viewed from the drive side of the bike. Because the hub is connected to the rim by spokes, these spokes “pull” the rim along with the hub. Thus, a torsion load is applied to the wheel through the turning force initially applied only to the hub.

There are two types of spokes in a wheel laced in a standard crossing pattern. Look at any rear wheel laced with such a tangential pattern. Look at the spokes at the top half of the hub. Notice that half of the spokes extend backward in the wheel, and half of the spokes extend forward in the wheel. The spokes that extend backward are called “pulling” spokes because they “pull” the rim in the forward direction. The spokes that extend forward are called “pushing” spokes because they “push” the rim in the forward direction. Many wheel builders feel that the spokes should be laced into the wheel in a manner such that the pulling spokes will be “heads in”, that is, the heads of the spokes should be on the inside of the hub flange which puts the body of the spoke itself on the outside of the flange. In that case, the pushing spokes will be the opposite, that is, “heads out”. There is no agreement on this point, and it is probably not critically important to the strength of the wheel. It is certainly a very minor point compared to the proper tensioning of the spokes. For rim brake wheels, I tend to lace them so that the pulling spokes are heads in because I think that that is the “conventional wisdom” or consensus held by most wheel builders, but I know many wheel builders whose skills I respect who lace the pulling spokes heads out. With either lacing, the maximum torsion load on a rim brake rear wheel occurs during hard peddling. Braking places no reverse torsion load on...