Spokes, hubs, rims, brakes and torques

Spokes on wheels with disc brakes and rim brakes

There is an immediate visual difference between road bike wheels that use disc brakes and those that use rim brakes and it is not only in the pads or the discs, would you know which of these pairs of wheels (same brand and model) ) corresponds to a disc brake and which one to a rim brake?

The difference is evident and it is in the distribution of the spokes of the front wheel.

Traditionally the spokes of the rear wheel have been placed with a different crossover than the front wheel. In many cases, the spokes of the front wheels had an exact “radial” layout, that is, they went from the center of the wheel (the hub) outwards and reached perpendicular to the rim.

Mechanical stresses on the wheels

The reason for this is that the mechanical stresses to which the front and rear wheels are subjected are different, especially the stresses on the transmission. Specifically, the rear wheel must transmit to the ground the torque (moment) of the force of the chain on the sprockets, which in turn is transmitted to the hub and from the hub to the rim and tire through the spokes. So the spokes are the “pillars” that transmit the spin from the axis of the wheel to the ground.

As the ground opposes resistance to the advance and rotation of the wheel, mechanical stress is generated on the spokes.

Mechanical stresses on wheel spokes

The main stresses to which a spoke is subjected will be compression-traction and bending, but how do these affect the mechanical behavior of the spokes? Compressive/tensile stresses act in the same direction as the major axis of the radius and will tend to shorten it (compression) or lengthen it (tensile). Bending stresses are the result of forces that act perpendicular to the main axis and will tend to bend it, so they are the ones that will have the greatest effect on the radius bending. We must consider that the ends are not free because in this case they will produce rotation and/or translation movements. We can imagine that the radius is "momentarily" fixed at the point of contact with the ground and imagine it as a rod fixed at that end. The axle-hub of the wheel is applying a force on that radius in the opposite direction.

If we think about how much force is necessary to deform a rod of any metal, we surely conclude that it will almost always be easier to break a pencil by holding it by the ends and bending it than by resting it on the table and pressing on one of the ends.

Let's see how the radios are affected by these forces according to their disposition.

A spoke that is perpendicular to the ground at the point of contact will be subjected to forces in the opposite direction at each of its ends that will almost exclusively cause bending stresses, which are those that have the greatest effect on the spoke bending. In the case of a “crossed” radius, at the moment of contact with the ground, the force applied by the bushing has a bending component and an axial compression component, with which part of that total force will be absorbed by the material. Compressive forces have a much less significant bowing effect on a rod.

Another of the forces that act on these same planes of a wheel are the braking forces and let's not forget that effective braking occurs at the point of contact of the wheel tire with the ground. In the case of a rim brake, the force applied to stop the wheel is applied to the rim and this transmits it to the tire, located very close, so it is the rim that supports almost all the efforts with very little repercussion on the wheels. spokes. However, in the case of disc brakes, the braking force is applied to a disc attached to the hub, so this force must be transmitted to the rim and tire, the spokes being those force transmission elements. This is one of the reasons that today the front wheels of road bikes that mount disc brakes have abandoned the radial arrangement and adopted the traditional crossed spoke arrangement of the rear wheels as well.

Could this be the real reason for the broken spokes on Wilco Kelderman's disc brake wheel at the 2022 Giro d'Italia, when he lost almost 11 minutes on the descents? Some attributed it to the "heat" of the spokes due to the disc brake, although it seems difficult that so much heat could be transmitted from the disc to the spoke and that it would destroy it. Perhaps those spokes simply couldn't withstand the intense and repeated flexing stresses of hard, long downhill braking.