The Physics of Wheel Building

Wheels are one of the more impressive aspects of a bicycle. At a relatively low weight, they can withstand immense forces and are a decisive factor in defining your bike’s riding characteristics. If you would like to build custom wheels and go deeper into the process, or if you just want to better understand why system wheels are made with certain properties, we’ve put together some theoretical knowledge that is certainly worth considering.

The strength of spoke lacing

Spokes made of thin wire are much stronger tension-wise than in compression. The rim, on the other hand, is quite pressure-stable. Tension spokes experience an elastic change in length while in use. That is why the spokes must be pretensioned enough to allow the system to compensate for short-term stretching. The spokes relieved at the lower part must not lift off from the rim base. Such loosening and re-tensioning would cause a notch effect between spoke bend and hub flange – and weaken the spoke at this point. For this reason, the preload must be as high and as uniform as possible. With sufficient pre-tensioning of all spokes, peak loads during sprints, jumps and full braking actions are distributed evenly over all spokes. Nevertheless, try not to go overboard with spoke tension. Carbon fibre rims in particular are somewhat sensitive in this regard during assembly, and are more likely to break than bend. So be sure to follow the manufacturer's instructions for hubs, nipples, spokes and rims, and use a tensiometer if in doubt, but especially if you are lacing carbon rims. Theoretically, the connection between spoke and rim makes the perpendicular wheel stable in itself, so that it can absorb vertical forces. In reality, however, the wheel must also withstand extreme torsional and lateral forces. The absorption of these forces is ensured by at least two hub flanges. The support on both sides results in the angle of the spoke discs, and can absorb high forces – comparable, for example, to the mast of a sail boat, which is held in place laterally by pre-tensioned shrouds. If one spoke breaks, the other spokes are initially able to compensate for the forces that occur. In the long run, however, this can lead to overloading of the neighbouring spokes, which is why the damaged spoke should be replaced as soon as possible.

Radial vs. tangential forces (lacing pattern, drive and brake side)

With radial lacing, the spoke is the shortest and most direct connection between the hub and the rim – its imaginary extension is aligned through the centre of the wheel axle. Radial wheels are therefore laterally stable and lightweight. When absorbing the torsional torques caused by acceleration or braking (disc brake, back-pedalling), radial storage is not very helpful, as it can only transmit small amounts of torsional force. That is why radial wheels are best suited for wheels without brakes (e.g. on a trailer) or as front wheels with rim brakes. The drive side of the rear wheel and the disc brake side on both front and rear wheels should therefore not be spoked radially. In order to better absorb rotational forces, the spokes must have flatter contact with the hub – ideally, at right angle to a line that intersects with the centre of the axis. The hub flange acts as a lever: the larger the flange diameter, the more stable the wheel. The ideal balance between weight and stability is offered by triple crossing, which is why it is most widely used on bicycles. The number of crossings indicates how many spokes a spoke crosses between the hub and the rim. In classic triple crossing, each spoke crosses its first, third and fifth neighbouring spokes. It is above the first two crossing spokes, but below the third.

Stiffer = better? Considerations for targeted flex

When you’re building custom wheels for your mountain bike, you can use different factors to influence whether the wheel should have maximum stiffness or a certain degree of elasticity. There are different philosophies here, which can also be found in system wheels. The approach transferred from motocross motorcycles to modern e-mountain bikes, for example, states that the rear wheel should be as stiff and wide as possible and may also be smaller, as it transmits the driving force and is exposed to higher lateral forces in corners, and should therefore be more agile. The front wheel, on the other hand, can be a little more "compliant" to provide maximum traction. That's why it's a little narrower and bigger. Many “mullet” wheel combinations, i.e. a 29 inch in the front and 27.5 inch in the rear, take this consideration into account as well as system wheelsets in the Crankbrothers Synthesis family. The opposite point of view argues for stiffer front wheels for maximum steering precision and somewhat more flexibly-designed rear wheels that better follow the ground surface. An extreme example of this philosophy was the first Mavic Crossmax Enduro wheelset with 24 spokes in the front and only 20 in the rear, developed in collaboration with Fabien Barel. To achieve full on compliance and targeted flex to maximize traction, Zipp has come up with the 3Zero Moto line, which features so-called single wall rims instead of the usual hollow rims, which allow the wheel to specifically twist around the nipple seat, increasing the contact area of the tyre to the ground. Zipp calls this ankle compliance in reference to the mechanics of the human ankle.

Only you can decide which approach is best for you and your riding style. And that's the beauty of an individually assembled wheel: You decide, within the framework of the physical possibilities, which criteria are important to you and can thus build the best wheel for you. You can find lots of tips on this in our other articles on wheel building: