September 1998

Hubs hang from the rim!

Background

The wheel on the bicycle consists essentially of four parts: the tire/tube, the rim, the spokes, and the hub. The spokes connect the hub to the rim, usually in some sort of criss-cross pattern.

Cyclists (at least the ones on the internet, we might be called cyclo-geeks) like to argue about all sorts of bizzarre things. In this case the argument is, what holds the hub in place. Well, the obvious answer is the spokes, but the real question is, which spokes.

If you look at a wagon wheel from the old west, you'll see that it is made of a set of wooden spokes connected directly from the hub to the rim. By directly, I mean they don't cross each other. The important thing to observe here is that if you have a load on the wheel (e.g. a wagon full of a family's posessions), the wheel doesn't collapse. More importantly, you could actually cut away many spokes and the wheel would continue to not collapse (while standing still, that is), if you picked the right spokes. In the case of the wagon wheel, you could cut away every spoke except for the one or two on the bottom of the wheel.

So?

So, only the bottom spokes are actually supporting the wagon. The ground pushes up into the rim, which pushes up into the spoke, which pushes up into the hub. The hub pushes back down on the spoke. The spoke is said to be in compression, because both ends are being pushed in towards each other (compressed).

Wire spoked wheels

What about bicycle wheels? Well, there is one very important difference between wooden wagon wheels and bicycle wheels. On a bicycle wheel, the spokes are thin pieces of wire. They can't support (very much) compression, without buckling and collapsing. They can however support lots of tension (stretching). Bicycle wheels are built to take advantage of tension rather than compression.

When a bicycle wheel is built, the spokes all start out loose, then they are gradually made tighter and tighter. When complete, every spoke pulls the hub towards the rim, but all the spokes are in balance, so the hub and the rim stay put (if you screw this process up, the rim usually folds over to one side, possibly ruining it). This process is occasionally referred to as pre-tensioning, because you are putting tension in the spokes, even before they wheel has to support any load.

It's hard to visualize (I suggest looking at a bicycle wheel), but every spoke is pulling on the hub simoultaneously, in all directions. The tension in every spoke is (very close to) identical. What's interesting is what happens to the wheel when a load is applied (someone gets on the bike). If you measure the tension in all the spokes, only those spokes in the bottom of the wheel change tension significantly - the tension decreases. In other words, the bottom spokes become more loose, all the other spokes remain unchanged.

In mathematical terms it is possible to describe the bottom spokes as being in compression. The have less tension than they had before, so if you count the starting (pre-tensioned) state as the zero state, you have put them "in compression". The reason they can support this compression is that the spoke has been pre-tensioned.

This looks a lot like what happens in a wagon wheel. The bottom spokes go into compression, while all the other spokes remain unchanged.

Wrong!

Many people have used this description to explain how it is that the lower spokes are the ones that support the bicycle wheel. But you have to be careful to examine what is really happening.

The "compression" of the bicycle spokes is really a mathematical fiction. They are compressing only in that there is less tension than there was before. Relative to the starting (pre-tensioned) state, they have compressed, but relative to the totally slack state, the spokes are still in tension. In other words the lower spokes which are described as being in "compression" are still pulling downwards on the hub. Clearly, pulling downwards can not have the effect of holding up.

Only the upper spokes are actually pulling upwards on the hub. This is why I still say, without any doubt, that the hub hangs from the upper spokes. Oddly this does not contradict the following statement, that the lower spokes play the most dynamic role in supporting the load.

As far as the true test, of cutting away spokes to see which ones are more or less important, I haven't tried it (I can't afford to, can you?). I suspect that you could get away with cutting more lower spokes than you could upper spokes (when there is a load on the hub). But because of the pretensioning process, all spokes play some role in supporting the hub at all times.

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