Perhaps you are wondering about the mechanics behind the bicycle wheel, how spokes work, what lacing patterns can be used and so forth. More likely, perhaps your not. Regardless, here are a few words on the fascinating topic of spokes.
The spoked wheel was first conceived by Mr George Cayley who was an early pioneer of flight. As well as being successful in producing a working glider, he invented the wire spoked wheel in his quest to design a light and durable wheel for his flying contraptions. Spoked wheels were around much earlier than this, chariots and horse drawn carts had spoked wheels, but these wooden masterpieces were not stressed like the wire spoked wheel. On the wooden wheels the weight of the vehicle was simply supported by whichever spoke was at the bottom. The spokes had to be strong enough to support the weight of the vehicle plus its load and with excess strength to resist impacts.
Wire wheels came later and were significantly more advanced because they were kept in tension with the hub being effectively suspended from the top of the rim rather than propped up from the floor. Construction of this manner allowed thinner, lighter spokes to be used resulting in a much lighter wheel capable of the same performance as a wheel with thick spokes used in compression.
From a mechanical perspective, a material in compression has to be designed strong enough to resist buckling; but interestingly, placing the same material under a pure tensile load removes any risk of buckling and the part can therefore be much thinner.
For a wheel which doesn’t have to resist hub torques, a radial spoke pattern can be used. This is probably what George used on his gliders as his wheels had to transmit no drive or brake forces. Radial spoking produces a wheel which is strong to radial and axial loadings but one which is unable to transmit much torque. Radial spokes are still used on some bikes today but are really only suited to front wheels where a rim brake is used.
The diagram shows how the spoke tension on a radially spoked still allows movement of the spoke in the direction of an applied hub torque. This is because the torque is acting at 90° to the tension force.
If we are to use hub brakes of any kind and allow drive forces to be transmitted then we require a different spoke pattern. Most bikes will employ a tangential lacing pattern in the spokes. This allows the wheel to transmit braking and drive torque from the hub much more efficiently. In a tangential spoke pattern it can be seen that the torque force transmitted to the spoke is in line with the spoke tension rather than being 90°. This allows the wheel to transmit torque with much less stress on the spokes.
However the lacing pattern shown above (known as spiral lacing) can be further improved. Spiral lacing will put the spokes into tension under one direction of torque and then in compression in the other direction. We already know that the spokes need to be kept in tension to avoid buckling; in fact in a properly built wheel, spokes could theoretically be replaced by pieces of cable and the wheel would still be as strong.
The logical conclusion to all this is a lacing pattern that has become standard for bicycle wheels for centuries. Wheels can have anything from 16 to 48 spokes and the spokes can be laced in a cross 2, 3 or 4 pattern, but all these variations provide a wheel which can resist massive axial loads, modest side loads and torque in either direction.
There are two interesting side benefits of the crossed spoke patterns that are used today.
There is an increase in rigidity when we compare how much hub movement you get for a given increase in spoke length. In the diagram below we can see that for a spoke extension of 1% on a radially spoked wheel, the hub movement would equate to 32mm. But on the tangential spoke pattern, the hub movement is much less for the same spoke extension, specifically 13mm hub movement for the same 1% increase in spoke length. The result is a more rigid wheel.
A second benefit of crossing the spokes is that they are more likely to remain in tension and as already discussed they must be in tension to avoid buckling. The ability for them to remain in tension occurs because where the spokes cross they effectively push on each other. If one spoke should become loose then a tighter spoke resting on it can push on the looser spoke to restore some of its tension. If we build a cross 3 or 4 spoke pattern then this tension sharing benefit is multiplied further.
The wheel on a cargo bike take will take heavier loads than a solo bike and so component selection is important as well as a knowledgeable wheel builder. Tandems bikes which also carry more weight often favour the use of a 48 spoke rear wheel to provide extra strength but this restricts the number of hub and gear options available.
On the front of our box bikes we favour the use of a 20″ wheel which is inherently stronger than a larger rim and we use 36 spokes to allow the use of disc, dynamo and roller brake hubs. The smaller wheel also helps to reduce the overall length of the bike and allows the use of a shorter/stronger fork leg.
At the back we use a 26″ wheel which allows the fitment of a broad ranges of tyres and again we use 36 spokes for maximum choice when it comes to hubs. A smaller wheel might be stronger but the gears would be lower unless a larger chain ring was used on the front and this would have implications for chain guard compatibility and chain ring clearance.
In any case, we feel that a 36 spoke wheel, if properly constructed, will be more than strong enough for a box bike rider. As a general rule, a properly made bicycle wheel will happily take 400 times it’s own weight on a regular basis and will not break until 700 times it’s weight is exceeded.
Bill Mould famously mounted a bicycle wheel on the back of a Toyota Corolla and drove it. You can view his video at https://www.youtube.com/watch?v=sZ7dtrRrSTg
The car is driven at parking lot speeds and of course side loads should be avoided but Bill measured the spoke tension in the lower spokes and proved that this remained positive as the car was lowered. This meant that even when the weight of the car was on the wheel, the lower spokes were in tension and therefore not prone to buckling.
The practical science of wheel building is a topic for another time; there are different spoke thicknesses and materials, rims types and hubs to consider, also different lacing patterns and hub offsets. The wheel must be built, trued and tensioned correctly to perform well and this is why we trust our wheel building to the Chipping Sodbury Cycle Workshop (CSCW). Adam at CSCW knows a thing or two about how to build a wheel.