Bike wheels are so common that few of us ever stop to think how remarkable they are. A pair of wheels that weighs under 1500g can carry a bike and rider that weighs over 100kg and will withstand loads many times greater when, for example, the rider hits a pothole.
The key to the success of the wheel is the tension – stretching force – in the spokes. For millennia all wheels worked by supporting the hub by pressing down on the spoke. Engineer George Cayley invented the much lighter wire-spoked wheel in 1808; its first widespread, practical use was on early Ordinary ('penny-farthing') bicycles of the 1870s.
When you sit on a bike, the tension in the spokes between the hub and rim is reduced. This is equivalent to compressing a wooden spoke, but because a metal spoke is far stronger than a wooden rod, the wheel can be much lighter.
Modern bicycle rims come in two types, named for the tyres that fit them: clincher and tubular. The most common type, clincher rims have a hook shape on the inside of the rim where the tyre sits. Unless you race, you almost certainly have clincher rims (and tyres) on your bike.
Rims for tubular tyres don't have a hook for the tyre. Instead, the tyres are glued into place. The rim and tyre are both lighter as a result, and this weight saving is one reason why tubulars are still popular for racing. It's also possible to ride a well-glued tubular tyre when it's completely flat after puncture, which means a rider can carry on until mechanical assistance is available.
Most rims are made from aluminium alloy, but the pursuit of light weight has brought in advanced carbon fibre composites.
The tensioned wires connecting the hub and rim are usually made from stainless steel. Lightweight wheels may use aluminium or even carbon fibre, but it's interesting to note that when strength and reliability are paramount, in events like the punishing Paris-Roubaix classic, most racing teams use tried-and tested steel-spoked wheels. Spokes may be thicker at the ends than in the middle to save weight and, by distributing the loads round more of the rim, make the wheel stronger.
Spokes are joined to the rim and hub by various methods, but it's usually possible to adjust the tension in them with a special spanner – a spoke key – to keep the wheel aligned. A few exotic all-carbon fibre wheels can't be adjusted because they are moulded in one piece.
Free-turning, durable bearings are vital, and top-quality bike hubs use finely-polished bearings so they turn with a minimum of force. Instead of steel balls, the very best hubs now use ceramic balls which are closer to perfectly round, completely corrosion-resistant and harder.
The hub also includes a freewheel mechanism and splines for the sprockets.
Almost all racing wheels are held in the frame with a quick release skewer. The mechanic can remove the wheel by simply flipping the lever, so if the rider needs a replacement – in the event of a puncture for example – he can be back on the road in seconds.
More than the sum of the parts
The best bicycle wheels are designed so that the hub, rim and spokes work together to achieve the designer's aims. Designers have to trade off weight, durability, aerodynamics and cost. In the last few years wheel manufacturers have emphasised the importance of aerodynamics, and we have seen blade-shaped spokes and deep, v-section rims that undoubtedly make for faster riding. For a technology that's over 200 years old, the spoked wheel is still developing rapidly!