A brief overview of bicycle geometry, the complex science underlying the design of frames and forks, which affects every aspect of a bike's ride.
What do we mean by bicycle geometry? It’s a complicated subject but, putting it as simply as possible, geometry is about the dimensions and angles of the frame and forks, which are at the heart of every bike. I certainly hope reading about it will help, and that the diagrams accompanying this article are also illuminating, but the best way to get a real feel for the difference that geometry makes is to ride a number of different bikes. The differences in the way they ride can be startling. We’re talking about three main aspects here:
- efficiency of pedalling;
- handling, particularly steering;
- comfort
Geometry isn’t the only thing influencing how a bike rides. Changing the tyres, or even just the tyre pressures, can make a big difference. But geometry sets certain limits. Superlight wheels with tyres pumped to 100 psi wouldn’t make a shopping bike into a race machine. Ride quality is also affected by the materials from which the frame and forks are made.
Let’s look at the anatomy of a frame and forks, and how these relate to geometry and the way a bike will ride. It will help to look at Diagram 1 here. You might also like to refer to one of the diagrams from an earlier article, which name the essential component parts (these names are italicised here).
One fundamental measurement is wheelbase. This can be measured between either the wheel centres (the hubs) or the contact points on the ground; these distances must be equal (assuming the wheels are round!). It’s A-C on the diagram. A longer wheelbase tends to make a bike more stable in a straight line but less nimble in turns, and may feel more comfortable, or at least softer, to ride (partly because longer frame tubes will tend to have more flex than shorter ones).
Two other crucial factors are seat angle and head angle. These are measured against a horizontal axis, so they’re not affected if a bike has a sloping top tube. Head angle (also called steering axis angle) is particularly important for steering and handling, while seat angle is important in relation to a rider’s position and balance on the bike.
Small variations in these angles - a degree or less - make a big difference to the way a bike rides. On most road racing bikes both angles are usually around 73º; time trial and track bikes may have steeper angles, but it’s rare to find anything exceeding 76º.
Mountain bikes will have slightly shallower angles; a head angle around 67º is typical on downhill bikes, which need more stability, while cross-country bikes, especially those for racing, can be a little steeper.
A final item to consider, in this brief tour of bicycle geometry, is trail - C-D on the diagram. Increasing trail makes a bike more resistant to turning. To achieve the right amount of trail, most forks have some amount of offset (crudely indicated by the kink in the forks in the diagram).
Some of the theoretical physics behind bicycle geometry is mysterious even now, and much of what is known is derived from experiment and experience. It’s impossible to cover the field adequately in a brief article like this, but at least it may give some insight into what people mean when they refer to geometry, for instance in detailed bicycle reviews. As a shorthand, some manufacturers’ catalogues (Specialized, for example) refer to nimble, neutral, or stable geometry.
Of course it all gets even more complicated when you start looking at mountain bikes with front or full suspension: as the suspension compresses, the geometry can change. At least it helps to explain why there are so many different designs out there. It also suggests that buying a mountain bike without a decent test ride is a chancy thing to do.
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