Performance Factors
Though fit is related only to saddle height, angle and reach there is more to a bike than its size and its frame angle. There are several further elements of the shape of the bike (as opposed to the materials) which affect its handling and performance.
Head angle
The angle between the steerer tube and the horizontal. For almost all bikes this is between 72 and 74 degrees and is normally the same as the frame angle. A steeper angle results in more responsive but less stable steering. i.e. for straight line riding a shallow angle is better, for very technical riding a steeper one. Think of a chopper motorbike with the front wheel out in front for a shallow angle - turning the bars hardly changes the position of the wheel on the road at all. For a steep angle think of a unicycle!
Rake
Rake is the perpendicular distance of the front hub axle ahead of the steering axis (best illustrated with a diagram). It provides cushioning from road vibration and is normally achieved by curving the fork blades. More rake leads to more cushioning. Smoother roads mean that modern bikes have less raked forks than older ones.
Trail
This is a factor of head angle and rake which determines how much the bike will tend to steer itself. A small amount of trail (4-6cm) is essential for stability With no trail, or negative trail, it would be impossible to take your hands off the bars. With too much trail steering becomes hard work.
Chainstay length
The chainstays are the tubes that run from the bottom bracket to the middle of the back wheel. If they are ‘short’ there is only just room to fit the wheel in. Short chainstays lead to a stiffer and more responsive frame. Because more weight is over the back wheel traction is also improved. Longer chainstays give a more comfortable ride and allow for larger tyres and mudguards. Supershort chainstays are a nuisance - they can make the wheel difficult to get in and out and limit tyre choice. Some frames are built with curved seat tubes to allow even shorter chainstays.
Front Centre
This is the distance between the bottom bracket and the centre of the front wheel. It’s dictated by the length of top the top tube, the head angle and the rake. The only thing that it really affects is toe clearance. Toe clearance, whether your feet can touch the front wheel when they are on the pedals, is only an issue at very low speeds.
For the majority of cyclists these issues are relatively unimportant. Most frames are built to a fairly standard pattern such that the handling remains neutral and predictable. Problems start to arise for small frames (under 52cm) and large frames (over 60cm). This is primarily becase of the limited variation in wheel and crank size.
Small frames
Frames smaller than 52cm should really use 650c wheels instead of the typical 700c. With 700c wheels the seat angle is forced to be steep and the head angle to be shallow. If manufacturers use the same tubes throughout their range the small bikes tend to be very stiff and give a very harsh ride - not ideal for small, light riders.
Large frames
Cranks longer than 175mm tend to be difficult and expensive to obtain. Large frames tend to have shallow seat angles to compensate for proportionally shorter cranks. If manufacturers use the same tubes throughout their range the large bikes tend to be too flexible - especially for large and powerful riders.
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