Journey by Bike: Framebuilding 2 (Trail)

A randonneuring bike is quite different to a racing bike. It’s designed for long days in the saddle, possibly over rough terrain, in the dark and in all weathers; so it must perform well in many different circumstances. One of the primary characteristics of a comfortable bike like this is that it must steer well – that is, it goes where you point it without thinking too hard. If you have ever gone cyclo-touring with a rear rack and panniers bulging with everything but the kitchen sink you will know what I mean – the steering becomes extremely light and unpredictable because of the increased weight on the rear wheel. A well designed randonneur bike doesn’t have this problem because the load is put on the front wheel . . . but in order to do this the front end geometry must be designed to prevent over-steering. My research had indicated that the design criteria which are most important in this respect are head tube angle, fork rake and trail. In order to understand these concepts take a look at the schematic diagram of a bike in Figure 1. Head tube angle (α) is the angle between the head tube (line CD) and the ground; fork rake (r) is the distance the fork ends are pushed forwards from the line made by the head tube axis, often produced by a pronounced bend in the fork near the bottom; and trail (t) is the distance between the contact point of the wheel with the ground, directly below the axle at F, and the point on the ground projected by a line through the axis of the head tube. These dimensions are shown in Figure 1.

Fig 1

Figure 1 Seat tube = AB; top tube = BC; down tube = AD; head tube = CD; fork = EF; head tube angle = α; seat tube angle = β; rake = r; trail = t.

The head tube angle and fork rake affect the steering characteristics of the bike because, between them, they determine the amount of trail (Eqn. 1):

Eqn. 1

Eqn 1

In order to grasp the concept of trail, imagine what happens if you vary either the head tube angle or the rake. You can see from Figure 1 that if the rake (r) is increased then point F moves further forwards and the trail (t) decreases. So, the purpose of fork rake is to move the steering axis forwards toward the contact point of the wheel on the ground by curving the forks forward slightly, thereby reducing the trail. Alternatively, if the head tube angle (α) is increased then this will also decrease the trail (t). Racing bikes tend to have steep (larger) head tube angles (and low trail) because this makes the steering sharper and more responsive, but can also make the ride somewhat unforgiving because there is less flex in the more steeply angled fork. Touring bikes tend to have shallower head tube angles and more trail because they are used at lower speeds and the steering does not need to be as responsive, and this results in a less harsh ride. So handling is influenced both by head tube angle and rake. To further complicate matters, steering is also affected by the tyre size and whether the bike is loaded at the front or back, and the geometry must be adjusted to account for this. Traditionally, randonneur bikes had very large tyres and front-loaded racks, so a low trail geometry was specifically designed for this to make handling as responsive as possible without being unstable.

Having satisfied myself that the bike fit was as good as it was going to get, I called upon those long-unused rules of trigonometry – after all, a bicycle frame is just a collection of geometric forms defined by the lengths and angles of the tubes.

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