Frame Geometry 1

Frame geometry parameters (www.bikecad.ca)

When you set out to buy a bike you usually have some idea of the type you want; is it to be a mountain bike, a dropped handlebar racer or a folding bike for a commute. Sometimes this is all you need to know and apply these simple criteria at the shop. It gets trickier when you've been riding for a while and want a bike for a specific purpose, be it racing, randonneuring, cyclocross or extended tours; and this is where the subtleties of frame geometry come in.

The bicycle is an intrinsically simple yet brilliant design made of a frame, fork, two wheels, saddle, handlebars and drivetrain (pedals, cranks, chain and rings). How these parts are put together will affect the utility and handling of the bike, to the extent that getting it wrong will make for an unpleasant ride at best and dangerous at worse. The diagram above, rendered using BikeCad (BikeCad, 2013) shows the basic parameters of bike geometry.

At this point it is worth making a distinction between geometry and fit. Parameters which primarily affect bike geometry are:

• bottom bracket (BB) height;
• head tube (HT) and seat tube (ST) angles;
• fork length (FL) and fork rake (FR);
• trail (T);
• wheel size (W);
• chain stay (CS) length.

The parameters which affect bike fit (though it must be borne in mind that all these things interact) are mainly:

• head tube (HT), top tube (TT), seat tube (ST) and down tube (DT) lengths;
• saddle height (SH);
• stem length (SL).

Bottom bracket height

BB height can vary between 24 to 31 cm, depending on what the bike is to be used for. Cyclocross and mountain bikes will require a high BB to give plenty of clearance, while a touring bike will require a low BB to give a low centre of gravity and a more stable ride. Hence, the starting point of bike geometry is often to choose the BB height, however, it is worth noting that this has consequences for subsequent angles and dimensions when building a lugged frame.

Head tube angle, fork rake and trail

This combination is the most imprtant aspect of geometry:

• HT angle is the angle between the HT and the ground;
• fork rake is the perpendicular distance between the straight upper part of the fork and the fork dropouts;
• trail is the distance between the contact point of the wheel with the ground, directly below the fork dropouts, and the point on the ground projected in a straight line through the axis of the HT.

The HT angle and fork rake affect the steering characteristics of the bike because, between them, they determine the amount of trail. In order to grasp the concept of trail, first consider a bike with a perfectly straight fork in (one which has no rake). If the HT was perpendicular to the ground (a HT angle of 90 degrees) the point of contact of the wheel with the ground would be directly under the steering axis and there would be no trail. However, the HT is usually angled between 72 to 75 degrees so, in the scenario with our straight fork, as the HT angle decreases the contact point of the wheel directly below the dropouts falls behind the steering axis (the contact point projected through the HT) - this is the trail. So, as the HT angle decreases trail increases. The purpose of fork rake is to move the contact point of the wheel below the dropouts back towards the steering axis by curving the forks forward slightly, therby reducing the trail. Racing bikes tend to have steep (larger) HT 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 HT 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. Raking the fork reduces the trail, so the handling is actually influenced by both HT angle and rake.

It should be noted that complications arise when considering tyre size and whether the bike is to be loaded front of back, because this also affects the handling characteristics and the geometry must be adjusted to account for this. A particular case in point is the low trail geometry of French randonneur bikes which have large tyres and front racks and have geometry specifically designed for this to make handling as responsive as possible without being unstable. (Heine et al., 2007)

All that said, a HT angle of 73 degrees is a good compromise for the beginner in framebuilding using commercially available forks, and when there are no complicating factors with regard to large tyres and loaded panniers.

References

BikeCad (2013). Available at www.bikecad.ca [accessed 07/092013].

Heine, J., Vande Kamp, M., and Wetmore, A., (2005). Bicycle Quarterly, 5(3), 42-47.