The clearance post showed how bottom bracket drop and crank arm length are directly related to safety on the track. Here, I’ll talk a bit about angles and how they impact ride quality and safety.
Bike shops today are filled with a near limitless selection of bicycles which can make it difficult to choose your new bike. As a result, we often make the decision on the based on colour and style only. Quite likely we’ll be happy with our purchase but maybe it wouldn’t hurt to understand how our bike is going to perform before we buy it.
Seat Tube Angle
The angle of the seat tube with respect to the ground is called the seat tube angle (imagine that). On road bikes, the angle may be as low as 72 degrees while on a triathlon bike it may approach 80 degrees. Although this doesn’t sound like a huge difference, it has a huge impact on both comfort and the muscles that get used when riding. A low seat angle necessitates a longer wheelbase and puts the rider in a more seat-back position (which is good for climbing) while a more upright seat tube allows the rear wheel to move forward somewhat (shortening the wheelbase) and puts the rider over the pedals more (allowing better power transfer when in a time trial position).
Head Tube Angle
Measuring the angle with which the head tube makes with the ground, the head angle of a bike, to a great extent, affects comfort. The steeper head angle requires a fork with less rake to keep the same steering characteristics which results in less compliance in the front end. On the road, a 73 degree head angle may give an acceptable ride over long distances whereas a track bike with a 75 degree head angle will probably make you very sore, very fast if ridden on the road.
Rake and Trail
The amount by which a fork curves away from the straight line through the steerer tube is known as rake. This, in concert with the head angle, determines whether a bike will have twitchy or neutral steering. Both have their place but it’s good to know which one you’re getting before you purchase a bike.
So how do rake and head angle relate to steering? Well, they relate through something called ‘trail.’ Trail is the distance of the tire contact point (with the ground) behind the point at which a line through the centre of the steerer tube would touch the ground. The greater the rake, the smaller the trail (for a given head angle) and vice versa. Increase the head angle and the trail decreases for a given rake. So, what is ideal? It depends what you mean by ideal but, if you want a bike that has neutral steering, then you want to aim for a trail value of about 60mm. If you want things to be a bit twitchy (to get out of trouble on the track) then you might want a trail value of about 55-57mm.
So, in summary, a shallow seat tube is more comfortable (although this may be subjective) then a steeper one. The same applies to head angle (not subjective) however rake and trail also need to considered to understand how a bike is going to steer. Look for a trail close to 60mm if you want bike that has a neutral feel. If, on the other hand, you ride on the track and need to avoid that inevitable crash in front of you, look for trail less than 60mm.
