Geometry, not school geometry, exciting geometry, it has bikes in it.
Frame geometry is a complex and delicate balancing act, adjust one part of a frame and you run the risk of unbalancing another. It is the thing that defines the character and capabilities of a bike, but what are each of those numbers doing, in this article we are looking at head angle and fork trail?
The head angle is the headline geometry number, its the probably the most commonly looked for number after frame size. The head angle affects how a bike will climb, descend and handle in turns.
The rule of thumb is, the slacker the head angle, the more capable a bike is whilst descending. The fork is pointed more directly downhill and the rider is pitched less over the front when the fork is fully compressed, increasing trail composure. Conversely, the rider is moved further back and has to weight the front wheel to stop the bike wandering during climbs.
But the head angle is only part of the story, it works in tandem in fork offset to create whats called trail.
So what is trail and what are the variables that alter it?
Head angle, is the measurement taken from the horizontal to the imaginary line running through the centre of the steering axis.
Fork offset (sometimes referred to as fork rake), is the measurement taken between an imaginary line through the centre of the steering axis and the centre of the hub axis.
Fork Trail, is the measurement taken if we draw a line through the centre of the steering axis to the ground and one from the centre of the hub axis vertically down to the ground. The distance between the points were these meet the ground is called the trail.
If we think of these three elements as a triangle with the trail as the base, altering either the head angle or offset will adjust the width of the base.
- Shortening the offset – longer trail
- Slacker head angle – longer trail
- Lengthening the offset – shorter trail
Steeper head angle – shorter trail
So we know how to change the trail, but how does the trail alter the bikes handling?
Longer trail results in a slower steering bike, eventually becoming almost lazy and needing more input from the rider to initiate turns. Shorter trail will produce a bike that is faster steering with a lighter feeling at the bars but will result in a more twitchy feeling bike.
A bike with a longer trail will be significantly more stable over rough terrain in a straight line compared to one with a shorter trail. This means you will be able to hold speed and confidence and ultimately, go faster.
A bike with a shorter offset will also perform well in turns, typically requiring less mid turn adjustment as the hub is effectively moved rearwards. This results in a slightly shortened wheelbase which creates steering characteristics typically associated with steeper head angles (faster and lighter steering).
A slackened head angle is still arguably the best way of improving downhill performance. It places the steering axis in a straighter line down the hill and when the fork compresses (effectively steepening the head angle) the front wheel is still in front of the rider and not tucking underneath them.
This means the rider does not have to shift there weight so far back on the bike, which in turn ensures that the front wheel stays weighted and maintains traction.
However trail is also affected by wheel size, a larger wheel inherently has greater trail, the axle is higher off the ground so the gap between the steering access and contact patch is larger. To compensate for this, 29ers usually have steeper head angles and or longer fork offsets compared to similar length travel 650B bikes.
Fork manufacturers therefore adjusts the standard offset for each of the three main wheel sizes. However, some manufacturers offer a variety of offsets within a set wheel size, such as the RS Pike which in 650b is available in 47mm and 51mm offsets.
So what does this all mean? Well it means that the head angle is only part of the story and the fork offset has a significant part to play in a bikes handling. This often overlooked figure should be taken into consideration when speccing a custom build or upgrading the forks on an existing bike.