Head angle, fork rake and trail
If there is a close relationship between the headtube angle and seat-tube angle, then the link between head angle, and the rake and trail of the fork is inextricable; indeed adjusting the headtube angle will have the same effect as adjusting the fork rake: a favoured solution for designers of mountain bikes.
A classic headtube angle for a racing bike is 73 degrees. A steeper angle (higher number) is likely to make the handling of the bike more responsive; a slacker angle (lower number) to offer greater stability.
It is unhelpful, however, to consider the angle of the headtube in isolation. For Steward, “we can’t talk about head angle without taking into account fork rake and trail.”
“There’s not enough importance placed on trail,” he continues, referring to the distance between the centre of the contact patch of the tyre and the steering axis of the headtube. “Most people will look at the fork rake and either presume it’s going to be a fast handling, or slow handling bike without taking into account the relationship with head angle and the resulting trail it will give and therefore the feel of the bike.”
Again, we are talking about fine margins. Gribaudo says that the difference in rake between Lapierre’s race-ready Xelius EFI and ‘performance’-oriented Sensium is just 4mm – 43mm and 47mm respectively. He talks about finding the best ratio for delivering handling and comfort, and says a longer rake will offer greater comfort by way of greater shock absorption.
Fork rake holds a direct relationship to trail and head angle, Olsen explains. “56mm to 58mm trail is often quoted as your ideal neutral and that’s generally what you would get from 73-degree head angle and 43mm fork rake.”
“I won’t say that’s the perfect geometry,” he continues, “but it’s probably the most common that people design to. If you want to make a standard racing bike that you won’t have any problems with, you go for a 73-degree head angle and 43mm fork rake and it will work fine. But a little more trail can help if you want a bike that’s a bit more stable, a bit less fidgety. ”
For Steward, there is no written rule governing the selection of headtube angle, fork rake and trail. “A lot of traditional frame builders will say a 73-degree head angle with a 45mm rake fork is the sweet spot and everyone has their own thoughts on it,” he says.
The bike’s intended purpose will also have a bearing on the designer’s choice of angles. Steward continues: “For the touring bike that we’re working on, you’d ideally pitch for a low trail figure compared with a conventional road bike. You need to carry front loads and that’s where the trail figure comes into play – so you don’t get too much shimmy, and so the bike doesn’t feel too slow when loaded.”