Cyclist
What is a bike’s head angle?
What is the head angle of a bike? It’s not uncommon to hear terms such as ‘slack’ or ‘aggressive’ to describe a bike’s geometry and its subsequent handling traits. Almost always what that refers to is the frame’s head tube angle.
In very basic terms, a steeper head tube is thought to make a bike’s handling more direct and responsive, while a shallower head tube angle makes a bike more stable and predictable in its steering.
That said, a quick look at some popular road bikes’ geometry charts reveals head angles do not vary greatly, usually falling within the range between 72.5° (slack) to 74° (aggressive), while gravel bikes are often a degree or two slacker.
Most people wouldn’t be able to see the difference, so can a single degree of head tube angle really make such a difference to how a bike rides?
How is head angle measured?
Head angle is measured from horizontal to a virtual line running down the centre of the fork steerer tube. This means that if your head angle was 90°, your fork would point straight downwards (and your bike would be very twitchy indeed). Obviously, no bike is like this. In fact, even the most extreme head angles fall within a range of just a few degrees.
‘Even half a degree is meaningful, absolutely,’ says Tom Sturdy, former head of education at the Bicycle Academy.
‘A very small change in angular dimension at the head tube makes a big change when you project that all the way to the ground.
‘It’s complicated, though, because the head tube angle as an isolated dimension doesn’t give the whole answer.
‘It’s just one component in what governs trail, and it’s that which makes all the difference to what you feel when you’re riding. The other key component is fork offset.’
Rake, trail and fork offset explained
If you shone a laser beam of light down through a bike’s head tube and another beam vertically downwards from the centre of the front wheel, the horizontal distance between the two spots of light on the ground would be the trail.
‘Trail provides rotational stability to the steering axis, that is to say it produces a force that, together with gyroscopic effect, means the wheel will have a tendency to remain straight and stable,’ says Sturdy.
‘Trail is why shopping trolley wheels will always spin round to point in the stable direction.
‘Assuming the fork offset remains unchanged, slackening the head angle [tilting it further away from the vertical] increases trail,’ Sturdy adds.
‘More trail means the force that keeps the wheel trailing behind its steering axis will be stronger, so the likely resulting sensation is that the bike will feel more stable – you would feel safer taking your hands off the bars.’
That is perhaps why, according to Sturdy, the majority of riders tend to feel more confident on a bike with more trail. It will feel less twitchy as essentially the front wheel is holding itself straight, rather than the rider doing all the work.
Why then would we not want as much trail as possible?
‘Some riders, myself included, prefer a bike with less trail because it means they take more control of the steering and it’s easier to change a bike’s line,’ Sturdy says.
‘But in this scenario, you need to keep your concentration because the bike will need to be constantly managed.
‘Plus, there is a counterargument that suggests when we ride a bike we’re continually making a series of small adjustments and if the bike resists that it can actually feel less stable.
‘Wheel flop is also a consideration with slacker head angles,’ he adds. ‘The slacker the head angle the more the wheel will want to flop from one side to the other when the bike is leaned. This is often very noticeable, and potentially disconcerting, at slow speeds.’
It gets more complicated…
As with almost everything relating to how a bike feels to ride, even this thorough explanation by Sturdy is not without its caveats.
‘The rider is very much part of the vehicle,’ he says. ‘Body position and weight distribution relative to the front wheel have a big effect on how a bike handles because when a rider drops their weight forward, such as when getting into a tucked position on the drops, it exaggerates the effect of trail.
‘Things such as handlebar width also matter, as that changes how much leverage the rider has to steer with. It’s the reason why mountain bikes need wide bars to aid steering as they have a much larger amount of trail.
‘At the other end of the spectrum, most TT bikes also have a relatively long trail, typically because of slightly slacker head angles.
‘On paper that should be more stable, but I don’t know anyone who has ridden a TT bike and found that to be the case, because of the position of your hands relative to the steering axis.
‘Track bikes also feel very weird as they have lots of trail but also very narrow bars.’
So, a bigger (steeper) head tube angle may suggest a racier bike, a smaller one may suggest a cruising bike but, as with everything to do with geometry, nothing is as simple as it seems.
Understand how a couple of degrees on the headtube can alter how your bike feels on the road? Why not learn about the effect of stack and reach in the next of our series on bike fit variables?
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