Head Tube Angle
What is Head Tube Angle?
The angle of the head tube (also called steering axis) relative to the horizontal plane.
Lower numbers are often described as being more "slack" (shallow) and higher numbers are described as being more "steep" (approaching perpendicular to the ground).
Why does it matter?
Head tube angle plays a significant role in how a bike handles. In general a steep head tube angle results in quicker-feeling steering while a shallow angle results in more stable straight-line tracking. However, this is a generalization and the Fork Offset, Wheel Diameter, and Tire Width are all factors at play
Traditionally, performance road bikes have had steeper head tubes while recreational and utility road bikes have more moderate angles. Mountain bikes are commonly more slack than road bikes and have a greater range of angles depending on the use case.
In an ideal world, a bike’s desired handling characteristics would be the only factor considered when specifying a head tube angle. However the tendency for toe overlap (the tendency for the rider's toes to catch on the trailing edge of the front wheel when wide turns are taken) in smaller frame geometries sometimes results in a design compromise where the head tube angle is reduced to place the front tire farther from the rider’s feet. This means that handling can be compromised for smaller sized bikes compared to larger sized bikes of the same model.