WHEEL TRAJECTORIES (wheel centre and contact patch centre)

When a wheel hits a bump, forces are transmitted to the rider through the suspension. The direction in which the wheel moves in response to hitting the bump (along with the stiffness and damping of the suspension) determines the magnitude of the force transmitted. If the wheel centre moves in an upwards and rearwards direction, away from the direction of the applied force, the shock transmitted to the rider is reduced. In the clip (and subsequent animation clips), the trajectory of the wheel centre is illustrated by the bar along which the wheel is constrained to slide.

Conversely, the shock is increased if the motion is upwards and forwards.

The conventional front fork gives an upwards and rearwards trajectory of the wheel centre – which is a good comfort feature.

A trailing arm type of rear suspension with a high pivot point also has this advantage.

Having a favourable front wheel trajectory (upwards / rearwards) helps not only to reduce shock loading on the rider but also helps to protect the tyres and rims from damage. It also helps to keep the front tyres in contact with the ground over bumpy terrain to the benefit of steering control.

In a similar way, the direction in which the tyre contact patch moves when the brakes are applied influences the degree to which the bicycle pitches in response to the application of the brakes. If the contact patch moves forwards as the suspension is compressed, the bike pitches less than it would if the contact patch moves rearwards. This property of the suspension is called anti-dive...and it can be positive (i.e. it helps to prevent pitch)...or negative (promoting pitch). Here the slider bar illustrates the tyre contact patch moves when the suspension is compressed with the brakes applied).

A conventional fork has negative anti-dive, which makes pitching due to braking worse...and so the suspension has to be stiffer to compensate.

At the rear, the opposite is true. So a trailing arm with a high pivot point helps to prevent pitching when the brakes are applied.

So for a front suspension, what tends to be good for comfort is bad for preventing brake dive, but at the rear, the requirements of comfort and preventing pitching due to braking are complimentary.