Front wheel swept away

Can you explain this particular type of crash? I was going slightly downhill on a tarmac cycle track. Ahead was a closed steel gate, but to the right, just off-piste on the grass, was an open cattle gate. I went to turn across the kerb onto the grass for the open gate, but I didn’t turn: the front wheel touched the kerb, then ran down that straight into the steel gate. BANG!
How does it happen? The nearest analogy I can think of is trying to drill into a steel plate at an angle. The drill skids off. But what are the physics?
It’s clearly best to get the front wheel to cross rails, pipes, roots and kerbs as near to a right angle as possible. Another tactic I have used in the forest is to throw my weight right back. But on occasions, I have jumped the first root and then landed on a second that led me at enhanced speed into a tree trunk.

Martyn Dyer-Smith

The maths needed to describe the physics of such crashes is, er, challenging. A simple explanation is that a single-track two-wheeled vehicle will begin to fall to one side as it rolls, and will topple unless corrected. It is kept upright by steering into the direction of lean; this shifts the tyre contact patch sideways under the mass of the rider and bike, making them then lean to the opposite side, which is corrected in the same way, etc. This only works provided the front (steering) tyre has enough grip on the ground to shift its contact patch in the required direction.

If grip is insufficient, or if there is an obstacle such as a kerb preventing the front wheel from moving in the direction of steer, the contact patch won’t travel sideways
to correct the initial lean and the bike and rider will fall to that side. The obstacle doesn’t need to be high; I once fell off when my front wheel refused to climb over a wet, slippery bead of bitumen road sealant raised about 5mm proud of the road surface.

If you hit a log, kerb or similar at an acute angle, the effect is to push the front wheel to the side, creating a lean that the wheel can’t correct. If you hit the obstacle at around 90°, there’s little or no sideways deflection. Lifting the front wheel over an obstacle removes the immediate need to use its surface to gain steering and balance traction but merely postpones the moment grip is required until the tyre touches down; if it lands on something slippery, the result is usually a fall.

Richard Hallett

​​​Cycle’s Technical Editor

This Q&A was published in 'Cycle' the magazine for members of Cycling UK. To contact the experts, email your technical, health, legal or policy questions to or write to Cycle Q&A, PO Box 313, Scarborough, YO12 6WZ