Front wheel swept away
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.
Cycle’s Technical Editor