Keeping Your

End Up

The Science of Hydroplaning

Hugh Hunt explained how Sir Isaac’s First and

Second Laws of Motion ruled the roost when

it came to hydroplaning.

EWTON’S   First Law, remember, states

that an object, whether it has parked

itself in one place or is on the move, will

carry on doing what it’s doing unless an outside

force causes the situation to change.

N

In the lake, the water, having gathered and been

there for a while, will want to stay exactly where

it is. The motorbike or, more specifically, the

tyres on the motorbike (because they are first

to come into contact with the water) will be

moving forward at, say, 40 mph and wanting to

carry on doing that. They can’t both have it their

own way. In fact, neither of them can.

Just as we had to consider forward thrust as

a force when I was providing the thrust by

pedalling Britain’s Fastest Bike, and also when

I was pedalling to turn the propeller in the

SUHPA, we have to think about the forces

that the motorbike represents. It has forward

thrust provided by the engine powering its rear

wheel and also the downward force of gravity.

On dry land the downward force that gives the

motorbike its weight helps its tyres to grip the

ground, giving it traction. On water the same

forces are at work. Rather than thinking about

the two forces working at right angles to each

other – thrust straight ahead and weight straight

down – on water it is easy to see how the two

forces work at an angle, slanting forwards and

downwards. But the water will resist that force,

146  HYDROPLANING MOTORBiKe

in accordance with Newton’s Third Law: for

every action there is an equal and opposite

reaction. The water will push back on the tyre.

When this happens to a vehicle on the open

road it causes a huge problem. The grooves in

road tyres are designed to squeeze water out

from beneath the tyre, letting the tyre stay in

contact with the road. When the car, truck or

motorbike hits the water at speed, however,

the water pushing back on the wheel creates a

wedge-like bow wave of pressure in front of the

tyre that pushes it up off the road. The vehicle

is then skimming along on what may only be

a few millimetres of water but is enough to

prevent it getting any purchase on the road

surface. With no traction, the driver has no

control and the vehicle will go wherever the

forces at work propel it.

On the road, the vehicle will carry on

hydroplaning until it passes over the puddle

and the tyres hit the tarmac again, or until

another of our old adversaries from the

previous challenges comes into play – drag.

Drag in the case of a vehicle hydroplaning will

be from air resistance and water resistance.

If the vehicle’s tyres can no longer drive it
forward, drag will ultimately bring it to a halt