It's down to timing

In his letter regarding acceleration over road humps to minimise jolting (Letters, 13 November) your correspondent P Field asks if the laws of physics apply. This overcomplication of things is typical of an engineer — I know, being one myself.

The reason the theory works is simple.

When accelerating, the front suspension is extended, so when the front wheel makes contact with the hump there is more suspension travel available to absorb the impact.

If you brake, you pre-compress the suspension units, therefore there is less travel available in the suspension, which bottoms out quicker, sending a thud through the car as it occurs.

The same can be applied by braking sharply before a hump and releasing the brake. The car will rise on the rebound. If you time it right you can travel over certain speed humps without feeling them.

M Leybourne

Darlington



In response to P Field's point, when I studied automotive engineering design at Coventry polytechnic — now a university — we did the force analysis of a suspension system riding over a bump to look at the accelerations and forces of each of the components.

The conclusion was that decelerating while hitting a bump in the road greatly increased the forces throughout the system. This was a complex (and enjoyable) calculation covering more than four pages of A4 paper.

I always remember those findings when approaching speed humps and make sure my foot is off the pedal when the front wheels hit them.

From my own practical experience I also know that pre-1985 Renaults, which have softer suspension than we have today, were always fantastic at speeds in excess of 30mph over bumps — much better than speeds below this.

S Firth

Haslemere, Surrey

Regarding 'Drive your way out of a hump', off the top of my head I would guess that the state of the front suspension system is the key.

Under-braking, spring and strut will be pre-compressed as the nose of the car dips. With no acceleration, the suspension will be roughly where is should be.

Under light acceleration the nose of the car will rise slightly, but the level of acceleration will be such that the speed of impact will be lower, as will impact forces. High acceleration means a higher impact speed which makes the experience worse.

The resulting pitching due to the speed hump (bearing in mind that the suspension will recover as the axle goes over the top of it) is worst when the car brakes before the hump, best when the car is accelerating and middling when between these two extremes.

This assumes that all other things are considered equal.

Glenn Miles

Future Concepts Lockheed Martin UK INSYS,
Ampthill, Bedfordshire