How 4 axels cause more damage on the road a 3

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

12 October 2008

What causes more damage: a heavily laden lorry with three rear axels or a moderately loaded one with four? This is the debate going on between the Ministry of Transport and trucking companies.

If a lorry weighing 20 tonnes is loaded with 30 tonnes of goods, the total weight is 50 tonnes. But this whole weight does not act on one single spot; it is distributed over all the tyres of the truck.

Suppose the truck comprises of a prime-mover with ten tyres on three axels pulling a trailer with three axels. If the 50 tonnes total weight was evenly distributed, each axel would carry 8.3 tonnes. But of course, the distribution is never uniform; the rear wheels always carry more than the front ones…perhaps over ten tonnes each.

This would be a problem for the transporter because the law allows a maximum of 8 tonnes per axel. But if the trailer had four axels, each would take an average of 7.1 tonnes and that would be within the weight limit.

This is the reason why the transporters have added a fourth axel to their trailers. They can carry a lot more total load without breaking the limit. However, excessive weight per axel is not the only thing that damages a road: traction is even more devastating than vertical load.

Traction is the horizontal force applied on the surface. On a road, it comes into play when a vehicle is starting, stopping and when going round a corner. It pulls the tarmac particles apart and breaks it like grains on a grinding stone.

When a vehicle is going round a bend, the amount of traction depends on the distance the two farthest non-steering axels. To appreciate this, imagine a ten-wheel lorry with the third axel located at the centre of the body; what would happen when it goes round a corner? The central axel would be dragged towards the centre of the curve while the rear one would be pushed outwards.

This always happens on ten-wheelers but since the two rear axels are close together, the amount of pulling and pushing is greatly reduced.

When a fourth axel is added to a trailer, the distance between the rearmost and the front no-steering set of wheels is increased. This increases the traction force by a bigger margin that the reduction in the weight per axel. When that lorry goes round a bend even with half of its load capacity, the wheels will break the tarmac. If you want proof, check any “U” turn section between the Airport and Nyayo Stadium.

And, by the way, they still haven’t discovered a road surface material that can withstand heavy traction – even concrete break easily under traction.