May the force be with you

When selecting a new vehicle it is wise to take cognisance of the forces that impede a vehicle’s ability to perform. One also needs to endeavour to minimise the effects of these forces, suggests VIC OLIVER.

The forces of resistance (opposing forces) that act upon a truck are:
Gravity – Gross vehicle or combination mass (GVM/GCM)
Rolling resistance – Type of road surface
Grade resistance – Hills or gradients
Air resistance – Wind pressure

Newton’s third law of motion states that the action of every force is accompanied by an equal reaction in the opposite direction. This means that, if an object is to be moved, a force greater than the forces acting upon the object must be applied to the object in order to obtain movement. The distance moved will depend upon the duration of the greater force.

Therefore the ability of a truck to move a load (its ability to overcome the sum of all the resistances) depends upon the following contributing factors:

• Engine torque and horsepower
• Transmission ratios
• Rear axle ratios
• Tyres and tyre rolling resistance

There are a number of factors that can impede a truck’s ability to perform:

The first restraint that impedes the movement of a truck is the force of gravity, which, related to a truck, is the GVM or the GCM of the vehicle. All operators wish to maximise payload – a low tare mass (the empty mass of the vehicle before the load is added) allows them to do so.

Most vehicles available on the South African market offer a competitive tare mass, and there is not much that can be done to lower the basic chassis tare mass. However, a lot can be done to ensure that the truck body that is fitted to the chassis and/or trailers that are pulled behind the vehicle are well designed and not too heavy so as to increase laden mass.

Traditionally semi- and full-trailers manufactured in South Africa have been over designed, to accommodate operators who in the past overloaded their vehicles. Carefully check the tare mass of any new trailer that you intend to buy, and make sure that the tare mass is as low as possible and is competitive with other modern trailers that are available on the market.


The second restraint that impedes your truck’s ability to perform is rolling resistance. The road surface is the one factor that robs the power of the vehicle. The smoother the surface, the less power required to move the vehicle forward. Using radial tyres and ensuring optimum and correct pressures are used will significantly reduce rolling resistance.

The third resistance factor is gradients on the route. When planning a trip route try to use the route with the least gradients.

The fourth factor is wind resistance that, as the most significant factor, robs the vehicle of its power and ability to perform whilst increasing fuel consumption. There are a number of elements that a potential buyer of a new truck needs to consider to ensure the lowest possible drag is created by the front of the vehicle and which can dramatically affect wind resistance.

The dimensions of the truck’s front profile should be carefully selected. The height and the width should, wherever possible, be of the smallest dimensions given the constraints of the application.

Curtains and tarpaulins fitted to truck bodies and trailers should always be kept as tight as possible to reduce drag. The load profile should also be kept as low as possible and should be well secured. End tipping trailers should be fitted with tarpaulins to stop turbulence inside the trailer and stop the tailgate acting as an air brake.

Understanding the forces that impede your fleet’s performance will help you to select the right vehicle for the application. This will also enable you to buy a vehicle that offers high productivity through increased efficiency.


One of this country’s most respected commercial vehicle industry authorities, VIC OLIVER has been in this industry for 45 years. Before joining the FOCUS team, he spent 15 years with Nissan Diesel, 11 years with Busaf and seven years with International.

Published by

Focus on Transport

FOCUS on Transport and Logistics is the oldest and most respected transport and logistics publication in southern Africa.
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