Why Trailers Extra Wheels Don't Touch the Ground
Somewhere toward the centre of a truck or a trailer, there are always a set of wheels that most people tend to believe serve no purpose.
Those extra tyres which are a foot off the ground are part of what is known as a lift axle. They perform part-time roles when required by the driver.
Often, they are operated using a special remote that lowers them to the ground while simultaneously lifting the axle.
Extra tyres on a tractor are called pusher axles, while those on trailers are known as tag axles. Usually, they do not have any drive from the engine.
A trailer moving with a set of wheels raised above the ground.
For a trailer to fix the tyre, it must be inspected and cleared by relevant authorities, including the National Transport and Safety Authority (NTSA).
How They Work
According to car experts, the extra-hoisted wheels were introduced to help spread the weight.
For instance, instead of 50,000 tonnes of goods being ferried in a trailer pressing down on three sets of tyres, the extra tyres are lowered, making the goods press down on four sets of tyres.
Other than spreading the weight, the tyres help contain wear and tear due to the extra weight exerted by the load in the trailer.
“The retractable axle has an automated control that ensures the remaining axle or axles and axle set or axle sets in contact with the ground remain within the mass limits,” driving test expert Darren Cottingham explained.
Countries have different policies and regulations regarding using extra tyres. For instance, in Maryland, US, truck drivers can carry 55,000 pounds if they have three axles and 70,000 pounds if they have three axles plus a lift axle.
However, the extra tyres have some disadvantages. When lowered without a trailer carrying a heavy load, they increase the rolling resistance, which uses extra fuel and wears the tyre out.
Many hanging tyres also affect the general movement of trailers. They make it difficult for trailers to negotiate corners due to the effect of scrubbing sideways.
Trucks along the Northern Corridor highway from Mombasa to Malaba.