Why do some trailers on lorries not use all their wheels? Like some are raised up, why not use them all?
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The wheels can be lowered if needed to support the load. If the load is light enough, youll save fuel by lifting the wheels
They are usually for heavier loads on roads that have lower axle weight limits. There’s no point in using it unless you need to as that’s unnecessary tire wear, and truck tires are *expensive*.
Wheels rolling on the ground will wear them out. However, more wheels touching the ground spreads the weight of the load so each wheel carries less.
For light loads, only a small number of wheels will touch the ground. More weight pushes the other wheels down to help spread the load over multiple wheels.
Its all about weight distribution. The heavier the load, the more wheels on the ground are needed to distribute the weight evenly among the tires. When the load it light, the unneeded wheels are lifted to increase fuel economy. As the loads get heavier and and exceed the load rating on the tires, more wheels are lowered to accommodate the load correctly.
They’re available for heavy loads. If the load isn’t heavy enough to require them, they’re pulled up to save on costs.
You have multiple axes on trucks so you can support more weight so they are down when required.
The drawback with more wheels is that the rolling friction increase so lifting them up save on fuel.
If you look at their behavior when you take a turn it will be around the center point between a par so if they do not turn there will be some scrubbing against the ground. So the tire wear will be higher with an extra axis on the ground.
The result is that the cost for the operator is lower if they lift up an axis when it is not needed.
Weight also effects braking on commercial trucks. A fully loaded truck will actually stop sooner than a empty truck traveling at the same speed. Keeping those wheels raised will keep the appropriate weight on the other axels.
Most people have already given the answer (efficiency… which is just a fancy way of saying money), but I’ll just add that weight limits on big trucks are more a result of regulation rather than physical design capability. The more weight you put on one wheel, the more stress you put on the road surface and the quicker the road breaks up, or rather, the quicker the taxpayer needs to pay to fix that road. Therefor the government regulates a maximum amount of weight that can be carried per axle. If you have more axles, you’re allowed to carry more weight. But more axles mean more expense so a sometimes axle can give a little more flexibility.
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