Sometimes trucks will have an axle that they can lift up - a retractable axle. If it’s on the tractor unit it’s often called a pusher axle while if it’s on a trailer it’s often called a tag axle. Retractable axles are sometimes called dead axles, lift axles, cheater axles or drop axles. These are freely rotating - they don’t have any drive from the engine.
Why Dual Tyres On The Rear Axle Of Trucks & Buses : • ट्रक और बस में पीछे दो...
A retractable axle or lift axle is a way in which the axle can be raised so that its tyres aren’t touching the road surface. This improves fuel economy, and reduces maintenance and tire wear, although it increases road wear.
The advantage of a lift axle is that it provides more scope for carrying heavier loads without having negative impacts when the truck and/or trailer are empty.
Every tyre has a rolling resistance - it’s the force required to make it roll on the road. Burning fuel is the way this is overcome. The rolling resistance heats the tyre up and that heat plus friction from the road surface is what degrades the tyre.
On a tri or quad trailer (i.e. three or four axles) the rearmost axle scrubs sideways around corners. Trailers may have a rearmost axle which turns to reduce this, but lifting is a better option to reduce the scrubbing which wears the tyre out more quickly.
Trucks have multiple axles with two or four tyres per axle to spread the weight over the road surface. There’s a maximum weight per type of axle, therefore to carry heavy weights, trucks need multiple axles. But, when the truck or trailer is empty, there’s no reason to have those axles rolling when the weight is not there to justify it.
While this does create slightly more damage to the road surface than spreading the weight over a large number of axles, road authorities allow for retractable axles under certain conditions.
Trucks carrying a heavy load must spread out that load over the road surface to stay within the safe operating limits of the tyres and to reduce the amount of damage caused to the road surface.
However, any tyres that are touching the road create rolling resistance which uses extra fuel, and it wears the tyre out. The three rear tires on a tri-axle semitrailer create about 57% of the rolling resistance. As rolling resistances contribute about 15% of the total fuel costs (depending on the type of driving), lifting one of those axles could save 2-3% in fuel costs. Therefore when carrying a light load, it makes sense to try to reduce the number of axles with tyres touching the road surface. You can often see this with empty logging trailers stacked on top of one another.
#TruckDropAxle #LiftAxle #TagAxle
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