Why do some trucks have a spare set of wheels hanging? Wouldn’t it better to just have all the wheels on the road?
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Less wear on the tyres and bearings on the raised wheels. They are only needed to bear extra weight so when it is light they raise them for less wear and tear. It might not sound like much but they can do half the distance of the other wheels in some circumstances and if they are driving thousands of kilometres it makes a big difference.
Having the extra set of wheels down adds transmission drag (and thus reduces fuel economy) and also causes more damage to the tyres when turning, so unless you absolutely need them to support the weight of the load, it’s better to have one out of the way.
Those wheels and axles are needed to carry a heavy load. When not carrying a heavy load, they are not needed. If you left them down, those tyres would get extra wear and tear. In some circumstances, it also allows the trailer to be turned around tighter corners.
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It’s called a drop axle. They are used in various applications. Often seen on vocational dump trucks. When a truck is unloaded it doesn’t need the support, so it’s lifted to maintain fuel economy and total cost of ownership (tire wear, rolling resistance, transmission and axle losses). Thre are also more common in the US in Northern states where you have lots of “Bridge Laws” that regulate the load per an axle.
Those wheels are there to take load from the other axels when heavy loads are being moved. Because the other axel simply can’t handle the load without breaking. Just as if you need to get on weak ice, dont walk because you will go through, but go on your stomach and roll. Just like you are told to do if you get out of falling through ice. Pressure is force on surface area; increase surface area and pressure goes down. In engineering we think all forces on material as pressure, since that is true in a way that makes sense once you accept it.
On the downside they add extra friction. This neand you need more power to accelerate and sustain speed which in turn means burning more fuel.
Ideally with any vehicle you want the least amount of wheel touching the road, while having best efficiency and most control. This is one reason why trains are efficient, you don’t need much control while on a track, just to accelerate and retard the train. Between the track and wheel there is very little surface area causing friction.
Logistic compete with fuel efficiency anf efficient use of machinery. Drivers and trucks cost about the same locally and customers want the margins, so you want to get yours and that is with fuel efficiency.
If you want to make lot of money, develop a method to increase logistics efficiency, whether route planning, engine, truck, motor, kind of fuel. Even achieving 1% will bet you a fortune when you sell it. A 3% fuel saving is enough to justify replacing whole fleets of machines, even planes and ships.
Another reason is that it improvs turning circle. For instance timber trucks with trailers can then turn unloaded on a small patch on a forest road. After turning, they lower the wheels and start loading. The wheels needs to be down to not exceed max axle loads.
Watch the last axle of the trailer
I’m not sure if they automatically lower themselves above a certain weight now, but in the past truckers used to keep them up when they should have been lowered, to save on tyre wear and fuel.
Problem with this is, having too much concentrated weight increases wear and tear on the roads.
Serious question, I have never seen tire (US English) spelled tyre; is this a regional thing (UK English?) or perhaps specific when referring to tractor tailors? Five different posters used the spelling tyre, so I’m assuming it’s not just an auto-correct thing.
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