On tractor trailers the axles are high enough where chains will not drag on the road. You may not see them on the axle, hanging on the axle is just a way to store them while not being used. If they need to be used then you already have the inside chains connect and all you have to do is pull the cain taut on the tire outside. It is not that complicated if you use chains routinely. They also hang them on emergency vehicles,like fire engines, again for storage and it is a time saving routine when they have to respond to a call and need to have chains.
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Jim
Sometime life gets in the way of living.......
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