Why do transport trucks sometimes have many spare tires hanging from their undercarriage, almost as many tires as those being used? Surely they don’t expect almost all tired being used to break down at once?

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Why do transport trucks sometimes have many spare tires hanging from their undercarriage, almost as many tires as those being used? Surely they don’t expect almost all tired being used to break down at once?

In: Engineering

Yes. It can be that the truck is carrying cargo which must reach a destination within a certain amount of time and therefore can’t afford to lose a few hours.

Imagine you are going to a party, do you bring 4 napkins because there are only 4 people attending? Or do you bring a whole pack incase something happens?

I believe what you are talking about are tires that can be lowered to the ground and used as needed. Typically they are there to be used to distribute the payload across more tires. Let’s say a lighter load can use 6 tires and a heavier load will require 10 tires due to the extra weight.

Every tire a vehicle has on the road decreases its fuel efficiency and increases vehicle wear. Trucks can have an extra axle which is only lowered to the ground when the cargo is load is enough to exceed the rating of the other axles already on the ground. This axle is always raised unless needed.

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You might be thinking of lift axles – axles that can be raised or lowered depending on the situation.

They affect turning radius, traction, weight capacity and distribution, and to some degree fuel efficiency and of course the tire wear depending on whether they’re raised up or lowered so that the tires contact the road.

More than 1 spare is very understandable. 6 spares? Probably overkill.

For you, having a blowout means an inconvenience while waiting for AAA.

For them, a blowout means less time to get to destination. 2 blowouts would maybe put the cargo at risk. If they don’t deliver to destination within the time window, they could stand to lose many thousands of dollars.