MadSci Network: Engineering

Re: Why do heavy vehicles like trucks and buses have four wheels at the rear?

Date: Fri Apr 19 00:36:22 2002
Posted By: Aurelio Ramos, Grad student, Computer Engineering, Not a member of any institution
Area of science: Engineering
ID: 1018260998.Eg

Short answer: Because a tire can only carry so much weight on it, and by 
using more tires, the load is distributed.

Long answer:
Trucks and buses have most of their weight towards the rear (in buses, 
most of the weight is because of the engine.) Keep in mind that those 
vehicles are designed to have their weight biased towards the rear axle 
(or axles). This is done for many reasons, particularly because the rear 
axles have the driving tires, and those tires need a good amount of 
weight above them so that they don't just spin. Another reason for the 
rear weight bias is that steering becomes lighter, and the amount of 
power steering required is reduced.

The weight of a bus or a truck is so much that more than two axles are 
needed, regardless of what tire configuration is used, and it would not 
be practical to put two front axles (because of steering). So rear 
placement is preferred.

I hope that answers your question,

Your mad scientist,

Aurelio R. Ramos

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