Tires: Tire Size, passenger car tire, inflation pressure

Question
QUESTION: Why do trucks have larger tire size than cars?
what about a tractor?
and could you describe how the size of the tire is determined

ANSWER: M Sharifirad,

The load carrying capacity of a tire is determined by 4 factors:

1)  The volume of air contained within the tire (Its size)
2)  The inflation pressure
3)  The intended maximum speed (slower allowing more load)
4)  The surface the tire is intended to roll on.  (the rougher the lower the load carrying capacity)

With these factors, tire standardizing organizations set the load vs inflation pressure relationship for a given type of tire.  These are in the form of a table and the contain all the approved tire sizes for that type of tire.

Needless to say, different types of tires had different needs.  For example, tires for commercial aircraft need to be light weight and small compared to load they need to carry.  Plus aircraft tire only need to operate for a short time and distance on fairly smooth surfaces.  So aircraft tires are highly inflated (some over 300 psi), but allowed to use a lot of load compared to a comparable passenger car tire.

Tractors, on the other hand, operate on soft surfaces (so low pressure helps with traction), for hours on end, but at slow speeds.

The tire standardizing organizations take all of this into account when they set the load table.  These load tables are based on a theoretical study done decades ago (before the use of computers), but have many decades of experience behind them now and they basically work!

Here's a bit more on the subject:

http://www.geocities.com/barrystiretech/tirestandardizingorgs.html



---------- FOLLOW-UP ----------

QUESTION: could you describe more about the first factor and also Is the inflation pressure dependent to tire size?
Is there any thumb rule between vehicle maximum weight(with passengers or load) and tire size?

Answer
M Sharifirad,

I'm not sure what you mean by "describe more about the first factor".  If you looked at the web site, there was a page on tire size and that described the diameter and the section width for a given tire size and for practical purposes that is enough information to reasonably guess at the contained volume within the tire.

Inflation pressure is not dependent on tire size.  

And your question about rule of thumb?  There is a reserve load capacity that is a trade secret for each vehicle manufacturer.  I can make educated guesses based on what the vehicle manufacturer publishes, but what I don't know for sure is the actual weight of the loaded vehicle.