Barry's Tire Tech

This is a series of articles on the technical aspects of tires, their care and usage.

My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day.

..........and since tires is just about the only thing I know about..........

Please drop me a note if you have a topic you want to see:

Barry@BarrysTireTech.com
The Physics (and Math!) behind Tires - Part 2:

L = K * P0.5 * Sd1.39 * (Dr + Sd)

Where:

  • L = Maximum Load on tire
  • K = Coefficient for Service Conditions
  • P = Inflation Pressure
  • Sd = Reduced tire equivalent section width
    • = [0.34848 + 0.6497(A)]S.70
    • which is derived from .96S.70 - 0.637d
    • Where:
      • d = Design section height of round tire
        • (0.96S.70 minus maximum section height of tire having depressed crown
        • = .96S.70 - [1.02(A * S.70)]
          • A = Aspect Ratio = H/S.70
  • Dr = Diameter Code

Whew! That formula is something else!

If you read Part 1, then you've seen the formula to the left.

I hope it is obvious that the larger the tire, the more load the tire can carry.

I hope it is also obvious that the higher the inflation pressure, the higher the load carrying capacity of the tire.

I'm now going to talk about K, the Coefficient for Service Conditions.

The K value varies depending on the service conditions.

Speed:

The higher the speed the lower the K value, so the lower the load carrying capacity.

Below is a table taken from a page of the 2005 Tire and Rim Association Yearbook - the Truck-Bus section.

Yes!! It includes pressure increases as well, but please note the general trend of more speed = lower load carrying capacity! If you do the math so you are looking only at the same pressure, this relationship to speed holds through the range!

Throughout the TRA Yearbook there are similar tables and discussion points.

Side Note: If you do the math, the above table says that a truck tire can carry 170% of the rated load if it is staionary.

What kind of vehicle would you use this information for?

How about a mobile crane - and if I was a mobile crane design engineer, that would be good to know!

BUT!

Certain kinds of tires have fairly well defined operating conditions and those tires don't have provisions for speed adjustment.

Passenger car tires have such an operating condition. There are speed limits throughout the US, so it is common not to reference speed adjustments.

Truck/Bus tires are similar, but if you dig a bit, you can find speed adjustment charts.

Rough Roads:

You have to dig very deep to find it, but the TRA Yearbook also references a load reduction for rough roads. The good news is that this load reduction is offset by the load increase due to the reduced speed. In other words, it's close to a wash!!

The only reason I am including this in this discussion is for completeness. Normally the designation of the tire handles this and the load tables are correspondingly different.

For example, it is common for tires for have the letters NHS after the size to tires designed for those operating conditions and the load tables reflect this difference in the K value.


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