"Tundra Tires"?

[Paul Z]

There are several things that play into the tire diameter.
First, 21" is the tire diameter, not the "Tire Height".

I would bet if you went and measured the molds the tires are made in at room temperature they are 21". When they form the tire the mold is hot, the bias ply belts are hot, and the rubber is hot. As they cooled down there is shrinkage. I've never worked with the molding of rubber, but I have with both plastics and aluminum. The are all effected by the coefficient of thermal expansion (CTE) as thing heat up they expand, and as they cool down they contract.


Also if you measured the diameter of the tired after installing it on the rim, air them up when they are mounted to the wheels, and sitting on the flat on the floor and measure the diameter of the tire, without any weight on the tires. I would bet the tires would measure about 20 3/4" up to 20 7/8".

With the 300+ lbs on each of the tires the weight of the aircraft the aircraft it flattens the tire. Mine measure 20" with the planes weight on the tires, with 15 PSI air pressure.

With less pressure the tires get flatter, if you measure the tire with 15 PSI from the floor to the top of the tire you will get a different measurement than if you put in 18 PSI.

I would venture to say the tire diameter is measured in the Free State, with no weight on the tire, laying on its side, and aired up to a engineering specific PSI Air Pressure. I have no clue what pressure they may have specified on the engineering design specification. Also, per the ASME Dimensioning and Tolerancing Standards it states measurements apply in the Free State, unless otherwise specified on the engineering drawing, or specification.