>Ok another dumb question. When I look at the charts for my
>LT225/75R16 LRE . They list "overall diameter" as 29.4. Then they
>list the "Loaded Radius" as 13.7 inches. Now if I double that number
>I come up with a diameter of 27.4.
Right. There are a few factors at work. The loaded radius is when the
tire's contact patch is maximized under the load and pressure constraints
of your Michelins (recommended pressure and maximum load). Using the loaded
radius of 13.7 inches to calculate revolutions per mile assumes that the
tire has a single point of contact. Remember that the tire under load is
more or less round with a flat spot for the contact patch. Instead of
forming a circle with one side flattened, it actually causes the tire to
bulge out in other places, tending to keep the tire tread length about the
same (there will be some small amount of shortening as the rubber
compresses). So, every turn of your Michelins takes you about 92.4"
FWIW. the loaded radius measurement can be used to find the optimum
inflation pressure of your tires without measuring the wheel weight. Just
inflate until you reach the loaded radius for each wheel. Measure the tire
pressure and keep them inflated to this amount. Note: this DOES NOT take
into account any requirement for sidewall stiffness for the GMC rear tires.
>Then Steve tells me the tires will
>"grow" at speed. Does anybody know what is real with all of this?
Remember in the days of your misspent youth watching dragsters burnout? The
slicks got noticeably bigger as they spun up to speed. Other tires
experience a similar
effect.But keep in mind that we don't run up to 200MPH
and our tires are steel belted. The belts limit the amount that the tire
can expand. Instead the force increases the effective stiffness of the tire.
>I am thinking of going back to being just fat dumb and happy.
Once the djini is out of the bottle ...
Henry
