Re: Tire pressure - this can\'t be!!!
Guys,
The common metric units for expressing tire inflation pressures are either in "Bars" or "kilopascals - kPa".
One Bar is equal to 14.5 pounds per square inch (psi).
One kPa is equal to 1/100 of a Bar or 0.145 psi.
I think the easiest way to remember the conversion is the inverse form, i.e. to convert from psi to kPa, multiply the psi by 6.89 to get the pressure in kiloPascals. I usually just remember that the factor is about 7.
Another consideration is the meaning of the pressure imprinted into the sidewall of a tire. It is the Maximum pressure allowable and corresponds to the Maximum load the tire is supposed to carry. For most, but not all automobile and truck tires, the tire has been sized to the vehicle so that the Maximum pressure is near what is actually required. In other words, the tire is just barely large enough to support the expected weight it carries at the rated speed (also imprinted on the sidewall.) So you are safe if you inflate to the pressure shown on the sidewall.
In contrast, almost all tires on tractors of all horsepower levels, are sized to carry much larger loads than the tractor normally imposes in "normal" operations. So if blindly inflate the pressure shown on the sidewall, it could be much more than is actually required. You should go by what the operators manual says instead.
Now when using a front loader, the front tires will have to be inflated to maximum or close to it in most cases when lifting the maximum amount the loader is rated for. Also if you are using an extremely heavy rear 3 point hitch implement, the rear tires should be inflated to the pressure required to carry the load with the implement raised. For most compact utility tractors with typical tires that are used for both loaders and rear implements, the front inflation pressures will typically be higher than the rear. Typical numbers might be something like 18-22 psi in the front and 12-16 psi in the rear, but, of course, the specific tire sizes and loads carried will determine the actual required pressures.
Tractors that are only used to operate mid-mount mowers can usually be set with lower front and rear inflation pressures. This is the most important consideration for minimizing soil compaction, rutting, and turf damage. It is not just a matter of weight--it is a matter of how that weight is carried. The lower the inflation pressure, the lower the pressure on the ground in the contact patch of the tire. The general rule of thumb is that the Average contact pressure over the entire contact patch area is only about 1-2 psi more than the inflation pressure in the tire. When inflated properly for the load, you will see a long contact length front to rear and the lugs in the contact patch will be touching all the way out the their ends. If you can stick your finger partially under a lug touching the soil, the tire is overinflated in most circumstances.
Probably more that you really wanted to know, but it is something I have been helping farmers understand with regard to optimizing tractor performance for many years.
JackIL
