Tyre Pressures when loaded

[mattlittle]

Question that I have wondered about for a while:

On the fuel flap, there are two tyre pressures listed - one for unloaded and one for a heavily loaded car.

I am going to be carrying some heavy loads in the car soon and want to make sure that I have the correct pressure, so my question is this:

Is the pressure for a heavily loaded car what I should put the tyres at before loading the car or is that the pressure that I should see from a correctly inflated tyre after the car is loaded (the load effectively increasing the measured pressure)?

In other words, before I load the car, do I add more air or not?

[dubber36]

Strange question? What difference do you think it would make? Are you thinking that the tyres will collapse on your drive when you are loading the car, or that there will be so much weight on them when it is loaded, that your pump will struggle to get any more air into them?

The weight of car has no effect on the pressure of air in the tyre. The pressure of air in the tyre is what is required to support the weight of the vehicle without the tyre deforming beyond it's design tolerances. The higher the pressure of air in the tyre, the less is will get squashed by the weight of the car.

Seriously, you are over thinking a problem that isn't even there. You're not my wife are you?

[mattlittle]

The question was to avoid adding extra air, loading car, finding out that load caused pressure to read higher and then having to let air out.

[dubber36]

As I said, the pressure of air in the tyre won't change, even if you do add extra weight to the car.

If you think about it, you put a given amount of air into a tyre, when you apply a load to that tyre, it will deform, but even tho' the shape of the tyre has changed, its volume will remain the same. This means that there is still the same quantity of air in the tyre which is still the same size as before, therefore the air/volume ratio, i.e. pressure has to still be the same.

[pango1in]

Tyre pressures will increase when the car is loaded, but this shouldn't impact the end result. The difference could be, say, 0.1bar, but this shouldn't change the handling etc.

[fredgroves]

By Boyles Law the pressure increases if volume decreases, but this tends to only apply to a piston type scenario - which makes the assumption that the cylinder doesn't deform as the piston descends.

I can't see how a car tyre is the same as this model.

The extra pressure you put in is to stop the wall deformation (which is what happens normally) and maintain the same optimal tyre shape under an increased load.

There's some better, more detailed descriptions here:

http://physics.stackexchange.com/questions/132892/does-car-tire-pressure-change-with-weight-of-car-load

Basically, the answer is that it will increase slightly, but the actual psi value is not what you are trying to achieve, you are trying to achieve optimum tyre shape no matter what the numbers show you.