# Tire psi-heated debate

tire psi--heated debate

here is my question:

does the pressure (psi) in a tire change if it is on the ground attached to the car with the extra weight of the car added, or if it is suspended in the air by a car lift?

my roommates and i are having a heated debate on this topic and any advice/input would be appreciated

Related Other Physics Topics News on Phys.org

Yes. When there is a load on the tires, they will compress, and their pressure will go up.

mgb_phys
Homework Helper

Yes it does - in fact you can use the tire pressure and the area of the contact patch to measure the weight of the car.

thank you very much for your responses, do you know if the change would be significant enough to worry about?

for instance if i were to fill my tire up to 35 psi on a lift would the increased pressure of a 2k lb car spread over 4 tires pressing down on any one (or all) of the tires be something to be concerned with?

note the manufacture recommends that the tire be filled to 35 but anything over 39 is considered dangerous

Load/increased load is taken into account, which is why tire types should be matched to the vehicle type/vehicle use.
2 different tire types with the same psi spec can have very different load ratings due to differences in tire construction.

stewartcs

thank you very much for your responses, do you know if the change would be significant enough to worry about?

for instance if i were to fill my tire up to 35 psi on a lift would the increased pressure of a 2k lb car spread over 4 tires pressing down on any one (or all) of the tires be something to be concerned with?

note the manufacture recommends that the tire be filled to 35 but anything over 39 is considered dangerous
Probably not enough to worry about. If you are worried however, just simply measure the pressure in the tires after you lower the car to make sure it is not over the 39 psi.

CS

in fact you can use the tire pressure and the area of the contact patch to measure the weight of the car
Oddly, I don't think that actually works: A tire can be 35psi or 25psi and yet appear to have the about same surface contact area...I guess the stiffness of the tire masks what should be a valid approach??

mgb_phys
Homework Helper

It's deceptive - remember the tire is spreading out in 2 directions, so a 20% increase in the length of the flat spot is a 50% increase in area.
For most vehicles at normal pressures the tire mechanical stiffness isn't a factor.

again thank you for all your responses, i plan on doing a test on this in a local shop with an electronic gauge, if anyone is interested in the results i will post them here

Ranger Mike