Tires like to go in a more or less straight line when they are rolling. Thomas Gillespie explains this very well in his book “Fundamentals of Vehicle Dynamics.” Slip Angle No matter how it happened in Patrick’s case, in order to understand how incorrect inflation pressure would have caused the behaviors that Patrick noticed in his Gladiator, we need to understand a little about how tires work - in particular how tires generate cornering forces. I have seen some OEM’s inflate their tires as high as 50 psi to help prevent flat spotting and part of the dealer prep is to reduce the pressures back down to the factory recommended levels before the car is delivered to the customer. In most cases the flat spotting will disappear after driving for a few miles, but in extreme cases it can completely ruin a set of tires. The tire develops a flat spot where it contacts the ground and will create a thump-thump sound as you then drive it. This helps to prevent flat spotting which can happen in any tire when a car that sits in one place for a long time, like on a transporter or in a storage lot. Many OEM’s will ship their cars from the factory with much higher than normal tire pressures. One possibility might be that the dealer prep on this vehicle was not performed correctly. What might have happened here? How can the tire pressures in a new vehicle be so much higher than the factory recommendations? Why An Automaker Might Over-Pressurize Its Tires Before Sale The truck rode better, the cross winds didn’t bother it so much, and overall it handled like he expected from a vehicle of this type. When he pulled off the highway and adjusted pressures back to the recommended levels, the difference was instantly noticeable. When he checked the Tire Pressure Monitoring System reading it showed all four tires inflated to about 41-42 psi while the recommended pressure is 37 psi. It followed road grooves excessively, and he felt the handling was actually pretty scary. It was sensitive to wind and drafts from passing semi trucks. He had just recently bought himself a shiny new Jeep Gladiator, and when he first took it on the highway it just didn’t feel right to him. This week I thought we could talk about tires again since we had so much fun with them the other week.Ī reader named Patrick asked me a question about recommended tire inflation pressures. Hello Autopians and welcome to another edition of Ask an Engineer.
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