Hello Autopians and welcome to another edition of Ask an Engineer. This week I thought we could talk about tires again since we had so much fun with them the other week.
A reader named Patrick asked me a question about recommended tire inflation pressures. 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. It was sensitive to wind and drafts from passing semi trucks. It followed road grooves excessively, and he felt the handling was actually pretty scary.
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. When he pulled off the highway and adjusted pressures back to the recommended levels, the difference was instantly noticeable. 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.
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
One possibility might be that the dealer prep on this vehicle was not performed correctly. Many OEM’s will ship their cars from the factory with much higher than normal tire pressures. 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. The tire develops a flat spot where it contacts the ground and will create a thump-thump sound as you then drive it. 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. 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.
How Tire Pressure Can Affect Handling
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. Thomas Gillespie explains this very well in his book “Fundamentals of Vehicle Dynamics.”
Tires like to go in a more or less straight line when they are rolling. When we turn the steering wheel in a car, we are forcing the tire to point in a direction other than straight ahead, and this difference in the direction the car is going and the direction the tire is now facing creates a sideways force in the tire.
The force is a product of the friction between the tire and the road, and it pushes the car in the new direction the tire is facing. When we look in detail at what is happening between the tire and the road surface we see that the flexibility of the belts inside the tire and the rubber in the tread blocks causes the portion of the tire in contact with the road (called the contact patch) to deform slightly. Like this:
Here we see how the contact patch has deformed because of the side force created by a left turn (this image represents a tire in the middle of a turn). The deformation causes the tire to roll in a slightly different direction (in this case, a bit more to the right) than it is pointed. This is called the slip angle and it is critical to how a tire behaves in a corner and how the car handles, especially as you approach the limits of adhesion.
Imagine an element, or piece, of rubber in the tread of the tire as the tire rolls down the road. As Gillespie explains:
“As the tire advances and the tread elements come into contact with the road, they are undeflected from their normal position and therefore can sustain no lateral force. But as the tire advances further at the angle of its direction of travel, the tread elements remain in the position of their original contact with the road and are therefore deflected sideways with respect to the tire. By this process the lateral force builds up as the element moves rearward in the contact patch up to the point where the lateral force acting on the element overcomes the friction available and slip occurs.”
The slip referred to above occurs because the lateral force overcomes the friction between the road and the tread element, and also also because the curvature of the tire is starting to pick the element up off the road so it is not being pushed down as hard as it was earlier in the rotation of the tire.
Let’s look at this in more detail. Imagine two spots on the tire tread shown below as points A and B.
Image 1: Neither points A or B have reached the contact patch yet and are sitting in part of the un-deformed tread.
Image 2: As the tire rolls forward (i.e. towards the right in the image above), point A comes into contact with the road while point B is still on the un-deformed tread. Because the tire is being pushed sideways, Point A has already started deflecting slightly to the one side. At this point, point A can only move a little bit sideways since it is physically connected to the rubber just ahead of it that has not come into contact with the road yet and is still un-deformed.
Image 3: The tire has rolled a little further to the right and now both points A and B are in contact with the road. Since the tire is continuing to be pushed sideways by the cornering force, both points A and B have now deflected to the side although point A has gone further since it has been in contact with the road longer.
Image 4: The tire has rolled even farther to the right and now point A has reached the point where the pressure in the contact patch has reached its maximum. The friction between the tire and the road cannot get any higher and point A is deflected to the side as much as it can before it will start to slip.
Image 5: Point A has slipped completely back to its original spot on the tread since the curvature of the tire has lifted that part of the tread off the road again, reducing the pressure at that point and eliminating the friction. This happens quite quickly and you can see how little the tire has to roll to the right before point A goes from maximum deflection to zero deflection. Point B is still stuck to the road, however, and has reached its maximum deflection.
Image 6: Both points A and B have gotten to the end of the contact patch, have been lifted off the road, and have returned to their original un-deflected positions on the tread.
Another way to think about this is to picture each element of the tire as a spring. As the element comes into contact with the road, the spring is not deflected yet and so it has no force in it. But, as the element is pushed sideways in the tread by the slip angle, it is as if we are pushing more and more against that spring. It takes more and more force to do it. The friction between the element and the road is what pushes on the spring. But as the friction reduces towards the back of the contact patch because that part of the tire is starting to get lifted off the road again, it can no longer push on the spring and it lets go. Take all the elements that make up the contact patch and add up all the forces in those little springs and you get the total side force the tire generates.
One thing to note is that the slip angle shown in the images above is greatly exaggerated for illustration purposes. An average tire has a cornering stiffness around 150-200 lb/deg, although this value is heavily dependent on the load the tire is carrying, the inflation pressure, and the tire construction, among other things. If we take a 4,000 lb car cornering at 0.5G (which is a higher than the average driver will ever corner in their life!), this means the tires have to provide a cornering force of 4000 x 0.5 = 2,000 lb. If we assume all four tires are doing the same amount of work, then each will need to provide 500 lb of cornering force. If the tire cornering stiffness is 200 lb/deg then the slip angle will be 500 / 200 = 2.5 degrees. You can see we are talking about small numbers here, not the large angle shown in the drawings above.
Now you might think that more slip angle would be better because all those little springs get deflected more and create more force, but that would not be correct. A soft spring will deflect more for a given force than a stiff spring, so the “stiffness” of the contact patch matters. The softer the little springs are, the more the contact patch has to deflect before it can generate the side force needed to steer the car, meaning a larger slip angle. T
The deflection of these little springs also takes time which means there is a delay between when the tire is steered to the time it can generate the required side force. This delay can be felt in the responsiveness of the car. A stiff tire will respond much faster to changes in direction than a soft tire because each element doesn’t have to deflect as far to get the cornering forces you need to navigate a turn. If you can make a tire with very stiff little springs, then it doesn’t have to deflect as much for a given side force and will be more responsive to steering input.
So what does this have to do with the inflation pressure problem Patrick had in his Jeep? For that answer we need to understand another characteristic of how tires work and it is shown in this graph:
Notice that as the inflation pressure increases, the cornering stiffness increases, meaning that the slip angle decreases. It takes more pounds of cornering force to get a degree of slip angle.
The increased inflation pressure will also lead to an increase in the vertical stiffness of the tire. This will make it ride harder and less able to absorb small imperfections in the road. Imagine the extreme where the tire is as hard as concrete. It wouldn’t ride very nicely
All of this translates into a stiffer tire that will respond to changes in the road more quickly and ride more stiffly, just like Patrick noticed in his Gladiator.
It is tempting at this point to conclude that increasing the inflation pressure of your tires will make the tire stiffer and therefore more responsive. To a certain degree you would be right but be careful. Every road car is designed to have a certain amount of understeer, which is the tendency of the car to want to keep going in a straight line when you corner. The opposite is oversteer, which is the tendency of the rear of the car to step out in a corner and cause the car to spin out. Understeer is best for most drivers since if you lose control you will go off the road nose first. An oversteering car will want to spin and go off the road sideways or backwards. The nose of the car is the strongest and designed to best absorb energy so that is what you want to hit something with, not the doors or trunk.
Getting back to our tires, keep in mind that the cornering stiffness of your tires works in conjunction with the springs, dampers and bushings in your suspension to create an overall lateral stiffness of the suspension and is a large contributing factor to the amount of understeer or oversteer your car has. Changing the cornering stiffness of your tires can upset this balance and lead to handling problems, so be careful.
I Generally Advise Sticking With Manufacturer-Recommended PSIs
You can, however, make some significant changes in the way your car handles by playing with pressures. If you participate in track days with your car then you are probably already well aware of this especially if you play with the front and rear pressures separately. It is also a great way to get yourself into trouble so doing this at a track is a great way to learn in a controlled and relatively safe environment. You don’t want to find out you’ve gone too far in an emergency situation on public roads.
While it may be tempting to start playing with the tire pressures in your car, you will need to work within some limits. Every car now has a tire pressure monitoring system (TPMS) as standard equipment. These systems monitor the pressures in each tire and will give a warning to the driver if any tire falls below a specified minimum pressure. In the U.S., this is governed by regulation FMVSS 138 which became a requirement in 2007 in response to the Ford Explorer Firestone issues. It states that the warning must be given when the pressure in any tire drops below 75% of the recommended level, or below a minimum level as stated in table 1 below, whichever is higher. The result is that no tire will be more than 25% below the pressure of the other tires before a warning is made to the driver. The handling impact of an underinflated tire is thereby minimized.
Let’s look at an example of what FMVSS 138 requires. Here is one of the forms the OEM is required to fill out which sets the TPMS trigger pressure.
I own a 2015 Mustang with P265/35R20XL tires. The recommended pressure is 35 psi, or 240 kPa. Here is the form filled out for that car:
According to the regulation, my car should light the TPMS light if the pressure in any tire drops below 160 kPa or 26.25 psi. I know from experience that for my vehicle this is indeed the case.
Because the TPMS trigger pressure is not something we as owners can change, if you do decide to use tire pressures that are different from the recommended levels, you will be running at pressures that are either closer or farther from the trigger levels set by the OEM. If you go lower in pressure then you will be closer to the trigger point and you may find false activations of the TPMS light in colder weather. If you go higher in tire pressure then you will be farther away from the trigger point and it will take a much larger drop in pressure before the system will give you a warning. This means that any one tire can be more than 25% underinflated before you get a warning and the handling impact of having one underinflated tire will be that much greater.
My recommendation as an engineer is to stick with the recommended tire pressures, but if you do decide to experiment with your car’s tires, please do it in a safe way. As Patrick found out, tire pressures can have a significant impact on the way a car performs both in a positive and a negative way.
Please keep those questions coming to firstname.lastname@example.org!