I don't know why, but for some reason, this has been on my mind this evening. I think I read a post where someone said something about tire pressures affecting tire diameter. Anyway, I thought I might start a discussion and see if anyone has read or performed any actual tests to determine how much tire pressure changes how far a wheel/tire travels per revolution. In my mind, the tire is rolling on the tread which does not change length with tire pressure changes. What do you guys think?
Tire pressure vs rolling circumference
- Thread starter JoshH
- Start date
Easiest way to think of it to me would be to imagine 2 tires on a rearend with a spool, lower the pressure in one tire enough and it's going to slip. A low tire puts the wheel closer to the ground. At least that's how I think about it.
I'm having a hard time visualizing how lowering the tire pressure will effect the circumstance of the tire. The same amount of rubber surrounds the tire. It matters not how close the wheel is to the ground. Think about a track. The track will move the vehicle no matter how much of the track touches the ground and how much does not. Neither does it matter how close the drive wheel is to the ground. The same amount of links= same circumstance= same relative speed.
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From what I have seen, a higher pressure causes a tire to "baloon" a bit in the center of the tread. So instead of the tread being flat and making even contact with the ground, only the center takes the load, and wears quicker. The inverse is true if the pressure is too low: the center can't support any weight, and the tread near the sidewalls bear the load (and wear).
This may not be a big effect for drag racing, but I notice it over thousands of miles. If I have too much air in the rear tires for the current weight, the centers wear out quickly. If I have too little for the weight, the edges wear quickly. Get the right amount, and the tires wear evenly and seem to hook up the best (at least on the street).
Now keep in mind I am running a 315/70R17, so it is a relatively wide tread with relatively tall sidewalls. I imagine these effects get smaller with narrower tires and shorter sidewalls.
This may not be a big effect for drag racing, but I notice it over thousands of miles. If I have too much air in the rear tires for the current weight, the centers wear out quickly. If I have too little for the weight, the edges wear quickly. Get the right amount, and the tires wear evenly and seem to hook up the best (at least on the street).
Now keep in mind I am running a 315/70R17, so it is a relatively wide tread with relatively tall sidewalls. I imagine these effects get smaller with narrower tires and shorter sidewalls.
In turn the higher the pressure the more the center stretches, and gets taller than the side walls making the diameter larger in the center correct? Like when you blow up a Baloon and it gets bigger around, only there's steel belts trying to keep it from stretching.
I know for sure on an atv, if you have uneven tire pressure in the rear and ride a wheelie it will turn to the side with the lowest pressure.
I know the 10plys aren't as bad but nitto 420s will definitely wear out the centeres fast as it's just a 4 ply so in high speeds the middle baloons.
This exactly..
However it might not be what josh is looking for.. But the tire is taller in the center with more air obviously because of wear.. Now wether it's an inch or quarter inch I don't know.
Also... We used to mount/ balance a lot of drag car slicks.. My step dad had a car and it was pro street record holder at sikeston for a long time.
When mounting up slicks you have to air up both of them to the exact same pressure when seating the beads. Slicks grow and if you only air one up to 10 lbs and one to 30 while seating beads then the 30psi will be taller, even though you have both of them aired up to 15psi racing.
When mounting up slicks you have to air up both of them to the exact same pressure when seating the beads. Slicks grow and if you only air one up to 10 lbs and one to 30 while seating beads then the 30psi will be taller, even though you have both of them aired up to 15psi racing.
It is all about the tire circumference. Increases in the tire pressure will stretch out the tread a bit, but with steel belts in most street tires, the changes will be very minor. However, some tires, specifically race slicks, grow not only from tire pressure, but centrifugal force as they spin faster down track. For instance, the slicks on top fuel drag cars grow 50%-75% at speed.
The drag that some are attributing to a change in circumference on an under inflated tire, is mostly from rolling resistance. Under inflated tires do not roll as easily as properly inflated tires.
The drag that some are attributing to a change in circumference on an under inflated tire, is mostly from rolling resistance. Under inflated tires do not roll as easily as properly inflated tires.
I've been told that top fuel dragsters only need one gear and are direct driven, using a special series of clutches to slip for staging and at the start, and that the tires grow so much with centrifugal force it actually changes the final drive drastically, so much so that as the car goes down the track, the tire is like a "gear" in itself, as the tire is getting taller and taller going down the track it allows the car to go faster and faster...
as for inflation/pressure relates to a street tire...the same theory has some (very little) application on our trucks (as mentioned they are belted, keeping the tires shape/integrity in tact) but more so relates to the weight on the tire...unladen and sitting on the mounting machine with say, 20psi in the tire, the tire may have the specified dia/circumference...now, mount that bitch up and you're going to squat the hell out of it and obviousally lose ground clearance...if you were to drive down the road like this it will affect the speedometer....hence, the circumference has changed, the tire now has more contact patch and therefor, less circumference, regardless of how much material is there for creating the circumference of the tire, it's not being applied to creating or meeting the potential circumference...in this reference, much like the dragster tire, at speed the centrifugal force will make the tire round out a little...this is also why guys that do burn outs see the center portion of the tire disappear faster than the outer tread portion
As Dutch pigheaded self-declared tirepressure-specialist, I google dayly for it, and came to this topic. Registered to give an answer.
I think to know how it works. The surface length on the ground and the line from centre of wheel to the ground is important.
The centre of wheel makes a sertain distance, this is the surface length on ground, and during this the tire makes a sertain angle . rolingsircumference can be calculated by deviding 360 degrees /angle the tire makes, and multiply it by the surface length on ground.
Then I also can estimate the line from centre to ground is called loaded radius, is unloaded radius minus deflection, and can estimate the influence of pressure on deflection. This all together I can pretty accuratly difference in rolingcircumference and pressure.
I think to know how it works. The surface length on the ground and the line from centre of wheel to the ground is important.
The centre of wheel makes a sertain distance, this is the surface length on ground, and during this the tire makes a sertain angle . rolingsircumference can be calculated by deviding 360 degrees /angle the tire makes, and multiply it by the surface length on ground.
Then I also can estimate the line from centre to ground is called loaded radius, is unloaded radius minus deflection, and can estimate the influence of pressure on deflection. This all together I can pretty accuratly difference in rolingcircumference and pressure.
That makes a lot of cents. Do your work at NAPA?
Edit: Should read "do you work at NASA?"
If your tire is 104" in circumference, each revolution, you travel 104" regardless of how much is flattens on the bottom.
as for inflation/pressure relates to a street tire...the same theory has some (very little) application on our trucks (as mentioned they are belted, keeping the tires shape/integrity in tact) but more so relates to the weight on the tire...unladen and sitting on the mounting machine with say, 20psi in the tire, the tire may have the specified dia/circumference...now, mount that bitch up and you're going to squat the hell out of it and obviousally lose ground clearance...if you were to drive down the road like this it will affect the speedometer....hence, the circumference has changed, the tire now has more contact patch and therefor, less circumference, regardless of how much material is there for creating the circumference of the tire, it's not being applied to creating or meeting the potential circumference...in this reference, much like the dragster tire, at speed the centrifugal force will make the tire round out a little...this is also why guys that do burn outs see the center portion of the tire disappear faster than the outer tread portion
no jason..... not how it works
I agree with this. Granted it is a soft atv tire and not a truck tire. The theory that the circumference of the tire remains the same makes sense, but it's hard to ignore personal experiences.
in regards to what James?
I get fingers reference to if the tire is 104" the tire travels 104" per revolution, but when I aired my jeep down on the trail it always changed the rpm to mile per hour ratio for a given gear when I hit the street, vs having them properly aired up...as far as the dragster and burn out reference goes and the science behind centrifugal force, yes James, that is how it works
I get fingers reference to if the tire is 104" the tire travels 104" per revolution, but when I aired my jeep down on the trail it always changed the rpm to mile per hour ratio for a given gear when I hit the street, vs having them properly aired up...as far as the dragster and burn out reference goes and the science behind centrifugal force, yes James, that is how it works
Just to help you out, I believe your engine RPM's will vary with MPH related to PSI...Low PSI (offroad; say 15 - 20 PSI) creates a greater rolling resistance causing the motor to work harder to turn the tires. Properly or even overinflated tires (say 65 - 80 PSI) will lessen the power required to turn them.