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410 Posts

ps mines got firestone 245's

**Edited by: bob camire**

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ps mines got firestone 245's

When unloaded, I run 80psi front, and 43psi rear. I have a pyrometer, and these pressures gave the tires their best contact patch. I also verified that the rolling radius of the rear EXACTLY matched that of the front.

Notsdale said:Empty I'm running 50 front and 40 rear.

Amric, how do you verify the rolling status of a tire?

Mark each wheel with chalk pointing directly to the ground. Roll the truck forward until the the front tire chalk again is pointing directly to the ground (one exact revolution). Look at where the rear tire chalk line is. Less than one full revolution, more PSI is need. More than one revolution, less PSI is needed.

Doing this really helps keep the wear on the tcase down when doing 4WD 1/4 mile runs. It also helps increase handling.

I'll have to try this out for myself. Thanks for the info.

Most people don't get so carried away, because the difference is small, and not worth worrying about in 2WD. But it also helps with the rear locker to keep the radius of both back tires as close to identical as possible.

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Would it be safe to say more air in the rear makes my truck faster!!!!

Mackin....here's the answer to your prayers. Next speeding ticket, just tell the judge someone over-inflated your tires making your speedometer off by 33%

OK I am just kidding around, but I do like the idea of matching the rear to the front for perfect rotation and maybe a better ride.

Leaving for Vegas in the morning, I am going to air up/down tonight and see if it "feels" any different on the way over and back from Vegas. 590 miles each way should take me about 8 hours each way and only 1 fuel stop needed both ways ha ha ha

Anybody on I-5 and I-15 early tomorrow morning, please stay to the right! LOL

Regards,

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How far ahead of the axle is your hitch centerline? I just had a hitch installed and it is ahead of the axle but not by much. You look like your truck is overloaded by 100 lbs or so which is where I'll be. Does your truck set level when loaded?

Thanks

towing a Carriage F32RIK3 ( or will be ...picking up this weekend)

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218 Posts

My hitch pin centerline is 2" ahead of the axle centerline. Both the truck and the trailer are level. A unit I would recommend is the TrailAir hitch pin unit. It reduces the effect of trailer bumps being transmitted to the truck, resulting in a much smoother ride. The other thing I swear by is the Jordan brake controller.

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Amric said:Let me add one fact that should help explain this trick. circumference = pie * radius * 2. The radius is measured from the center of the wheel to the GROUND. The weight on the wheel is trying to decrease the radius, the the PSI is holding the weight, and maintaining the radius. If you don't believe the rolling radius trick, just measure from the ground to the center of the wheel, and then let out a good amount of air (say 33%) and measure the radius again. It will be down in essence reducing the circumference.

Amric,

First let me say that mathmatically I fully agree with you. However, while some things are correct mathmatically, physically they may not support what we are trying to prove.

If I measure my tire from the spindle to the ground, increasing or decreasing the tire preasure will increase or decrease the radius, just as your formula states. Problem is, we're only measuring the radius from the ground to the spindle. The distance from the spindle to the top of the tire remains unchanged. Using your formula, we're assuming that the entire circle or circumference of the tire is decreasing equally, which it isn't.

I had to physically prove to myself that while I thought I was correct, I wanted to make certain. I started out with 60 psi in the front and 40 psi in the rear. I marked the top and bottom of the tire so that I had a good reference point. I then rotated the tires 360 degrees and the marks lined up perfectly as I suspected they would. I then lowered the rear tires to 30 psi. The radius decreased 1/8 inch. When I rotated the tires 360 degrees again the marks still lined up perfectly. As they should.

When I started typing up this response to you I suddenly realized the falicy of your thinking. As stated above, I always agreed with you mathmathically but I also knew the circumference of the tire was not changing. So, where was the problem. The problem was the fact that the formula assumes a perfect circle. As such, if the radius decreases, it decreases equally all the way around. On a tire, the decrease is only from the spindle to the ground.

Dale

Previously I was just eye balling front and rear tires and inflating them so it looked like they were fairly equal in height. Never considered putting a tape measure to the tie and checking the radius. I was close.

NotsdaleIf said:I measure my tire from the spindle to the ground, increasing or decreasing the tire preasure will increase or decrease the radius, just as your formula states. Problem is, we're only measuring the radius from the ground to the spindle. The distance from the spindle to the top of the tire remains unchanged. Using your formula, we're assuming that the entire circle or circumference of the tire is decreasing equally, which it isn't.

The only accurate measurement of tire radius used to determine rolling radius is the center of the axle to the ground. I like the plumb idea, as it is more accurate that than my previous stated method. The only way the radius to the TOP of the tire would have any impact, was if the sidewall would not compress as that side of the tire came to the bottom.

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