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Tire Pressure Formula


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Mike, I saw on Cummins Forum, your/the tire pressure formula.

(Axle weight / 2) / Tire Capacity weight ) x Tire Max Pressure = Infation Pressure.

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So my 'normal formula is:

Dual Rating:

((4200 /2) / 3415 x 80 = 49.19 psi

Single Rating:

((4200 /2) /3085 x 80 = 54.45 psi

My question is do you add the load weight over the axle, ie. pin weight, or w/e weight is on the axle? If not, why? The tires still bare that added weight to the truck.

So would the the formula look like this with pin weight of 3000 lbs.?

Single Rating:

((4200 + 3000 /2) / 3085 x 80 = 93.35 psi?

Dual Rating:

((4200+3000/2) / 3415 x 80 = 84.33 psi?

Second, do you use the Single rating or the Dual Rating and why?

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First off your number are very dangerous...

Lets get the information in one spot...

Formula...

(Axle weight / 2) / Tire Capacity weight ) x Tire Max Pressure = Inflation Pressure.

Tire rating...

Single 3,415 pounds @ 80 PSI

Dual 3,085 pounds @ 80 PSI

Assuming the 4,200 is your rear axle weight dry and we are going to calculate rear axle for a dually.

(4,200 / 2) / 3,085) x 80 = 54.45 or say 55-60 PSI for the duals empty truck.

Now adding assumed pin weight of 3,000 over the rear axle...

(4,200 + 3000 / 2) = 3,600 pounds! (Tire rated for 3,085 pounds)

(4,200 + 3000 / 2) / 3,085) = 116% (This number should never exceed 100% but for safety margin it shouldn't cross about 90-95% so you not killing the tires)

Danger Icon DANGER!
Your now in a very dangerous place. Your now over step the rating of the tire and I highly suggest you don't tow this load or you upgrade to a higher load range tires. Not to mention your rear axle weight might be getting close to its limits.

I would highly consider weighing said trailer and find out the actual axle weight in total loaded. since the trailer and the load is unknown you might be capable of shifting the load off the pin by moving load rearward of the trailer axle. But once again I'm not sure what I'm working with. :shrug:

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Yes I realize I don't have the numbers, the scales I stopped in at where broken.So those are figurative numbers. But my question, is do I add the weight of the load to the formula? It sounds like I do. Second question was, which of those rates do I uses and why? Dual is for Dually? Single for single tire? Sill wondering on that. When I get my rear axle weight empty and my trialer pin weight, i'll adjust to my tire psi. However, I am guessing 75 - 80 psi is where I need to be with 2000-3000 lbs pin weight, and I bet my axle is more like 3,800 lbs but i don't know. The reason I bring this up is I see people towing heaving with only 55 - 60 psi in tires and that doesn't seem right. So I like the formula, and when I get hard numbers I'll put them in there and find my psi, but i needed to know if i add the load weight to the formula, and which of the two ratings to use, and why.

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Ok... A for rear axle weight I'm right at 2,860 dry weight in the truck and full fuel tank.But it would be best to weight hitched up and again just the truck. Weigh your front axle, rear axle, trailer axle and a total weight. I usual find a close truck scale, farm co-op, or a public scale and get my weights. No getting these weights might be tough or difficult depending on the size of scale pad. I've seen both small singles and large singles then seen multiple large pads.Typically I would weight the front axle (stop), pull up weigh the rear axle with the front (stop), then pull up and get a total with trailer...You should be able to do some math and separate the weights.Front will be right on...Rear axle will be minus the front on the second number...Total truck weight is the second number...Total truck minus total GCW will be the trailer axle.Now after you come back and scale again without the trailer is easy. Front (stop), total (stop), rear (stop)...Now with the rear weight now and the rear weight with trailer minus the numbers and you'll have your pin weight.Hopefully this helps...

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Next time you take off with your trailer stop in at a truck stop with a cat scale. It cost me $10 and they weigh the rig all at once, both truck axles and the trailer axles. You get a print out giving the wieght of each individaul axle on the truck and the weight on the trailer axles.[ATTACH]4212[/ATTACH] here is mine from last year.

cat scale 10-3 -11.pdf

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Ok, got some rough info.

http://www.rvcountry.com/IRV/Files/205988_1.pdf

GVWR: 17,000 (the 7,000 lbs axle)

Hitch Weight: 2,400 (2,650 in all honesty what I bet it is with my load.)

Unloaded Weight: 12,050 Lbs. (7,000# axles)

[i got a bunch of empty rear axle weights off this site and Cummins Forum and the avg. is about 2,800, so i'm using this number as my rear axle weight until i hit a scale]

Tire formula: Empty

(2,800/2)/3415 x 80 = 32.79 PSI????

(2,800/2)/3085 x 80 = 36.30 PSI????

These psi seem VERY low, but buy this formula this is where they are supposed to be.

Tire formula: Loaded

(2,800+2,650 / 2) / 3415 x 80 = 63.83 PSI

(2,800+2,650 / 2) / 3085 x 80 = 70.66 PSI

The second one seems closer to what I would think than any of the others... maybe 55-65 psi in the front.

So with this I am way under load max for my tires.

- - - Updated - - -

First off your number are very dangerous...

Lets get the information in one spot...

Formula...

(Axle weight / 2) / Tire Capacity weight ) x Tire Max Pressure = Inflation Pressure.

Tire rating...

Single 3,415 pounds @ 80 PSI

Dual 3,085 pounds @ 80 PSI

Assuming the 4,200 is your rear axle weight dry and we are going to calculate rear axle for a dually.

(4,200 / 2) / 3,085) x 80 = 54.45 or say 55-60 PSI for the duals empty truck.

Now adding assumed pin weight of 3,000 over the rear axle...

(4,200 + 3000 / 2) = 3,600 pounds! (Tire rated for 3,085 pounds)

(4,200 + 3000 / 2) / 3,085) = 116% (This number should never exceed 100% but for safety margin it shouldn't cross about 90-95% so you not killing the tires)

Danger Icon DANGER!
Your now in a very dangerous place. Your now over step the rating of the tire and I highly suggest you don't tow this load or you upgrade to a higher load range tires. Not to mention your rear axle weight might be getting close to its limits.

I would highly consider weighing said trailer and find out the actual axle weight in total loaded. since the trailer and the load is unknown you might be capable of shifting the load off the pin by moving load rearward of the trailer axle. But once again I'm not sure what I'm working with. :shrug:

What did you mean calculate for a dually, is this a formula for a dually?
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The formula can be used for either dually or single. Just got to remember when your figuring for duals there is 2 tires holding up the weight.As for the unloaded value I would bump to 40 PSI for the rear. But once again that is here say for numbers...front axle 4,400 poundsRear axle 2,860 poundsTypical Load range E tire is 3042 single @ 80 PSI(4400 / 2) / 3042) x 80 = 57 PSI front (60 PSI front)(2860 / 2) / 3042) x 80 = 37 PSI rear (40 PSI rear)Now trailer loaded.(4040 / 2) / 3042) x 80 = 53 PSI front (60 PSI front) - This offset is from my weight dist bars.(4280 / 2) / 3042) x 80 = 56 PSI rear (60 PSI rear)Now on my Voma Solid Trac Load Range G's which carry 3750 pounds at 110 PSI(4040 / 2) / 3750) x 110 = 59 PSI Front (60 PSI front)(4280 / 2) / 3750) x 110 = 63 PSI Rear (70 PSI rear)Really does help when you have actual weighs to work with. Because guessing at weight number tend to screw with the formula big time. All the (xx PSI) are what I actually inflate to for my purpose. I tend to round up to the next whole ten pounds. Just a few extra PSI isn't going to screw it up but when hauling heavy I prefer a solid tire over a mushy tire. But in the same sense there is no need to run the tire pressure all the way to MAX inflation. Just with the rears at 70 PSI its a very rough ride... :ahhh:Also take notice the front axle really never changes much at all. It the rear axle pressure that goes up and down all the time.This is set at 60 PSI front and 60 PSI rear...post-2-138698189478_thumb.jpg

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