Diesel Place banner

1 - 20 of 20 Posts

·
Registered
Joined
·
410 Posts
Discussion Starter #1
Guys, this may seem like a silly one, however, if you read the mfg recommended tire pressure on the door, it states to run 60 in the front and 80 in the rear. This seems dumb to me, whereas most of the weight is in the front of the pickup. That dmax and allisson weigh a hell of alot more than the rear axle and pickupbed. Just for the hellofit, what are you guys running? Thanks, Bob


ps mines got firestone 245'sEdited by: bob camire
 

·
Registered
Joined
·
122 Posts
I run 55lbs front, 50lbs rear empty and 65lbs front and 75lbs rear when towing my 8k lb fifth wheel


Steve
 

·
Registered
Joined
·
704 Posts
I think the factory recomended tire pressure is to reach the maximum recomended GCWR.


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.
 

·
Registered
Joined
·
76 Posts
Same here. 55lbs. front 50lbs. rear. Their recommendation made no sense to me either.
 

·
Registered
Joined
·
704 Posts
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.
 

·
Registered
Joined
·
68 Posts
Okay, this is a new one on me. I was always under the impression that the circumference of 4 identical tires, other than difference in tire wear, was equal. I don't see how the air pressure in a tire can change the revs turned in reference to the front and rear axle.


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

·
Registered
Joined
·
61 Posts
I weighed mine. While towing the 5'er, my rear tires carry 4400 lbs, the front carry 4100 lbs. Empty the front carries 4050, the rear 2900. Towing I keep it at 55 lbs front, 60 lbs rear. Empty, 55 front, 45 rear. Gotta check Amric's rolling radius trick real soon though. Edited by: Sneaks
 

·
Registered
Joined
·
704 Posts
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.


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.
 

·
Registered
Joined
·
704 Posts
A 1" change in radius makes for a 6.28" change on EVERY revolution. Thats 171 FEET every mile. Edited by: Amric
 

·
Registered
Joined
·
361 Posts
I always figured the extra air in the rear applied when hauling a full load. Typically I run 42 all the way around But it sure makes sense to up load the fronts when empty.
 

·
Registered
Joined
·
432 Posts
AMRIC, I like that, makes perfect sense, probably explains why my rear tires wear faster than the front.....it's all that extra mileage the rear of my truck is doing ha ha ha


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,
 

·
Registered
Joined
·
218 Posts
As you can see from my signature, I tow a 37ft Titanium weighing well over 11,000 lbs. According to the local CAT scale, my front and rear axle weights are 4310 lbs and 3120 lbs when driving without the trailer. With the trailer, these loads change to 4260 lbs and 5140 lbs respectively. (Yes, my hitch centerline is ahead of the rear axle)Using to the Goodyear inflation chart, for these loads, I run at 50 psi (front) and 35 psi (rear) when not towing, and 50 psi (front) and 65 psi (rear) when towing. Have now covered 11,000 miles (3,000 of them towing) and the wear on all tires is the same, and even across the tread. I use the stock Firestone 245's.
 

·
Registered
Joined
·
132 Posts
Railbuff


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)
 

·
Registered
Joined
·
218 Posts
Hi Rmax,
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.
 

·
Registered
Joined
·
361 Posts
I had to run 300 miles up I-75 to deer camp today so I thought I would do a little experiment while I was at it after reading this about the tire pressure. I have always ran all 4 at 45 PSI and was useing Power Service DFA on the way up. 71 mph cruise all the way. 17.5 mpg no load . Returning I switched to FPF DFA and 8+ Cetane Boost, I then adjusted the front tire pressure to 55 and the rears to 50. Returned the same stretch of I-75 71 mph, no load.....19.7 MPG
 

·
Registered
Joined
·
68 Posts
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
 

·
Registered
Joined
·
68 Posts
One final comment, as mentioned above, I am now running 60 psi front and 40 psi rear. This gives me an equal radius on the front and rear tires in a resting position. I would assume that the tires now have a more equal rolling resistance. Can't prove it, just an assumption.


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.
 

·
Banned
Joined
·
6,539 Posts
Yes the tire is always deformed and the distance from the centerline of the spindle to the top of the tire will always be slightly larger than to the bottom. The proper method to determine actual rolling diameter is to make a plumb vertical chalk line with a level through the centerline of your tire. Mark the tire and the ground..... roll one revolution, stopping with the line plumb and measure the distance between the two marks. This would be your running circumference. Most likely that number will match up with using the distance from the centerline of the spindle to the ground as the radius in the circle formula.


Edited by: hoot
 

·
Registered
Joined
·
704 Posts
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. Edited by: Amric
 
1 - 20 of 20 Posts
Top