Super Single Tire Pressure / Static Load Radius

[MilSpec78]

So I have not been happy with the tire pressure on my deuce. The rears appear to have too much pressure and the fronts too little. By sight, I set the fronts to 32 psi and the rears to 24 psi. It still didn't seem right but it drove fine.

I have Continental 18 Ply 14.00-20 tires with NATO tread. I found their specs online and I noticed the Static Load Radius (SLR) is 23". If I understand SLR correctly, that is the distance from the center of the hub to the ground when the tire has the correct psi pressure for its particular load.

My particular tire also has a 9,960 lb max load and a max psi rating of 95. That is about a 104 lb to 1 psi ratio. Considering the estimated weight of the front end of the duece, I guessed the psi for the front would need to be 30 to 40 psi and the rears which have much less weight (the truck is bobbed and has no bed currently) should be between 15 and 20 psi. These were just guesses not knowing the true load of each tire.

Today I measured and measured and measured the wheels. I also adjusted air pressure. The driver's front was at the correct SLR at 38 psi and the passenger's front was there at 44 psi. However, I made it down to 10 psi in the rears and the center hub was at about 23.5". I did not want to go below 10 psi.

I took the truck for a drive and it felt fine. A little more sluggish though. Is 10 psi too low for the rear? Does anyone else run their rears this low?

I saw a spec that these tires can be run as low as 8 psi. They also look like they are sitting properly. See the attached pics. At 24 psi the edges are rounded up but at 10 psi the bottom appears to sit correct.

Thoughts?

 

[Scarecrow1]

My 14-20,s are set at 30 psi per instructed but they are on the 923 series and not a deuce . I Know 10 psi in the back is not near enough pressure to run in them. Look on the side of the tire and it should tell you what to run in them regardless of what they are on.

 

[MilSpec78]

The side of the tire shows 95 psi. That is the max psi for the max load. It is not correct for this application.

 

[emr]

You are correct on max pressure, there are allot of pressures that one should run for every reason under the sun, No Bed ? finish the truck and do it again, unless i am missing something, Just like max pressure is for max loads, lowest air is for extreme slow driving conditions and you must act accordingly. But if you are running a truck way lighter than the tire is designed to ride on, You must make drastic decisions and live with the results, they may be unsafe at road speeds though, good luck,

 

[MilSpec78]

I believe you are right. I was researching pressure versus load formulas and there are many. In the end it appears a linear formula is easy and conservative. That is... load/max load = pressure/max pressure. In this case a 10 psi result for the rear seems reasonable.

The tires are designed for a much heavier load. The pressure/load tables provided by Continental only go down to 70psi and that is with a load of 7,500 lbs per wheel or 15,000 lbs per axle.

The vehicle these tires came from (Husky MRAP) wasn't nearly that heavy and ran the psi at 30 to 35 for highway driving. Or, at least, those are the specs I found.

I need to build the bed and extend the cab back. This will provide more ballast for the back. In the meantime, I am wondering if the correct psi to load (10 psi in this case) is fine to drive at 55 mph. According to the formulas I was reviewing you don't adjust the psi unless it is for high speeds (i.e in excess of 100 mph). Of course, this might not be the case for these large tires. I do not know.

i know low air for offroad purposes calls for slow speeds. However, this is for tires aired down half of what they should be for normal road purposes. In that case the 10 psi rear would drop to 5 psi.

I am really curious about this issue. The low psi is good for tread pattern. I could run at higher psi but the middle tread will wear faster than the outer edge. Extra ballast (bed and extended cab) may add another 5 to 10 psi.

 

[MilSpec78]

By the way, the 395s, 14s and 16s from Michelin and Goodyear have even higher max load ratings than my tires. They are run on bobbed deuces all the time. For those of you with those configurations, what psi are you running and does your tread wear evenly?

 

[juanprado]

10 psi on a belted bias tire at highway speed does not sound safe at all. The less psi, the more rubber contact on the road and even more heat generated from highway speed sounds like a problem waiting to happen.

 

[The King Machine]

By the way, the 395s, 14s and 16s from Michelin and Goodyear have even higher max load ratings than my tires. They are run on bobbed deuces all the time. For those of you with those configurations, what psi are you running and does your tread wear evenly?

I'm using the 395 xml's on HEMTTs. On pavement I run 50 psi, any less and she gets real tough to steer and my fuel mileage goes way down. Off road I run 10 psi.

 

[MilSpec78]

10 psi on a belted bias tire at highway speed does not sound safe at all. The less psi, the more rubber contact on the road and even more heat generated from highway speed sounds like a problem waiting to happen.

I agree it does not sound safe. However, if you look at the pics, there appears to be the correct tread contact at 10 psi. Plus there is no side bulge.

The walls on these tires are so stiff I think the tires with 0 psi or close to 0 may be able to support the weight of the light rear end. Later today I may air the tire down completely and see if they sit similar to 10 psi. If so, I will then air them up until the tread pattern indicates over inflation. This will give me a good idea of the pressure needed for this particular application.

By the way, I found this excellent article covering this very topic. It provides his formula to calculate psi for load to max load ratios. It also provides examples on how to check if your psi is correct through tread patterns, psi increases and heat.


http://www.4crawler.com/Diesel/Tires.shtml#LOAD_INDEX

 

[porkysplace]

I agree it does not sound safe. However, if you look at the pics, there appears to be the correct tread contact at 10 psi. Plus there is no side bulge.

The walls on these tires are so stiff I think the tires with 0 psi or close to 0 may be able to support the weight of the light rear end. Later today I may air the tire down completely and see if they sit similar to 10 psi. If so, I will then air them up until the tread pattern indicates over inflation. This will give me a good idea of the pressure needed for this particular application.

By the way, I found this excellent article covering this very topic. It provides his formula to calculate psi for load to max load ratios. It also provides examples on how to check if your psi is correct through tread patterns, psi increases and heat.


http://www.4crawler.com/Diesel/Tires.shtml#LOAD_INDEX

Just because they look like they are hold the truck up does not mean the sidewalls are not being damaged . Low tire pressure may be alright at low speeds , but it will overheat a tire at higher speeds . Also keep in mind the max speed rating on these tires is 55 to 65 mph. Get a infared tempeture gauge and drive a few miles at different p.s.i.'s and do heat checks .

 

[Ford Mechanic]

I'm using the 395 xml's on HEMTTs. On pavement I run 50 psi, any less and she gets real tough to steer and my fuel mileage goes way down. Off road I run 10 psi.

Do you notice alot of tread wear in the center that way? I have aired mine down (on 395's) to 35 front and 30 rear trying to get full contact with the tread all the way across so they wear evenly. But mabey I'm barking up the wrong tree? I'll try it at 50 psi and see how it rides and drives. My thinking was that your passenger cars and trucks tend to have full contact the whole way across the tread so I was airing mine up acorddingly.

 

[doghead]

The "problem" is your putting way too high a load rated tire on a 10000lb truck.

 

[rosco]

That is a super article on tires. While on a slightly different vehicle/subject, I run load range E tires on my Toyota. I run about 20 psi in the rears (empty), with adjustment for cold tempertures. Probably your 10 psi is appropriate for the weight you are carrying on the tires. I have gotten criticism though, from the tire dealer, for not running much more pressure in the 10 ply tires. But there is that proportioning valve in the vehicle, that regulates the brake application pressure, front to rear too. The question is, on your vehicle, about safety and stoping on the highway, and how your vehicle will preform in a panic stop. As if there aren,t enough variables in the tire equation, already. Off road, issues on safety are different, but on the highway, I would be afraid that your truck would swap ends and be unstable, if you get on the brakes hard, regardless of tire types, with no load on the rear. I would not be afraid though, of doing damage to the tires, with the 10 psi inflation pressure. I'm sure though, that you are working at a different body style.

Thanks for the great subject!

 

[The King Machine]

Do you notice alot of tread wear in the center that way? I have aired mine down (on 395's) to 35 front and 30 rear trying to get full contact with the tread all the way across so they wear evenly. But mabey I'm barking up the wrong tree? I'll try it at 50 psi and see how it rides and drives. My thinking was that your passenger cars and trucks tend to have full contact the whole way across the tread so I was airing mine up acorddingly.

At 10 psi off road the truck rides like its on clouds....almost. 50 psi on the pavement is a good all around number for me. Having used the truck several times for driving 4 hours plus air pressure makes a big difference in fuel consumption. The more air in the tires the less drag.
For the longer hauls I've even aired them up to 70, with a 2000 lbs load.
As far as tread wear, these tires were designed for a much heavier application, running on a deuce I would expect them to wear well under such light working conditions regardless of tire pressure.

 

[Cycletek]

I have a M813 that I drive daily and I run 70 lbs in the front and 55 in the rears in my 14.00 x 20 Michelin XL's singled, they wear fairly flat depending on the roads I am traveling, but these tires are a compromise in design. While they work pretty well whether on or off road, but it is nearly impossible to build a tire that works in both area's of use and carry the load that these trucks require. This air pressure keeps the truck from being squirmy under hard braking and wandering while going through the canyon that I live in. Plus it keeps the temperature rise of the tire minimal.

 

[Scarecrow1]

The one thing unless I missed it be brought up is the tire it's self. You are running a tire that wasn't meant to on a deuce to start with. You can air down a tubed tire to a fairly low pressure with out it breaking the bead due to the tube is creating the seal at low speed as well as high speeds. The 1400,s are tubeless if you run them at high speeds with low pressure you are elongating the tire and creating stress on a tire that is known to be unstable at high speeds to start with. Also the tire was designed for off road use as it's proving ground not the public highways..... It is a safe bet that you will be happier with the tires at a middle ground of 30 psi and able to control it than to have good tread wear and you lose control and kill someone. Thing to keep in mind is that tractor trailers run the same air pressure empty and bob tail as they do hauling a heavy load and that is simple enough to understand with out a math book or a calculator .................

 

[rosco]

[Thing to keep in mind is that tractor trailers run the same air pressure empty and bob tail as they do hauling a heavy load and that is simple enough to understand with out a math book or a calculator .................[/QUOTE]

Yes, but this is a copromise! In fact, it might be said, that air pressure, in general, is a compromise, as well as the application the tires, wheels, and vehicle is in, wether its on-road, or off-road. It takes a lot of experimentation, trial and error, to find the sweet spot for the given application.

 

[welldigger]

The biggest issue you are having is you need more weight in the rear. Your going to have to compromise on ride quality vs. damaging sidewalls from running too little air pressure. As DH stated these are high load tires and you have a very light (relatively speaking) rear end. The rear is going to be bouncy even with a bed so subtract 1500 pounds give or take its really gonna be stiff and bouncy.

 

[MilSpec78]

Actually the ride is pretty good with the m105 springs. It is a little bouncy.

i took the rear tires down to 0 psi and the tires looked just a little low. Those are some stiff sidewalls!

i aired back up to 20 psi where it will stay as a compromise until the bed and cab are complete. The truck is mostly just for offroad right now anyway.

In short, too stiff of a tire for too light of rear end...but it makes me smile when I see it!

 

[Scarecrow1]

This the TM for the 1400-20s on the 923a1 tm 9-23- 220-272-10 it is for a 5ton. In reading it I found out I was wrong on my own psi I was told 30 empty but, the tm stated 60 psi on the road. We live we learn that is what this site is all about .......