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Old 09-16-2015, 10:41 PM   #1
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Tire Pressure, how low can you go?

Hi,

We recently flew to TN to pick up our new to us motorhome, that story is on the site under "Class A Resurrection".

Our first stop was a tire dealer in Knoxville to replace all 6. The price was great, $2745.00 for Firestone FS561 255/70/22.5's mounted and balanced. All last 2 month date codes and a preset appointment, in and out in a little less than 90 minutes. So far so good, when we left, the handling appeared to be a bit loose and I figured it to be the new tread needing a few miles to break in.

250-300 miles later, it wasn't getting any better, so when we stopped at our next camp site, I borrowed a tire gauge to check the pressures.
The fronts were 110psi and the rears at 100psi. I went online to the Firestone inflation chart page and looked up the range for our factory axle ratings. Not to much help, but I felt comfortable dropping the fronts to 95psi and that helped a lot. However, I still need quite a bit of steering correction to keep her in the lane.

Once we arrived home, we loaded up the MH and went to the weigh station. Full fuel, full water and half and half on the holding tanks. pretty much loaded with all the necessary equipment we would travel with. The scale only weighs the front and rear, no corner weights. I'm still looking for a local scale with that ability.

Here is the question,

Front axle 7000lbs
Rear axle 14240lbs
Gross weight 21240lbs

According to Firestone it appears that the front inflation for a single tire would be approximately 4190lbs per tire @ 80psi
The rear inflation for a dual tire application would be 4110lbs per tire @ 85psi.
This would give a load carrying weight of 8380lbs for the front axle and 16440lbs for the rear axle for a total of 24820lbs.

I understand that without a 4 corner weigh it's harder to calculate the heavy side and distribution, so some additional pressure may be needed.
We also tow a trailer sometimes and the tongue weight is about 350-400lbs, so that needs to be factored in as well.

I have seen several posts about tire inflation on the site, I just need a little help from someone who has a better handle on this than I do.
80psi front and 85psi rear just seems a little low. I want to find the happy medium for best handling and safety.

Here is the Firestone Load and Inflation Tables.

http://commercial.firestone.com/cont...s_web_2014.pdf

Any one want to chime in and give me a hand?

Thanks,

Hooptie
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Old 09-16-2015, 10:51 PM   #2
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Michelin says that it takes about 30,000 miles for truck size tires to break in and give the best handling and fuel mileage.

Never go lower in pressure than the lowest shown on a tire chart.

I see 80 psi front and 80 rear. I'd go 5 psi over that since you don't have actual axle end weights.
80 psi single is 4190 each for 8380#'s
80 psi dual is 3970 each for 15580#'s
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Old 09-16-2015, 11:10 PM   #3
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I had a similar issue years ago when I owned a kenworth and was contracted to FedEx ground. FedEx loads are very light & my truck was riding like the axles where bolted to the frame. I made a call to Michelin and gave them my typical axle weights. I was told by them to lower my pressure from 100-110 to 80-85 Lbs. It made a marked improvement in my ride comfort. The tires lasted just as long as the prior set did in the past at the higher pressures. About 70,000 miles on the steer tires and 250,000 miles on the drive tires. Your tire manufacture knows all about what pressures their tires need to support a given load. I would head their recommendation.

Also different tire brands and differing models within a brand will give differing rides and handling. A ribbed highway tread will have less squirm than a tire that has a lugged tread. Your FS561 is most definitely a highway rib style. Was the old set worn down or just out of date? As a worn tires tread will be much firmer simply because of not being as deep as new tread will be. How is your alignment? I always had a full alignment done at ALL tire changes. It does not take long to establish a wear pattern on a new tire because of an out of spec rig alignment.
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Old 09-16-2015, 11:13 PM   #4
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G or H Load Range?

By not having four corner, you really are risking if you go by a straight 50/50 division of the weight per axle. You might try the owners group for this coach, and ask if others will share their four corner weights for your year and model coach. This will give you some perspective on if one side of an axle is dramatically higher then the other (Can happen more then you'd think:(!).

In absence of some kind of input to see if one side an axle is heavier then the other, I think that I would take the 50% of the weight, and then add 500 lbs to it, and use that to consult the chart. (Again, Load Range is important when looking at the chart.) And then I personally add a 5 PSI safety margin on top of that. (If you search for thread by Tireman9, you will find that he recommends a contingency percentage on top of the the charts recommended PSI. If I recall right it was 10%).

For sure, until you get four corner weights, it is better to error on the higher side, then it is on the lower side...

Best of luck to you,
Smitty
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Old 09-16-2015, 11:22 PM   #5
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Quote:
Originally Posted by Smitty77 View Post
G or H Load Range?

By not having four corner, you really are risking if you go by a straight 50/50 division of the weight per axle. You might try the owners group for this coach, and ask if others will share their four corner weights for your year and model coach. This will give you some perspective on if one side of an axle is dramatically higher then the other (Can happen more then you'd think:(!).

In absence of some kind of input to see if one side an axle is heavier then the other, I think that I would take the 50% of the weight, and then add 500 lbs to it, and use that to consult the chart. (Again, Load Range is important when looking at the chart.) And then I personally add a 5 PSI safety margin on top of that. (If you search for thread by Tireman9, you will find that he recommends a contingency percentage on top of the the charts recommended PSI. If I recall right it was 10%).

For sure, until you get four corner weights, it is better to error on the higher side, then it is on the lower side...

Best of luck to you,
Smitty
Pressure chart doesn't show any difference for the load range for the 255/70 22.5's until you get to the higher weights/pressure.
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Old 09-16-2015, 11:24 PM   #6
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Hi Super7pilot,

The old tires were 13 years old, the rig was parked for the last 5 years and had not been started in over 2. It was scary to drive it 42 miles into Knoxville to get them changed. Front end was done at the same time and I'm hoping that the reduced air pressure will make for a more comfortable drive. When we left the tire dealer it seemed like super power steering. 200 plus mile of that will wear on you nerves.
We are home now and I hope with input like yours and the others that it will be a whole lot better at a reduced pressure.

You"re right, BMW's are cool, Super 7's are Bats**t crazy.

Thanks,

Hooptie
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Old 09-16-2015, 11:35 PM   #7
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Hi All,

When I posted the axle weights, the fronts @ 80psi give an additional 1,380lbs of capacity and the rears @ 85psi give 2,220lbs of capacity.
Would I be correct if I added 500lbs per side front and 5% cushion to the fronts for a total of 8,400lbs. That would be right at 80-85psi.
The rears at 500lbs per side plus 500lbs for trailer tongue weight plus 5% would be @ 85-90psi.
Am I doing this correctly?

Hooptie
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Old 09-16-2015, 11:43 PM   #8
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Hi All,

When I posted the axle weights, the fronts @ 80psi give an additional 1,380lbs of capacity and the rears @ 85psi give 2,220lbs of capacity.
Would I be correct if I added 500lbs per side front and 5% cushion to the fronts for a total of 8,400lbs. That would be right at 80-85psi.
The rears at 500lbs per side plus 500lbs for trailer tongue weight plus 5% would be @ 85-90psi.
Am I doing this correctly?

Hooptie
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Old 09-16-2015, 11:44 PM   #9
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I don't mean to sound to uppity. But with 1.6 million miles of tractor trailer driving under my belt. That getting into the axle loads from side to side is rather getting into the picking of nits. My semi was always carrying a differing load. With over 300 gallons of fuel on board. I could spend most of my trip playing with pressure adjustments to compensate for the variances from fuel usage or topping up alone. Yes weight can vary side to side. But if your rig was SO heavy on one side to the point of needing any sort of tire pressure differences side to side. Then you have a whole set of different issues to sort out. But by all means one should er on the side of high rather than lower pressures.

Your right hooptie. My seven is bats**t crazy. But it's a hoot to drive. had a zed 4 when I lived in Germany. I took the zed to the Nurburgring. Boy was that a blast. For those that don't know what the Nurburgring is. Here you go.
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Old 09-17-2015, 06:50 AM   #10
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I think you need to check alignment. Sounds like you have more going on than just a few pounds of air pressure.
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Old 09-17-2015, 08:55 AM   #11
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Michelin have special load tables for RV application that tries to allow for real-world situation of rarely being balanced from side to side.
Tyre dealers always put 100psi in because it is a nice round figure and easy to remember. Others might pump every tyre up to maximum sidewall pressure because that doesn't require remembering anything at all.
Having the correct pressure means your tyres can act as part of the suspension and absorb a lot of the shocks caused by road joints and such, plus they should wear evenly across the tread. Win win.
Pays to check tread wear across the face every year as it will confirm correct pressure or show that you should alter it.
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Old 09-17-2015, 09:21 AM   #12
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Our old T28 Bounder, had substantial differences on weight from side to side. And I recall reading of other RV's from different manufacturers that were pretty high in the differences from side to side.

Mr D. - Yep, I missed that the G and H PSI's differences were in the higher weight ranges. (I was going off of memory on these tires, being available in two different load ranges.)

RV Tire Safety

Good reading from Tireman9 above, retired tire engineer and I find his site to be outstanding with solid information on all things tires!

Good luck OP, hope the new ride sorts out for you,
Smitty
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Old 09-17-2015, 10:14 AM   #13
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Don't get hung up trying to calculate side-to-side variations or load variations, but DO allow for more than the absolute minimum weights. Axles are unevenly loaded, and the loaded weight of the coach does change a bit, all the time. You handle those variations with an extra safety margin, typically in the 5-10% range. You can add the margin to the weight before you look up the pressure in the tables. It is also wise to go above the minimum shown in the table for your weight, so accommodate varying day-t0-day temperatures. Experts like Tireman9 recommend going up one table entry so that you don't have to worry about being underinflated if the weather sudden gets colder.

The table shows that the minimum for your tires is 80 psi, regardless of load, and at that minimum they carry 4190 lbs (single). That's far more than sufficient for your front axle, so 80 psi is sufficient for the fronts.

For the rear, you will want enough combined capacity for the 14,240 plus that extra margin, so lets call it 15,240. Divide that by 4 and it comes out to about 3810 per tire. Looking at the (dual) values in the table, we see that the 80 psi minimum will carry 3970 lbs, which is sufficient even with the safety margin. 80 psi is adequate, then, for the rear axle as well.

To avoid futzing around with pressures when the weather gets colder, I would use 85 psi in those tires instead of 80. That way you should not go below the 80 psi minimum even if cold weather drops the pressure a bit. An extra 5 psi will have negligible effect on ride quality or tire wear, and might even gain a tiny bit of fuel economy.
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Old 09-17-2015, 10:07 PM   #14
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Last January I put a set of Firestone FS 561"s on my coach. The tire shop put in 110 lbs. all the way around. They felt a little squishy so I checked the air pressure and dropped them to 105 on the front and 100 on the rear. After about 1000 to 1500 miles they now ride and handle great. I personally wont go lower then that.
I have had that same thing happen with various sets of tires on trucks that I have driven and it just takes a few miles and they settle down. After a few miles if they don't then you need to look at alignment etc.
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