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Old 09-18-2015, 11:43 PM   #29
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Quote:
Originally Posted by l1v3fr33ord1 View Post
Tireman9-

A question came to me as I looked at the manufacturer's tire loading charts for a Class A I'm considering.

Suppose the measured (i.e., scale-weighed) weight on an installed tire was lower than the lowest charted pressure for the tire?

I know this is an uncommon problem. I can see four ideas right off:

1) Run the installed tires at lowest charted pressure
2) Run the installed tires below charted pressure
3) Install tires that can run within their pressure chart at those weights
4) Increase the coach weight (e.g., carry liquids)

Each of these has drawbacks. If smooth ride was the goal and money not a problem, I'd pick 3), but I'm interested in your take- and have probably overlooked some other options.

Thanks!

Mark

P.S.- Here are the actual values that have me asking this question:

Monaco Monarch 30PDD SVE (Ford F53 chassis)
Gross vehicle: 20,500 lbs
Gross combined: 25,500 lbs
Front gross axle: 7,000 lbs
Rear gross axle: 13,500 lbs

Michelin tires
Size: 245/70R 19.5
Max. speed: 75 MPH
Minimum charted pressure: 80 psi
Single tire, 80 psi: 3640 lbs
Dual tire, 80 psi: 3415 lbs

These values are from the Owner's Manual. But, they also match the Michelin Truck Tires Data Book, for this size tire, models XRV (LRF) and XVE (LRG).

2 x 3640 lbs = 7280 lbs (exceeds front gross axle)
4 x 3415 lbs = 13,660 lbs (exceeds rear gross axle)

Maximum tire load at minimum pressure exceeds axle load maxima in both cases.
In your example you should
1.Run the installed tires at lowest charted pressure 80 psi

on truck wheels that have 15 degree taper bead seat there is a need for minimum pressure to keep the tire seated. This is different than with the 5 degree seen on LT, ST and P type tires.
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Old 09-19-2015, 12:00 AM   #30
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Quote:
Originally Posted by twinboat View Post
I read this over and over, from some on this site, but when I read my tire sidewall, I read this;

LOAD RANGE E
MAX LOAD SINGLE 1215 kg ( 2680 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS
MAX LOAD DUAL 1120 kg ( 2470 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS

To me, this means no more then 2680 lbs of weight on the tire.

It also means that I should NOT put more then 80 lbs of air pressure in that tire.

I can't find anything about Minimum pressure.

They are 16" light truck tires on my Class C, MH, maybe larger tires have something else on the sidewalls, but I have never seen it.

The pressures in the load & Inflation tables are in fact the minimum inflation needed for each stated load. So at the highest level of inflation for each Load Range the Minimum inflation is also the Maximum inflation.
This is why it is better to load a tire to less than it's max capability as then when you inflate it to the inflation that is greater than the minimun needed to carry the laod, you still have a tolerance or margin between the Max and Min.
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Old 09-19-2015, 06:20 AM   #31
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Tireman9-

Please bear with me- I'm just trying to get to an "optimum," as you said.

In the example I gave, the minimum inflation pressure is 80 psi (for the installed tire). At that pressure, the maximum weight the tires can support is 20,940 pounds. The GAWR (loaded weight on the axles) maximum limit is 20,500 pounds. If I weigh the loaded motorhome and it comes out at 20,000 pounds, I use 80 psi cold tire pressure all around. Hunky-dory.

But, what if I was driving the motorhome completely unloaded, say at 17,800 pounds? That's 2,700 pounds less than the GAWR. For the moment, assume equal weight on all tires. That 2,700 pounds translates to 450 pounds less load on each of six tires than what the load would be at 80 psi. Correct me here: Aren't the tires at 80 psi overinflated for this condition? But, the tire chart (and design, per your post) does not allow me to lower the tire pressure to match the motorhome weight.

Upshot: If I drove frequently below the maximum weight, wouldn't I be better off replacing the 245/70 tires with 225/70 tires? The 225 has a load range (single) from 2755 to 3950 pounds, wider than the 245s, at 3640 to 4080 pounds.

Thanks for your patience!

Mark
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Old 09-20-2015, 10:42 PM   #32
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Quote:
Originally Posted by l1v3fr33ord1 View Post
Tireman9-

Please bear with me- I'm just trying to get to an "optimum," as you said.

In the example I gave, the minimum inflation pressure is 80 psi (for the installed tire). At that pressure, the maximum weight the tires can support is 20,940 pounds. The GAWR (loaded weight on the axles) maximum limit is 20,500 pounds. If I weigh the loaded motorhome and it comes out at 20,000 pounds, I use 80 psi cold tire pressure all around. Hunky-dory.

But, what if I was driving the motorhome completely unloaded, say at 17,800 pounds? That's 2,700 pounds less than the GAWR. For the moment, assume equal weight on all tires. That 2,700 pounds translates to 450 pounds less load on each of six tires than what the load would be at 80 psi. Correct me here: Aren't the tires at 80 psi overinflated for this condition? But, the tire chart (and design, per your post) does not allow me to lower the tire pressure to match the motorhome weight.

Upshot: If I drove frequently below the maximum weight, wouldn't I be better off replacing the 245/70 tires with 225/70 tires? The 225 has a load range (single) from 2755 to 3950 pounds, wider than the 245s, at 3640 to 4080 pounds.

Thanks for your patience!

Mark
Please read post #26 in this thread and stop trying to find an "optimum" inflation pressure when you only talk about tire loading.

No you should not run a pressure lower than the lowest pressure for your tire as shown in the tire manufacturer's charts.

No I see no reason or advantage to change tire size so you can have a lower margin on load capacity.

I would not be concerned about being 5 or 10 or even 15 psi higher than the minimum needed to support the actual load on the tires.

My normal recommendation is to run at least 10% ABOVE the inflation found in the tables for your measured load as long as you are not exceeding the wheel rating.

I would stop estimating tire loading and make the effort to confirm your actual load for each corner of your motorhome. I understand you are presenting examples but dealing with the actual numbers is of much more value.

Again your wheel design is different than P or LT type and maintaining at least the inflation in the table is important for vehicle safety.
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