I think the charts _only_ take the weight into account, _not_ the dynamic effect of a tall 'house' on handling. The chart would have you inflate the same for an equal weight of steel plates on a flat-bed truck as a tall moho.
I _know_ that my former SRW class B would have been fine according to the scale and charts with about 45 & 55 psi. I tried it, just once, briefly. The handling was so soggy as to be hugely unsafe. It rode 'nice' if you like body roll and no road feel, but that's all. I didn't even get as far as I-40 (which I can see from my porch) to try its response truck-wind response; I came home and put it back up to the Roadtrek recommended pressures (60-65 / 80). _Much_ better.
IMO the charts should be regarded as a _minimum_, not a sacred absolute. Pretty much the same with the sidewall; that's a maximum. The corrrect pressure lies in between, and may be lose to the coach makers' recommendation. But use common sense.
Reaching for my Nomex skivvies now....
Jim, "For every action, there is an equal and opposite objection."
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison ('Loafer's Glory'); '07 Forester 2.5 ( the 'HANDBSKT'); '95 Toyota SR5 V-6 4x4 pickup, ARB locker, Bilsteins, Warn hubs & M8000, etc;
'94 968, M030 swaybars ('DOPPLER')
Handbasket wrote: ..IMO the charts should be regarded as a _minimum_, not a sacred absolute.
No..., they are the correct tire pressures for the weight on the tires, regardless of what kind of box that rides on them. One decides the type of tire based upon the type of vehicle, the pressure based upon the weight on the tires.
Note: The chassis/MH manufacturers does not have "recommended" tire pressure - they give you the tire pressures based upon the GAWR and the tire weight tables. That's easy enough to verify.
If you choose an "soft" tire (not very stiff sidewalls), you'll get more wallowing than with a stiffer tire (like the XPS Ribs). I noticed the difference immediately when I switched from the Firestone Steeltex to the Michelin XPS Rib. Same tire pressures, but what a difference!!
It's simply not a good idea to inflate over the tire pressure designated for the weight on that tire. Personally, I think the tire manufacturers like Michelin actually do know what they are talking about when they put out these tables and even an RV Tire guide that tells us to use them.
I trust them far more than anyone else.
As for your 45/55 psi, it seems almost unbelievable that your MH was so light that the factory specified psi for full load of 65/80 was that far above the psi specified by weight.
Using the LT225/75R16 table, you would have been loaded to only 77% in both front and rear, and that's incredibly light for a fully-loaded MH. I wonder if you made a mistake someplace in reporting those numbers. They are hard to believe. Most MHs end up loaded up way closer to the tire limits than you are reporting. I consider my rig to be rather lightweight, relatively, and I am about 89% of the limits when fully loaded for travel. Maybe you could explain your numbers more fully? I am very interested.
On my Tioga 29V I run 60 in the front and 75 in the rear. These pressures are based on the weight over each axle and the chart from the tire manufacturer. Before I got it weighed, I ran 65 in the front and 80 in the rear. The RV drove like a chariot. It wanted to follow every little crack and grove in the pavement. After I adjusted the pressures for weight, the ride smoothed out and the handling improved. It now tracks well and provides a comfortable ride.
The B tires were 245-75 x 16's, load range E... a SRW Chevy Express 3500 Roatrek 190. It's long gone, and so is the scale ticket & the Michelin chart I worked from. That is indeed the Roadtrek, Pleasureway, and other B makers' recommended pressure for SRW Chevy 3500, older Dodge 3500, and AFAIK, E-350 chassis. Mostly driving here in the mountains, the tire wear was even.
Handbasket wrote: The B tires were 245-75 x 16's, load range E... a SRW Chevy Express 3500 Roatrek 190. It's long gone, and so is the scale ticket & the Michelin chart I worked from. That is indeed the Roadtrek, Pleasureway, and other B makers' recommended pressure for SRW Chevy 3500, older Dodge 3500, and AFAIK, E-350 chassis. Mostly driving here in the mountains, the tire wear was even.
You do whatever you want. I'll do it my way.
Jim, "Proud to be a carbon-based life-form."
You were only at about 80% of capacity (pretty amazing). I wonder if the real issue was poor tire selection for your use (probably not your fault)?
It doesn't take a physics professor to understand that when a tall (high profile vehicle) goes around a corner or gets leaned over in a cross wind that the outside wheels get more of the weight placed on them. Extra pressure (up to the tires limit) is a very practical and accepted way to reduce body roll.
mikeyairtime wrote: It doesn't take a physics professor to understand that when a tall (high profile vehicle) goes around a corner or gets leaned over in a cross wind that the outside wheels get more of the weight placed on them...
Sure, but any heavy vehicle going around a corner will shift considerable weight over to one side, and that is taken into account on the data tables. Class C motorhomes are hardly unique - they are much like thousands of delivery trucks all around the country.
However, Michelin does have a guideline on when to use higher pressure.
Michelin Truck Tire Service Manual wrote: For optimized tire performance, it is usually best to use the tire inflation pressure shown in the application data books for the particular axle load. Exceeding this pressure could result in reduced traction and tread life.
However, for steering axle tires, it is often acceptable to use inflation pressures greater than that shown in the application data books for the particular axle load. However, when operating the tire at maximum rated load never exceed the maximum sidewall specified pressure by more than 10% unless technical approval is obtained from Michelin North America, Inc. Never inflate to cold pressure beyond the rated capacity of the rim.
Following are two examples of applying the previous considerations to an operation where the user mounts new 275/80R22.5 XZA-1+ tires on a steer axle (emphasis added) and desires to increase the air pressure in order to see if this will help alleviate the occurrence of irregular wear. (emphasis added)
Example 1: If the axle load is 10,310 lbs., then the table in the data book specifies a corresponding pressure of 85 psi. So, the user can increase the pressure 15-20 psi above that to 100 or 105 psi.
Example 2: If the axle load is 12,350 lbs., then the table in the data book recommends 110 psi. As this is the maximum load of the tire, only a 10% pressure increase is permitted. Thus the adjusted
pressure would be limited to 120 psi.
murffdog wrote: Type "Tires" in the search box at top left, you have reading for a month. The orginal poster only asked a question, he doesn't want a beating.
True enough, though no one is "beating up" the orginal poster in any way. I think everyone is trying honestly to ensure he gets the best advice.
gerrytee wrote: I just picked up our 2000 Tioga on Ford 450 Superbody V10 engine from its second owner. The first owner Cruise Canada in their sign off sheet listed all tires to have 80 PSI pressure which the second owner faithfully maintained. The Spec sheet with all the weights pasted in the unit cubboard states the tire pressure to be 65 PSI. Since the front axle (GAWR=4600 lbs) and the rear axle (GAWR=9450 lbs) I believe that the correct tire pressures must be 65 to 70 front and 80 psi in the dual rear tires. The Wrangler E tires are rated at 80 PSI max.
Am I correct or could somebody give me better advice? I have not been able to weigh the unit yet.
The tire pressures on Ford's sticker on the door post should say 65 psi front and 80 psi rear, which correspond to the GAWRs.
A couple of days ago, there were postings of what people's rigs were weighing on the Ford chassis, and it seemed to be the front axles were generally under loaded. So, my advice is to go with Ford's recommendation until you get it weighed at all four corners.
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