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Old 09-20-2016, 02:54 PM   #1
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Tire Inflation??

Hello,

Last week I took my Motorhome with Toad attached to a CAT Scale ( Certified Automated Truck Scale) and had it weighed on a 3 platform scale.

Here are the results:

Steer Axle.........4440 lbs
Drive Axle.........9840 lbs
Toad................3600 lbs

The Motorhome has a GVWR of: 14500 lbs ans a GCWR of: 22000 lbs

The tires are Michelin LTX M/S2 LT225/75R16 Load Range E

going to the Michelin Web Site I can't find those tires listed on the RV nor Truck section. So looking at the RV chart that lists the RV XPS RIB Tire, it shows that the Steer Axle (Front) should be inflated to 62 +/- psi & the Drive Axle inflated to 80 psi. Meanwhile the metal tag attached to the post at the Drivers Door says 75 psi for Fronts and 80 psi for the Duals.

Does dropping from 75 to 62 +/- seem too much?? Comments??

Thanks in advance, you Guys are teaching me more than I thought I was going to have to learn.
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Old 09-20-2016, 05:14 PM   #2
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Man are you close to your GVWR rating .
You've given the scale weights for the axles and the GVWR. , but not the individual axle ratings , to compare to the weights , you could be over on an axle and have to do some serious weight distributing.
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Old 09-20-2016, 07:30 PM   #3
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You're over your rear axle weight rating by 240#.
Your drive axle weighs 2,020# more than my 32 1/2' fully loaded class C on an E450.
Do you have a rock collection ?
Have you tried driving with the front tires at 75# ? If the ride isn't that bad stick with that. I wouldn't go any lower than 65# in the front. 70# would give a little bit more of a cushion over the weight you're carrying.
You're only 40# under what those tires on your drive axle are rated for, at 80#. Can you move or remove weight over your rear axle ? Are you traveling with a full fresh water tank ?
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Old 09-21-2016, 06:05 AM   #4
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prndl-

As an overarching principle, unless you can get four-corner (i.e., individual axle-end) weights, you're better off using the coach manufacturer's pressures, as listed on the label. That's because axle weights do not take into account side-to-side differences, which can be significant.

That said, as "Mich F" points out, it appears your coach is overweight on the rear axle by 240 pounds. The Ford Body Builder Layout Book, available at this link, says the rear-axle GAWR (Gross Axle Weight Rating) is 9,600 pounds. This is a safety issue. Before anything else, your concern should be to reduce the rear-end weight of your coach to get it below the rear-axle GAWR, preferably by several hundred pounds, so you can carry fluids or shift things around in small ways and not worry about frequent reweighing.

The most common tactics to reduce rear-end weight are to carry little fresh water and to shift heavy items forward.

If you are able to get four-corner weights, there is another problem: As you point out, Michelin does not list an LTX M/S2 in the 225/75R16 Load Range E size, at least in the four Michelin Truck Tire Data Books I've downloaded. I would check the dates marked on the tires, the model (an M/S model is listed in the books), and the size. It's possible you've misread one of those. Although Michelin inflation tables group tire models by type (e.g., light truck), size and load range, I'd be wary if your tires appear nowhere in the official documents.

Finally, as weighed your coach is only 220 pounds below the GVWR. That's equal to around 28 gallons of water. You don't say if you weighed your coach with full fresh, black or grey water tanks. If any of those tanks were empty when you weighed, then running around with half-full tanks will put you over the GVWR for the coach. It's be wise to get rid of several hundred pounds of "stuff" in your coach, and shift some of the remainder from rear to front, then re-weigh.
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Old 09-21-2016, 06:45 AM   #5
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You are looking at the right chart, assuming you're looking at the 16" XPS Rib tire which is the same size as your tires. Michelin says on that chart -
Quote:
To select the right load and inflation table for your commercial tires, locate your tire size, then match your tire's sidewall markings to the table with the same sidewall markings.
Your M/S2 tire replaced the M/S, many of which were recalled and replaced by Michelin.
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Old 09-21-2016, 08:27 AM   #6
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Off topic a bit but,,

I was curious to see what was being manufactured these days with 14,500 gvwr so went and looked at the Thor website. On their 2017 Class C models, they list a few 32 footers, with slides, on this chassis. Them things have to be close to being over when they leave the factory,,, or am i missing something?
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Old 09-21-2016, 08:40 AM   #7
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hi there
i am in a similar quandary on making sense on tire inflation. i dont want to hijack this thread, but is also dont want to start a new one since there are so many of these. so, here i go.

2016 thor 22fe class c motorhome

dry weight per thor was from factory was 9852

GVWR is 14500

had it weighed on 8/29 fully loaded for camping except the DW and plus 120# great pyr. full fuel, lp. empty black/ gray and fresh water tanks


front GAWR 5000
CAT weight 3880

rear GAWR 9600 dual
CAT weight 7300

no toad but GCWR 22000

front tires lt225/75r16e 115/112r
rim 16 x 6.0 k

rear tires lt225/75r16e 115/112r
rim 16 x 6.0k

door sticker says
75 psi front
80 psi rear

this assumes max weight evenly distributed at all times

this chart from Michelin states
Load & Inflation Tables | Michelin Truck

for my weights
front 50 psi at 3880 lbs
rear 55 psi dual at 7300

am i reading this wrong and if not which one is right Ford or Michilen

thanks in adance
patrick
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Old 09-21-2016, 08:50 AM   #8
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Quote:
Originally Posted by Mudfrog View Post
Off topic a bit but,,

I was curious to see what was being manufactured these days with 14,500 gvwr so went and looked at the Thor website. On their 2017 Class C models, they list a few 32 footers, with slides, on this chassis. Them things have to be close to being over when they leave the factory,,, or am i missing something?
Nope probably haven't missed anything. RV companies do their best to just pass the minimum requirements. After all they don't sell RV based on load capacity but on how many slides & TVs they stick into the RV. Load numbers and tire info is boring.
Bells, lights and mirrors are easier to sell.


RE OP question and some of comments.
1. RV should be weighed when it is heaviest. Don't unload then get weighed then load up.
2. IMO as a tire engineer I would suggest use a 47/53 side to side load split if you only can get full axle weight on a scale. Many CAT scales are now making it impossible to set one side weights. In fact that is against company policy.
3. I cover how to weigh and calculate tire inflation, including margins in a number of posts on my blog. Please note that trailer and motorhome info is different.
4. You should ALWAYS have a margin and be above the minimum inflation needed by 10% for motorhomes and certainly below the maximum axle weight (some suggest at least 10%) Trailers should have at least a 15% margin on tire loading and should ALWAYS run tire sidewall inflation. Never lower to lower chance of tread/belt sep due to Interply Shear.
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Old 09-21-2016, 06:02 PM   #9
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Quote:
Originally Posted by Mudfrog View Post
Off topic a bit but,,

I was curious to see what was being manufactured these days with 14,500 gvwr so went and looked at the Thor website. On their 2017 Class C models, they list a few 32 footers, with slides, on this chassis. Them things have to be close to being over when they leave the factory,,, or am i missing something?
My 32 1/2' Class C, has a 3,275# OCCC, which is more than a lot of Class As out there that I've seen. It doesn't have levelers and stuff like that, but it also isn't carrying around the weight of all that stuff. It does have one slide out, about 12' long.
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Old 09-21-2016, 06:51 PM   #10
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Quote:
Originally Posted by Mich F View Post
My 32 1/2' Class C, has a 3,275# OCCC, which is more than a lot of Class As out there that I've seen. It doesn't have levelers and stuff like that, but it also isn't carrying around the weight of all that stuff. It does have one slide out, about 12' long.
Well, there ya go. Our little 28 footer is a bit above 12,000 so i kinda assumed (you know what they say about that) that a longer model would weigh even more.

Sorry to be off topic,,, Back to tire pressure!
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Old 09-22-2016, 09:13 AM   #11
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When looking at tire pressure charts it should be understood that these are minimum pressures for the weight on each tire...they are not "pressurize to exactly this pressure for this weight." The effective weight that each tire "sees" changes dramatically as the vehicle bounces down the road. Most tire pressure wear and tire safety incidents occur from under inflation.

I target my tire pressures to the door sticker cold tire pressure values. Lower pressures *may* give a better ride, but the tradeoff is more stress/flex on the sidewalls...and the sidewalls are what carry the weight of the vehicle. For my cars I keep tires right at the the door sticker values; for the MH (with it's higher pressures) I keep them within a few psi...if under by more than 2-3psi (cold) I will inflate immediately, and when temperatures vary during a trip (this was fairly dramatic on a recent trip from Dallas to Yellowstone) I'll adjust during the trip after rest/fuel stops.

I am just as guilty as anyone else from trying to rationalize my choices in RVing based on what I *want* versus what I *need* (nobody *needs* a MH). But I always ground myself with the philosophy that it's better to be safe when driving down the road with 10K/20K/30K lbs of weight...blowouts can affect not just me, but others as well.
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