tire & tire pressure question

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tire & tire pressure question

July 14, 2016, 12:49:00 am

Have 36ft 2002 295 the gvwr is 34,500 lbs, the gawr for the front is 13500lbs and the rear gawr is 20000lbs. The coach was fitted with 2016 Michelin 275/80R22.5G xza3+Evertread tires. The tires read at max pressure of 110 pounds the max load is 6175 lbs single tire and 11350 for dual tires. The plate by the drivers chair indicates the recommended tire pressure front should be 112 and rear 94. I'm new at this but am I correct that at the recommended pressure I will be fine in the rear BUT the max stated pressure of the front tire (and the rim itself) is 110 not 112 and at the max tire pressure of 110 I am still under the gawr for the front axle correct?
When I accepted delivery of the coach I was told all tires were filled to 95 pounds. According to Michelin at 95lbs of air the max axle end load for the front axle is 5510lbs....that puts me at 2500lbs under the gawr for the front axle is that safe?

So bottom line I should run 110 in the front and leave the back at 95 right?

Never knew you had to be a math wiz to put air in your tires!...wow...nothing is easy on these things!
Thanks everyone!

Re: tire & tire pressure question

Reply #1 – July 14, 2016, 01:02:59 am

Quote from: dke1955 – July 14, 2016, 12:49:00 am

Have 36ft 2002 295 the gvwr is 34,500 lbs, the gawr for the front is 13500lbs and the rear gawr is 20000lbs. The coach was fitted with 2016 Michelin 275/80R22.5G xza3+Evertread tires. The tires read at max pressure of 110 pounds the max load is 6175 lbs single tire and 11350 for dual tires. The plate by the drivers chair indicates the recommended tire pressure front should be 112 and rear 94. I'm new at this but am I correct that at the recommended pressure I will be fine in the rear BUT the max stated pressure of the front tire (and the rim itself) is 110 not 112 and at the max tire pressure of 110 I am still under the gawr for the front axle correct?
When I accepted delivery of the coach I was told all tires were filled to 95 pounds. According to Michelin at 95lbs of air the max axle end load for the front axle is 5510lbs....that puts me at 2500lbs under the gawr for the front axle is that safe?

So bottom line I should run 110 in the front and leave the back at 95 right?

Never knew you had to be a math wiz to put air in your tires!...wow...nothing is easy on these things!
Thanks everyone!

Front 13,500 Rear 20,000 = 33,500 not 34,500 something is wrong with the math.

Re: tire & tire pressure question

Reply #2 – July 14, 2016, 01:07:20 am

Quote from: craneman – July 14, 2016, 01:02:59 am

Front 13,500 Rear 20,000 = 33,500 not 34,500 something is wrong with the math.

Yep...this is what the plate in the coach states....I read it over and over to be sure I wasn't missing something....great huh!!

Re: tire & tire pressure question

Reply #3 – July 14, 2016, 05:27:43 am

Quote from: dke1955 – July 14, 2016, 12:49:00 am

Have 36ft 2002 295 the gvwr is 34,500 lbs, the gawr for the front is 13500lbs and the rear gawr is 20000lbs. The coach was fitted with 2016 Michelin 275/80R22.5G xza3+Evertread tires. The tires read at max pressure of 110 pounds the max load is 6175 lbs single tire and 11350 for dual tires. The plate by the drivers chair indicates the recommended tire pressure front should be 112 and rear 94.

When I accepted delivery of the coach I was told all tires were filled to 95 pounds. According to Michelin at 95lbs of air the max axle end load for the front axle is 5510lbs....that puts me at 2500lbs under the gawr for the front axle is that safe?

So bottom line I should run 110 in the front and leave the back at 95 right?

The pressures Foretravel puts on that plate are probably what puts the tire load capability at the same number as the axle limit. Will you be safe with that? Yes IF you aren't overloaded.

But your ride may suffer.

By far, the best thing to do is get the coach weighed as loaded for travel (including passengers). Individual wheel weights are best, if not, at least individual axles. Then get the load and inflation table FOR YOUR SPECIFIC TIRES (brand, model, size, and load range). Find the pressure that matches the heaviest end of an axle and add 5-10 pounds safety margin (to allow for loading variances). That is the appropriate inflation for your axle. Not what someone else does (unless they have your same weights).

RVSEF Wheel Position Weighing and Escapees' Smart Weigh SmartWeigh are two well-known options. National Indoor RV in Lewisville, Texas has also added 4-corner weighing capability if you happen to be anywhere near them. In the interim, you can get axle weights at truck scales.

Re: tire & tire pressure question

Reply #4 – July 14, 2016, 09:34:10 am

At about the time the coach was built many states were limiting single axle loads to 20,000 lbs. So that is what the plate says to comply even though it is a 21,000 lb axle. As Michelle suggests the best practice is to weigh each corner individually and set tire pressures accordingly. Second best is to weigh the front and rear axles individually and if there is room drive around and to the left or right of the scale so that only one side is on the scale then do some math. Load range G tires will give you a max load capacity of 12,350 lbs at 110 psi in the fromt and 22,700 lbs at 110 psi in the rear. So they are OK in the rear for the GAWR but undersized in the front. So it is very important that you know your actual weights especially in the front. If you do not know what the weights are I would run 110 in the front and 100 in the rear until yoiu know for sure. These are good tires if you are not overloaded relative to the tire capacity. Probably not but you could be close in the front.

We had LRG on our 36' coach and were within the load rating on all four corners, just barely in the front. We just switched to LRH and can run lower pressures with plenty of room to spare.

Re: tire & tire pressure question

Reply #5 – July 14, 2016, 09:35:39 am

Dave,
You can't get anywhere until you know what you weigh, so off to the scales.
Then set the pressure by the tire chart.
You could run five psi higher than chart settings but that will effect the ride.
A lot of us do that. I tried that but didn't like the change in ride so ran chart pressure and never had a problem.
I did send my tire pressure gage to the manufacturer and had it set to be accurate at 100psi.
When I got my gage back it had a picture of my gage next to the master gage. Both at 100!

Re: tire & tire pressure question

Reply #6 – July 14, 2016, 09:53:26 am

A friend turned me on to the fact the Washington State Patrol weigh stations are left on, even though it is not manned. That's how we weighed our coach. It takes a little moving around to get all fours separately, but I'd bet other States have the same deal.

Re: tire & tire pressure question

Reply #7 – July 14, 2016, 10:19:47 am

Quote from: mike leary – July 14, 2016, 09:53:26 am

A friend turned me on to the fact the Washington State Patrol weigh stations are left on, even though it is not manned.

Same thing in Oregon - scales left on all the time. Some are arranged so you can do "4-corner" weighs.

Re: tire & tire pressure question

Reply #8 – July 14, 2016, 11:54:35 am

Quote from: Chuck & Jeannie – July 14, 2016, 10:19:47 am

Same thing in Oregon - scales left on all the time. Some are arranged so you can do "4-corner" weighs.

Anyone traveling west from Portland, will find the one at Dundee Oregon is always on and has a huge amount room to turn the coach around and do 4 corner weights.

Re: tire & tire pressure question

Reply #9 – July 14, 2016, 12:52:15 pm

It is good to try to do all and one side or all four corners on the same scale. Sometimes that just doesnt work. Scales are close but the ones in OR read in 50 lb increments so assume you weigh 50 lbs more than the scale says and then add some for a cushion.

Re: tire & tire pressure question

Reply #10 – July 14, 2016, 01:09:52 pm

As suggested, your true weights are needed.
We loaded as we travel, and added full water, diesel, and propane. We were able to do a CAT scale, at a local truck stop. We weighted each axle, then told the voice in the speaker we were going to go around the parking lot for a re-weight. Then we did one side only, to get side to side corners.

I have to laugh at recommended tire pressures numbers posted on the data plates. How in 1998, did the coach, car, or motorcycle factory know what tires I was going to buy last month? We had a 1987 Toyota Supra for 15 years, and bought many different brand Z speed rated tires, over those years. I had side wall molded numbers that ranged from 28 to 42 as the correct for the loads.

Re: tire & tire pressure question

Reply #11 – July 14, 2016, 01:10:29 pm

Fill the tanks. Check the Michelin guide. Add 5LBs of air above 4 corner weight figure. Done

Our 280 is at 75 front and 90 rear per Michelin.

Re: tire & tire pressure question

Reply #12 – July 14, 2016, 01:15:59 pm

My DW controls the stern section, all her art supplies, papers, thumb drives, live back there. Next time we are about to weigh, when she's out, I'm going to root the area. I hope (since we're both rockhounds) that the "Lucille Ball Syndrome" has not affected her!

Re: tire & tire pressure question

Reply #13 – July 14, 2016, 01:21:29 pm

I have to laugh at recommended tire pressures numbers posted on the data plates. How in 1998, did the coach, car, or motorcycle factory know what tires I was going to buy last month? We had a 1987 Toyota Supra for 15 years, and bought different brand Z speed rated tires, over those years. I had side wall molded numbers that ranged from 28 to 42 as the correct for the loads.

So true. Per Michelin you want the proper tire footprint on the road to get the intended ride and handling characteristics of their tire. I believe to default to the highest pressure listed on the sidewall does not work best. IMHO.

Re: tire & tire pressure question

Reply #14 – July 14, 2016, 01:21:42 pm

If you're driving down the road and see a weigh station closed but that left the scales on do you wait two or three hours for the tires to cool as recommended before weighing or do you guestimate what they would be if you'd waited? What is the ambient temperature? Did you remember to take into account which side was on the south for solar gain? We seem to be worried about the 50 lbs the scale could be off and whether our tire gages have been calibrated correctly, etc, etc, etc. Ten miles down the road after waiting to weigh for the recommended time the tires are going to heat up with 5 or 10 psi and then when you check them in the morning they'll be way down again. Had a friend who checked his tires so often that he decided he had a leak and had them each taken off aI'm all for trying to stay safe and get as much out of my tires as I can but I've really got lots of other things to do that I belive are really far more important than 5 psi either way. Safe travels.

Re: tire & tire pressure question

Reply #15 – July 14, 2016, 02:25:33 pm

Larry, weighing the coach at a weigh station has nothing to do with tire temps or outside air temp. Once you know your weights you can do a cold tire pressure adjustment the next morning.

Tire pressure stated on the data plate is for the OEM tires. The weight capacities at different air pressures for a give load range tire probably haven't change much over time.

Outside air temps do make a different. When we have left Hastings at -30 °F with tires inflated for that temperature and have been in Texas two days later there can be a 100 °F temp difference. If we were at max air pressure when we left we would have been overinflated two days later.

Most tires we use on our coaches are considered flat at 80 psi or 20% below normal pressure.

I have had an RV since 1974. Tire pressures get measure before we move. Good habit.

Re: tire & tire pressure question

Reply #16 – July 14, 2016, 02:58:02 pm

The only thing I would add to what Roger just said, is that instead of measuring tire pressure the old fashioned way before leaving, I use my Pressure Pro. That way, I don't lose a little bit of air each time, and end up needing to add air every month or two.

The data plate is a safe starting point, you mite be overinflated if the coach is not fully loaded, but that is better than under inflated, The data plate is based on a fully at max weight coach using the OEM tires. After I had my coach weighed, four corners, I found my new weight was the same in front but slightly reduced in back. I am near the max on the front axle, but well under on the rear. Side to side weights are nearly identical. Foretravel makes well balanced coaches.

Re: tire & tire pressure question

Reply #17 – July 14, 2016, 03:17:59 pm

Tom, I have a TPMS that is on all the time. One time it alerted us to a slow leak (bad stem valve) a couple days before we were ready to leave so we had time to do something about it. I still check each tire -- force of habit sort of thing. And it makes me do a walk around and look at things while doing so. Double check bay doors, awning locks etc. I think after so long now I would feel like something was not done and feel unsettled as we set out. Sometimes I will check the evening before. Before we got our FT we never had a TPMS or any of the fancy stuff we have now.

Edit

I have read that tire pressure will increase about 1 psi for each increase 10° increase in temp.

Re: tire & tire pressure question

Reply #18 – July 14, 2016, 04:30:44 pm

Before we went out today, checked our tire psi, it's been in the 100's here. Our fresh Michelin 255/80R/22.5/XRV tires all showed higher psi than when we were up north. I don't figure the slight difference a big deal, since the coach tracks so well. I still have my rubber mallet from the logging days, but a "knock knock" on the tires tell me what I need to know.

Re: tire & tire pressure question

Reply #19 – July 15, 2016, 12:05:40 am

Quote from: Bob & Sue – July 14, 2016, 01:10:29 pm

Fill the tanks. Check the Michelin guide. Add 5LBs of air above 4 corner weight figure. Done

Our 280 is at 75 front and 90 rear per Michelin.

I would assume that's backwards?

Re: tire & tire pressure question

Reply #20 – July 15, 2016, 12:17:38 am

I have had many experiences as a Foretravel manager with tire pressures/ride quality issues.

One that comes to mind was a customer who traded in a 78 35' dodge with its air tag for a 29' Chevy grand villa.

Rare. Extremely. To go smaller.

Had to order in the hydraulic jacks from Mike grimes so two weeks later they brought their 29' in for the install.

Their old coach was in service not sold yet and they asked to see it again.

DW mentions how nice the coach was "but it rode so bad"

Light came on over my head. OMG. Kicked a tire and almost broke my toe on the 8r/19.5.

They left and I took a tire gauge with me back to service and all the tires were full sidwall pressure.

Drove the coach and everything rattled.

Dropped the pressures and it was like on pillows.

Perfect

Told a lot of customers the story

Re: tire & tire pressure question

Reply #21 – July 15, 2016, 07:21:42 am

Quote from: Caflashbob – July 15, 2016, 12:05:40 am

I would assume that's backwards?

I agree.

Re: tire & tire pressure question

Reply #22 – July 15, 2016, 12:30:41 pm

Quote from: Bob & Sue - 23 hours ago
Fill the tanks. Check the Michelin guide. Add 5LBs of air above 4 corner weight figure. Done

Our 280 is at 75 front and 90 rear per Michelin.

I would assume that's backwards?
More...Like Quote

I will be re weighing the coach today but per Michelin chat the pressure is over some.

Re: tire & tire pressure question

Reply #23 – July 17, 2016, 09:46:22 pm

Quote from: wolfe10 – July 15, 2016, 07:21:42 am

I agree.

I don't understand why this would be considered backwards, the MFG plate shows the pressure for the MAX GAWR to be 85psi single and 90psi dual for the GVWR of 28,000 or 10,000 front and 19,000 rear. The OEM tires were 275/80 LRG. Using a LRH rated tire you should be able to run slightly less pressures for the same weights as the LRG.
Maybe I am missing something?

Re: tire & tire pressure question

Reply #24 – July 18, 2016, 03:56:34 am

First time I have seen a Michelin chart to my memory where the tire pressures do not drop the same percentage of max as the load.

Thanks for bringing this up. Michelin is saying that at a certain percentage below the max for load and pressure that the pressure decouples the ratio between the two and the pressure drop starts increasing faster than the load is decreasing.

big difference in the front axle weight on our u320 at 12k versus your u280 at 10k.

Our rear is 20k yours is 19k.

Five percent difference on the rear and nearly twenty on the front.

I would have overinflated a u280 fronts without the chart.

I would wonder if using "h"s and the corresponding even lower pressures might show different handling. Squirm?

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