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Old 05-06-2021, 08:09 AM   #1
US1
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Rounding up tire pressure?

I went and weighed my RV last weekend to get tire pressure set. Full gas, full water tank for a max load we will carry. RV was loaded with what will be our normal load personnel belongings wise I don’t anticipate carry much more. Only variable is I was alone. Add 105# for the wife.
Michelin tires, so I’m using their chart which use per axle ‘end’.

Front is 3220lbs per axle end. Add 55# for the wife, call it 3275lbs
Rear is 6460 lbs per axle end.

Using the chart:
front comes in at under 70 psi ( which is min tire psi)
Rear under 75psi.

I’m understanding this to recommend 75psi for fronts and 80psi for the rears?? This allows a little safety margine, but not the 10-15%.
Before I weighed it, I put 85 in front and rears, thinking I’d lower the pressure when I got to campsite once I looked over the charts. The RV rode like a tank! Every crack was felt. When I picked it up from dealer 2 weeks ago, all tires had 75psi and it rode nice. (Drivers placard recommends 90psi. Way to rough!)
I really liked how it rode with the rears at 75, but not sure if that’s right on the edge.
Are my 75/80 numbers inline, or could I use 75 all the way around?
Thanks
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Old 05-06-2021, 09:08 AM   #2
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I add 5 PSI to what the charts show!
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Old 05-06-2021, 09:59 AM   #3
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The other consideration is if you only did axle wts and not individual wheel wts using the avg may be off.
It is not uncommon to have some variation side to side and you ◊ant to use the highest wt for figuring pressure and use that with whatever safety factor you choose for both sides.
10% +/- side to side not uncommon.
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Old 05-06-2021, 10:33 AM   #4
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Your MH is on a leaf spring chassis it will ride like a truck regardless. Remember, when you use the minimum pressure for the load you are operating your tires at 100% of their capacity.
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Old 05-06-2021, 12:16 PM   #5
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The charts show the inflation required for the actual load. If you check the weight before every travel day and adjust the psi for the current actual weight, the ambient air temperature and air pressure, you can use that psi directly.


Some RVers do that, but the majority do not. Instead, they inflate to a bit more than the chart values to allow for possible increase in weight and daily variations in external temperature and air pressure. That way they don't need to fiddle with air adjustments every day or two. The extra is often referred to as a "safety" margin, but its really a "convenience" margin.


If I knew I was traveling from the plains to the mountains or to a different climate, I might add a substantial extra cushion, but for most uses and extra 5 psi is probably plenty to insure pressure is ok without frequent checks. And if you have a TPMS, you can easily keep tabs on actual pressures.
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Old 05-06-2021, 02:24 PM   #6
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Thanks all. Appreciate the comments.
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Old 05-06-2021, 02:36 PM   #7
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Check/Adjust tire pressure on AM of Travel Day

Run 75 Front and 80 Rear

See how that 'rides'

When you have time/place....preferably asphalt/concrete surface
Get some sidewalk chalk (big hunk of chalk)
Wipe it across the threads on all tires...big wide stripe
Drive 25' straight forward then have a look at the chalk stripe

Chalk missing in the middle...too much pressure
Chalk missing on inner/outer edges....too low pressure
Chalk evenly smeared across threads...just right
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Old 05-07-2021, 09:26 AM   #8
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I'm pretty sure my MH came with G rated tires and now it has H rated so less pressure than stickers .
I wonder if G rated tires were more common in the years past. I don't even see G rated tires in 22.5 or 24.5
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Old 05-07-2021, 11:12 AM   #9
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Quote:
Originally Posted by US1 View Post
Before I weighed it, I put 85 in front and rears, thinking Iíd lower the pressure when I got to campsite once I looked over the charts. The RV rode like a tank! Every crack was felt. When I picked it up from dealer 2 weeks ago, all tires had 75psi and it rode nice. (Drivers placard recommends 90psi. Way to rough!)

I really liked how it rode with the rears at 75, but not sure if thatís right on the edge.
Do you use a TPMS? I often see a comment like yours and think to myself that it's not making much sense to me. The "Perfect ride!" to "OMG! It's HORRIBLE NOW!!!!!!" for what is actually a smallish percentage change in the tire pressure.

Why?

Because you're setting the cold pressure and the tires will see an increased pressure as you drive. I see a 10 PSI or slightly higher increase on mine, even a bit more on the sunny side than on the shady side.

So setting 75 PSI and driving on the highway a bit should give about the same ride as setting 85 as the cold pressure and the "starting out" ride, if you understand what I'm trying to reconcile in my mind.

If you have a TPMS You could equate the ride to the pressure so it's not quite so subjective.

Also, the sticker inside, the 90 PSI, legally must specify the tire pressure required to support the maximum amount of weight that each axle is designed to hold. That pressure has nothing to do with comfort or ride or anything except assuring a non-overloaded vehicle will always have adequate air pressure to be safe.

Your signature lists two motorhomes, a Brave and a Bounder. Which one was weighed to see an as-weighed weight of 19,360 pounds?

Have you done any chassis suspension maintenance? If that's the 2007 Bounder and you're on original shocks and your leaf springs are losing some of their "springiness", the lower tire pressure could be compensating for worn suspension components.

Just a thought,

Ray

PS: If the Bounder, the GVWR is 22,000 pounds like mine. My tire sticker also is 90 PSI on all tires. I set my cold pressure to the upper 90's to give me room for when the temps drop or in case I develop a slow leak. I'm never starting off at less that 94 PSI after the temps have really dropped here and sometimes in Florida I'm starting off at 100 PSI to head back north to the snow. I feel no difference in the ride either way. My highway pressures are normally 106 to 110 PSI and occasionally 112 PSI on the sunny side on a hot day.
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Old 05-07-2021, 08:17 PM   #10
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Quote:
Originally Posted by 153stars View Post
I'm pretty sure my MH came with G rated tires and now it has H rated so less pressure than stickers .
I wonder if G rated tires were more common in the years past. I don't even see G rated tires in 22.5 or 24.5

Plenty of Load Range G tires around in the 22.5" diameter. The Michelin and Goodyear RV Guides show several of them.


Further, nearly all LRH tires uses the same pressures as an LRG of the same size & type. The LRH tire simply adds some additional pressure bands above the max for its LRG brother. Simply put, you can inflate the LRH to a higher psi and carry more load, but the psi is the same for any load that is within the range of either one.
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Old 05-08-2021, 07:19 AM   #11
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Quote:
Originally Posted by NXR View Post
Do you use a TPMS? I often see a comment like yours and think to myself that it's not making much sense to me. The "Perfect ride!" to "OMG! It's HORRIBLE NOW!!!!!!" for what is actually a smallish percentage change in the tire pressure.

Why?

Because you're setting the cold pressure and the tires will see an increased pressure as you drive. I see a 10 PSI or slightly higher increase on mine, even a bit more on the sunny side than on the shady side.

So setting 75 PSI and driving on the highway a bit should give about the same ride as setting 85 as the cold pressure and the "starting out" ride, if you understand what I'm trying to reconcile in my mind.

If you have a TPMS You could equate the ride to the pressure so it's not quite so subjective.

Also, the sticker inside, the 90 PSI, legally must specify the tire pressure required to support the maximum amount of weight that each axle is designed to hold. That pressure has nothing to do with comfort or ride or anything except assuring a non-overloaded vehicle will always have adequate air pressure to be safe.

Your signature lists two motorhomes, a Brave and a Bounder. Which one was weighed to see an as-weighed weight of 19,360 pounds?

Have you done any chassis suspension maintenance? If that's the 2007 Bounder and you're on original shocks and your leaf springs are losing some of their "springiness", the lower tire pressure could be compensating for worn suspension components.

Just a thought,
The Bounder is current RV. Only owned it 3 weeks. Didnt see any mention of new shocks in work orders, and we have every service work paper since day 1 by original owner. We are now the 2nd owners. The shocks 'appear' original. Only thing Ive done so far is to replace both swaybars bushings with polys, since 1 rear was missing. This is on my hit list as we go along. No TPMS at the moment. Yes, I understand your start out and driving psi differential explanation.
To clarify, we picked up this RV a Fri morning at our local dealership, 3 hrs later we were on our way to a campground 1 hr away for a shake down. RV rode nice, I was pleased. Tires were at 75psi all 4 corners when I checked them a few days later. Last weekend, we went back to same campground, driving the same road, same distance, same speeds, same load. Knowing I was going to stop at truckstop mid point to get weighed, I bumped the psi up to 85 all the way around with the intention to lower them the next day with the now known weights. This is when it rode rougher and every street crack could be felt and heard. Was surprised 10psi made that much difference. Using the charts, with my weights and recommendations, I'm thinking 75/80 also. Our usage has been so far to stay in IN, KY, and TN, so I should be ok.
Thanks
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