Monday, August 8, 2022

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RV Tire Safety: Setting inflation pressure step-by-step

While I have posted a number of times on how I would set tire pressure, this topic seems to be a recurring question. So here is a slightly different approach to the answer where I use information on a specific example.

First off, you will need to adjust for your RV, your weights and your size tire. This should work for owners of motorhomes but not for towables or 5th wheel RVs. I will try and cover them in a couple of weeks, once I get some real weight information from a trailer/5th wheel owner.

So here is the question I got.

Sorry to bother you again, but I just realized I should have asked you about CIP (cold inflation pressure) and ambient temperatures. I was hoping to set my new tire pressures late this evening or early tomorrow morning when it cools down.

As I stated in my first email, we live in the California High Desert and the lowest temperatures we are going to see today, tonight or in the wee hours of the morning is a cool 75 degrees. If I remember correctly in one of your posts, you stated CIP of 65 – 70 degrees. I am sure folks in Arizona and Nevada have the same issue, where low temperatures may not get below the 80s or 90s in any 24-hour period for weeks, if not months.

Do we just apply the 2% per 10-degree formula when initially setting our tire pressures? Meaning a 90 psi would then be 91.8 @ 80 and 93.6 @ 90 degrees?

For us tonight and tomorrow morning:
90 @ 7 PM
83 @ 10 PM
75 @ 6 AM
79 @ 7 AM

Here are my numbers:
2018 Winnebago Vista 29VE
Class A
Two axles, dual rear wheels

Scale weights:
Total: 16,540 lbs.
Front: 5,620 lbs.
Rear: 10,940 lbs.

Goodyear G670 RV, 245/70R19.5G

Tire sidewall information:
Max. Load Single 4540 lbs @ 110 psi cold
Max. Load Dual 4410 lbs @ 110 psi cold

RV placard information:
GVWR: 18,000 lbs.
GAWR: FRT 7,000 lbs. 82 psi Single
RR  12,000 lbs. 82 psi Dual

Goodyear chart:
Goodyear Load/Inflation information for Unisteel G670 RV:

Max Speed 75 mph
  80 85 90 95 100 105 110
S 3640 3740 3890 4080 (F) 4190 4335 4540 (G)
D 3415 3515 3655 3970 (F) 4115 4265 4410 (G)

 

Again, thank you for the wealth of information and your incredible blog.

My reply

Yes, you can use axle weight numbers until you can get “4 corner weights” (weight of each tire position).

I suggest you assume one end of the front axle is supporting 52% of that axle. Similar for the rear. So based on your numbers that would be:

2,922 lbs. on the front tire
5,689 lbs. on one end of the rear axle or 2,845lbs. on a tire. Yes, I always round up when calculating loads.

Using your chart, the minimum inflation for the front would be 80, and the minimum on the rear would also be 80.

BUT don’t forget I also strongly recommend we use at least +10% on inflation to avoid the day-to-day temperature changes that will change the tire inflation. A +10% on inflation means you could experience a change in temperature of 50° F and not be forced to get out and adjust the tire inflation. Don’t forget that some folks are where it might be raining in the morning and don’t want to be on their hands and knees messing with tire inflation if they can avoid it.

Add 10% to 80 and you get 88 psi. I see nothing wrong with rounding up to 90 psi.

At 90, a front tire would be rated for 3,890 lbs.; 3890/2922 = 133. So you would have a good 33% reserve load.

At 90,  a rear tire would be rated for 3,655 lbs.; 3655/2845 = 128. So you would have a good 28% reserve load

You should be good to go.

I hope this real world example shows you that learning the proper CIP is not complex.

Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1063

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Bill Tucker
9 days ago

The door placard on my c class has the pressure levels for the front and back. what happens if I just set the tires to those values? My tires are Michelin Defender LT225/75 R16 E  115/112R.

Roger Marble
8 days ago
Reply to  Bill Tucker

If you have learned the actual load on the Front & Rear axles by stopping at a truck scale to confirm you are below your GAWR I see nothing wrong with following the inflation numbers on your Certification label. Just remember that if your scale readings are close to the GAWR figures you may have no Reserve Load margin. Only you can decide how close to the limit you want to operate. Most people do not know that most cars come with tires and inflation numbers give 25% to 30% Reserve Load, while most RVs have less than 10% margin with a majority running with their tires overloaded which in my opinion is asking for tire problems.

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