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Friday, July 29, 2022

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 realize I should have asked you about CIP (cold inflation pressure)  and ambient temperatures. I was hoping to set my new pressures late this evening or early tomorrow morning when it coos down, but.
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. 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 80's or 90's 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:
18,000 LB
FRT  7,000 LB 82 psi  Single
RR  12,000 LB 82 psi Dual

Goodyear chart:
Goodyear Load/Inflation information for unisteel G670 RV:
Max Speed 75 mph
S3640374038904080 (F)419043354540 (G)
D3415351536553970 (F)411542654410 (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# on the front tire
5,689# on one end of the rear axle or 2,845# on a tire.  Yes I always round up when calculating loads.
Using your chart the min inflation for the Front would be 80  and the min 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 AM 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. and I see nothing wrong with rounding up to 90 psi

At 90 a front tire would be rated for 3,890#    3890/2922 = 133 so you would have a good 33% reserve load
at 90 a rear tire would be rated for 3,655#    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.

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