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:
GVWR:
18,000 LB
GAWR:
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 |
| | 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# 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.
##RVT1063
