Tire Pressure Calc, Check newbies math please!

[Happytown]

Got our first RV a few months ago, 2002 Safari Sahara, first thing we did was put 6 new Sailun S637 275/70R22.5 on her, installer put 115psi all the way around and that's how we've left it so far.

Today I finally got around to getting weighed on CAT scales, got 2 axle weight, couldn't do four corner. At the end of all this research and calculating, when I went to Sailuns chart, I find that the lowest pressure they give on the chart (80psi) for my tire size is more than sufficient, with plenty headroom for the unknown imbalances.

HERE'S THE MATHS

STEER AXLE
9000LBS ÷2 = 4500LBS × 1.05 = 4725 per tire
Sailun says 80psi is good for 5205LBS single,
even giving me an extra 480LBS margin per wheel

DRIVE AXLE
16560LBS ÷2=8280 × 1.05=8694÷2=4347 per tire
Sailun says 80psi is good for 4805 dual, giving an extra 550LBS margin per tire.

Does this check out? It just makes me a little nervous to go drop from 115 to 80psi all the way around the rig right before our "maiden" voyage (going from Baton Rouge to Terlingua Tx over Thanksgiving). Appreciate your feedback

[amosnandy]

Originally Posted by Happytown

Got our first RV a few months ago, 2002 Safari Sahara, first thing we did was put 6 new Sailun S637 275/70R22.5 on her, installer put 115psi all the way around and that's how we've left it so far.

Today I finally got around to getting weighed on CAT scales, got 2 axle weight, couldn't do four corner. At the end of all this research and calculating, when I went to Sailuns chart, I find that the lowest pressure they give on the chart (80psi) for my tire size is more than sufficient, with plenty headroom for the unknown imbalances.

HERE'S THE MATHS

STEER AXLE
9000LBS ÷2 = 4500LBS × 1.05 = 4725 per tire
Sailun says 80psi is good for 5205LBS single,
even giving me an extra 480LBS margin per wheel

DRIVE AXLE
16560LBS ÷2=8280 × 1.05=8694÷2=4347 per tire
Sailun says 80psi is good for 4805 dual, giving an extra 550LBS margin per tire.

Does this check out? It just makes me a little nervous to go drop from 115 to 80psi all the way around the rig right before our "maiden" voyage (going from Baton Rouge to Terlingua Tx over Thanksgiving). Appreciate your feedback

115# cold sounds high based on your load chart, but I don’t know I would drop it all the way back to 80.

I scaled mine for the first time after I put new Bridgestones on. My weights are very close to yours, and by there charts I settled on 103 cold all around. That put me in about a 10% margin. Still rides good and after an hour on the road they settle in at 115-117.

[High Road Travelers]

I ran 80 lbs. in my Michelin tires on my 24K chassis for about 2 1/2 years, which was safe and good per my tire chart and weights. I now run 85 lbs. after doing some suspension adjustments and would never consider running much over that for my weight and tires.
Thanks,

[Mike-NM]

What does the placard state on the air pressure for the motorhome at maximum weight? I would run air a few LBS. higher than what the placard states. I would never fill it to maximum cold pressure indicated on the tire for a whole host of reasons.

[Country Road]

Originally Posted by Mike-NM

What does the placard state on the air pressure for the motorhome at maximum weight? I would run air a few LBS. higher than what the placard states. I would never fill it to maximum cold pressure indicated on the tire for a whole host of reasons.

Placard info is important. However, remember that tire inflate information has probably changed a lot over the last 20 years. What used to be good info for inflation on yesteryears tires may not be relevant today.

The OP has gotten some good recommendations. Have a good one and be safe.

[Jake21]

Originally Posted by Mike-NM

What does the placard state on the air pressure for the motorhome at maximum weight? I would run air a few LBS. higher than what the placard states. I would never fill it to maximum cold pressure indicated on the tire for a whole host of reasons.

I disagree with this. Go by the actual weight and the tire chart.

My sticker says 120 front and 100 rear. My weight and tire chart calls for 90 front and 85 rear. I add 5 psi and roll. 120 or higher would be a bit too harsh of a ride.

[Ray,IN]

Originally Posted by Mike-NM

What does the placard state on the air pressure for the motorhome at maximum weight? I would run air a few LBS. higher than what the placard states. I would never fill it to maximum cold pressure indicated on the tire for a whole host of reasons.

I always use the Federal Tire Placard listed air pressure, it is the pressure required when the vehicle is loaded to GVWR. BTW, I run that same Sailun tire.

That eliminates over-concern about how much stuff we put in the MH since we know what was in it when we had it weighed.
For others, Goodyear and several other premium tire brands state reducing air pressure does not appreciably improve ride quality.

[Max Headroom]

Originally Posted by Happytown

Got our first RV a few months ago, 2002 Safari Sahara, first thing we did was put 6 new Sailun S637 275/70R22.5 on her, installer put 115psi all the way around and that's how we've left it so far.

Today I finally got around to getting weighed on CAT scales, got 2 axle weight, couldn't do four corner. At the end of all this research and calculating, when I went to Sailuns chart, I find that the lowest pressure they give on the chart (80psi) for my tire size is more than sufficient, with plenty headroom for the unknown imbalances.

HERE'S THE MATHS

STEER AXLE
9000LBS ÷2 = 4500LBS × 1.05 = 4725 per tire
Sailun says 80psi is good for 5205LBS single,
even giving me an extra 480LBS margin per wheel

DRIVE AXLE
16560LBS ÷2=8280 × 1.05=8694÷2=4347 per tire
Sailun says 80psi is good for 4805 dual, giving an extra 550LBS margin per tire.

Does this check out? It just makes me a little nervous to go drop from 115 to 80psi all the way around the rig right before our "maiden" voyage (going from Baton Rouge to Terlingua Tx over Thanksgiving). Appreciate your feedback

Two axle weight is fine for tire pressure purposes.

To answer your question of does this seem OK, it probably is.
Now you didn't say what condition the coach was weighed in.
Best is full fuel, propane, fresh water, empty gray and black, and loaded as you would for a big trip.
After you have had the coach for a while, load up for a trip, and go get a new weight. Coaches seem to gain weight as you use them.

A lot of people on this forum will encourage running well over the chart pressure. Don't listen to that. All you get is a harsh ride, and a tire that is not flexing enough. Follow the chart, and if you want to add a bit for a margin for extra load, that's fine, just don't grossly over-inflate. It's as bad as under-inflation.

[Ray,IN]

max, when you use a load/inflation chart, which shows the minimum inflation pressure for the corresponding load, your tires are operating at 100% capacity. For comparison, auto and pickups are running with approx. a 35% inflation safety factor over minimum inflation to support the actual weight.
Tireman0 if a retired tire design engineer, he recommends 15% over chart-listed pressure.
Michelin, Firestone and Goodyear all have a paragraph in their literatuare to never use less than Federal Tire Placard listed pressure.

[Mike-NM]

Originally Posted by Jake21

I disagree with this. Go by the actual weight and the tire chart.

If you have not had it weight then they should use the placard and hopefully the right type of tire. Once weighted and the true numbers are available then i would agree to you the tire chart provided by the manufacturer of the tire.

[Happytown]

Thank you ALL for your input. I know this subject gets brought up a lot on the forum, I appreciate all the different viewpoints, it is a healthy useful discussion. Thanks again.

[Mudfrog]

Originally Posted by Happytown

HERE'S THE MATHS

STEER AXLE
9000LBS ÷2 = 4500LBS × 1.05 = 4725 per tire
Sailun says 80psi is good for 5205LBS single,
even giving me an extra 480LBS margin per wheel

DRIVE AXLE
16560LBS ÷2=8280 × 1.05=8694÷2=4347 per tire
Sailun says 80psi is good for 4805 dual, giving an extra 550LBS margin per tire.

No one has asked so I'm assuming it's just me that doesn't know. What is the 1.05 number?

As far as deciding on pressures,, I'd have to do some experimentation to see what pressures gave the best ride. Although 80 psi shows that it supports the weight now, I would not run that low. A nice even, easy to remember, 100 psi per tire might be a good place to start.

Edited to add: I see that the 1.05 comes out to 5%. I get it now.

[Crasher]

Originally Posted by Mudfrog

No one has asked so I'm assuming it's just me that doesn't know. What is the 1.05 number?

As far as deciding on pressures,, I'd have to do some experimentation to see what pressures gave the best ride. Although 80 psi shows that it supports the weight now, I would not run that low. A nice even, easy to remember, 100 psi per tire might be a good place to start.

Edited to add: I see that the 1.05 comes out to 5%. I get it now.

As I would calculate it, the 5% is a factor used to account for more than likely uneven weights side to side, when you do not have actual position weights. That number would then be the MINIMUM psi for that axle. An additional 5-10% would needed for the recommended Safety Factor.

[Happytown]

Originally Posted by Crasher

As I would calculate it, the 5% is a factor used to account for more than likely uneven weights side to side, when you do not have actual position weights. That number would then be the MINIMUM psi for that axle. An additional 5-10% would needed for the recommended Safety Factor.

This exactly. And just coincidentally, the load rating Sailun gives at 80psi works out to around 10% OVER the 5% already factored in for imbalance.