Smart Weigh results?

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Smart Weigh results?

June 30, 2021, 08:54:10 pm

Previous coach had Michelin tires. I ran pressures according to the Michelin tire psi/load chart. My current coach has Uniroyal tires. The link below (hope it is a link) is where my info comes from for the tires I have on my coach. If I'm reading the chart correctly, I can safely run 70psi in all tires. But I know very little about tires so was hoping those more educated in tires could comment on the data I see from the chart. Am I reading it correctly?
Currently running: Steer, 105psi, Drive, 105psi, Tag, 80psi. When I purchased the coach, these are the pressures that were in the tires.

https://www.eastbaytire.com/wp-content/uploads/Uniroyal_Truck_Data_Book-2.pdf

Re: Smart Weigh results?

Reply #1 – June 30, 2021, 09:12:12 pm

You will get opinions all over the graph.. you've weighed the coach so you know how close to maximum load recommendations you are. I go by the cold psi recommended stamped on the side of the tire. Who knows better than the manufacturer.

Re: Smart Weigh results?

Reply #2 – June 30, 2021, 09:24:54 pm

Seems quite low to me especially for steer tires. I've always run 110 psi all around but with our new H rated 295/70s, I run 105 psi. Record hot weather raises tire temperatures and high temps are the enemy of tires. To reach the maximum load capacity, I would have to inflate to 120 psi. We have a 10,500 lb capacity front axle but you have a 13,800 lb front axle in your coach. Your pressure recommendation is stamped on the side of your tire along with the weight maximum.

Good read here: Are You Putting Enough Air In Your Steer Tires? - Equipment - Trucking Info

Pierce

Re: Smart Weigh results?

Reply #3 – June 30, 2021, 09:28:32 pm

According to the chart for your axle weights you minimally need:

Front 115 lbs (with no cushion)
Rear 80 lbs (with no cushion)
Tag 75 lbs

Re: Smart Weigh results?

Reply #4 – June 30, 2021, 09:30:17 pm

So the uniroyal chart show 6610 Lbs load rating at 120psi and the 9000 pounds rating on the third chart is higher than I think it actually is. Look on your actual tire sidewall and it will have posted its actual rated single and dual rating that the ticketing officer will also be looking at. That's what they are rated for. 70 psi might be okay for your tag but is too low for everything else dangerously low IMO
Scott

Re: Smart Weigh results?

Reply #5 – June 30, 2021, 11:00:46 pm

Front Axle is 12,100 total, 2 x max side is 12,200. For single tired axle use 110 or 115 for some cushion,
Rear axle is 16,900 lbs total, 2 x max side is 17,100. For a dual tired axle us5 80 minimum or 85 for some cushion.
75 for the tag is probably OK. A higher pressure will maybe reduce side wall flex if you are skidding the tire around a corner.

Re: Smart Weigh results?

Reply #6 – June 30, 2021, 11:15:46 pm

The way I read the chart is that the weight shown is a single tire on both sides of one axle, or dual tires on both sides of one axle.

Your front axle weight is 12,100#, so by the chart you should run 110psi in the fronts.
Your drive axle total is 16,900#, so the chart says you should run 75psi in the drive duals.
Your tag axle total is 6,650#, which is off the scale on the low size. 70psi is the minimum pressure on the chart.
These values assume no margin or extra loading (like trailer tongue weight).

I have the same tires on my '03 U320 38' with tag.
For comparison, my steer axle is 11,480#. The chart says I should run 100psi and I do.
My drive axle is 16,380#. The chart says 70psi, but I also plan to tow a trailer which can add some weight. I added a 20% margin to the weight and I run 90psi in the duals.
My tag axle is 7,020#, again below the chart limit, so I run 70psi in them.

I generally see about 7-10psi increase in pressure as I drive for the drive and tag axle tires. The fronts go up 12-14psi. In 115*F temps here, the highest I've seen is 124*F tire temps at those increased pressures.

Re: Smart Weigh results?

Reply #7 – June 30, 2021, 11:18:28 pm

Bill,
For me, I think it depends. Did you weigh your coach close to the actual load you expect? (i.e. yourself, all your traveling (human and pet) companions, "usual" refrigerator, pantry, clothes, storage bays "stuff" as well as "normal for you" fresh water and waste tanks levels?
Pressure across your front axle (most critical due to single tire between you and trouble):
12,100 (your weight) + ? Fuel + ? Propane (Diesel weighs 7lbs/gal, water 8.3lbs/gal).
12,100 + 400 (approx people/pets/propane) = 12,500 front axle total
Pressure across your rear axle: 16,900 + wetbay liquids.
16,900 + 830 (50 gal fresh and 50 gal waste 100 x 8.3 = 830lbs) = 17,730 rear axle total
100-115 front/80-85 rear/70-75 tag (higher number for cushion). Also note the max 75MPH tire speed rating.

Re: Smart Weigh results?

Reply #8 – June 30, 2021, 11:21:10 pm

My 40' U 320, after weighing (full fuel and water, empty sewer), loaded for full timing, H rated continental.

With changes in ambient temps, I use a range of pressures (ok anywhere in the range, depending on cold tire at ambient)

107 - 113 steer, 80-90 drive, 75 - 80 tag.

More exact: 110, 85, 75 prime.

The 75 in the tag is min I run, not based on weight.

Re: Smart Weigh results?

Reply #9 – July 01, 2021, 07:50:19 am

I get what everyone is saying, but nowhere on the tire chart does it talk about axle weight. Each tire was weighed with the coach set up for travel...full fuel, water and all our stuff. I don't see on the chart where it says axle weight. So I look for 6000 lbs on the chart. Am I to assume the chart is for axle load and not tire load? I agree, the axle load pressures seem more practical than the tire load pressures. All of you have been doing this much longer than I, so I will heed your advice and inflate accordingly. As always, much appreciative for your advice and opinions.

Re: Smart Weigh results?

Reply #10 – July 01, 2021, 08:09:59 am

Tire pressure charts are the pressure you set when the tire is "cold". That is like in the morning without sun on the tires. When you weight each corner us the maximum of either side x 2 to get axle weight for the purposes of reading a tire pressure off the chart. Find the first column that exceeds your axle weight, that is the minimum, i usually go up one more column for some cushion. Tires and the air in them heat up while driving and in the sun. Pressures are set based on "cold" conditions.

If my coach is going to sit for a few months I usually air up to the max. When it is 10° out in December it is easier to let air out to get to the pressure I want than to add air. A few days later where it is 65° in the morning I let air out back down to where I want to be and then we are good. I use a water trap on my compressor air line to keep moisture out of the tires.

Re: Smart Weigh results?

Reply #11 – July 01, 2021, 08:32:54 am

Why try to run the tire pressure so close to the minimum requirements? I would bet 99% of folks could not tell the difference in tire pressures if they did not know. I run 100 in the rear 120 in the fronts Toyo 177 LR H . Never a tire pressure adjust due to temperatures.

Re: Smart Weigh results?

Reply #12 – July 01, 2021, 09:33:58 am

Quote from: bbeane – July 01, 2021, 08:32:54 am

Why try to run the tire pressure so close to the minimum requirements?

To get a softer ride.

Re: Smart Weigh results?

Reply #13 – July 01, 2021, 09:34:39 am

I've always maxed out the pressures like my FD and by bus/trucking companies everywhere. With our new H rated (M speed rated) tires I've dropped down to 105 all around. Since we don't full time, we probably weight 24,000 plus. If we are having a heat wave and we've got to go down I-5 with all the potholes, I would not hesitate to bump up to 120 cold. Our front axle is rated at 10,500 so the H rated tires are well under their maximum rating.

Pierce

Re: Smart Weigh results?

Reply #14 – July 01, 2021, 09:46:12 am

Quote from: Travelin' Man – July 01, 2021, 09:33:58 am

To get a softer ride.

That's it in a nut shell. Everyone wants a soft ride and will search everywhere for something in writing that lets them have peace of mind running low pressures. No one wants to think they are putting their family and others on the road in danger.

In the fire service, we took the advice of Firestone and Cooper and ran all at the maximum pressure. In responding to calls, I would petal to the metal to our "maximum" city speed, wait until the last second at a red light where I could not see traffic and them brake as hard as possible just short of locking all six up and then max accelerate to the next intersection where this would repeat itself. If the response was any length at all, after stopping, there was a pretty good brake odor. And yes, any modern fire truck can easily lock up all tires. No screech but a low pitched howl.

Pierce

Re: Smart Weigh results?

Reply #15 – July 01, 2021, 10:16:38 am

DWMYH Pierce

Re: Smart Weigh results?

Reply #16 – July 01, 2021, 10:55:57 am

Quote from: Travelin' Man – July 01, 2021, 10:16:38 am

DWMYH Pierce

Exactly!

P

Re: Smart Weigh results?

Reply #17 – July 01, 2021, 11:05:11 am

And let others do the same.

Re: Smart Weigh results?

Reply #18 – July 01, 2021, 11:18:30 pm

My goal is to properly inflate my tires for safety and longevity. I did not agree with the pressures from the chart I found online so came to the forum. The chart I was using did not mention axle weight, just tire load, same as stamped on the tire. I don't know how everyone knows that. It isn't a mater of looking for documentation to make me feel better, but a mater of looking for documentation that gives me facts.

Re: Smart Weigh results?

Reply #19 – July 02, 2021, 04:08:20 am

inflation

Re: Smart Weigh results?

Reply #20 – July 02, 2021, 08:13:47 am

Thanks T-Man. The load on the "axle end" is the same load on the tire if I read correctly. So why would you double the heavier side to get your tire pressure, when it says to weigh the "axle end" and use that weight to determine the correct pressure.

Re: Smart Weigh results?

Reply #21 – July 02, 2021, 08:21:22 am

You want the tire pressures on the same axle to have the same pressure. Take highest corner weight and double it so one side is not under-pressurized.

Re: Smart Weigh results?

Reply #22 – July 02, 2021, 08:58:19 am

Not that you need another opinion but,

If the documents presented are correct, here is what I would do.

Based on highest weight on front axle of 6100 lbs and chart is showing the lowest pressure of 70 lbs for 9000 lbs I would at least run 80 PSI, This way I would have a reserve air pressure if I did not catch a slow leak right away.

Drive Axle I would run the same 80 PSI for same reason above.

Tax Axle 75 psi, Still more then what chart says is needed but would make me feel better.

Re: Smart Weigh results?

Reply #23 – July 02, 2021, 10:03:39 am

Quote from: Bill B – July 01, 2021, 11:18:30 pm

My goal is to properly inflate my tires for safety and longevity. I did not agree with the pressures from the chart I found online so came to the forum. The chart I was using did not mention axle weight, just tire load, same as stamped on the tire. I don't know how everyone knows that. It isn't a mater of looking for documentation to make me feel better, but a mater of looking for documentation that gives me facts.

The data charts can be a bit confusing as the first chart you posted was a single tire maximum load at maximum cold pressure ratings. The 2nd chart is axle load ratings at various cold inflation pressures. I searched online for the Uniroyal RS20 specifications and above the inflation pressure tables it says "Maximum Loads Per Axle at Cold Inflation Pressures". So the charts are correct but you may have missed the switch from single tire to axle ratings.

Re: Smart Weigh results?

Reply #24 – July 02, 2021, 10:16:01 am

Different steer tire pressures on each side will result in different tire compression when you go over any bumps. This causes the vehicle to have side to side movement on uneven pavement or dips (vados) on desert highways.

Pierce

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