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Old 04-18-2018, 01:07 PM   #1
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Big Tire Air Pressure

Just bought a VIAIR portable compressor for my 01 HR Imperial. Since I'll be keeping my tires aired up myself, what is the best air pressure to maintain?
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Old 04-18-2018, 01:15 PM   #2
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What ever the tire manufacturer and vehicle manufacturer recommend.
My CC owners manual says to only run the pressure on the Federal Weight sticker and there is no condition that would change that. Then Michelin publishes weight/pressure charts allowing lower pressure but that only affects the front tires since the drive and tag are already at the Michelin minimum.
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Old 04-19-2018, 05:31 AM   #3
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There are plenty of threads on this topic, but like Grady says, get a tire manufacturer's PSI/weight chart, then:

1. Weigh each coach axel separately (CAT Scale) -- normal load of fuel, water, etc.
2. Choose a PSI capacity from the chart that adequately supports the actual axel weight (I shoot for around 75% ratio: actual axel weight/gross capacity of tires at selected PSI).

Note that I reviewed this approach with the REV Group support team and they approved.

I have a VIAIR -- really happy with it except it has a tendency to blow out hoses if you run it for an extended period of time,

Good Luck!
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Old 04-19-2018, 06:11 AM   #4
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Originally Posted by thomaspryan View Post
There are plenty of threads on this topic, but like Grady says, get a tire manufacturer's PSI/weight chart, then:

1. Weigh each coach axel separately (CAT Scale) -- normal load of fuel, water, etc.
2. Choose a PSI capacity from the chart that adequately supports the actual axel weight (I shoot for around 75% ratio: actual axel weight/gross capacity of tires at selected PSI).

Note that I reviewed this approach with the REV Group support team and they approved.

I have a VIAIR -- really happy with it except it has a tendency to blow out hoses if you run it for an extended period of time,

Good Luck!
75% is not a ratio. I believe what you are attempting to achieve is an axle weight that when supported by either 2 or 4 tires is equal to 75% of the tires maximum load rating.

It certainly isn't necessary from a safety standpoint and it wouldn't be worth buying new tires to achieve.
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Old 04-19-2018, 07:09 AM   #5
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To determine what the right tire pressure is for you, you need to weigh your MH. Load your MH as you would for travel, fill you gas and water tanks and then get your MH weighed.

If you can't do 4 corner weighing, this is the procedure to follow:

1. Weigh the coach axles
2. Add 5% to account for possible uneven weight side to side.
3. Divide by the number of tires on each axle.
4. Look to tire chart for your brand to find tire pressure.
5. Add 5 psi for safety
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Old 04-19-2018, 07:46 AM   #6
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Originally Posted by veraken View Post
To determine what the right tire pressure is for you, you need to weigh your MH. Load your MH as you would for travel, fill you gas and water tanks and then get your MH weighed.

If you can't do 4 corner weighing, this is the procedure to follow:

1. Weigh the coach axles
2. Add 5% to account for possible uneven weight side to side.
3. Divide by the number of tires on each axle.
4. Look to tire chart for your brand to find tire pressure.
5. Add 5 psi for safety
The first 4 steps seem reasonable as long as you find a chart for the correct brand and size. Why would you add 5 psi for safety?
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Old 04-19-2018, 12:57 PM   #7
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For what it is worth, I weighed the coach, then looked at the tire manufacturer's recommended pressure, then added 10psi.

A hard tire is not a problem. A soft tire will flex too much and heat up. Yes, you can argue that I may get more center wear on the tire, but I have not, and even if I did, these tires will time out long before they wear out.
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Old 04-19-2018, 06:21 PM   #8
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For what it is worth, I weighed the coach, then looked at the tire manufacturer's recommended pressure, then added 10psi.

A hard tire is not a problem. A soft tire will flex too much and heat up. Yes, you can argue that I may get more center wear on the tire, but I have not, and even if I did, these tires will time out long before they wear out.
The question remains. Why?

Is there a tire manufacturer that recommends overinflating their tires?

What would make the practice as safe or safer than following the manufacturers guidance for the correct pressure?
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Old 04-20-2018, 04:36 AM   #9
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Quote:
Originally Posted by RVPioneer View Post
75% is not a ratio. I believe what you are attempting to achieve is an axle weight that when supported by either 2 or 4 tires is equal to 75% of the tires maximum load rating.

It certainly isn't necessary from a safety standpoint and it wouldn't be worth buying new tires to achieve.
Huh?

Ratio/schmatio: works for me. In an attempt to positively contribute to this thread, I canít help feeling like I just had my essay graded by an overworked shop teacher.

Give us your solution or save the platitudes.
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Old 04-20-2018, 09:11 AM   #10
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Huh?

Ratio/schmatio: works for me. In an attempt to positively contribute to this thread, I canít help feeling like I just had my essay graded by an overworked shop teacher.

Give us your solution or save the platitudes.
Well, that was certainly positive.

A solution requires a problem. There isn't a problem to be solved when it comes to tire pressure in an RV.

Weigh the coach.
Inflate to the recommended pressure on the chart.

May these words be deemed positive and helpful and assist you in enjoying safe and happy travels.
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Old 04-20-2018, 11:10 AM   #11
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The question remains. Why?

Is there a tire manufacturer that recommends overinflating their tires?

What would make the practice as safe or safer than following the manufacturers guidance for the correct pressure?
Tire manufacturers set minimum and maximum pressures. The recommended pressure is the minimum they recommend. Lower than that and you risk excess tire flex and catastrophic blow out. If you exceed their maximum, or the maximum supported by the wheel manufacturer, you also risk catastrophic failure.

Higher pressures support greater weight because it reduces sidewall flex. Manufactures also use the recommended pressure as a way to avoid paying warranty claims for premature wear when the center of the tread is worn. This is not an issue if you are inflating a little above the recommended pressure, and on our coaches none of us are going to wear out our tires, unlike commercial busses, our tires time out.

So, softer tires (i.e. at the manufacturer's minimum) will provide a plusher ride, but also result in tires heating up more due to greater sidewall flex. Slightly harder will give you a little harsher ride, but also lower tire temps.

Having said that, knock your self out.
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Old 04-20-2018, 12:56 PM   #12
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Well, that was certainly positive.

A solution requires a problem. There isn't a problem to be solved when it comes to tire pressure in an RV.

Weigh the coach.
Inflate to the recommended pressure on the chart.

May these words be deemed positive and helpful and assist you in enjoying safe and happy travels.
Sage advice. I rest my case.
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Old 04-20-2018, 02:15 PM   #13
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Manufactures also use the recommended pressure as a way to avoid paying warranty claims for premature wear when the center of the tread is worn. This is not an issue if you are inflating a little above the recommended pressure,

Having said that, knock your self out.
That statement is incorrect. The center of the tread will wear more than the outside of the tread on an over inflated tire. The opposite occurs on an under inflated tire. Manufacturers caution against both conditions.

If and when a tire manufacturer recommends adding 5-10psi more than recommended, I'll consider it.
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Old 04-20-2018, 04:06 PM   #14
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Originally Posted by veraken View Post
To determine what the right tire pressure is for you, you need to weigh your MH. Load your MH as you would for travel, fill you gas and water tanks and then get your MH weighed.

If you can't do 4 corner weighing, this is the procedure to follow:

1. Weigh the coach axles
2. Add 5% to account for possible uneven weight side to side.
3. Divide by the number of tires on each axle.
4. Look to tire chart for your brand to find tire pressure.
5. Add 5 psi for safety
Veraken: So I'm going to try and do the math here and hopefully this example will help everyone.

My tires I bought in 2014 - Hercules H-901 235/85 R16.

Weighed Front Axle:4960 plus 5% (248) = 5208 lbs *Weighed at Flying J
Weighed Rear Axle: 10660 plus 5% (533) = 10893 lbs *Weighed at Flying J

Two tires on front - 5208/2 = 2604 lbs per tire
Four tires on back - 10893/4 = ~2723 lbs per tire

Hercules Tire Inflation Chart:(Chart goes to 110 PSI)
@ 85 PSI - 3170 Single, 2885 Dual
@ 90 PSI - 3300 Single, 3005 Dual
@ 95 PSI - 3415 Single, 3085 Dual
@110 PSI - 3760 Single, 3415 Dual

So according to this I only have the following amount of weight to play around with:
@85 PSI on Front 3170 minus 2604 = 566 lbs additional each side
@90 PSI on Front 3300 minus 2604 = 696 lbs additional each side
@95 PSI on Front 3415 minus 2604 = 811 lbs additional each side
@110 PSI on Front 3760 minus 2604 =1156 lbs additional each side

@85 PSI on Rear 2885 minus 2723 = 323 lbs additional each side? Or do I double that?
@90 PSI on Rear 3005 minus 2723 = 563 lbs additional each side? Or do I double that?
@95 PSI on Rear 3085 minus 2723 = 723 lbs additional each side? Or do I double that?
@110 PSI on Rear 3415 minus 2723 = 1383 lbs additional each side? or do I double that?

I JUST did the math today. Prior to this (Like a total of 3.5 years now) I've been running 90 PSI in the front and 85 PSI in the back as I was guessing.

I'm surprised by the small amount of extra weight I can carry. I'm also nervous about inflating the tires to 110 PSI! Especially since the chart I found on IRV2 here seems to be cut off and I don't know what those letters mean in the parenthesis. 3042(E), 3760(G), etc.

Here's the link: Lesson Learned - Tires.

One thing I will have to check on is what the max pressure is that is stamped on the side of the tire. I just don't have that info at the tip of my fingers.
**Update - Actually I found that info, see info at bottom

So there you go. Looking at this I might bump my tire PSI on the all the tires up to 95. But again, I guess that depends on what the max is stamped on the tire.

Thanks.

PS: Did a quick search and found these numbers for the H901:
Single Max Load @ Cold Inflation Pressure: @110 PSI - 3760 lbs
Dual Max Load @ Cold Inflation Pressure: @110 PSI - 3415 lbs
Per this site:https://www.herculestire.com/tire-details/Hercules/H-901%20LT

Wish I could talk with a Hercules guy.
**Update: Found this email: info@herculestire.com, and this phone number:1.844.432.9729
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