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Determining Optimal Cold Tire Pressure


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#1 QuiGonJohn

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Posted 05 June 2012 - 02:58 PM

I have read a lot on here, as well as web searches and I am trying to figure out how does one determine the tire pressure to use for their RV.

We bought our 2006 Winnebago Aspect 26A in April and had to get tires on it right away. We knew this, and we negotiated a pretty good deal on the RV, but could not get new tires thrown into the deal.

Anyway, I was hoping the shop listed the pressure they set the tires to initially, but they did not. So far, we have only driven about 600 miles since then.

I looked at the GVWR Decal and it shows GVWR 14050, GAWR F 4600, GAWR R 9450, and lists pressures for the Front as 65 and the Rear as 80.

I'm thinking I just need to visit a scale to be sure I am not exceeding either axle of total weight, and then if I am not, just set 65 F and 80 R. Would this be correct?
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#2 wolfe10

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Posted 05 June 2012 - 04:05 PM

Safest way is to weigh each wheel position and then go to your tire manufacturer's Inflation Chart to determine the MINIMUM PSI for that weight (that is what the chart shows). Most of us add 5 PSI to that minimum so every shopping trip to Walmart does not mean stopping to add air.

If all you have is axle weights, you might add 10 PSI to account for both Walmart and left/right imbalance.

The PSI on the GVWR sticker assumes, as you suggest, that each axle is loaded to its GAWR-- not over or under AND that that weight is distributed perfectly left to right. Lots of assumptions there!

Brett
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#3 AndyShane

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Posted 06 June 2012 - 07:03 AM

Safest way is to weigh each wheel position and then go to your tire manufacturer's Inflation Chart to determine the MINIMUM PSI...

Brett

Didn't Michelin just change the verbage in their new materials from "minimum" to "recommended"? In fact, I think either their engineering or legal staff made them purge the term. That said, I still add an extra 5psi as a technique, just to get my running temps down and to hedge for a time when I'm a few pounds low.

Their latest is at this link, the inflation and load tables; and, all their safety stuff for RVs.

http://www.michelinr...s/new-tires.jsp
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#4 wolfe10

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Posted 06 June 2012 - 07:30 AM


Didn't Michelin just change the verbage in their new materials from "minimum" to "recommended"?

Andy,

It may be just a matter of semantics. If your weight is XX, a certain PSI is required. If it weights more, you move to the next higher PSI increment. That is why I said the minimum PSI for that weight-- as any more weight requires more air.

Brett
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#5 QuiGonJohn

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Posted 06 June 2012 - 10:40 AM

I know what you're saying about adding a bit extra makes sense. But does it hurt to have too much extra, (within the tire's max rated, of course).

Let's say the tire say 90 lbs max. and the chart, for your weight say 65, would it hurt to run at, let's say 80. Does running with higher pressure also cause excess heating?
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#6 wolfe10

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Posted 06 June 2012 - 11:29 AM

I know what you're saying about adding a bit extra makes sense. But does it hurt to have too much extra, (within the tire's max rated, of course).

Let's say the tire say 90 lbs max. and the chart, for your weight say 65, would it hurt to run at, let's say 80. Does running with higher pressure also cause excess heating?

The chart shows the amount needed for that weight. If a Walmart "stock up" trip in in your near future, or you are going to dry camp so decide to top off potable water, etc, adding 5 PSI to that PSI makes sense so you don't need to recheck after the the grocery carts of stuff comes on board.

Can you get "too much of a good thing"? Sure. Look up the load your tire at 80 PSI can carry. Are you ever going to carry that much weight-- perhaps if starting a new rock collecting hobby???! Truly excessive PSI causes loss of tire footprint, wear in tire center and really rough ride. Of course, if your chassis/coach maker did not oversize the tires/load range of the tires, you will never get into that "too much of a good thing" PSI as you may be close to the max carrying capacity/highest recommended cold PSI to begin with.

Brett
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#7 QuiGonJohn

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Posted 06 June 2012 - 11:49 AM

I'm trying to find the info now. My tires are Nexen Rodian HT size LT225/75R16. One chart on the Nexen website only shows maximums and lists SINGLE 2680 and DUAL 2470 and Max Press of 80 psi. So if I understand this right, they say the tires can handle up to 5360 for the front axle, (although Winnebago lists max at 4600) and 9880 for the rear axle, with dualies, (again Winnebago lists max at 9450}.

But so far, no chart showing lighter loads and recommended or minimum pressures. I will email Nexen Customer Service and see if I can get any info.
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#8 QuiGonJohn

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Posted 13 June 2012 - 10:26 AM

No response to my emails from Nexen, but in one post Tireman said for a given size, they're pretty much the same, so I found a chart for Goodyears.

If I'm unable to weigh each tire individually, then do I take the axle weight and divide by 2 for the front and use the single and divide by 4 for the rear and use the dual? It would seem so. Also, when one gets their RV weighed for each tire, do they usually weigh each tire of the dualies separately or just the position, RR or LR and divide that by 2?
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#9 wolfe10

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Posted 13 June 2012 - 10:51 AM

If I'm unable to weigh each tire individually, then do I take the axle weight and divide by 2 for the front and use the single and divide by 4 for the rear and use the dual? It would seem so. Also, when one gets their RV weighed for each tire, do they usually weigh each tire of the dualies separately or just the position, RR or LR and divide that by 2?

Yes, if all you have is axle weights, divide by two, BUT you will want to put 5 PSI or more to cover your ASSUMPTION that left and right weigh exactly the same thing.

All wheel position weighing weighs each "axle end" rather than individual tires on the rear-- same as DOT.

Some tire manufacturer's charts are for individual tires, some for "axle ends/both tires in a dual application". Just read what the chart says.

Brett
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#10 Larrydyork

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Posted 20 June 2012 - 06:24 PM

My 2008 Itasca Suncruiser 35L door placard calls for 100 PSI for the front axle tires, and 90 PSI on the rear. I have meticulously maintained the tires at those pressures for 39,000 plus miles. I had the coach weighed for the axles, as 4-point scales were not available.

The front axle is 7100 pounds and rear is 13560 pounds, both below the GAWR. Michelin's Tire Pressure chart for my tires calls for 75 PSI front and 80 PSI rear.

Should I go by Winnebago's placard (which appears to have the tires grossly over-inflated) or by Michelin's chart (and adding a little for axle inequity)?
I've read so much in FMCA about not having the tires underinflated that I am concerned about deflating to the Michelin chart values.

Any thoughts are appreciated.
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#11 wolfe10

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Posted 20 June 2012 - 06:32 PM

Larry,

First, let's be clear: The inflation information on your vehicle's GVWR plaque is based on each axle being LOADED TO ITS GAWR.

Were I armed with that information, I would compromise between the two.

If the Michelin inflation table based on your coach loaded as it goes down the road specifies 75/80, that ASSUMES that that inflation is the minimum for your actual weight based on the heavier wheel position on each axle. So, I would add 5 PSI to the minimum to account for those big Walmart trips and moderate ambient temperature range and 5 PSI for side to side imbalance (until you get 4 wheel position weights).

So I would go 85/90. Then modify it based on actual wheel position weights. This should improve your ride quality.

Brett
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#12 RVerOnTheMove

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Posted 20 June 2012 - 07:31 PM

While not as good as getting a 4-point weighing, you may be able to get on a set of scales where you can weight the front and rear axles and then by adjusting a bit to the right or left, weigh the left and/or right side and subtract the difference. You really need to know if one side of the coach is heavier than the other as the inflation tables are based upon the heaviest weight on any tire on the axle.
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#13 QuiGonJohn

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Posted 20 June 2012 - 08:19 PM

I will be going to a Pilot Travel Center Friday, that has CAT Scales and plan to get my Class C weighed. Do scales like that usually have the capability to do 4 point or only front and rear axles?
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#14 wolfe10

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Posted 20 June 2012 - 08:26 PM

I will be going to a Pilot Travel Center Friday, that has CAT Scales and plan to get my Class C weighed. Do scales like that usually have the capability to do 4 point or only front and rear axles?

Some have the ability to weigh individual wheel position, but most are not level from scale to side to allow you to put just one side on at a time. Also, many have rails to keep you from doing this. Clearly, you can get individual axle weights.
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#15 QuiGonJohn

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Posted 20 June 2012 - 08:49 PM

You would think that it would be fairly simple to have a scale with 4 large scale areas, 2 front, L & R and 2 rear, L & R. And some sort of mechanism to lock one side or the other to take weights of each side. Then give you a print out of each position, as well as each axle and of course the total.
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#16 RVerOnTheMove

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Posted 20 June 2012 - 08:54 PM

While it is simple it's not the reality. I have used the CAT scales to do as I have described with a motor home and I don't know why it should be any different with a Class C. I would go in and talk to the clerk at the desk and explain what it is that you need to accomplish before you get started.
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#17 AndyShane

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Posted 21 June 2012 - 11:13 AM

... I would add 5 PSI to the minimum to account for those big Walmart trips and moderate ambient temperature range and 5 PSI for side to side imbalance (until you get 4 wheel position weights)...


I just ran for two hours with 5 psi over Michelin's recommended value, was pleased to note a 10F drop, on the average, in my sidewall temps. No noticeable difference in ride quality, wife agrees.
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#18 Briarhopper

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Posted 21 June 2012 - 11:53 AM

My 2008 Itasca Suncruiser 35L door placard calls for 100 PSI for the front axle tires, and 90 PSI on the rear. I have meticulously maintained the tires at those pressures for 39,000 plus miles. I had the coach weighed for the axles, as 4-point scales were not available.

The front axle is 7100 pounds and rear is 13560 pounds, both below the GAWR. Michelin's Tire Pressure chart for my tires calls for 75 PSI front and 80 PSI rear.

Should I go by Winnebago's placard (which appears to have the tires grossly over-inflated) or by Michelin's chart (and adding a little for axle inequity)?
I've read so much in FMCA about not having the tires underinflated that I am concerned about deflating to the Michelin chart values.

Any thoughts are appreciated.


Since you have kept the tires at the same pressures and traveled for 39,000 miles, take a few minutes and measure your tread wear on each tire and each tread. The treadwear will give an idea of how the load is being carried and also if you are grossly overinflated. If all tires are wearing more in the middle of the tread, confirms over-inflation. If one side is wearing differently that the other, it is an indication of load imbalance. Above assumes proper alignment of wheels and axles.

BH
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#19 Larrydyork

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Posted 21 June 2012 - 07:08 PM

Thanks Brett.

I come from the wonderful world of aviation, where the placard serves as an operating limitation, so its kinda scary to wander away from it.

I'll try your suggestion - maybe the ride will be smoother.

Thanks again.
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#20 TBUTLER

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Posted 21 June 2012 - 11:16 PM

Larry,

I am also familiar with the world of aviation. Rest assured that Brett has the correct information. Think of this as being like figuring a weight and balance for an aircraft. You have to calculate the W/B for the aircraft with empty fuel tanks and then with full fuel tanks. The center of balance must remain within specified limits at both extremes of fuel. Weigh and balance for the aircraft is not a static thing, it changes as you fly and consume fuel. This is the same for motor homes.

Each motor home is designed differently and this has to be factored into your tire calculations. In our motor home we have three tanks up front that are full width tanks so drawing from those tanks doesn't affect the side to side weight. With three tanks literally sitting over the front axle it can result in significant changes in the weight on the front tires. The three tanks are the fuel tank, 127 gallons diesel at 7.3 pounds per gallon for 927 pounds, 90 gallons of fresh water at 8.3 pounds per gallon for 747 pounds and 44 gallons of propane at 4 pounds per gallon for 176 pounds. So my front axle weight can vary by about 1850 pounds. That is 925 pounds per tire. So when I weigh, I want to know with some precision what the level of each of my tanks are so I can compute the additional weight each tank can carry. I use this calculated weight to set my tire pressure as Brett mentions. Sometimes I'm at my limit for pressure, sometimes my tires are overinflated. They should never be underinflated.

My rear axle supports the black and gray water tanks. The black tank on the passenger side of the coach at 40 gallons figured at 8.5 pounds per gallon is 340 pounds. The gray water tank on the driver side of the coach is 60 gallons at 8.3 pounds per gallon is 498 pounds. So the rear axle can vary by about 850 pounds, or 215 pounds per tire. The rear axle can also vary from side to side if only one tank is full and the other is empty. So again, I calculate the weight of the coach from the actual weighed values to adjust for full tanks. Again, my tires are sometimes overinflated but never underinflated.

Knowing where your liquid storage tanks are and the levels in the tanks when you have the coach wieghed is important in getting the tire pressure adjusted properly. Also it is impossible to always be on the knife edge of the correct pressure. For this reason, the pressure should be set at the weight of your coach with full tanks and a full load of passengers as well as your usual equipment, clothes and food. If this number is less than the sticker that is fine. The tire manufacturers schedule for weight and pressure will give you the correct inflation for your motor home.

Adding af few extra pounds (5%) as Brett mentions insures that temperature variations and shopping trips don't affect these numbers enough to cause the tires to be underinflated. How often do I check my tire pressures? Every morning we are driving, before the sun hits the tires. I run my front tires at 112 pounds, + or - 5 pounds, and my rear tires at 92 pounds, + or - 4 pounds. Outside of that range, I'll be adjusting the pressure before we start out for the day. My maximum tire and rim pressures are 120 pounds cold.

I get our coach weighed every time we are at an FMCA convention where the RV Safety Education Foundation offers weighing. They give you individual wheel position weights and the paperwork to help you calculate all this. The reason you don't see this offered at truck scales is that truckers don't have the side to side variations like motor homes do. Their load is almost always equally distributed from side to side.
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