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Old 08-27-2015, 08:37 AM   #1
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Trying to calculate tire pressure.

I got my coach weighed on my way home from our week long trip. So fully loaded even with water as I filled to 2/3 before leaving the campground.

I have a 16k chassis f53 2015 Brave 26a by winnie. IT has Goodyear GS670 tires at a size of 245/70R19.5.

My weights came in at

5520 in the Front (that axle is rated for 6500)
9980 in the Rear (that axle is rated for 11000)
15500 overall (coach is rated for 16,000)


I'm trying to figure out what pressure I should be running based on this information.
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Old 08-27-2015, 09:02 AM   #2
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Here is Goodyears RV tire pressure chart. Just follow the instructions on how to figure the pressure you need.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
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Old 08-27-2015, 09:10 AM   #3
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You may have to wrangle with the Goodyear dealer ("It's written on the side dude"), but each type of tire has a chart that specifies pressure verses weights.
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Old 08-27-2015, 09:27 AM   #4
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SOmewhere in your coach is a printed label stuck to a wall (inside a cabinet ofer the driver's seat in our MH) that shows the manufacturer's recommended tire pressure. This is tire pressure for which the suspension was designed. In the case of our MH, the tire pressure is lower that the "rated tire pressure " on the sidewall of the Michelin tires.

I pumped up the tires to the sidewall numbers and the MH was VERY difficult to control. It would jump left and right depending on the peaks and valleys in the road.

After I set the pressure to the "manufacturer's expected pressure the coach is a dream to drive.
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Old 08-27-2015, 10:13 AM   #5
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Remember folks, those are either the tires max tire psi and caring capasity or the numbers on the coach placard is for the max design weight of the coach with stock tires. The recommended psi by Goodyear for the tires you have on the coach (you may not have the original tires) is based on the actual weight on the tire. The best ride, handling and proformance are achieved using the recommended pressure recommended by the tire manufacturer.

A Good example is my coach, the original tires were 255/80r22.5. When I purchased and still today my tires are 275/70r22.5. The recommended tire pressure for the 255's are 102 psi and max RV design weight would be 110psi.
The Goodyear G670RV tire recommend pressure is 85psi and at max RV design weight would be 90psi.
The max tire pressure is 125psi and can support 6940lbs. If you inflate these tires to 105 psi the coach rattles your teeth every time you hit a gap on concert roads and this is an air ride coach.

IMHO use the tire manufacturer recommend pressure plus maybe 5psi. This will give you a good ride, proformance and handling on a safe tire.
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Old 08-27-2015, 11:14 AM   #6
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Quote:
Originally Posted by dc8cappie View Post
Here is Goodyears RV tire pressure chart. Just follow the instructions on how to figure the pressure you need.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

So, if I have 5500 lbs on the front then I would only need to run 80 psi in the front and roughly 90 in the back? I'm sure I'm not reading this chart correctly.
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Old 08-27-2015, 11:48 AM   #7
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I called Goodyear and talked to a Rep who suggested I run 80psi on all tires based on my load.
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Old 08-27-2015, 12:48 PM   #8
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OK, lets take this step by step:
Your front axle has two tires that now carry 5520 lbs but might be asked to carry as much as 6500 lbs (the max rating). You cannot assume the weight is evenly dived between the two either, so need to allow a bit extra in case one is more than 5520/2. So the "right psi" for the front is the table entry somewhere between 5520/2 = 2760. Since the absolute minimum shown in the Goodyear table for the 245/70R22.5 is 70 psi, and that carries a total of 2 x 3640 = 7280 lbs, you can safely us 70 psi for the front tires under all load conditions.

Now for the rear axle. Your are carrying 9980 on four tires, or 9980/4 = 2495 each. Again, the load may not be evenly balance, and some day you might carry a bit more, up to 11,000 lb (11,000/4 = 2750/tire. Looking at the dual tire line in the chart, we again see that the absolute minimum pressure for the 245/70R22.5 is 70 psi and that carries 4 x 3415 = 13660, more than enough. So you can safely use 70 psi in the rear.

Nothing wrong with using the greater 80 psi the GY guy recommended, but one wonders why he doesn't believe in the table published by his own tire engineers? The ride will be a tiny bit rougher at 80 psi, and traction slightly reduced as well, but probably not enough to really notice. Still, using 70 psi has plenty of extra safety margin already and 80 psi is just overkill.
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Old 08-27-2015, 01:02 PM   #9
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Originally Posted by Gary RVRoamer View Post
OK, lets take this step by step:
Your front axle has two tires that now carry 5520 lbs but might be asked to carry as much as 6500 lbs (the max rating). You cannot assume the weight is evenly dived between the two either, so need to allow a bit extra in case one is more than 5520/2. So the "right psi" for the front is the table entry somewhere between 5520/2 = 2760. Since the absolute minimum shown in the Goodyear table for the 245/70R22.5 is 70 psi, and that carries a total of 2 x 3640 = 7280 lbs, you can safely us 70 psi for the front tires under all load conditions.

Now for the rear axle. Your are carrying 9980 on four tires, or 9980/4 = 2495 each. Again, the load may not be evenly balance, and some day you might carry a bit more, up to 11,000 lb (11,000/4 = 2750/tire. Looking at the dual tire line in the chart, we again see that the absolute minimum pressure for the 245/70R22.5 is 70 psi and that carries 4 x 3415 = 13660, more than enough. So you can safely use 70 psi in the rear.

Nothing wrong with using the greater 80 psi the GY guy recommended, but one wonders why he doesn't believe in the table published by his own tire engineers? The ride will be a tiny bit rougher at 80 psi, and traction slightly reduced as well, but probably not enough to really notice. Still, using 70 psi has plenty of extra safety margin already and 80 psi is just overkill.
Thanks! Now, what does the placard in the coach recommend........morbid curious, I am......
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Old 08-27-2015, 02:18 PM   #10
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Also please note that you will notice there is a minimum pressure for any particular tire. Never go less then the min recommend tire pressure. Your then asking for real trouble!!!!!!


The placard on my coach was the required pressure on the original 255/80r22.5 tires that came on it at GVWR which was 105*. For safety reason they use the GVWR weight so you can't blame them that you under inflated the tires based on their numbers. Another words, LAWYER requirements, or least common denominator rule.
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Old 08-27-2015, 03:15 PM   #11
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Quote:
Thanks! Now, what does the placard in the coach recommend........morbid curious, I am......
The RV manufacturer gets to apply his judgment on the placard pressure, within the limits set by the tire manufacturer. For legal reasons, the RV maker nearly always "recommends" a tire pressure that will support a fully loaded axle, i.e. the axle GAWR. He doesn't know how much weight you will really carry, so he "recommends" enough pressure for the max allowed axle load (the GAWR). Anything less exposes him to liability if you did in fact load to the GAWR limits (which are also shown on the placard), so they rarely do that (except by mistake). Unfortunately, placard mistakes are not all that rare - there are typically several recalls every year just to replace the placard!

The good news is that the placard pressure is typically on the high side of real need, so it's a safe value to use.
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Old 08-27-2015, 03:27 PM   #12
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Lots of mostly correct advice.

Some observations from a tire engineer.

Use the placard inflation until you at least get the actual load on each axle.

As others have pointed out few RVS have actual tire loads split 50/50 side to side on an axle. A good rule of thumb if you only get the total for an axle is to calculate a 47/53 to 45/55 side to side split and use the heavier end number for your estimate until you get actual "corner loading"

With your heavier load number look up the MINIMUM cold inflation needed to carry that load in the tables. Knowing the minimum Cold Inflation Pressure needed I would add 5% to 10% to that number by rounding up to a convenient 5 psi number and then inflate all tires on thet axle to that pressure. This extra will mean you don't have to worry about being a couple psi under your minimum when the temperature drops.

Don't forget the tire pressure changes by 2% for each 10F change in temp.

I have many posts and a number of examples of how to do the calculations on my blog (see my signature)

Remember have a good time with your RV.
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Old 08-27-2015, 04:09 PM   #13
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Still doesn't answer the question......

WHAT does the 2015 Winnebago Brave Tire Placard say?????????????
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Old 08-28-2015, 04:43 PM   #14
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The 70 psi is only on these tires manufactured before 2006. After 2006 the chart has no weight rating for anything lower than 80psi.

I can't find the Placard anywhere. I'm in the RV now looking.
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