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Old 07-19-2021, 11:36 AM   #1
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Axle ratings and tire pressure

On our drive home yesterday with a fully loaded coach, I got a weight at a Pilot Travel Center. It's only front and back with no corner weights. The front weight was 12,900 which is fine, as our coach had one of the rare 14,000 pound front axles. The rear came in at 20,880 pounds. Since we're not a tag axle, that's 880 pounds over the rating. We had a full tank of water and were not towing. Our tires are all upgraded to Michelin 295/80R XZA2 Energy that have plenty of ability to handle the loads, but it's the rear axle weight that worries me.

We also realized we're over inflated on our tires when compared to the weights observed on the scale. This probably contributed to our rough ride - our front tires were about 10-15 psi higher than needed to hit the weights and the rear 4 tires about 10 psi over what was probably needed.

Any advice would be appreciated on the rear axle rating and inflating tires to what is needed for the weights (vs. over inflation to be "safe" and end up with a rough ride).

Thanks!
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Old 07-19-2021, 12:04 PM   #2
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Always inflate your tires to proper pressure. There's no need, and it's a bad idea, to over or under inflate. Despite the many anecdotes about how a small change in pressure will change your ride, it won't.

The GAWR is the maximum weight the axle can support. You want to stay below that weight. You might be able to just move some stuff forward since your front axle isn't at its limit yet. Or better yet, find some things to leave behind. If you don't need the water, leaving the tank less full will help also.
Changing pressure in the tires won't make up for an overloaded axle.
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Old 07-19-2021, 01:39 PM   #3
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I choose to run my tire pressures 5 to 10 PSI higher than the dictated minimums specified by the tire manufacturer for my loads. The loads are determined by my 4 corner weights ( NIRVC is capable of doing 4 corner (6 corner in some owners terminology)) weights at all of their locations and at all/most FMCA/big rallies.

I’m plus 10# in the 365 fronts and plus 5# in the 315 drive and tag tires.

Gary
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Old 07-19-2021, 03:47 PM   #4
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I hate to mention this but your are over 880 lbs on that rear axle as far as being legal. The legal weight for your single rear axle is 20k.
But I guess if you have had the coach for nearly 10 years and haven’t had any problems in that regards it’s probably not an issue.

I would be interested to know what the Spartan’s ID tag behind the drivers seat shows for the rear axle. Should be something like RS19144.

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Old 07-19-2021, 03:58 PM   #5
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I hate to mention this but your are over 880 lbs on that rear axle as far as being legal. The legal weight for your single rear axle is 20k.

Richard

NO. 24,000 per axle for motorhomes




Change in Law for Motor Home Axle Weight on National Network Highways

Effective October 1, 2012

In 2011, Congress passed the Moving Ahead towards Progress in the 21st Century Act (MAP-21; Public Law 112-141), which re-authorized U.S. transportation programs, including those for highway and bridge building, maintenance and improvement. As part of that bill, the Recreation Vehicle Industry Association (RVIA), with the help of the Indiana Congressional delegation, was successful in obtaining an exception to the federal law that limits vehicle weight on roads and bridges (the “bridge formula.”) The amendment provides an exception, which is the same as the bus industry has had for several years, that permits motorhomes to carry up to 24,000 lbs. on a single axle (previously limited to 20,000 lbs.).
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Old 07-19-2021, 04:04 PM   #6
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I would start with trying to get my weight down on that rear axle. I like to travel with water near full just in case but I have the weight capacity.

I don’t put my tires at max capacity though.

Here is the process I follow for tires.
Get the coach weighed on your 4 corners.
Use the heaviest side of the axle and then use that weight in the Michelin chart.
I add 8 pounds over that recommendation weight but do not go over the max for the tire.. (That way if I add some weight to the coach it is no big deal). It is worse to be under inflated.

Measure and fill tires when cold. In the morning after sitting all night. Be sure tires are not in the sun. Even the morning sun can change the air pressure reading.
Keep the tire pressure the same across the axle.
Use a quality professional tire pressure gauge.

Good luck.
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Old 07-19-2021, 06:34 PM   #7
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Quote:
Originally Posted by wolfe10 View Post
NO. 24,000 per axle for motorhomes




Change in Law for Motor Home Axle Weight on National Network Highways

Effective October 1, 2012

In 2011, Congress passed the Moving Ahead towards Progress in the 21st Century Act (MAP-21; Public Law 112-141), which re-authorized U.S. transportation programs, including those for highway and bridge building, maintenance and improvement. As part of that bill, the Recreation Vehicle Industry Association (RVIA), with the help of the Indiana Congressional delegation, was successful in obtaining an exception to the federal law that limits vehicle weight on roads and bridges (the “bridge formula.”) The amendment provides an exception, which is the same as the bus industry has had for several years, that permits motorhomes to carry up to 24,000 lbs. on a single axle (previously limited to 20,000 lbs.).


Thanks Brett for the correction,
I knew there was a change for motor coaches but was not aware it applied to motorhomes.

I was hoping the OP would share what rear axle was under his coach, not necessarily the rating but the Meritor or Dana Spicer number that’s on the Spartan ID tag behind the drivers seat.
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Old 07-19-2021, 07:35 PM   #8
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Congress can pass any law about bridge axle weights, but has nothing to do the design limit of specific axles. As an example if a smaller coach only had 15,000 for the rear axle the 24,000 loading would result in failure. That being said the OP only is over less than 5 percent. I would not be concerned.
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Old 07-20-2021, 08:48 AM   #9
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My previous RV was a 40' National RV Islander which had a rear axle rated at 20,000 lbs. Some other RV manufacturers were using the same axle and rating it at 22,000 lbs. Your tires can handle the extra 880 lbs and obviously your air bags are holding up. I am not aware of any axle failures due to being over loaded - just tire failures. You are under weight on the front which is great. Engineers typically build in a safety factor for axles and tires. My advice is get the weight down if you can, but don't worry about it since your GVW is under - allowing your brakes to stop the rig as designed. Adjust the cold rear tire pressure to support 21,500 lbs and enjoy the ride.
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Old 07-20-2021, 08:52 AM   #10
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Quote:
Originally Posted by wolfe10 View Post
NO. 24,000 per axle for motorhomes




Change in Law for Motor Home Axle Weight on National Network Highways

Effective October 1, 2012

In 2011, Congress passed the Moving Ahead towards Progress in the 21st Century Act (MAP-21; Public Law 112-141), which re-authorized U.S. transportation programs, including those for highway and bridge building, maintenance and improvement. As part of that bill, the Recreation Vehicle Industry Association (RVIA), with the help of the Indiana Congressional delegation, was successful in obtaining an exception to the federal law that limits vehicle weight on roads and bridges (the “bridge formula.”) The amendment provides an exception, which is the same as the bus industry has had for several years, that permits motorhomes to carry up to 24,000 lbs. on a single axle (previously limited to 20,000 lbs.).

I was under the impression that this law applied to federally supported highways such as the interstates but not necessarly applicable to state highways, hope I am wrong on that? Any comment?
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Old 07-20-2021, 09:42 AM   #11
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I believe you have the Meritor RS-19-144 rear axle under your coach. That same axle was used under a number of other coaches. Once the Federal Bridge Law was amended to allow 24K axles many of these axles were recertified at up to 22K for RV use due to the lighter duty use by RVs compared to trucks that put on many miles. Personally, I wouldn't be concerned about the extra 880 lbs. Granted it's over what the placard states but a small percentage and these things are built with a bit of overhead in them.

As far as tire pressures go I would adhere strictly to the manufacturer's inflation charts. This requires an accurate weighing. Four corners is best because oftentimes you'll see one side heavier than the other due to location of batteries, cargo, etc. You always look at the higher weight of the sides and use that one to make sure your pressures are adequate. Always keep the same pressure on both sides of the axle. If you don't have a four corner weight and are using an axle weight then I would add in a bit extra to allow for any left-to-right imbalance, Plus it never hurts to add another 5 PSI to allow for any increases of weight if you add more stuff after you've gotten your scale readings.

If you inflate to the full sidewall rating keep in mind that this number reflects the maximum PSI and the maximum load. You don't need to inflate to that. In addition to a rougher ride you'll also put more wear on the middle of the tire due to the crowning caused by excess pressure for that weight. On an RV that won't necessarily wear out your tires any faster because they die from age rather than miles. But you will have less traction because you are riding on the center of the tread. If you inflate to what the pressure tables shows you'll have more of the tread on the road, which helps with braking and handling. Lastly, and this is more applicable to a tag axle coach, is that you should never go below 85 PSI. Tag axle coach tires typically can handle way more weight than what the coach can put on them. When you check your axle weights against the inflation charts they are way below. But if you go below 75-85 PSI you run the risk of popping the bead of the tire off the rim if doing a tight maneuver. Really, you won't notice any rough ride at 85 PSI so there's no need to go any lower.
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Old 07-20-2021, 03:40 PM   #12
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Thanks everyone. I have not been able to get the axle number, but I do feel better about the extra weight. Some years ago I spoke to someone at Spartan who said they can't "officially" condone going above the axle rating, but they also build in quite a fudge factor. We were as heavy as we ever get on this recent drive coming back from a month on the road and also having bought stuff (wine, gifts, lots of fresh fruit for freezing). The coach goes back to Indiana this week to finish up some maintenance and get our windshield replaced. We'll be traveling MUCH lighter. I'll check into corner weight stations and see if anyone can do that between Chicago and Wakarusa, IN.
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Old 07-20-2021, 04:56 PM   #13
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They used to be able to do corner weights in Middlebury at the factory.... not sure now. They surely would know where they can be done in the general area.

Gary
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