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Old 11-11-2020, 02:21 PM   #1
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Inflate 365 steer tires to 125 psi

Need advice on best way to inflate my steer tires (365 x 70) to the Entegra recommended 125 psi. My onboard just can't get there, and looking into buying compressor that will do the job. Will the Viair 450rv be able to do the job?
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Old 11-11-2020, 02:30 PM   #2
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If you have the space, I would go with a separate 150lb air compressor. They are inexpensive, you don’t start up the coach and they are there for other things like bicycle tires and it is portable so you can take it anywhere. JMHO
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Old 11-11-2020, 02:38 PM   #3
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Viair will have no problem.
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Old 11-11-2020, 02:39 PM   #4
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Viair says max tire size is 275 80 R22.5 up to 150psi.

Might work on your 335s, might not...

It will be deathly slow in any case.

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Old 11-11-2020, 03:43 PM   #5
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Quote:
Originally Posted by Rocky40067 View Post
Need advice on best way to inflate my steer tires (365 x 70) to the Entegra recommended 125 psi. My onboard just can't get there, and looking into buying compressor that will do the job. Will the Viair 450rv be able to do the job?
I’m not sure why Entegra would be recommending 125 PSI in 365/70 tires, other than it being the max pressure Michelin recommends for that tire.

According to the Michelin inflation table:

Steer Axle Weight 14,700 – 80 PSI
Steer Axle Weight 15,420 – 85 PSI
Steer Axle Weight 16,140 – 90 PSI
Steer Axle Weight 16,860 – 95 PSI
Steer Axle Weight 17,560 – 100 PSI
Steer Axle Weight 18,260 – 105 PSI
Steer Axle Weight 18,960 – 110 PSI
Steer Axle Weight 19,640 – 115 PSI
Steer Axle Weight 20,400 – 120 PSI
Steer Axle Weight 21,000 – 125 PSI

I highly doubt you have 21,000 pounds on your steer axle. The last time our 2018 Cornerstone was weighed it had 8,190 pounds on the driver’s side steer tire and 8,610 pounds on the passenger side steer tire for a total of 16,800 pounds. And we don’t travel anywhere close to light!

Doubling the weight of the heaviest side, 8,610 x 2 = 17,220. That falls between 95 PSI and 100 PSI. Rounding up, I go with 100 PSI.
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Old 11-11-2020, 04:02 PM   #6
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First, you should know that it makes no absolutely difference what size tire you are inflating. What matters is the PSI rating of the compressor. If you need to get to 125 PSI, you don't want any compressor that does less than 150.

The best compressor choice is a little subjective, but there is definitely big differences between tankless 12V models and the 110 VAC tank models. Do you want to add a few pounds quickly, or wait hunched over for several minutes per pound?

Pancake compressors rated at 150 PSI can be purchased at all the home stores for about $150. They will fit through a DP bin door.

12V compressors ( I also have one I use for my car) run the whole time the switch is on. Compressors with a tank, fill to the setting you want and then shut off.

Owning both I recommend the pancake compressors by a large margin.
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Old 11-11-2020, 04:09 PM   #7
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Quote:
Originally Posted by LWBAZ View Post
I’m not sure why Entegra would be recommending 125 PSI in 365/70 tires, other than it being the max pressure Michelin recommends for that tire.

According to the Michelin inflation table:

Steer Axle Weight 14,700 – 80 PSI
Steer Axle Weight 15,420 – 85 PSI
Steer Axle Weight 16,140 – 90 PSI
Steer Axle Weight 16,860 – 95 PSI
Steer Axle Weight 17,560 – 100 PSI
Steer Axle Weight 18,260 – 105 PSI
Steer Axle Weight 18,960 – 110 PSI
Steer Axle Weight 19,640 – 115 PSI
Steer Axle Weight 20,400 – 120 PSI
Steer Axle Weight 21,000 – 125 PSI

I highly doubt you have 21,000 pounds on your steer axle. The last time our 2018 Cornerstone was weighed it had 8,190 pounds on the driver’s side steer tire and 8,610 pounds on the passenger side steer tire for a total of 16,800 pounds. And we don’t travel anywhere close to light!

Doubling the weight of the heaviest side, 8,610 x 2 = 17,220. That falls between 95 PSI and 100 PSI. Rounding up, I go with 100 PSI.
What Larry said.

For confirmation, load up the coach, fill with fuel, and head to a CAT scale. Consult the table Larry posted. You'll probably like the change in ride and handling if you're anywhere close to 125 PSI already.
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Old 11-11-2020, 04:43 PM   #8
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I should have mentioned what follows in my previous post, but since I didn’t I’ll post it now.

It’s not that easy to get corner weights. I got ours at Spartan in Charlotte, Michigan but there are relatively few organizations around the country that have the equipment to do it. Some owners feel you have to have corner weights to accurately set tire pressures, but unless you have a coach that is seriously out of balance (which is pretty rare these days), you can proceed as follows:

Go to a CAT scale (available at most of the major truck stops) and get axle weights – steer axle, drive axle and tag axle (if applicable). Recall from my previous post that our 2018 Cornerstone weighed 8,190 pounds on the driver’s side and 8,610 on the passenger side for a total of 16,800 pounds. A few months before we got those numbers at Spartan, I weighed the coach at a Pilot truck stop here in the Phoenix area. At that time the steer axle weight was 16,580 pounds – not all that different than the 16,800 at Spartan a few months later.

So let’s say all I had was the total steer axle weight. In that case, the recommended approach (from a friend who spent 40 years in the truck tire industry) is to add 5% to the axle weight to allow for side-to-side weight variation. So using that approach, 16,580 + 829 (5% of 16,580) = 17,409. Comparing 17,409 to the inflation table numbers I posted above, you can see it’s between 16,860 (95 PSI) and 17,560 (100 PSI). Rounding up, we’d go with 100 PSI, which is the same pressure I came to using the individual corner weights.

That isn’t just luck. I’ve found the results to be pretty much the same with our other motorhomes as well – that getting axle weights and adding 5% results in virtually the same numbers as when we’ve been able to get corner weights.
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Old 11-11-2020, 04:51 PM   #9
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Larry
I like your approach. There is too much burden to get 4 corner weights. I think it might intimidate some from getting weighed at all. Axle weights would seem to be far better than no weights.
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Old 11-11-2020, 04:51 PM   #10
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Actually, size does matter...

... in that it directly correlates to volume. Viair style compressors, while wonderful, generally aren't designed to fill multiple high-volume tires to high pressure. The unit in question, the 450P-RV, has a max pressure of 150 PSI, which is what is needed, but the duty cycle is only 40 minutes above 100 PSI.

It will easily fill a tire or two (or maybe three) to 125PSI. But filling 6 (or 8) and depending on where you're starting from, will be a very slow process and is likely to require one or more cool-down cycles which can be 30 minutes or more long. It does have thermal overload protection, but it's never a good idea to let it get so hot that it shuts down, so you may need to take more breaks to prevent that.

The Viairs also have a nice feature in that they only run when the trigger on the fill-gun is depressed. Once it's closed and pressure builds in the hose, they stop, which helps a bit to keep them cool.

The Viairs are great, and I love mine, but if I routinely had to fill multiple high-volume tires to 100PSI or more, I'd be buying a good pancake compressor. But I'd also still carry my Viair for everything else!

Regards,

Randy
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Old 11-11-2020, 05:11 PM   #11
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Quote:
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Larry
I like your approach. There is too much burden to get 4 corner weights. I think it might intimidate some from getting weighed at all. Axle weights would seem to be far better than no weights.
Tim, I agree.

I recall years ago talking with a guy from IBM who was conducting research of some sort. At the time, I was the I.T. Director for a reasonably large aerospace company. Whatever the guy was asking me, I had only the vaguest idea of an accurate answer. When I told him that, his reply was:

"Well, in my line of work we find that some number is better than no number."

You'd be amazed how often that general principle comes to mind and how applicable I find it to be.
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Old 11-11-2020, 06:31 PM   #12
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To your original question, the Viair RV will be perfect. It is likely though, that once you weigh your Cornerstone, you will find the front needs less than 110. All the rears will be 85, the minimum Spartan recommends so your tires don't come off the rims in an emergency maneuver.
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Old 11-11-2020, 09:47 PM   #13
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I like my VIair compressor. I agree with checking your coach weight. My front tires run 115.

I have never tried to fill a tire from empty with my VIair compressor. But it you need to bring a front or rear tire up a few pounds the compressor will get it done.

Be sure to check and air up in the morning before driving with no sun hitting the tires. I learned the sun lesson the hard way.

It takes about a minute per pound of increase for the VIair on my front tires. Not bad in my view.

I have never tried airing up from my coach onboard air. Many use an air doubler with the coach air to get the pressure needed.

The VIair compressors are pretty small. They easily fit in your RV/ I ordered the kit that comes with the bag and everything you need like hose and extending hose, etc.... I believe I have the 400. $300 bucks from Amazon. I think the 400 has a little more volume than the 450. I am sure they both would do the job.
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Old 11-12-2020, 07:07 AM   #14
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Quote:
Originally Posted by Tim Fox View Post
Larry
I like your approach. There is too much burden to get 4 corner weights. I think it might intimidate some from getting weighed at all. Axle weights would seem to be far better than no weights.
Totally agree with you and Larry. I have a DOT scale on I-75 where I get on when leaving home. I weight there several times and always with a new coach after loading. The tire PSI has always been correct when I can finally find a 4 corner weigh.

I had a 4 corner weigh at NIRVC during delivery, but the coach was not loaded. On our first trip loaded I'll weigh at the DOT scales and adjust tire pressure, if needed. I'll add 5 psi after loading and I'll probable be right on.
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