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Old 08-21-2018, 04:39 PM   #1
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Tire Pressure Confirmation

Hi,
I've read through the many threads on tire pressure and think I have it figured out, but want to confirm my thoughts before I screw up my tires.

I have a 2014 Canyon Star with 255/80 R22.5 tires. I had the coach weighed on a CAT scale, front axle 7640lb, rear 15960lb.

When I look at the Michelin table for my tires on the single row, there are no values that go up to 7640. So I assume I have to divide that number by 2 to get the pressure for each tire. 7640/2=3820. 3820 equates to 70 psi. Add 10% buffer and round up, I figure 80 psi.

For the dual rear, I divide 15960 by 2 for 7980 on each side. 7980 equates to 85 psi. Add 10% buffer and round up, I figure 95 psi.

I know the math is right. Please confirm the reasoning. I got the CS in April as a first venture in camping so have no prior experience to fall back on. The tires have been at 100 psi all around and the ride has been very rough to say the least.

TIA for your thoughts and valuable opinions.
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Old 08-21-2018, 04:51 PM   #2
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Hi,
You sure about the 855? Can't find it in the Michelin charts.
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Old 08-21-2018, 04:56 PM   #3
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I said my math was right, not my typing It is 255/80 R22.5.
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Old 08-21-2018, 04:59 PM   #4
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Hi,
You are correct up tp the 10%. It is personal preference. I add 5 PSI. I have found this is plenty to handle the variety to temps and altitudes I travel. Too much air makes it hard on the coach, suspension and ride.
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Old 08-21-2018, 08:47 PM   #5
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Quote:
Originally Posted by dck22 View Post
Hi,
I've read through the many threads on tire pressure and think I have it figured out, but want to confirm my thoughts before I screw up my tires.

I have a 2014 Canyon Star with 255/80 R22.5 tires. I had the coach weighed on a CAT scale, front axle 7640lb, rear 15960lb.

When I look at the Michelin table for my tires on the single row, there are no values that go up to 7640. So I assume I have to divide that number by 2 to get the pressure for each tire. 7640/2=3820. 3820 equates to 70 psi. Add 10% buffer and round up, I figure 80 psi.

For the dual rear, I divide 15960 by 2 for 7980 on each side. 7980 equates to 85 psi. Add 10% buffer and round up, I figure 95 psi.

I know the math is right. Please confirm the reasoning. I got the CS in April as a first venture in camping so have no prior experience to fall back on. The tires have been at 100 psi all around and the ride has been very rough to say the least.

TIA for your thoughts and valuable opinions.
I am not an experienced RV'er yet, but have read on the excellent RV Tire Safety website RV Tire Safety: Load Inflation tableto use minimum 53% of each axle load to account for uneven loading side to side. Unless you have weights from the scale of each axle end. Makes sense, I hope to weigh mine this way if I can find a scale to do so.

I just did this with my MH. I believe your rear numbers are off, should have divided the rear axle weight by 4 to get per tire load in a dual application. Using the 53% rule, multiply the rear axle weight of 15960 x 53% = 8458 / 2 =4229 per tire. Use this load on the chart on up accordingly.
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Old 08-21-2018, 09:01 PM   #6
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Originally Posted by DutchStarLou View Post
I just did this with my MH. I believe your rear numbers are off, should have divided the rear axle weight by 4 to get per tire load in a dual application. Using the 53% rule, multiply the rear axle weight of 15960 x 53% = 8458 / 2 =4229 per tire. Use this load on the chart on up accordingly.
That was what I first thought, but the chart for dual tire axle starts with 7050 lb and 70 psi, then goes up from there. To get a value that appears on the chart I needed to divide the rear axle weight by 2. Your 53% calculation also comes up with the same value I did, 95 psi.
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Old 08-21-2018, 09:15 PM   #7
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That was what I first thought, but the chart for dual tire axle starts with 7050 lb and 70 psi, then goes up from there. To get a value that appears on the chart I needed to divide the rear axle weight by 2. Your 53% calculation also comes up with the same value I did, 95 psi.
I should have looked at the Michelin chart first, it's based on dual load numbers as per 2 tires, where as the Goodyear chart which is what I have uses dual load numbers per each tire. The Michelin chart does list the maximum load on sidewall to the right as each tire. Not very clear on each manufacturers charts.

Bottom line is I think your numbers are correct.
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Old 08-22-2018, 04:40 AM   #8
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In your correction to tiresise, you give same sise, or did you correct it at opening post also.
On site of Michelin, I found via link on RV tiresavety , this sise only goes down to 80 psi.
Mayby you used another list.

What I do when using my spreadsheet , is adding 10% to the weighed axle- loads and then calculate the pressure fot that with use of my determined extra safe formula.
Commes down to about the same idea, but gives it to 1 psi rounded up.
Then for instance when 71 psi outcome, you yourselfes can round it to 75 psi, but if you fill 70 psi, that 1 psi lower will do , because already some reserve is build in. This to your own yudgement. You cant read most pressure devices that acurate by the way, and acuracy is never 100%.

So confirm the sise and I will fill it all in my spreadsheet, and see what that gives. Best give whats written on sidewall , example " max load single/dual xxxx lbs AT yyy psi( cold) " or give loadrange ( found G-load/ lrg wich standard is AT 110 psi).
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Old 08-22-2018, 06:33 AM   #9
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you should really get all four corners weighed.
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Old 08-22-2018, 07:03 AM   #10
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Quote:
Originally Posted by jadatis View Post

So confirm the sise and I will fill it all in my spreadsheet, and see what that gives. Best give whats written on sidewall , example " max load single/dual xxxx lbs AT yyy psi( cold) " or give loadrange ( found G-load/ lrg wich standard is AT 110 psi).
I have a 2014 Canyon Star with 255/80 R22.5 tires. I had the coach weighed on a CAT scale, front axle 7640lb, rear 15960lb.

Thanks
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Old 08-22-2018, 07:09 AM   #11
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you should really get all four corners weighed.
CAT doesn't offer that service. When I am able to get the corners weighed I plan to. Until then front and back weight is better than nothing. IMHO
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Old 08-22-2018, 11:08 AM   #12
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Then looking back in the Michelin list ,I suddenly see 70 psi as lowest, must have had only part of list in view on mobile.

But OK, took the maxload over from them .
Single 5205 lbs AT 110 psi .
Dual 4805 lbs AT 110 psi

Your axle weights F 7640 lbs single, R 15960 lbs Dual.

Then ....oeps... it gives 88 psi front and 100 psi back.
I stil am convinced that this pressure gives acceptable comfort and gripp, and a comfortable reserve for things as , unequall load R/L, mis- or inacurate reading of pressure, pressure loss in time, etc etc.

Even when I leave the 10% adding away , it comes to 79F and 90R, wich then can be compared to the liadcapacity's in the Michelin list.

Max speed of your tires is most likely 75mh , wich allows more deflection then I assume in my spreadsheet, so mayby for trucktires, wich in fact your tires are,I can adjust the spreadsheet a little, and it would give slightly lower pressures.
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Old 08-22-2018, 11:24 AM   #13
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Yes, your numbers are probably right. I just went through this with my Winnebago Adventurer. It's a high-end gas MH just like the Canyon Star. Mine weighed pretty much the same as yours. And I did use the charts to lower my tire pressure and it has vastly improved the ride.

Thing is, you need each corner weighed and here's why. You need to use the heaviest side to determine the weight on the charts. So, if one side weighs 3620 and the other side weighs 4020 you end up with the same average 3820 per side. But you need to use the higher of the two numbers in the chart when determining correct tire pressure.

Most CAT scales have an apron on either side of weighing portion of the scale. To get each side independently you drive one half on the apron and one half on the scale. Take a reading and reposition with the other half on the apron and the other half on the scale. This will give you the weight on EACH side of the coach front and back.

Also, be sure that you have a full-tank of gas and a full-tank of fresh water. The coach should be loaded fully - or nearly so. If you travel with your wife and she's not at the weighing add her weight to the passenger side of the weights.

I was running 95 front and 100 rears before weighing and now run 80 front and 90 rear. It's made a big difference in ride, harshness, noise and driveability.

Here's a photo of using the apron to weigh each side independently:
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Old 08-22-2018, 11:38 AM   #14
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One more thing... if you haven't gotten the CAT Scale phone app you should do so right away. It's called "Weigh My Truck". Get the app on your phone. But before you use it go to the www.weighmytruck.com and set up an account and enter your payment info - it's much easier to set this up on your computer.

Then you just pull up onto the scale, open the app and it will find your location. Also the scale will have a #number on it to enter. Then click Accept and you'll have your first weight. Reposition your truck and tell the app it's a reweigh and click Accept to weigh the other side. That's all you need to do. It's something like $11 for the first weight and $2 for the reweigh.

Your weights will be sent to you in PDF form. OR you can go into the truck stop and pick up hard copies, too.

Oh, the app will ask you for a "Truck Number" enter any number you want. It doesn't matter.
Your credit card will be changed automatically and boom, you're done.

Here's a FUN video done by James and Stephanie of the FitRV on using the new app.

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