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Old 04-15-2016, 11:34 PM   #1
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Tag's tire PSI vs Drive

I've had a few RV owners, as well as a few tire professionals, advise me to run the same PSI in the Tags as in the Drives, as long as the weight load PSI on the tire manufactures chart for the Tag, was under that of the Drive.

They said it is a KISS on keeping PSI's set. As well as if one of a Drive wheels dual tires has a problem the Tag is better PSI'd up to assist the remaining wheel. Some also said it helped in overall stability of that end of the coach. (Now my thinking is that most higher end DP's chassis, the Tag and the Drives are basically separate entities, as shown by the difference in measured weight loads between the Tag and Drive. If they were equal, that would be a surprise!

So was wondering about what the gang was doing?

Do you run the PSI based upon the weight carried, for the Tag?

Or, do you match the Drive tires PSI?

TIA, and best to all,
Smitty
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Old 04-16-2016, 03:07 AM   #2
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Since the tag wheels are a single wheel, I run the single wheel air pressure max listed on the side of the tire.

As an ex-over the road trucker, I follow a little different philosophy than what I read in most responses to tire pressure setting questions you may find on this forum.

The norm in the trucking industry had been, 95-100 PSI in duel wheels, and always 110PSI in singles including steer tires.

New tires and newer technology may dictate different thinking for your situation. But for me so far, falling back to that old technique has worked well.

When we first got our coach, I did try the decreased air pressure settings called for according to coach axle weights. It didn't work for me with decreased fuel mileage and less than desirable handling characteristics, and the ride did not improve.

But again, that's just how it worked out for me, and that's why I run the tire pressure the way I do.

As far as the difference between tag axle weight and drive axle weight, the drive axles should always take more of the load. This way the drive axle is not losing traction due to irregular surfaces as you motor down the road.

As you make a tight turn, the tag should be the axle sliding not the drive. in practice, this shortens your wheel base in turns and enable you to maneuver in as if you were driving a shorter vehicle.

Higher air pressure in the single tag tires also help to keep the side forces from flexing the tag tires excessively.


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Old 04-16-2016, 04:08 AM   #3
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I fill the TAG's according to the weight they're carrying - I do not match them to the drive tires. My drives are 95 psi, my TAG's are 85.

If, in some extreme circumstance, the TAG weight dictated that they be filled far below the pressure needed in the drive tires - like say... 25 or 30 lbs less, which is hard to imagine - then I would consider filling them up to pressures closer to the drive axle for the reason you mentioned already... to help out in case there's a flat or blowout on the drive axle.
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Old 04-16-2016, 04:57 AM   #4
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As you can see Smitty77, Wryly Blithe’s statement is typical of how most motorhome owners run their tire pressures and his explanation is spot on.
However I still question the effect of running any 22.5 tire at 85PSI knowing that lower pressures in this range create more rolling resistance and heat buildup due to more flexing of the side walls than a tighter tire.
I do understand and didn’t mention as Wryly Blithe has reminded us, the tag axle carries less weight than would a tandem drive axle of a truck, so the differential in tire pressure to the lower side sounds like it makes perfect sense.
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Old 04-16-2016, 07:39 AM   #5
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Quote:
Originally Posted by Wryly Blithe View Post
...I fill the TAG's according to the weight they're carrying - I do not match them to the drive tires...
Same here...that's the way the front and dually axles are set.
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Old 04-16-2016, 07:49 AM   #6
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After weighing all axles, I run fronts at 115 and all six rear at 100. 43' Dutchstar - The weigh "guy" said that I could run 120 up front but my thinking is the summer highways are so hot that 115 covers the weight I'm running fine. He also said that I could have the tag ride height adjusted a little lower and that would take some weight off the front axle? Not going to do it as the front changes with fuel load anyway. hope this helps - mark
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Old 04-16-2016, 09:51 AM   #7
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Keep in mind that the percentage of rear weight that the tag carries is determined by the proportioning valves. Some are 60/40, some 70/30 and others, mine included, are 75/25. The tire shop telling you to run the same psi in the tag as the drive so that it can help carry the drive in the event one of them has a problem doesn't have a clue how it works. If one or both drive tires on one side go flat, the tag is still only going to carry the 40,30, or 25% of the load. There are so many different views and opinions as to what pressures to carry one has to make their own choice. The only wrong choice is running any tire psi lower than the minimum needed for the load it is carrying. A higher psi than needed may give a harsher ride, but possibly better mileage and will not heat up as much due to less flexing. Too low of psi will give a better ride, but lower mpg and lead to tire failure. I run the tag tires at 90 psi and the drives at 100 psi, both set when cold. These values are 10-15% above the minimum needed.
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Old 04-16-2016, 10:30 AM   #8
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Our four corner weights are:

L Front - 5800
R Front - 6100
(Combined) - 11,900
(Weight to be used for Axle Weight) - 12,200

L Rear Drive - 7800
R Rear Drive - 8250
(Combined) - 16,050
(Weight to be used for Axle) (Note: Drive often called Duals.) - 16,500

L Tag - 3800
R Tag - 4200
(Combined) - 8000
(Weight to be used for Axle Weight ) - 8400


Total Weight - 35,950

Toad Weight - 3475

Combined Weight - 39,425
================

I put this info together for my wife, after our Escapees Smart Tire four corner weights. (Loaded for travel, full fuel, LP, Water, Wine, and some Grey and Black in the mix as well. Very representative of our 'Full Heavy' for travel. (Actually a bit more, as I had my MIL stay in the coach with her gear, as she was visiting us for a few days of travel. Did this because we often have our daughter going us too.)

So per the Michelin Tire PSI Charts for our XZE 12R's, the Tags are 1000 LBS below the lowest line level on the chart. And the Duals/Drives are 1730 LBS below the lowest level of the chart.

PSI Single Dual
75 5200 9980

So, I should be able to run with a Minimum of 75 PSI. I personally add 10%, the round up to the next "0" or "5" ending double digit, for KISS on setting PSI's. 75 + 7.5 = 82.5, rounding to 85 PSI.

For all reasonableness in safety for these specific tires with the loads they're carrying, I should be very safe funning at 85 PSI.

That being said, I currently have the Dual and the Tag set at 100 PSI. (Down from before my four corner weight, when I ran 105 PSI of the last two years. I had added Solar Panels and much larger L16's AGM's all on the right side of the coach, since my last weigh in. So I bumped it up to 105 PSI until I could get the new four corner weights.) As FYI, my Steers I run at 105 PSI. (These are now BF Goodrich ST230's, but have the same PSI to weights on the Manufacturers Charts.)

Why do I run higher PSI's then what I recommend? Heck if I know! But, as mentioned by DTW and Crasher, higher pressures help on two critical fronts: 1) Less flexing of sidewalls, usually results in lower running temps; 2) Potential for higher MPG.

As far as ride comfort, I did notice that the ST230's ride a bit less stiff then the XZE's at the same PSI.

I'm getting 6 new XZA2's Energy next week, changing to 295/80, mounting them on the Steers and Drives, and moving the current ST230's to the Tag (Only one year old, added due to damaging two tires last year on a California pothole.) I've been told by many who have had these tires, that they seem to ride a little softer as far as transferring expansion joints, etc., into the coach. We'll see.

And spot on how a coaches chassis has been set up on the split between the Drives and Tags load carrying. Many variables.

I will start these off with the same PSI's I've been running, and go take a route drive that I know well. Getting some miles on the coach before I head back for the final wheel re-torque. After that, I may drop the PSI down by 5 all around, and do the same drive to see if I can notice a difference in ride comfort. If not, I'll go ahead and bump them back up.

Appreciate you all sharing your perspectives on Duals vs Tags PSI. Not surprising that we come at it from different perspectives!

My best to all, keep the rubber side down, and have fun,
Smitty
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Old 04-16-2016, 10:35 AM   #9
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Smitty
At my December '15 visit to the Gaffney Freightliner Service Center with an actual weight of 39.5K with good corner weights and front/rear distribution, and Michelin 22.5s (295 and 275) all-around my pressures were set at 120 steer and 90 drive/tag. Rolling pressures, temps, control and ride are all very good.

Safe travels ...
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Old 04-16-2016, 11:17 AM   #10
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Lots of information and opinions about tires and tire inflation.

My limited research seems to indicate that tire manufacturers design tires with a certain amount of sidewall flex included. Over inflating (or under) would seem to be something that should not be done.

Comparing RV to OTR tires is impractical. OTR tires are inflated to maximum load, because that is what they do about half the time. Changing the tire inflation every time they deadheaded could be done but at great time and effort. A coach weight however remains fairly consistent comparatively speaking so optimizing the tire pressures makes sense.

Some interesting facts:
- ribbed tires make better mileage than lug tires.
- partly worn tires make better mileage than new tires.

Who'd a thunk?
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Old 04-16-2016, 11:33 AM   #11
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I've been keeping it pretty simple ... 100 lbs of pressure all around. Seems to work pretty well. No discernable issues with tire temps ... no noticeable impact on wear patterns ... and well within the range of recommendations from both the coach and tire manufacturers.
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Old 04-16-2016, 05:33 PM   #12
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Quote:
Originally Posted by Dtwallace View Post
As you can see Smitty77, Wryly Blitheís statement is typical of how most motorhome owners run their tire pressures and his explanation is spot on.
However I still question the effect of running any 22.5 tire at 85PSI knowing that lower pressures in this range create more rolling resistance and heat buildup due to more flexing of the side walls than a tighter tire.
I do understand and didnít mention as Wryly Blithe has reminded us, the tag axle carries less weight than would a tandem drive axle of a truck, so the differential in tire pressure to the lower side sounds like it makes perfect sense.
Except that I believe the rolling resistance and heat build-up of a tire is directly proportional to the load it's carrying. If true - and I'm no TireGuy9 so it may not be - the rolling resistance / heat build-up of a lower inflated tire carrying less weight would be the same or similar to a higher-inflated tire carrying more weight.

Theoretically, in a perfect world where we assume the psi's were set to the appropriate amount for the loads, lower-inflated tires carrying less weight don't flex more than higher-inflated tires carrying more weight, so the rolling resistance / heat build-up would be the same or similar between them. If they were all carrying the same weight then of course they'd build up more heat. But they're not carrying the same loads, so they don't.

Or, at least, that's what I believe and have read in dozens of hours of research on the subject, though admittedly I have no measurable scientific evidence to back it up. Logically it makes sense, but scientific reality may very well prove that theory wrong.

All that said, Dtwallace's idea of inflating the tires to higher psi's isn't bad thing (within tolerance limits), and it suits many drivers very well. We're just talking about options here, so...
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Old 04-16-2016, 05:36 PM   #13
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Quote:
Originally Posted by Crasher View Post
If one or both drive tires on one side go flat, the tag is still only going to carry the 40,30, or 25% of the load.
EXCELLENT POINT! I did know that, but failed miserably at factoring it in to my argument. Thank you for the outstanding clarification!
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Old 04-16-2016, 05:43 PM   #14
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Tag's tire PSI vs Drive

Mine are set according to the Michelin load chart. 120 front, 85 drive, 75 tag. The temps are very close to even while rolling. The ride and handling are perfect
That's my story and I'm sticking to it.


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