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Old 05-29-2023, 05:58 AM   #1
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Just a double check on tire pressure

Our 2014 Bay Star had the original Goodyear G670s on it. I ran 85 psi on all tire positions. After 9 years and 14K miles, it was time to replace them. I went with the Michelin XZE LRH. Since I made the change (the only change) the front end seems to wander a bit, and requires more input from the driver (me). I have a front-end alignment scheduled for next week, but want to make sure my numbers are right. Even though I went from 14 ply LRG to 16 ply LRH the pressures are still set at 85 psi. I just want to make sure I'm not shooting myself in the foot. I have included Two scale tickets. One showing full steer and drive axle weights, and another with only the passenger's side on the plates, and lastly, the chart for the Michelin XZE tires. If I am reading it right, 85 psi should be good. Thanks for the double check!
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Old 05-29-2023, 06:23 AM   #2
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85 looks right to me.
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Old 05-29-2023, 07:18 AM   #3
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I just changed mine to Sailun 637ís H range from G as well and found it to wonder a bit. After a 500 mile trip they seem to be getting better. I read that they need some time to break in and the squirm will go away.

Let us know the results of the alignment, I am curious. Thanks.
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Old 05-29-2023, 07:26 AM   #4
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First, I'm presuming your stated weights are your normal travel weights. If not, comments below are not valid.

To me, the issue you have with the LRH rating is there is no 70psi inflation recommendation for your tire in the chart, so you start out with 75psi even though your front axle weight doesn't necessarily require it. As you are calculating now, 85psi is 13.3% above the 75psi number in the chart. I think that baseline number already has some cushion in it even before adding the 13%. If you are wandering with the 85psi inflation, drop the front to 80psi and see if that helps (still ~7% above the chart's recommendation). That's certainly not underinflated given your loading and the fact that there is not a 70psi listing.
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Old 05-29-2023, 07:48 AM   #5
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85 should give you enough margin that you won't need to chase tire pressures as the temperatures change. I take the axle weight, add 5% to compensate for side-to-side weight variations, and then do the load inflation chart lookup to find the minimum pressure needed in the tires when cold.


When going in for the alignment, be darn certain to tell the tech you want to have a copy of the BEFORE and AFTER numbers. Many will just give you the "after" and they may not even record the "before".

Unlike a car the Ford 53 really only has one easy front end alignment adjustment, the toe. Changing the caster requires that shims be added or subtracted. Changing the camber on a solid axle vehicle requires heating up the axle and bending it so no one will do that anymore.

Ours was aligned at the Forest River factory when new and we had the Ford Commercial Vehicle dealer that local motorhome dealerships use perform an alignment three months later. Their tech does all of the motorhomes so he knows how to set them for best driving, not for minimizing wear as many truck alignment shops will do or just setting things in the acceptable range.

In case you're interested I attached our results. 22,000 GVWR chassis and 242" wheelbase. Per the header it covers your GVWR chassis as well if you have the 20,500 GVWR unit.

If you look in the corners of the colored blocks you'll see small numbers that show the lower and upper limits. For toe the alignment actually could have the tires pointed outward slightly and still be within spec but it would wander a lot ("toe'd out")

Our toe was within Ford's limits from the motorhome factory alignment but the tech said he changed the toe to make it drive better and he also readjusted the Safe-T-Pus steering stabilizer for no additional charge. Even though the toe change he made was slight I could detect a bit less wandering on the drive home.

You'll note that the toe is green before and after yet that is what he adjusted. You'll also notice that the caster is in the red and high.

Increased caster causes a vehicle to track straighter at the expense of requiring more steering effort to turn. In practice the driver of an F53 doesn't notice the effort. A usual recommendation for a motorhome is to set the caster at the upper limit so it tracks better. When ours was aligned in 2019 (V10) the tech said he had been seeing motorhomes coming through with above-normal caster for about a year. He recommended that the caster be left alone because it will make the motorhome drive better.

That is the difference between a truck alignment tech and a motorhome alignment tech.


I presume you have the annual front end greasing done but if not you should have that done to avoid excess wear in the front end. Many a person has worn out their kingpins on the F53 by not greasing them annually and worn parts cause handling problems.

Good luck,

Ray
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File Type: pdf 2020 GT5 Front End Alignment Results.pdf (428.2 KB, 7 views)
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Old 05-29-2023, 08:13 AM   #6
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Did you notice a difference in ride?

A quick review seems to indicate you are high on the front. 65 - 31 = 34 so the front axle should be aired for 3400. That would give about 75+ for the front because that is the minimum.

Same for the rear. Highest weight is 6300 but the chart only goes down to 75 lbs.

I would drop the pressure to 80 to see if the steering improves. And then try 75.

Go what you are comfortable with but nothing below 75.
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Old 05-30-2023, 03:27 AM   #7
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With my made motorhome tirepressure calculator, with axleloads filled in in part 2 and axle-end loads filled in in part 3, both calculations come to
Front/steer/ 87psi, Rear/drive 87psi.
Is with maximum reserve, with still acceptable comfort and gripp.

So this is not that far off the 85 psi, you use now.
So would not explain the wandering.
So sooner expected an alignment issue or something wrong with tires or mounting of them.
I expect the second, because nothing else changed in loading, from old to new tires.

Notice the crossed weightdifferences between the axles, you see that yust as often as same side heavyest. Has to do with dynamic balance.
Not important for the pressure, and was the same with the old tires.

Edit: reading back I see I swiched the passenger and driver sides, not important for the pressure
Right is.
Axle/ axleweight/ R driverside/L passengerside.
Front/ 6540/3440/3100 lbs
Rear/ 12380/6060/6320 lbs
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Old 05-30-2023, 06:52 AM   #8
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Quote:
Originally Posted by jadatis View Post
With my made motorhome tirepressure calculator, with axleloads filled in in part 2 and axle-end loads filled in in part 3, both calculations come to
Front/steer/ 87psi, Rear/drive 87psi.
Is with maximum reserve, with still acceptable comfort and gripp.

So this is not that far off the 85 psi, you use now.
So would not explain the wandering.
So sooner expected an alignment issue or something wrong with tires or mounting of them.
I expect the second, because nothing else changed in loading, from old to new tires.

Notice the crossed weightdifferences between the axles, you see that yust as often as same side heavyest. Has to do with dynamic balance.
Not important for the pressure, and was the same with the old tires.

Edit: reading back I see I swiched the passenger and driver sides, not important for the pressure
Right is.
Axle/ axleweight/ R driverside/L passengerside.
Front/ 6540/3440/3100 lbs
Rear/ 12380/6060/6320 lbs
There is likely an error in the weight distribution because the coach would not be level when driving with only one set of wheels on the scale.
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Old 05-30-2023, 07:29 AM   #9
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Ditto what NXR said. A good tire alignment after installing new tires or rotating the tires should help. I just rotated and re-aligned our tires after 20k miles and it improved tracking and steering. Also, re-confirming that your new tires have the correct pressure for the load, plus 5 psi for load safety margin, should help steering.
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Old 05-30-2023, 11:42 AM   #10
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Quote:
Originally Posted by Gordon Dewald View Post
There is likely an error in the weight distribution because the coach would not be level when driving with only one set of wheels on the scale.
If for instance ground besides the scale lower, a bit less weight measured on the wheel on passenger side. But then for both axles.
So this crossed weightdifferences on the axles, would not be explained by that.

Saw I was double mistaken L is driver side side and R is passenger side, but in my slreadsheet part 3, I gave it right.
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Old 05-30-2023, 06:18 PM   #11
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New tires require about 500 miles for a "break-in period". After that they track and ride smoother.
There are a couple of things that should be done when mounting tires. Quote:
"

How do you align your tires to achieve concentric mounting?
Use the red and yellow dots you find on Bridgestone tires to help minimize radial runout and aid in initial static balance. Be sure to align any dots correctly. Then, making sure that you follow all OSHA, RMA, tire manufacturer and shop procedures, proceed to seat the beads.
How can you check your wheel and tire assembly for concentricity?
Use the tiny ring that’s molded into the tire near the bead, and check its distance from the edge of the bead flange. Check it in at least four locations, 90 degrees apart, and do this on both sides of the tire. The molded ring must be the same distance from the flange in all locations. If you can see a difference, the tire is not concentrically mounted."
source: https://commercial.firestone.com/en-...ntric-mounting
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Old 06-01-2023, 08:24 PM   #12
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I plan on dropping the psi 80 and take our next trip of about 250 miles. Iíll see how the tires react to the lower pressure. I donít think it an alignment issue, as nothing has changed, other than the tires. The only shop I doing that would do an alignment on an F-53 chassis charges $275+ I would like to exhaust other possibilities first.

Thanks for all of your input. It is appreciated.
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Old 06-02-2023, 03:09 AM   #13
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Still I would not exclude alighnment issue.
Can be that with jacking up something went of its place, wich influenced the alignment.
Remember that my advice was 87 psi for your given weights,assuming weighed fully loaded, so using 80 psi, will not give the needed reserves. Then certainly dont use high speeds. For your 19.5 inch tires the speed for wich maxload is given is 75mph.
So 65mph with 80 psi I advice as maximum.
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Old 06-02-2023, 07:11 AM   #14
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275 for a wheel alignment is not an amount I would hesitate to spend if you find that a lower air pressure does not solve. Uneven wear on the tires, reduced handling and other issues including driver fatigue make the 275 a bargain.
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