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Old 01-05-2006, 08:19 AM   #1
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Motorhome Tire Pressures

I thought this was already covered in another thread but I could not find it so I'm posting a question about tire pressures on Classic Motorhomes.

What are people running for tire pressures? I have 19.5 Alcoa wheels and Michelin tires. I suspect I'm running too much pressure in them based on the ride quality an some slight cupping beginning in the steer tires.
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Old 01-05-2006, 08:56 AM   #2
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"AccessMaster"

Steven:

Maybe the following thread is the one you were referring to :

http://www.airforums.com/forum...ures-4300.html

"AccessMaster" gave a great dissertation on "finding" the proper tire pressure for your rig.

Be careful not to exceed any of the manufacturers max/min numbers in the "search" for the proper pressure balance between comfortable ride and max inflation.
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Old 01-05-2006, 09:17 AM   #3
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I normally run 10 to 15 PSI over the rating plate on the firewall of the coach. It seems to offer a firm ride, but yet it is not too harsh. Most guys running the bigger MH's that I have spoken to advocate a 10 PSI bump for cooler running tires. The link Dennis posted is a good one.
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Old 01-05-2006, 09:23 AM   #4
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when I picked up my coach from the po I thought it was riding quite harsh. The airbags in the rear were deflated, and the ones in the front had less that the correct amount.I also checked the tires, they had close to 90psi.
I have been running with 60-65 ever since, and the ride is way better. I also have experienced little wear on what are older (5yrs+) Goodyear tires.
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Old 01-05-2006, 09:33 AM   #5
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I went to an RV tire seminar at an SOB dealer where I learned the importance of weighing each wheel if you own a motor home. The point was made that while all motor homes usually fall into meeting the axle load limits, some manufacturers do not always take into account the load on a specific wheel.

The guy doing the seminar noted that upon purchase of his vehicle he weighed each wheel separately and found out that when sitting in his driver's seat, the weight measured on the street side left front tire exceed the capacity of the tire by 400 lbs. A front axle weigh in did not show that discrepancy. When pointed out, the dealer contacted the factory who replaced the tires with higher rated ones.

They do seminars and individual wheel weighing at various rally's and dealer seminars and the overloading of a single tire on motorhomes happens far too often.


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Old 01-05-2006, 10:36 AM   #6
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cupping...

I can't specifically remember, but doesn't this have to do with bad shocks? Under or over inflation would affect how they wear (i.e. - low pressures with the outside wearing more than the inside and overinflation with the center wearing more than the outer tread). Feathering is an alignment problem.
I think!
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Old 01-05-2006, 10:46 AM   #7
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Marc,
Shocks can cause this, along with worn suspension, loose bearings, balance etc. Basically vibration of the wheel. I'm checking on the suspension (new shocks, wheel bearings, etc) but since my rid is so harsh up front I'm suspecting tire pressure as well.

The sidewall rating is set to 110 lbs. Obviously I'm not running that high but I'm wondering what other run for pressures

BTW - does anyone know the axle ratings on the 345 (Dennis, anyone, Dennis, anyone at all?)
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Old 01-05-2006, 01:13 PM   #8
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Quote:
Originally Posted by swebster
..does anyone know the axle ratings on the 345?
I'm pretty sure the front axle is rated at 4880lbs, and the max rim rating is 2780lbs. I need to confirm that (all tech support is in the MoHo).

I THINK the rear is rated at 10,000 lbs (thus, the necessity of the tag axle).

again, I THINK the tag is rated at 2,000 lbs.

I know the max weight is 16,500 lbs, but that limitation may be due to the brakes or the 475 transmission.

A most excellent description of the "P" chassis can be found at the following link:

http://users.sisna.com/cebula/P-Chas...entProcess.pdf

I'll try to remember to confirm the specific axle weight numbers the next time I go the the MoHo. I do know that all of the individual systems (brakes, tranny, wheel rims, suspensions, etc.) are pretty much maxed out at the 16,500 lb rating.

I DO know the W-22 Workhorse Chassis' that AS used at the end of 2004 were rated for a total weight of 22,000 lbs with 15,000 max on the rear axle, and 8,500 lbs max on the front axle....but that's another story.
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Old 01-05-2006, 03:39 PM   #9
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Dennis - thanks for the info.

Looks like I'm running about at least 10 psi over pressure on mine. I don't have exact weights on my corners yet, but just doing the back of the napkin math I think I'm running at least 10 psi over on the fronts and 5 psi over on the rears.

I take a cold and hot pressure several times on each trip and consistantly see 80 psi on the fronts and 75 on the rears.

Max pressure is 110 (Michelin 8R 19.5 XZA all around)

Based on my tire specifications and inflation chart I should be running 65 - 70 psi. I'm plan to run some road test and see what this does to my handling and vibrations.
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Old 01-05-2006, 04:57 PM   #10
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We just had all new Goodyear G670 tires installed on our 345. Size 235-70R19.5. Goodyear recommends 90 to 95psi for these tires. I am running 95 in the fronts, and 90 in the rears. Suprisingly the ride is better than with the old Goodyears. Our old fronts were 3 years old, but the rears were 13 years old. Scarry I know.
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Old 01-05-2006, 06:03 PM   #11
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Factory Set Weight on Tag Axle

Went through all of my reference material this evening -

The "Owners Manual" by Airstream explains that the weight on the tags is set by the factory at 3,000 lbs. It explains that the weight is set by the factory by raising or lowering the air ride (height) adjustment after actually weighing the weight supported by the tags.

FWIW, my tags now support only 2,000 lbs. I think that if I adjusted the air suspension any lower the rear end would be WAY too close to the ground.

The other weights I reported in the earlier post are pretty much dead on.

I did find one reference to a GCWR (Gross Combined Weight Rating) limit of 19,000 lbs due to the brake system. This closely ties into the AS limit of 2000 lbs on the hitch with a weight of 16,500 lbs total on the axles.

I would say that ALL of the systems (brakes, tranny, axles, engine, drive shaft, suspension) are pretty well maxed out at the 16,500 lb limit set on the 345's.
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Old 01-06-2006, 09:57 AM   #12
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FWIW, my 89 370 has a Gillig chassis, and the manual for that has a fairly thorough discussion of tire pressures and their corresponding ability to carry weight. Given the factory tire size of 8R 19.5, load range 'F', it shows how much weight the tires can carry with a given psi. I weighed my 370 on a truck scale, front and rear, side to side, to determine the approximate weight each wheel/tire was carrying. From that I was able to set each tire pressure based on that weight. I added about 10 psi per tire to account for added weight when loaded for traveling. My rig weighs a total of 16,900 lbs. lightly loaded, and has a GVWR of 18,500 lbs. After weighing the load on each tire I ended up with 80 psi in each front tire (slightly <3,000 lbs per tire), and 60 psi in each rear tire (the minimum recommended for these tires since the TOTAL rear weight was just over 11,000 lbs.) Using the chart in the Gillig manual (which corresponds with similar charts available on tire web sites and in tire stores) 80 psi can carry up to 3170 lbs, and 60 psi can carry up to 2445 lbs. There's roughly a 200 lb weight capacity difference each time you add or subtract 5 psi from my numbers. I have no complaints about ride nor any irregular tire wear. The tires do not heat up unusually either on long trips - another sign the psi is not too low.

I know this is a long explanation of a relatively simple issue, but it's hard to explain in short form. My Airstream Owner's Manual, which is the same one as the C30 Chevrolet Chassis units use, just calls for a flat 70 psi in the front and 60 psi in the rear, regardless of length, weight, or load. While that's probably not dangerous, it would have left my front tires somewhat under inflated. By the way, I'll be glad to share the psi/weight information I have if anyone feels the need for exact numbers. It covers the standard tires on these MoHos showing weight capacity for psi ratings from 60 to 90 in 5 psi increments.

Tim
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