Tire Pressure Question

[WayneLee]

Yes, it's another "age old" question about tire pressure.

I just had my new HR Endeavor weighed - I could not get a "4 corner" weight, but I did get the front axle and total weight. Anyway, the tires are the "new" Michelin XZA3 275/80R/22.5 Load Range H. I had 9840 pounds on the front axle and 19,540 on the rear axle with 7/8 fuel, full LP, and 1/2 tank water. Adjusting for additional load due to DW, clothes and food, and comparing the axle weight to Michelin's web site, Michelin recommends 90 psi front and 85 psi rear. HR recommends 115 psi front and 90 psi rear.

Can anyone tell me why such a difference between Michelin and HR on the front axle? And, assuming my figures are correct, which one would you go by? Also, any thoughts on why the actual rear axle weight is only 460 pounds less than the Rear Gross Axle Rating? That 400 hp Cummins ISL engine must weigh a lot!

[WayneLee]

Yes, it's another "age old" question about tire pressure.

I just had my new HR Endeavor weighed - I could not get a "4 corner" weight, but I did get the front axle and total weight. Anyway, the tires are the "new" Michelin XZA3 275/80R/22.5 Load Range H. I had 9840 pounds on the front axle and 19,540 on the rear axle with 7/8 fuel, full LP, and 1/2 tank water. Adjusting for additional load due to DW, clothes and food, and comparing the axle weight to Michelin's web site, Michelin recommends 90 psi front and 85 psi rear. HR recommends 115 psi front and 90 psi rear.

Can anyone tell me why such a difference between Michelin and HR on the front axle? And, assuming my figures are correct, which one would you go by? Also, any thoughts on why the actual rear axle weight is only 460 pounds less than the Rear Gross Axle Rating? That 400 hp Cummins ISL engine must weigh a lot!

[Drifter]

It's very hard to understand why both the factory and tire company are so far apart, but after working in the auto industry for 38 years I know why. It's safety from HR on a new coach that a new customer might overload. And early tire whare from the tire company, so they sell more tires. The best result can be found right here on this forum. My guess is to pic something in the middle and work from there.

[Tom and Patty]

Manufacturers make those inflation charts so that the tire at the correct psi for weight will flex at the proper point in the sidewall, and be able to absorb impacts more readily. It also assures a proper footprint for even tire wear.

On my rig I error on the higher side of recommended PSI by 5-10 pounds psi.

Try to find a scale on a Sunday that has room for you to put only the rt or left front tire on the scale. That way you can determine the different weights side to side. Always have the tires at the proper psi for the heaviest corner.

Here is a site with pdf's that you can download about weighing your coach.
http://www.bridgestonetrucktires.com...g/rv/index.asp

[Roland]

Wayne,

I think the difference may be simple. Your Endevor should be on the same chassis as an '07 36PDQ Diplomat. If so, the front axle is rated for 13,000 lbs. GAWR.

The pressure Monaco / HR gives will be based on a 13,000 lb. weight load; not a 9,849 lb. load. The manufacturer dosn't have a clue how we load our coaches, so they base everything on max load IAW the specs. Check your owner's manual for the GAWR and then check Michlein's load chart for that weight and pressure. That should account for the difference.

You are correct in weighing your coach. What you don't know is the weight difference from one side to the other on the same axle. My '01 Dip weighs 200 lbs more on the street side front vs the sidewalk side front. There are some coaches, out in the world, with over 1,000 lbs difference street side vs sidewalk side. You will need to adjust tire pressure based on the heavy side, of an axle. But, for what you know you seem to be doing well. Do a search for RV Safety Education Foundation (RVSEF). The do four corner weights and should be posting (on their website) what RV shows thwey will be working.

Green lights and soft breezes,

[WayneLee]

Thanks to all for your replies..

Roland: Good insight, I do have that same chassis. So, I looked up on the Michelin site the requirements for a 13,000 pound axle and it is right inbetween 105 and 110 psi. At 105, the axle weight supported is 12,870 and at 110, the axle weight supported is 13,360. Don't know why HR would say 115 psi though

I use to carry an air compressor for my gasser Class A's. But with the air chuck available to tap into the Endeavor's air system could mean that I would not need to carry my air compressor. However, the max reading on my dash gage is 110 to 115 psi which could be problems with the 115 psi HR suggests for the front tires. So, if I really need 115 psi, I will have to continue to carry my compressor with me. If I can run at 110 psi, then the compressor can stay at home and I can save 50 pounds!

[Roland]

Wayne,

If your coach had been shipped with Goodyear then the number would be 115 psi. But don't fret. The only time you need to be in the 110 - 115 psi range is if you are carring 13,000 lbs on the front axle. Based on your figures, I would consider running 95 psi all around. The extra allows for loss over time, changes in ambient temperature and occassional high speed use(over 65 mph).

As to the weight on the rear, in addition to the engine & trans, consider propane & fresh water just in ftont of the rear axle and batteries just behind the rear axle.

Green lights and soft breezes,

[WayneLee]

Thanks for the answer and suggestion. I think I will take your suggestion and run 95 all around.

[Diplomat Don]

WayneLee....I have pretty much the same coach as you. My weights are almost identical to yours and my Goodyears called for 85psi rear and 90 psi front. I wanted to run as close to the recommended numbers as I could to make the ride as nice as possible.

After running the above pressure on a few trips I could always smell the slight odor of rubber after several hours of driving. I knew I was running a shade low on air. I bumped the fronts to 95psi and the rears to 90psi. We just finished a 1000 mile trip and this is the perfect setting for mine.