1) Tire diameters should be are relatively the same but are not usually exactly the same.

2) Inflation pressures routinely differ from tire to tire

3) Road crown impacts loading

In order to compensate for the above and ensure that one tire is not overloaded, the tire manufacturers and The Tire & Rim Association derate the dualies load capacity.

http://www.michelinrvtires.com/michelinrv/report.do?ReportType=LoadAndInflation

Mac

Year’s ago as I started my career in the tire business, I was frequently ask to look at tires on RV’s. I found a good/level pad of concrete that I used. I would then measure the loaded radius (LR) of all tires on the RV. I would ajust the air pressure in each tire to give me the LR the manufacture listed in their data book.
I found a good/accurate tape measure in mm.

I then gave each “customer” an inflation sticker to post on his RV. I always cautioned them if they change the weight of the RV, this procedure would have to be done again.
I never had a second complaint!

To me there is a real issue with the RV manufactures using cheap/foreign tires. Many foreign manufactures use distributors and do not have dealers that can provide technical information. Information such as LR is not readily available.

Unfortunately, too many RV manufactures are only concerned with the cheapest tires they can find to be able to get their rigs rolled off the manufacturing pads and to the point of sale. Once there, they fill their obligation has been fulfilled.

The fact is that when tires are in a dual application the actual load on each tire is not exactly 1/2 of the load when you measure the two tires together. There are some technical reasons for this but the simplest way to think about it is that the two tires are not exactly the same size so the larger tire has to carry more load until the total load for that vehicle position is supported by both tires.

As you load a tire, it deflects. The more load the more deflection. The total deflection is the same but one tire might carry 1800 pounds per inch deflection at the point you are loading it and the other tire might carry 1925 pounds per inch deflection. So if on your motorhome you deflect both tires 1.2″ one is carrying 2160# and the other 2310#.
One other thing that is considered in the load equations. Any one tire does not deflect exactly the same amount at every point around the tire. This is called non-uniformity. Many of us have felt this as a uneven ride, like being out of balance even when the tire is exactly in balance.

The load formula and the adjustment for dual application was developed based on decades of real life experience and is used by the entire industry.

Let me know if you have other questions.

PS Any chance you are going to Vermont FMCA Rally 1st week August?

Thanks

Thanks

Filling tires with nitrogen rather than air is becoming a common practice in the replacement tire market. This service offers tire dealers another avenue for making money while also promoting safety. The claimed safety benefits often include the potential for reducing air loss compared to an air-filled tire. Maintaining proper inflation can help prevent tire overheating; promote optimum tread life; and reduce rubber aging and wheel corrosion. The use of nitrogen in large truck fleets and the commercial tire industry are well documented and support these claims.

The National Highway Traffic Safety Administration (NHTSA) has seen reduced aging of tires filled with nitrogen. Though the data does support that passenger car tires could benefit by all the claims made for nitrogen, tire manufacturers say that they already design tires to perform well with air inflation. And while nitrogen will do no harm, manufacturers say that they don’t see the need to use nitrogen, which generally adds $5 or more per tire charge.

Nitrogen_tiresaver_4 Consumer Reports wanted to find out if nitrogen is worth the price, so we purchased a Nitrogen Inflation System and checked out how well the inflation held up over a one year period. We evaluated pairs of 31 tire models of H- and V-speed rated, all-season tires used in our tread wear test from 2006. We filled one tire per model with air and the other with nitrogen. The test was quite simple: fill and set the inflation pressure at room temperature to 30 psi (pounds per square inch); set the tire outdoors for one year; and then recheck the inflation pressure at room temperature after a one year period.

The tires were filled and deflated three times with nitrogen to purge the air out of the tire cavity. We also used an oxygen analyzer to be sure we had 95-percent nitrogen purity in the tire–the claimed purity limit of our nitrogen system, which generates nitrogen gas from ambient air.

Nitrogen_mainchart_consumer The test started on September 20, 2006 and the final measurements were taken on September 20, 2007. The results show nitrogen does reduce pressure loss over time, but the reduction is only a 1.3 psi difference from air-filled tires. The average loss of air-filled tires was just 3.5 psi from the initial 30 pressure setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial 30 psi setting. More important, all tires lost air pressure regardless of the inflation medium, so consumers should check their tires’ air pressure routinely. No evaluation was done to assess the aging claim.

Bottom line: Overall, consumers can use nitrogen and might enjoy the slight improvement in air retention provided, but it’s not a substitute for regular inflation checks.