More On Tires
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Well I really stepped in it last week and as Sven pointed out about a minute after I posted that I did not answer the question in the title. That was what is the correct tire pressure? Well, if we hang in a bit I promise that I will answer the question. A hint is that there is no one answer but perhaps we can clear up the formula.
Larry wrote that the “air limit is stamped on the side wall of the tire”. That is basicly true but there is also a weight stamped with the PSI number and usually more to the statement. larry has it half right. The Firestone T559 tires sized 9R22.5 12PR on my old Foretravel says the following: “max load single 4540 LBS at 105 PSI cold,” and then a second sentence, “max load 4200 at 105 PSI cold.” First of all why the difference in the tire pressure between single and dual? According to my sources it is the clearance between the dual wheels to make sure the tire side walls do not touch during deflection.
The statements on the tire side walls says that you can carry 4540 pounds of weight at a specific pressure and the tire will have the correct foot print and side wall deflection for a combination of low heat build up, good ride and handling and good tread wear. The entire statement must be taken in context and does not mean that this is the correct tire pressure under all conditions for this tire.
Jerry wrote, “OK under inflated is not good. So where do I get an accurate tire gauge? Where do I find a air pump that will properly fill my tires?” Well Jerry I use a digital tire gauge that I bought from the Northern Tool catalogue. My compressor is a Campbell Hausfield 110 volt 100 PSI that came off the shelf at Wal-Mart. It is a bit slow but it does do the job.
Roger, a tire designer, wrote, “I am a tire design engineer with 38 years experience. I teach tire failure analysis ( Think CSI for tires) so let me correct a few minor points and answer some of the questions.
The correct inflation is based on the actual load you are running. The inflation information in your owners manual is based on some assumptions on how much weight you are carrying. I have heard of some vehicles being only capable of adding 5oo# over their empty weight before the tire is overloaded so the only way for you to know the real answer is to get your vehicle weighed for each position. Then contact a tire dealer and ask if he has a load inflation chart (or check the manufacturers web site). If he doesn’t find a different tire store. The correct inflation should be something below the max infl on the sidewall of the tire. Only check your inflation when it is cold – not in the sun and not driven on for at least 4 hours.”
Roger says it all. Weight the rig, at least each end and preferably each corner with your travel load, and then compare the weight with the makers tire chart and determine the correct pressure for conditions.
Now stay tuned for next weeks blog when we will hear Gordon say, “I recommend doing a Google search on Nitrogen in tires. The information will make the choice a no brainer”.
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Tires – Nitrogen air loss study
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.
Need info. Can my 2002 Class A RV have the new single extra wide tires in the rear replace the duel tires? Is this a good idea? Any ideas?
Thanks
Need info. Can my 2002 Class A have the new single, extra wide tires in the rear replace the duel tires? Is this a good idea? Any ideas?
Thanks
Hi Brad
Well I know about dual spacing (Having started my career in Truck tire Design) but whoever is giving you info about the reason for a lower rating for dual application is off the mark. Actually I have never heard that as a reason for the load difference. Dual spacing is for a different reason.
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?
Check this video
I think the best way to set the correct air pressure in any RV, is to set the inflation pressure that will provide the “loaded radius” the manufacture calls for in their technical data book. Maybe not all manufactures post this measurement.
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.
While this article has the attention of experts, let me put forth a nagging question. Trailer tires and truck tires are the same, most of the time. I use LT235-75R-16,LR_E on Fiver and LT265-75R16,LR_E on truck. Should (ALL) tires be balanced. I have been told that heat is generated on fiver by unbalanced tires as well as incorrect tire pressure. I balanced all my tires now.Is this correct? Also on comments on tire weight and pressure. My GVWR on fiver is 14000lbs.Does this mean that if my weight is less than that, I should reduce tire pressure? Tire installers have different opinions regarding this very important saftey issue
If you run Michelin tires, here’s a link to inflation charts for RVs.
http://www.michelinrvtires.com/michelinrv/report.do?ReportType=LoadAndInflation
Mac
There are three reasons why dual tires have less load capacity.
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.