Tire Inflation Pressure and Longer Life Tires

While reading Mark Quasius’s article about “Weighing Your RV” in my July 2014 issue of Family Motor Coaching, I was reminded of a learning experience I had a number of years ago when my motor home tires required replacement.  Many of you may also have noted that your tires’ treads show uneven wear via having the tread worn away on the edges, or no/little tread showing in the center portion of the tread, which respectively is representative of what happens when a tire is worn while being either under or over-inflated for supporting its load.  However, it was more interesting because I also noticed that one of my tires looked like it had perhaps another 60,000 to 70,000 miles of life remaining, which puzzled me because I was sure that I had purchased & installed the existing tires all at the same time.


Even after finding the origination paperwork,  I remained without an answer as to why one out of six tires had managed such a privileged life via evidencing less wear, while the others all looked ready for replacement!  This unexpected observation caused me to look more closely at this tire’s working life style specifics to see if I could determine why it enjoyed a privileged life.  The tire was located on the left front steer axle & I had always inflated it to 100 psi just like I inflated my right steer axle tire (per the advice of someone I thought to be an expert).  On my next trip out with my motor home (& my last trip with my 5 tired tires), I started checking my “hero tire’s” inflation changes as it began warming up by rolling & working.  (This task was quite easy, because I own & use a PressurePro TPMS which allows me to note my tire pressures at any time, while driving down the road &/or while parked.)  I noted that after approximately 35 minutes of rolling, my 100 psi cold running-pressure for my “hero tire” increased to 114 psi & that my right steer axle tire had its 100 psi cold-running-pressure increase to 119 psi.


Since I wanted to observe my steer axle tires up close to ensure that there wasn’t anything that I was over looking, I stopped to find that my left front “hero tire” had a tepid moderate temperature feel to it, but the right steer axle tire felt warmer.  It was obvious that my left front tire was happier than my right front tire relative to 35 minutes of work exertion.  I then wondered why my left & right steer axle tires were experiencing a different working pressure experience.  I then remembered that I had my approximately 500-600 lb. tool box located up front in the underneath bin compartment on the right side, which of course caused the right front tire to work harder to support this additional load that didn’t exist for the left front tire.  The left front tire was happy when its inflation increased 14% over its cold-running pressure of 100 psi & it had confirmed this optimal happy disposition by showing me an even tread wear surface after approximately 70,000 miles, while the 5 other tires showed near terminal tread deterioration.


Then I had a flash thought…could it be this simple to determine adequately the correct optimal tire inflation to consequently cause the correct side-wall stiffness for a given load, by merely looking for an approximate +14% inflation increase over the cold-running-pressure?  The pragmatics for implementing this “Inflation Methodology” would merely involve my simple calculator!  To determine if the problem could be solved this simply, I estimated & inflated my best guess for the ideal cold-running-pressure for each of my new tires & wrote down this pressure for each respective tire on a sketch of my RV’s tire locations on a tablet sheet that would remain in my driver‘s area.  I then jotted down the pressure increases for all of my tires after approximately 35 to 40 mins. of initial driving in the early AM & then checked the pressure differences with my calculator to find the tires’ respective percentage pressure increases.  More specifically & as earlier revealed, my “hero tire” would be a happy tire if its increase in pressure was 14 psi (14%): 100 psi X 1.14 = 114 psi.  If a tire’s pressure increased by 16 psi to 116 psi after approx.35 to 40 mins. of initial driving (a 16% increase), it would be suggesting that its cold-running-pressure was less than required to make it a happy tire & would consequently cause additional sidewall flexing that in turn would cause the tire to heat up to in turn cause the pressure to reach 116 psi for a 16% increase in pressure, rather than the desired & seemingly correct 14%  increase.  Therefore 1.14 X 102 psi = 116.28 psi (sufficiently close to 116 psi for our rule-of-thumb method).  If alternately, a tire’s increased pressure over its cold-running-pressure was only 12%, it would suggest an over-inflation by approx. 2%, which would indicate that the cold-running-pressure be reduced by approx. 2 psi; i.e…1.14 X 98 psi = 111.72 psi (again sufficiently close to 112 psi for our rule-of-thumb method).


In order to keep the pressure data collections uniform to result in the best corrections, I would only collect the initial tire warm-up pressures for tires that were not having the sun shine directly on them during the early AM initial driving.  Collecting & adjusting tire pressures for all of the tires would usually require a few early mornings to accomplish.  Retained tire pressure evaluation notes can be useful for future inflation corrections or changes to accommodate load additions, reductions or relocations.


I’ve run my motor home tires successfully for approximately 15 years inflated via the above determinations, with them displaying even tread wear!  A seemingly unrelated separate motor home involvement, coincidentally turned out to also offer convincing confirmation for the above methodology via a good testing/demonstration when I combined my motor home battery banks (joined starter & house batteries into one battery bank) & expanded my total motor home battery capacity to 1,200 AH via six 200 AH deep cycle AGM batteries.  Unfortunately, the involved batteries could only be accommodated in my motor home over my right rear dual tires.  While I didn’t know what the load was at the time (nor do I know now what it is for this same motor home) on these duals, I do know that it has been necessary for me to create a cold-running-pressure of 118 psi for these duals using the above inflation methodology, even though my tire manufacturer’s sidewall recommendation has a maximum cold-running-pressure of only 105 psi!  I’m happy to report that these tires now have approx. 60,000 miles on them, run cool with even tread wear & show me only an approximately 14% increase in pressure over their 118 psi cold-running-pressure (134.5 psi).  Therefore, I’ve learned that it is sometimes necessary to use a cold-running-pressure for a tire that is greater than the manufacturer’s side wall recommended cold-running-pressure to keep a tire happy supporting its load as it must exist.  I have accepted this reality as the desirable option, when I haven’t been able to conveniently lessen the tire’s load, because it’s with certainty the only way to prevent the ensuing uneven tire wear & inordinate high heat levels caused by excessive side wall flexing, which will create a shorter tire life, as well as perhaps a tire disintegration calamity.  I’m sure that these tires would have given me a very short life if I had inflated them to only a cold-running-pressure of 105 psi.


Relative to the above referenced “Weighing Your RV” article, I’ve never heard an explanation or been able to understand why tires on the same axle, supporting different loads, should be burdened to cause uneven tread wear by using the same cold-running-pressures.   Manufacturer’s suggested maximum cold-running-pressures for a given tire model are established with the realization that greater pressures will be the normal reality for the working tires & that it is necessary for users to understand that heat is always the determinate relative to the life of a tire.  Higher pressures can be used if tires are not exceedingly hot; i.e….tire rubber chemistry starts to be modified to create a shortened tire life for the tires that reach 225 degrees F.


In conclusion, specific tire RV weighing is perhaps a good thing to do initially to determine if any gross error weight distribution has occurred, but many may also find the above methodology reasonably quick & easy for the ongoing determining & maintaining of correct tire inflations for longer tire life to be considerably easier to implement (without the help of others) & offers a more on-going tire specific optimization solution.  Note: Additionally to accomplishing an optimized longer tire life, the creation of even tire-tread-wear for your RV’s tires will also improve safety by offering near maximum road surface tire contacts to effect optimized straight ahead braking, thereby avoiding a direction change via a side-slide (& perhaps a roll-over) due to a portion of an RV’s tires slipping rather than grasping the pavement!

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