Tire Pressures Observation and Question

[Mike Groves]

Now that its summer in southern Oregon tire temp and pressure are on my mind.  We're full-timing now so per advice here I'd told Les Schwab to put 120lbs in all my tires.  I guess the last one maybe the tech was in a hurry (his boss came out once to try to hurry him up), but one was at about 112, the others about 118 and it was about 57 degrees in Newport, Oregon.

By the end of the day before pulling in at an RV Park near Ashland, the low tire was at (and had never gone above) 130lbs and the others were right around 140 with temps around 100-105.

I've thought forever, that the main reason for blowout was that an underinflated tire would build pressure more quickly than an overinflated tire but either that's wrong or perhaps I should be running these tires at about 112 (57 degrees).

Just looking for opinions.  I have weighed the coach and we're pretty much at or near our maximum weight so 120 would seem to be dictated on our Double Coin 606+ tires.

Mike

[Steve Huber]

The attached chart, courtesy of Ed Buker, is a tire pressure v temperature chart that shows proper fill psi depending on ambient temp

We've got a tire monitoring system and I noticed a significant difference in tire temperature from our last coach. The 00 Marquis had Michelin XZE2 11R 22.5. The 07 Contessa runs Goodyear G670  295/80R 22.5.  Corner weights are close with the Contessa running a bit heavier.  Interestingly, while the Marquis tire temps on the road (in summer) would run in the 130s, the Contessa runs about 110-115 F. Not sure if this is due to air flow, tire design or? In any case, I'm not complaining.
Steve

[Jerry Emert]

Maybe I'm wrong because I'm certainly not an expert but... most people I have seen through the years have no clue about tire pressure.  They put air in their car, and yes, RV tires when they notice that the tire looks like it's going flat!  It cannot be so complicated or so vital that I adjust the pressure according to what I think the mean temp is going to be that day.  I am super sensitive to tire pressures now because of all the various forum posts about front tire blow outs and such.  I'm sensitive enough that i bought a TPMS for the Beaver.  I do keep an eye on them when I'm driving.  I set my tires before I drive for the day.  I have all my tires set at 120 until I can conveniently find a place to get a 4 corner weigh.  When the TPMS says that a tire has changed by about 5% (6PSI) I will reset them (on edit, I refer to resetting them before traveling for the day).  While driving on a hot Florida day they will go up about 15 PSI.  While out west this spring, the TPMS would alarm at about 5AM when the temps hit a point that the tire went below 10% of their set points (108).  By the time we leave in the morning they are back near 120.  Sorry for the long post but I just can't see that tire pressures are that complicated.  Set them for the pressure that you need before you have driven for more than a mile.  Monitor to make sure that don't lose pressure rapidly or get too hot!  Too hot according to my TPMS paperwork is 158 I think!  Mine usually run around 100 degrees +- 10.

[Gerald Farris]

Mike,
Your tire air pressure should be set with the tires cold (60 to 65 degrees F) and not deflated when the tires heat up. Under inflated tires do not fail from excess pressure generated by extra heat, but they fail from the excess flexing and excessive heat caused by the under inflation. An RV tire running at it's maximum inflation will have less sidewall flex, causing less metal fatigue in the steel sidewall cords (cables) and less tire damaging heat.

If you take a piece of steel and bend it back and forth repeatedly, it will break. The same problem exist with the steel cords in you tire sidewalls. The common term used for a sidewall failure caused by broken steel cords is a "zipper rip" and it is considered a non warrantable failure by tire companies because it is a result of under inflation/overloading.

Gerald     

[Edward Buker]

Steve,

Thanks for re-posting that chart, I had forgotten about it. As Gerald pointed out, the tire pressures are to be set to a value that handles the load weight based on the manufacturers recommendation, say 120 lbs (an example for a given load weight on an axle) at 65 degrees. You then compensate that 120lb value based on the current ambient temperature, when you are filling them, so that they will be at 120lbs at 65 degrees. Sometimes an IR gun is helpful to get the current tire temp if the air temp has risen and the tire temp is lagging like in the morning. Gerald and Steve thanks for setting this straight...my earlier post was incorrect.

Later Ed

[Mike Groves]

Thanks all.  I think then that I'll simply shoot for max 120 pressure for these tires using Ed's chart.  There couldn't be a downside to simply inflating them to their max as long as I do so properly, right?

I must do it myself, however, because it appears that Les Schwab (as an example) has gauges all over the place and even the same tech can't seem to put the same pressure in all the tires.  That's been my problem.  I ordered a Viair system and it will be arriving at Pacific Shores when I return tomorrow.  I tried using the coach's air along with the apparatus for such a thing but that will NEVER inflate a tire to 120lbs as I've discovered.

Mike

[Edward Buker]

Mike,

That is certainly safe and probably close to what is needed in the front tires. It is probably more air pressure then is what required in the rear tires which will stiffen the ride some. Some BAC members may have had their SMC era Marquis that are the same model as yours and could possibly share the corner weights. You could buffer that weight by plus 500lbs and use that value to get your tire manufacturers recommendation. That would be safe with that margin until you have yours weighed.

Later Ed

[Mike Groves]

All,

Now that I have a DC Air Compressor can I use that to refill my onboard tanks while parked to allow the auto-leveling to work properly.

The chuck in the engine maintenance bay I have used in the past to attempt to fill my tires.  Can I plug in the Viair Compressor to that chuck, turn on the compressor, and fill my air reservoirs?

Thanks,
Mike

[David T. Richelderfer]

Your auxiliary air compressor isn't working?  I would have thought if your coach has air leveling, especially auto air leveling, then it would have an auxiliary compressor.

[Mike Groves]

There is none that I know of on this coach and with this design.  Mine has the air over hydraulic and they recommend the air for short stays (I guess like while shopping) and the hydraulics for at campgrounds.  My biggest issue with the hydraulics is that the frame get's twisted and then basement doors don't open/close properly.   We at Pacific Shores for 4 days and right now we just have the air leveling set but our tank is already out and I just wondered if I could attach this to the air chuck since I just got it.

Mike

[David T. Richelderfer]

You have 3-point hydraulics... meaning one ram in the front center and two rams in the rear side-to-side?  Make damn sure the front ram sets first and lifts the front end by at least a couple inches to take the pressure off the front end air bags; the more front end lift you get the less twist the frame will have.  Then set the rear rams for side-to-side leveling.  The idea is to free-up the front suspension so that the front end of the coach can more freely pivot over the front ram.  This procedure will remove the tendency for the coach's frame to twist and put everything into an out-of-square bind... like the basement doors, entry door, slides, and importantly, the windshield.

When I use my hydraulic rams the windshield quickly gets in a bind and will leak if raining.  Thus, I try not to use the hydraulics at all IF the coach is on a side-hill tilt.

There have been many Forum entries about auxiliary compressors failing due to their placement near the front end where the road dust and rainwater can get to them.  You may have an auxiliary compressor and it needs attention.

[Steve Huber]

Mike,
If your on-board compressor is working and you don't have a significant air leak, your air leveling system should work for long stays with no problems. Also it will check and self adjust every 30 min or so to ensure teh coach stays level. Our 2000 Marquis has air and hydraulic leveling and we never used the hydraulic system.
Steve

[Gerald Farris]

Mike,
You can air up your air tanks through the service port that you mentioned, but it would be much better to repair or replace the 12 volt air compressor that is part of your air leveling system. It is probably located inside the frame rail, in front of the front axle on the passenger's side of the coach. Most likely, it is a Thomas air compressor that has open ports at the commutator end of the compressor that fill with dirt and road debris. This design does is problematic for the location that the compressor is mounted in. Therefore, I always recommend replacing it with a sealed compressor like the Viair 380, but in doing so you will need to change the wiring to include a relay because the Viair compressor draws twice the amperage of the old compressor.

Gerald   

[Joel Ashley]

Thanks Gerald for the explanation of the sidewall flex being the main culprit of low inflation;  I knew that but couldn't have explained it as well.  At rest stops and camp setups, I infrared gun the coach and toad tires.  Of note is the fact that inevitably the tires on the sunny side will be hotter than those on the shady side, whether I check them cold with a guage and infrared or hot with just the gun.  Even the wind on one side can make a difference, or if one tire is on asphalt and others on gravel or concrete.  So be careful when noting differences between tires of 3-10 psi.  I try to adjust air only when all are equally shaded and exposed and fully cooled down;  early morning commonly fits the bill.

Not sure why Mike couldn't get his coach inflation apparatus to work;  the one I made as per the popular YouTube video works just fine, and is faster than other modes I've tried;  admittedly I haven't headed north of about 115 psi, but it sure seemed to want to if I let it.  It could be his tanks are set to top out at 120, whereas mine seems to be closer to 130.  Otherwise I also would've purchased a ViAir compressor.  But as Gerald also notes, it seems Mike needs a new auxilliary unit up front under the passenger side, although he doesn't seem to believe he has one. 

I'm not familiar with coaches with the additional hydraulic leveling, but if his air leveling-only operation isn't compromised in any way by the hydraulic side, auto air level should mean an auxilliary compressor that maintains level as Steve mentioned.  Plus when air dumping or something during leveling diminishes my tanks much, I just restart the engine briefly to punch them back up;  as long as I don't hit the Travel Mode button, or have a slow leak, they maintain full and the coach stays where I leveled it.

Joel

[Mike Groves]

I had no idea that I had a supplemental 12 volt compressor.  I'll have to look for it and see what the heck.  That would be great if it works but I've never heard it if it has been working while we're parked.

Mike