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Tire pressure

On all my past coaches I have had the inside rear drive axle tires show a higher pressure than the outer, they all start at x and 30 miles down the road always 6-8 psi higher only on the inside all other go up the same amounts. I would think the load is the same inside and out. Looking for a wiser person than me. Thanks in advance for a good answer.
Ahall
2018 ih 45

Re: Tire pressure

Reply #1 –
I have not noticed this.  On our TPMS all 6 tires show about the same rise in pressure as they heat up.  I have noticed the tires on the "sunny" side of the coach get hotter than those on the shady side, with corresponding pressure differences.

Perhaps on your coach, the inside duals are more affected by the heat from your massive engine?
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"It goes without saying..."

Re: Tire pressure

Reply #2 –
That be a good question. On roads with a big crown in theory the D/s outside and the P/s inside should have slightly more load. But that still doesn't answer your question.
Bruce, Linda, and Macy
Zoey RIP 1/20/19
1999 U295 40' build #5400
2017 silver Jeep Wrangler, 1260 watts of solar on top
Moving around the country

Re: Tire pressure

Reply #3 –
It may be the only logical answer, certainly would explain the psi going higher as the weight should be more.
Ahall
2018 ih 45

Re: Tire pressure

Reply #4 –
Long Story, but I have a TPMS, just still need to change all the Batteries and seals in them, and One allows Air to leak, when I serviced the Battery, so I went back to pre-TPMS days, of Checking Temps at rest stops.  I did notice the Inner Tire about 7-10 degrees warmer with the same starting pressures, but I did not check pressures on this trip! I will watch to see if my pressures do increase, on the Inside as well, once I sort out the TPMS Sensors!
Tom & Barb Root
'95, 40' U-320C SE, bought from Dave Head, 8/2017  FT Motorcade # 18196 Coach Build# 4663 October '94
2015 VW Golf Sportwagen, 1991 Suzuki Samurai, Off-Road Custom Build-Up, W/Toyota 22re mill, and a 2005 Chevy 2500HD, Duramax/Allison TOADS Will also pull a MacGregor 26X Powersailor 2001 Model, owned since new!

Retired SR Rocket Mechanic, FT Sales Associate @  FT of California, Escondido CA.!  and Retired USMC Gunnery Sergeant (Hence Retired Guns! ) Avionics Tech, on A6E Intruder & OV-10 Bronco Aircraft!  V/N Era Vet, and Desert Storm Participant.  Happily Retired now!

Re: Tire pressure

Reply #5 –
Inners are closer to the engines heat.

My guru buddy only installs the $2k bendix trucker set up as it incorporates both temp and pressure.

Requires tire removal as it mounts on the rim
"Riding and rejoicing"
Bob & Susan
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Tire pressure

Reply #6 –
For a fraction of what the so-called guru installs TST TPMS shows both pressure and temperature.  My inside duals are almost always a bit warmer and a bit higher pressure.  I think it is from engine bay heat.  Set all tires on the same axle to the same cold pressure based on the load on that axle and the tire manufacturer's chart.  Many add up to 5 psi to be sure.  For me Cold means not in the sun, usually first thing in the morning

Re: Tire pressure

Reply #7 –
Quote from: Ahall –
On all my past coaches I have had the inside rear drive axle tires show a higher pressure than the outer, they all start at x and 30 miles down the road always 6-8 psi higher only on the inside all other go up the same amounts. I would think the load is the same inside and out. Looking for a wiser person than me. Thanks in advance for a good answer.

Ahall,
Factors that add heat (and therefore temperature/pressure) to a tire:
Ambient temperatures:
    • the delta (change) in temperatures, from sitting to driving, are significantly affected by ambient/road surface temperature.  The amount of change you will see at 30 degrees F versus 110 degrees F will be drastically different.  The higher the ambient, the more delta there will be in the following factors:

Airflow: 
    • laminar versus turbulent airflow.  Laminar flow just passes quietly over a surface and isn't as effective as turbulent airflow in "scrubbing" or removing heat from any mass such as a tire. This is by far the biggest reason that inners run warmer than outer duals.  The airflow under the coach is slightly compressed and is very laminar due to the flat belly and the flat road surface.
    • the "air-cooled or swept" surface of the inner dual is less than 1/5 the total height of the tire and the duals are near enough to the belly that the airflow is still very laminar.  Therefore the inner duals are very poorly cooled by airflow in comparison to the outer duals which have turbulent air over the entire height of their outer sidewalls.  The top 4/5ths of the inner duals are just rotating through a pocket of relatively quiet air that doesn't provide as much cooling.
    • engine heat is being swept aft by the airflow passing under the coach as well as being drawn in the side grills and being "sucked out" the back of the coach due to the low pressure volume following behind the coach.  Heat doesn't transfer efficiently through air, but the radiated heat off the engine can be a small heating factor with inner duals , if there is no tag.  If there is a tag axle, the turbulant airfow coming off the duals and the increased distance from the radiant engine/muffler heat sources combine to reduce the engine heating factor even further.

Weight;:
    • an iH will heat more than a U225, just due to the carried weight.  A heavily loaded coach will show more delta temperature change (sitting to driving) than a lightly loaded coach.
    • a drastically crowned road will heat curbside tires more than streetside
    • anything that "tips" the coach while in motion, like long curves to the left or right.
    • uneven weight distribution;  other factors being equal, the heavy side will always run warmer. 
    • ride height out of adjustment such that one side (airbag set) is carrying more weight than it should be and the opposite corner will also run warmer as well, due to the weight being transferred there.  An active air suspension aggravates this factor.

Sun:
As already expressed and logically, the sunny side of the coach/toad/trailer will always be warmer and may be most pronounced when there is no airflow (sitting still).
Speed:  More speed, more sidewall/tread flexing , more heat. more airflow (through speed) may help with dissipation but not on inner duals.

Exhaust Flow:
    • Most have probably noticed that tires on the coach exhaust side of your toad or trailer run predictably warmer than the non-exhaust side.  Even though the airstream is turbulent, the hot exhaust stream shows persistence in traveling down the side of the toad or trailer.  All of my trailer tire failures have been on the coach exhaust side of the trailer.  That hot exhaust stream just ages the rubber compounds more rapidly.
HTH,
Neal
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Neal (& Brenda) Pillsbury
'02 U320 SPEC, 4200, DGFE, Build #5984
'04 Gold Wing
'07 Featherlite 24'
'14 Jeep Grand Cherokee Summit
MC #14494
Exeter, NH & LaBelle FL
Quality makes the Heart Soar long after Price is Forgotten

Re: Tire pressure

Reply #8 –
The rim mount commercial trucker only unit is overkill for most unless you want the best.  It can catch wheel bearing issues causing heating by being a rim mount and tire heating at the stem from sun exposure is lessened.

I would not worry about this as Bendix will not knowingly sell to  Rver's.

My buddy has to have the local Cummins dealer order them for him. 

Very sophisticated monitoring and features..

Like I said your wallets are safe without some work as Bendix is not interested in rv's.

Did not say I use this as it requires a tire dismount.  Had not asked him about this until I saw the dash display in several of his customers 45's.

I have stem mounts on two of my cars,  work great.  Both also have factory TPMS already.

But both oems are stem mounts on the inside.  Denso.  The stem stuff works very well.

Buddy has had lots of large coach customers with tire issues from near or actual overloading.

Newell requires 140 psi on tires marked 130 max on several customers rigs.

He has a special compounding tire pressure setup to get to 140. 

After a failure a lot of his affluent customers demanded the best system available.  Scared.

http://www.bendix.com/media/documents/products_1/tpms_1/Operators_Manual_BW2799.pdf


"Riding and rejoicing"
Bob & Susan
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4