Tire Pressure—Am I Wrong?

[Arch Hoagland]

Originally Posted by tuckmf

First post here.
As a road racer we adjusted tire pressures by temperature reading across the tire tread. If the middle was hotter than the edges you decreased the tire pressure 1 - 2 lbs and vice versa when the tires were hotter on the outer edges. We used nitrogen to minimized moisture in the tire and make temps more predictable. Wouldn’t this work on a RV?

Mike

I shoot my tires every time I stop but had never heard of measuring the temperature from side to side of the tread.

Sounds like a great idea and I'll start doing it now.

Thanks.

[Woodman018]

WEIGH IT. I take mine to the local grain elevator and weigh each corner. Front tire on one side, pull up with front and drive axle on scale (1 side only), pull ahead again to get tag on scale (1 side only) , back up and repeat for other side the same way. Then I weigh the front axle with both tires on the scale, pull up with front and drive axle on scale, and finally weigh with the tag on the scale also. This method gives you some math to do but this way you know what each tire is carrying. When I do this I don't come up with all of the individual weights adding up to match the total weight exactly, probably because of slight ramp angle going up to scale and possible lean side to side but the total on mine was within about 400 Lbs so I think that is close enough considering the coach weighs over 38,000 Lbs. I weigh with full fuel tank, full water tank and all gear on board. Look at the tire chart and add 3 to 5 Lbs just to be on the safe side. It works for me. The grain elevator I use is about a mile from my home and the manager told me to just do it any time I want as they leave the scale on at night so I can get my readings, pretty sweet.
Dave

[Mark_K5LXP]

Originally Posted by Hooligan

Very simple to find a "CAT Scale" at a truck stop near you in Albuquerque.

Yep, there's one at the T/A truck stop just south of Candelaria on the east side of Universitiy and another one at the Love's on the south side of I-40 and 9 mile hill. Open 24 hours.

In my motorcycle days we adjusted pressure based on temperature rise, which probably goes in keeping with the temperature across the tread of a "square" tire (motorcycle tires have a round contact patch, for the most part). But it's not often you're near the GVWR of a motorcycle or car to consider minimum inflation pressures, so with an RV at a minimum I'd go with whatever's on the vehicle sticker. Yes, it might be too much on an unloaded RV but realistically they aren't unloaded very often, and it's not like you're going to add just enough air say, after taking on water. Having a TPMS I can watch the pressure rise under way and see which ones are warming more than others, and can surmise which corner might have a greater load. I always start out with a 5PSI extra margin and I can always bleed off if I think I could go a bit less.

Nitrogen? Total waste of time if you ask me. In a race situation where you take any performance edge you can get no matter how small, sure. On a passenger car or RV it certainly won't hurt but it sure isn't going to make any difference. Especially in an RV where you're in single digit MPG and tires age out, there's just nothing there to gain.

Mark B.
Albuquerque, NM

[Ramona]

Originally Posted by Mark_K5LXP

Yep, there's one at the T/A truck stop just south of Candelaria on the east side of Universitiy and another one at the Love's on the south side of I-40 and 9 mile hill. Open 24 hours.

I'll check it out. Thank you Mark!

[jadatis]

This " pigheaded Dutch selfdeclared tirepressure-specialist" calculated for you GAWR'S , because these are the only sertain values there are for the moment.
Calculated with use of spreadsheet I made, in wich I use a formula that comes to higher pressures then those of american tiremakers, and even higher the better european officially used formula.
Then , when only GAWR , adds 5% to front ( because seldomly overloaded, and rear a floting scale from 18% for small motorhomes, and 10% for big ones, because smaller often overloaded.
Your came to 11%
Even gave rear 130 loadindex, because only one substracted from single is for most tires not done( Eur 2 and USA 3 or 4 li-steps.
To my concusions this gives still no discomfort and bad gripp.

Front 7500+5% / 95psi
Rear 13500+ 11%/98 psi

When tou have weighed fully loaded as you go on trip, best per axle-end, second best per axle, I can recalculate.
Then this weighing might prove, that your estimations of lower weight was to low.

But OK , to give an idea, I filled in axle-weights 1000 lower for each.
F 6500+10%/ 86 psi
R 12500+10% = 89 psi.

Max speed of tires Goodyear gives as 75mph, and for that speed and the 110 psi, the maximum loads are calculated.

If you would go for maximum safety, with still acceptable comfort and gripp, the deflection of tires would be that as if maxspeed 99mph.
I found out the system the tiremakers use for that, rule of tumb is , for every 10kmh/6.5mpu higher speed , 1 loadindex step lower so 4 steps lower filled in.
So dual and single both LI 129.
Then
F 6500+10%/ 96 psi
R 12500+ 10%/ 92 psi.

So the 78 psi ( most pressure devices give to high pressure, so can even be lower in real), let them highen it up.
Most digital devices for normal cars, have sensors that go out of their accuracy above 60 psi, and then yust give lower pressure above that 60 psi, then in real.
If to low on even one tire, when high speed used, overheating of tire wich hardens the rubber permanent, and every next deflection cracks it further, untill mayby only after 3 years that far that tire blows or treathseperation, so serious matter.
The tires must have in their live not any moment of overheating. What you do after overheated long enaugh, dont help, higher pressure then even cracks sooner, and then high pressure is blamed.

I would go back to RV dealer, and ask them to measure pressures again , their devices are ( hopefully) calibrated.
If they have a testbank for the brakes, it also weighes.
It depends on the attitude of the dealer, if he( or she) then wants to swich it to axle-end weighing.
Then you have your free weighing, and pressure measured accurate ( we hope).
Go fully loaded then, some small things can be added afterward and estimated. On such a total weight even a overweight co- driver is a relatively small thing, for the goal of pressure-determination.

[veraken]

To determine what the right tire pressure is for you, you need to weigh your MH. Load your MH as you would for travel, fill you gas and water tanks and then get your MH weighed.

If you can't do 4 corner weighing (which would the best), this is the procedure to follow:

1. Weigh the coach axles
2. Add 5% to account for possible uneven weight side to side.
3. Divide by the number of tires on each axle.
4. Look to tire chart for your brand to find tire pressure.
5. Add 5 psi for safety

[Mark_K5LXP]

One other comment, this entire thread is based on the premise the dealer intentionally set the tire pressure under a specific set of conditions.

Never attribute to malice that which is adequately explained by stupidity.

I would bring into question the diligence and competency of the dealer that sold it. In my own experience, and reading the endless posts in the forums it's not a stretch to consider this RV arrived on the dealer lot and it went to the customer without a single check of the tires, regardless of what a checklist or inspection form says. There's just nothing in it for them to be that thorough.

If the only issue discovered is low air in the tires consider yourself lucky. Technically tire pressures are the responsibility of the driver every time the RV is put in motion, so regardless of what anyone else may have done, it's on the driver to check.

Mark B.
Albuquerque, NM

[Quincy]

JMHO On the tire the MFG has put a max pressure cold rating. I have always inflated to a few pounds below that pressure. I do not weigh my coach because I dont believe it is that big a deal in a MH. A travel trailer, that is another story, but a MH not so much.

I also subscribe to an above post regarding Nitrogen in tires, after all, air is eighty percent Nitrogen. Now racing tires where a tenth of a pound makes a difference and may even be noticeable by the driver is also another story. As far as checking the pressure across the tread of the tire. What a clever way to check inflation pressure and be exact.

Most people pay little or no attention their tires pressure, and dont check pressure in their tires until one of them reaches 0.

I understand people who are meticulous about tire pressure but my methods have worked since the early eighties and with six or seven different campers. Since I have never had a flat, I am sticking with them.

As far as the dealer setting the wrong pressure is concerned, it would not surprise me if the last time those tires were checked is when the chassis mfg. inflated them before shipping the chassis to the MH Mfg.

I picked up a brand new fifth wheel (2012) that had an inch of water in the basement area which held the inverter for the fridge and converter for the rig. Both failed shortly after. A leak in the supply line to the ice maker. Another dealer that lied and paid no attention to their product before delivery.

All the dealer wants to do is say good by to you and cash your check then move on to the next victim.

[tuckmf]

Originally Posted by Arch Hoagland

I shoot my tires every time I stop but had never heard of measuring the temperature from side to side of the tread.

Sounds like a great idea and I'll start doing it now.

Thanks.

Once you get it right as far as temperatures across the tread, check your air pressure the next morning while the tire is cool on each tire and that would be your new cold tire pressure. Filling with nitrogen eliminates the effects of a expanding moisture which will increase the hot tire air pressure.

Mike

[CJ7ole]

If you don't weigh it (most folks won't even after reading a thread like this), just go with the tire placard like Ray,IN said. That way you are safe and covered as long as you don't overload your rig, or get it grossly out of balance.

And a black tire sitting in the hot FL or AZ sun can get hotter than one running down the freeway at 70 degrees.

[GNICK]

One thing I never see addressed is how much variation is accepted between adjacent tires on a dual wheel setup. The outside tire (in the sun) can be several psi higher than the shaded one next to it.

[jadatis]

Originally Posted by GNICK

One thing I never see addressed is how much variation is accepted between adjacent tires on a dual wheel setup. The outside tire (in the sun) can be several psi higher than the shaded one next to it.

There has been a topic about that on this forum.http://www.irv2.com/forums/f258/tire...es-427574.html


goal and rule is to keep cold pressure on same axle the same , but up to 5 psi in the higher pressures was concluded acceptable .

[Hit_the_Rhod]

Originally Posted by Ramona

Yes, I'd like to do that but no where close to here for it. Also, since I am confident we are under weight, without doing a weight for each tire, is it really that helpful?

If you don't need individual tire weights, just axle weights, try a moving and storage company. I'm pretty sure that there's one close by. Tell the tech you need individual axle weights, you'll have to pay for two, or possibly three tickets (if a tag axle), but worth it. If a tag, you can't weigh just the forward of the two rear axles, but you can weigh the two rear axles combined, and then just the rear, subtract the rear from the combined rear to get the middle axle weight.


Good luck, and as stated above, at least you're asking the right questions!

[Hit_the_Rhod]

Originally Posted by Mark_K5LXP

Nitrogen? Total waste of time if you ask me. In a race situation where you take any performance edge you can get no matter how small, sure. On a passenger car or RV it certainly won't hurt but it sure isn't going to make any difference. Especially in an RV where you're in single digit MPG and tires age out, there's just nothing there to gain.

Mark B.
Albuquerque, NM

We use a special blend consisting of approximately 78% nitrogen, 21% oxygen, and 1% proprietary gases. Works for us! And it's easy to find all the places we've gone so far!


One last thought, the advantages of using 100% nitrogen are generally in the fact that the gas in DRY (no moisture in the cylinder) rather than the fact that it is 100% nitrogen. The same advantage would probably be obtained by using any compressor, then running through a refrigerated air dryer. And seriously, in my meticulously designed, streamlined, balanced and blueprinted, high performance, state of the art motor home, I would challenge anyone to actually experience (or be able to measure) any performance increase by using 100% nitrogen, dry or otherwise . . .