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Minimum Tire Pressure?


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#1 Allegiance40x

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Posted 03 November 2013 - 05:11 PM

I weighed my coach today, and according to the actual weight of 19,500, my 295/80R22.5 dually tires should only have 80 PSI of air in them.  That does not seem like enough air for a tire with a max capacity of 6,940# and 125 PSI of air.  At my GAWR of 20,000#, that's still only 85 PSI.  I've been keeping 100 PSI in them.

 

For comparison, the front axle has a GAWR of 14,600, and an actual weight of 13,160.  That weight calls for TP of 100 PSI, I've been using 105 PSI in them, and will probably continue to do so, since this weight is not ready to travel except for full fuel.  The max weight capacity as a single is 7,830#.

 

Comments?


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#2 wolfe10

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Posted 03 November 2013 - 05:58 PM

Best answer:  Weigh each wheel position with the coach fully loaded with fuel and with the stuff you travel with.

 

Take the heavier wheel position on each axle and go to your tire manufacturer's Inflation Table to determine the MINIMUM PSI for that weight.  All tires on an axle to have PSI determined by the heavier wheel position.

 

Now that you have the minimum for your actual weight most (including our coach) add 5 PSI as a safety cushion above the minimum. 

 

This is cold (before driving).  As ambient temperatures drop with the coming of winter, you WILL be adding air, as tire pressure drops roughly one PSI per 10 degrees F. Tires also loose a little air over time.

 

A good TPMS (Tire Pressure Monitoring System) is an excellent addition to any coach/toad.  A 10 sensor system allows you to monitor both coach and toad without pulling out the tire gauge each morning.


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#3 iweigh

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Posted 10 November 2013 - 02:33 PM

A rear axle weight of 19,500 of course, assumes that the coach is balanced side to side, with an equal distribution of 9,750 pounds left and right (19,500 / 2). A washer dryer on one side combined with a refrigerator/pantry, or poorly loaded basement on that same side could easily cause a noticable, and possibly dangerous imbalance.

For example, your left side could weigh 10,050 and the right side would be 9,450 pounds. While your total rear Axle weight is still 19,500 pounds, you have exceeded the axle end limit weight of 10,000 pounds. Remember, the axle end limit weight is half of the total axle gross weight rating. In this case, the Rear GAWR of 20,000 divided by 2 equals 10,000. The tires can handle the weight without any problem, but the axle, with associated bearings and suspension components cannot.

Now one would think that the manufacturer would account for this during the design and construction process, and they are getting better at it, but it is the operator who has the end responsibility of proper balance.

Weighing each wheel position will give the most accurate information, but it's not always available. A CAT Scale facility is better than not weighing at all, but it has limits. It's up to us to try and balance the load as best we can.

And follow the excellent advice of Mr Wolfe once you get those individual wheel position weights.


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#4 Wayne77590

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Posted 10 November 2013 - 04:47 PM

Oh oh! I'm going to get in trouble because I don't normally disagree with Brett. However.

 

For each 10 degrees of temperature change there will be a 2% increase/decrease in pressure. I beleive the misunderstanding is when one takes 2% of 35 psi it comes out to 0.7 psi and rounded up would be 1 psi. However if you take 2% of 110 psi, it comes out to 2.2 psi.

 

For every 1000 feet of elevation change there will be a 0.48 psi increase/decrease in temperature.

 

If you look at the inflation tables and take those measurements into consideration you would set your pressure between the low and high pressure that would give you the most comfortable ride and have the "fudge" factor.  I'm not going to go look up the tables, but let us say for example that your weight is 9500 pounds on the heaviest tire and the pressure to abosorb that minimum weigh is 85 psi. You could set your tire pressure to 90 or 95 psi and have a fudge factor of 5 to 10 psi.  Consider that at 2% of 90 psi is 1.8 psi so if the temperature dropped 10 degrees you would be at 90-1.8 or 88.2 still within the minimum pressure for the load.  If the temperature were to drop 20 degrees you would still be at 85.6 and withing load limits. Normally if you are decreasing in altitude you would be increasing in temperature so there is also a fudge in those factors.

 

Happy trails.

 

Editied: I did a search in the Goodyear tire chart guide and this is what it states:

 

 

"The effects of temperature and atmospheric pressure.
Air temperature and atmospheric pressure effect tire inflation pressure. If the outside temperature increases 10 degrees F, tire inflation pressure increases approximately 2%. Conversely, when the outside temperature drops 10  F, the tire inflation pressure lowers approximately 2%.
 
Tire inflation pressure increases approximately .48 psi for every 1,000 feet of altitude due to changes in atmospheric pressure. On the other hand, tire inflation pressure will decrease approximately .48 psi for every 1,000-foot decrease in altitude.
 
In other words, if there are changes in temperature or altitude during your trip, it’s important to check your tire
inflation more frequently."
========
 
Or use the fudge factor and you don't have to keep adjusting the tire pressures.
 
p.s., you did not say the make of your tires. What are they?
 
Here is a good link to anything you want to know about tires: Tireman's Blog  Roger Marble is an engineer that has worked with tire companies for many years.
 

 


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#5 wolfe10

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Posted 10 November 2013 - 05:09 PM

Wayne,

 

Thanks for the correction.  You are correct-- Change of 10 degrees F makes 2% change in PSI.


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#6 Allegiance40x

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Posted 12 November 2013 - 05:04 PM

Appreciate the input guys.  Any suggestions as to where I can get four corner/side to side weight?  BTW, my tires are Goodyear G670RV, 295/80R22.5


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#7 wolfe10

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Posted 12 November 2013 - 06:27 PM

http://rvsafety.com/weighing-schedule/


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#8 ahepburn51

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Posted 13 November 2013 - 12:43 AM

If you're in the San Jose, Ca area you can go to Leale's RV Service on 7th St - they'll do a 4-corner weight for free - I just had my Bounder done there last week while they were repairing an awning arm.


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#9 John_Harris

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Posted 13 November 2013 - 10:25 AM

I went to WV DOT weight station along interstate with full tanks during mid day less than peak travel time. They weighted front and rear full axle load, then allowed me to back up and reposition with only left side front and rears on scales. Gave me print out for each axle total and then left side only. Difference was the load on right for each.


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#10 deenad

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Posted 13 November 2013 - 11:37 PM

Here in WA (and OR) the scales are left on even when the scale house is closed. Usually there's enough clearance to do side to side weights with no problem.

 

BTW: the lowest cold pressure you should ever have in a tire no matter what your weights is the lowest shown on a weight/pressure chart for your brand/model/size tire.


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