Maiden voyage, Day one

[Mike Groves]

Thanks Steve,

I'd have to conclude from that discussion that there seems to be a choice.  Mine is to keep mine at 105lbs at 65 degrees.  The 105lbs is based on the weight rating of my coach, that I don't think I am coming close to (I haven't yet tried to weight it but will try soon), and the 65 degrees was just the number I'd read, but haven't found again.  Evidently Gerald found 60 degrees, and his tires spec out to 120lbs for his weight see -

http://www.rvtirepressure.com/assets/images/extrapages/Michelin_Rv_Load_Inflation.pdf (I assume this is Michelin's recommendations). 

If I am reading that right, then with Gerald's 295/80/22.5s (see other forum thread), 120lbs would allow a total weight of over 43,000lbs.  I believe my weight is considerably less, probably at about 33,000lbs.  So, it just all depends I guess.

I have the TST TPMS system with flow through valves, and supposedly it measures temperature and pressure, and if so, and if accurate I am seeing more than 1.7 PSI per 10 degrees difference.  But I don't honestly trust the temperature sensor portion of this setup, as I would think the temperature would be affected more by the heat generated by the road surface, when in fact, it appears to be affected by which side of the coach the sun is on.  In other words its measuring the temperature of itself, not the air in the tire.  When one thinks about it, how could it measure the temperature of the air in the tire?

Good discussion,
Mike

[Steve Huber Co-Admin]

Mike,
As Gerald noted earlier, weigh all 4 corners of your coach fully loaded.  Full fuel and water tanks. I also fill the black tank about 70% to simulate a loaded reefer and clothes, etc. The loaded weights can vary significantly from right to left. For example, on our 2000 Marquis; FL is 6440 and FR 6140. RL is 11880 and RR 10600.  Our 01 Contessa had similar  variances. FL; 5228,  FR 4866. RL 9904   RR 8520.
Steve

[Gerald Farris]

This is a tire pressure chart that Ed Buker sent to me that should answer your questions about the ambient temperature and pressure correlation.

[Jerry Emert]

This is going to sound sarcastic but I really don't mean it that way!  Anyway, these coaches are not designed for engineers and atmospheric scientists.  Surely They can't expect a dumb old street cop like me to do all these calculations every time I fill my tires?  Can they?  I do realize that this is important stuff.  I put 120 in my tires as the placard says but I don't take outside temp into account and I surely don't stop in 100 miles on a hot day to let some air out to get down to 120.  As I said above, dumb old street cop here, should I really do all that stuff?  I'm still learning tons of stuff from you guys and gals so keep it up.  Thanks.
Jerry

[Gerald Farris]

Jerry,
You missed the point that cold inflation is at 65 degrees, and you should not deflate your tires if the weather is warmer than 65. Also, you should never deflate a hot tire to the cold inflation point because that will result in a dangerously underinflated tire.

Gerald

[Jerry Emert]

Jerry,
You missed the point that cold inflation is at 65 degrees, and you should not deflate your tires if the weather is warmer than 65. Also, you should never deflate a hot tire to the cold inflation point because that will result in a dangerously underinflated tire.

Gerald

I warned you that I was a little slow!  I've seen these discussions on other forums and I always wonder why folks just don't put the correct amount of air in their tires and be happy.  I realize that there are two ways, 4 corner and placard, to set them.  So you have confirmed that I am not as dumb as I thought. 
Thanks

[Edward Buker]

Jerry,

Somehow the tire manufacturers have to calibrate the weight capacity vs the pressure that you need to support your weight by axle. The issue is, that value varies by temperature, so they set the cold inflation pressure at 65 degrees as a calibration reference.

So by the chart, lets assume that the tire manufacturer says you need 100lbs of air in the tire at 65 degrees. Lets also assume that you are checking the tires before leaving a campground and it is now 85 degrees outside. By the chart the cold inflation pressure you should be seeing is now 105 lbs to be equivalent to 100lbs at 65 degrees. So if you see 105lbs, that is perfect. If you see 100lbs you are 5lbs underinflated. If anything this chart has you not chasing a single cold inflation value at other temperatures when you do not need to.

When it is very cold or very hot this chart should be a help to know when pressure changes are a real issue or just a result of temperature change. Hope this helps.

Later Ed

[David T. Richelderfer]

I used to have Michelin 295/80R22.5 tires on the front and tried to maintain 120 - 123 psi cold.  At times after an hour or so on the road I have seen increases to 138 - 140 psi on my TPMS.  This past Fall before going South for the Winter I put Dunlop 315/80R22.5 tires on the front to increase the carrying capacity.  At 130 psi cold these tires carry about 9k pounds each as compared to the 295s at about 8.25k pounds each at 120 psi.  There is no noticeable ride nor noise difference between the two sets of tires.  The other six tires need to be scheduled out due to age later this year or next.

[Edward Buker]

David,

You may want to check your rims, some aluminum versions may have a max cold pressure of 120lbs written on them.

Later Ed

[Mike Groves]

This is a tire pressure chart that Ed Buker sent to me that should answer your questions about the ambient temperature and pressure correlation.

As I read the information below the chart, it appears to be saying that you should inflate your tires to the recommended cold inflation pressure regardless of whether the temperature is below 65 degrees.  I've been simply looking at mine, knowing I have at least 105lbs in at 65 and that all tires are pretty identical on the TPMS except 1, and if its 48 degrees outside, and all tires but one are showing 99-100, and the other tire is showing 95, I fill it to 100.  Should I be filling it to 105lbs?  That doesn't make sense to me but appears to be what the wording beneath the chart says.

Jerry, I agree with your point, yet, I see the wording on the side of my coach that says something like "proper wheel lug tightening is the owner's responsibility" or something like that, and its just funny, as I don't have the equipment to tighten or loosen the lugs.  I suppose the bottom line, as always, is litigation, or the possibility thereof, so there are lots of warnings, making it appear to the common individual that they have no chance of using one of these coaches in a responsible and safe manner. 

Mike

[Edward Buker]

Mike,

Without making yourself too crazy over tire pressure, I and most people know the weights of thier coach and have added in a 5lb to 10 lb safety margin into the 65 degree cold inflation pressure. If that is the case you would just bring the 95lb tire, that is different and low, up to be the same as the other one on that axle. Below 65 degrees in general they want you to use the cold inflation pressure value but at what temp you would care about that is up to you and your tire position weights and the air pressure vs load safety margin you may have chosen. If it was zero out and you are planning on traveling you may want to have all the pressures compensated some.

The way I look at this is I may compensate one time during the cold months to get the tires about right and in the warmer travel months you keep the tire pressure on the same axle even. You can glance at the chart and know by temperature what they should be reading and not over or under compensate if you think it is time to add air because the pressure would be too low at 65 degrees. I personally add air if any tire in an axle position is 5lbs low, given my safety margin and I try and keep them within 2 lbs of each other on an axle. Tires lose air at a very slow rate if all is well so a compensation over a few months is normal. If you have a tire that loses air at a rate that is worse than others or has changed its air loss behavior, you need to get that taken care of right away. You do not know why and what it means will happen next.

As always, know what is the right pressure and manage it accordingly. Low pressure in an RV tire is not at all like a car. It is a true hazard because low pressure causes the sidewalls to flex back and forth at greater angles. There are hundreds of steel cables in the sidewall spaced about 1/4 inch apart and with that more severe flex angle, it can cause them to fatigue and break. Bending a wire severely back and forth is a good way to visualize this metal fatigue issue. It only takes one low pressure drive to ruin an RV tire. So managing tire air and tire age is probably the single most important safety issue for you and your family while on the road.

At these pressures steel cables hold the tire structure together and rubber holds the air. If the steel cables fail the rubber rips apart. If you lose a front tire you may lose control of the coach and lose your life. A car tire has no cables in the sidewall and can handle this low pressure issue much better. Hope this helps.

Later Ed

[Matthew Harger]

Since we are on the topic of tires......Thought this was an amazing video to see that happened a couple days ago.... I nearly experienced the same with a left front blow out a year ago but luckily had another traffic lane to my left and not a grassy median....  Not sure what caused this guys blow out, but if checking my coaches weights and the tire pressures can help prevent that from happening I'm definitely not gonna short cut in that area ever.

Nobody was seriously injured as a result thank goodness.

http://youtu.be/9LkLeljt4t0

[Keith Moffett]

Great way to make a point Matt.  Thank God he was in an open area unlike some of the related posts on this forum. 

[Fred Brooks]

         I'm wondering if the driver in the video was ever advised of what to do in the event of a blow out. It appears that he hit the brakes which is a normal reaction. The correct thing to do is apply throttle until you know you have control and then slow down gradually. Braking just magnafies the sudden change in tire pressure and assures that the coach will go in the direction of the blow out. I put 12R bus tires on my coach because I don't ever want to be put in that kind of situation. Interesting conversation in this post so far.
      Regards Fred

[Larry Williams]

Wow! Interesting video. But Fred, I did not see the brake lights come on. It is amazing how quickly he lost control. Scary.