Tire Pressure

[Ray Davis]

Geoff here is a link to a chart by Smartire. According to this 110 -> 115 at 85 degrees

http://smartireusa.net/2006/pdf/manu...ure%20Card.pdf

Ray

[george frudakis]

I as well, just order the Raytek MT6 for 49.95 free shipping from Amazon. Thanks Michael & Dr. Vita Terry for the info.
George

[travelite]

Geoff here is a link to a chart by Smartire. According to this 110 -> 115 at 85 degrees

http://smartireusa.net/2006/pdf/manu...ure%20Card.pdf

Ray

Hi Ray,

I took the following from Smartire's website:

"Temperature Compensation,

No matter how hot it is outside or how hot a tire gets rolling down the road, SmarTire RV's temperature compensation will always provide accurate tire inflation information. After all, temperature is a tire's biggest enemy.

As a tire deflates, its rolling resistance increases and its operating temperature goes up. As its temperature increases, the air inside the tire expands so the tire appears to be operating at correct inflation. When measured using a non-temperature compensated gauge, a tire can be 30% under-inflated and still appear to be normal. SmartWave measures a tire's temperature in addition to pressure in order to do a temperature compensation calculation. The system provides deviation numbers in its on-demand tire information and first level alert showing exactly how many PSI under or over inflated a tire is at its operating temperature."

I don't think Smartire is suggesting that we set our tire pressures according to their Temperature/Pressure Correlation Chart. My take is that the chart functions as an illustration of the calculation that Smartire does when checking pressures on a tire that is heated or cooled from a base point setting of 65 deg F. None of the major tire manufactures specify temperature compensated pressures when setting tire pressure. The only specification that the tire manufacturer's make is load. If the load table says to use 100 psi, then the tire manufacturers would like you to use 100 psi whether it's 0.0 deg F or 100 deg F. I'm very interested in any additional information that Smartire may have furnished.

[Ray Davis]

David, I'm not sure that is correct. The SmarTire sensors do indeed account for temperature as they work. However, if it's 85 degrees out (to use Geoff's example) and it's been that way long enough that the tires are 85 degrees as well, then the pressure in the tire should be 115, not 110. If he then fills his tire to only 110, that would be 5 pounds underinflated.

Luckily the Smartire would tell you that it's 5 pounds under inflated as well. There are three display modes on the system. (1) absolute tire pressure, (2) temperature, and (3) a delta (plus or minus) from the expected pressure at a given temperature. #3 is what I tend to watch the most. I can easily look down and see whether I'm over or under inflated and by how much. Generally I'm running +/- 2 pounds on my monitor for all tires, regardless of temp, driving time etc.

Here's a quote from their manual, which I believe backs up what I am saying:

The general rule of thumb is to inflate a tire to a
proportionately higher starting value when the
ambient/tire temperature is hotter than 65°F (18°C) and to the
recommended CIP value at temperatures below 65°F.
The chart on the adjacent page shows equivalent inflation values
for a series of cold inflation pressures at various temperatures.

The chart referred to in that paragraph is the chart I posted earlier. To me I read that as when I am filling tires, I need to properly account for temperature of the tire, rather than simply filling it to the suggested pressure as per the manufacture tire charts.

This is what I've done for the last two years since purchasing the system, and when I compare to the tire monitor, it shows I'm generally right on the money.

Ray

ps: Perhaps put a different way. Assume I filled my tires to 110 and it was 65 degrees out, tires at the same temp. If I simply came out to the vehicle 5 hours later, and the temp is now 85 degrees, I would assume that if I checked my tires they would be somewhere around 115 pounds. I certainly wouldn't bleed off 5 pounds to get them back to 110.

[travelite]

Thanks Ray,

Your are correct, and I think you are following Smartire's directions faithfully. What occurs to me is that as long as you don't overinflate your tires (as specified on the sidewall), you'll be fine following the path you are on. A slightly higher pressure than what's required will do no harm. It seems that Smartire wants you to establish a baseline at 65 deg F (their Pressure Temperature Correlation works best here). You'll notice that under 65 deg F they recommend that you go by the tire manufacturer's Cold Inflation Pressures. This means that they're establishing a floor. The ceiling (the amount of pressure that you don't want to exceed when filling a cold tire) is on the sidewall of the tire. In order for Smartire to work properly with regard to the pre-established baseline psi at 65 deg F is for you to follow their "proportionally higher" inflation pressures if you happen to adjust your pressures with an ambient greater than 65 deg F. If you don't, then Smartire may give you a false low pressure reading, unless you are able to go into Smartire and reprogram a new baseline at the new ambient temperature and manufacturers cold inflation pressure. Smartire is allowing you to establish a floor, and assuming you don't travel to any place hotter than 115 deg F, you won't be more than 10% high on your cold inflation pressures as specified by the manufacturer's load tables, and if you travel to a locale colder than 65 deg F you're expected to add air to your tires. This is really an artifact of how Smartire wants pressures to read with regard to temperature. It doesn't violate the manufacturer's guidelines so they are correct, but if you are so inclined, it's also correct to wake up every morning, regardless of the ambient temperature, and adjust your tire pressures according to the manufacturer's cold inflation pressure guidelines.

[Ray Davis]

Luckily I've never been in the situation where I've been below 65, at least not a lot, and not when I'm filling tires.

I've read other posts from Poggers who have external sensors who have been woken in the middle of the night (cold night) with a low pressure alarm from their system. That's never happened to me, and I'm guessing that even though SmarTire suggests never filling less than CIP, it nonetheless does compensate downward with a lower pressure than the 65 degree pressure.

Next time it's less than 65 out, I will check the monitor, just to see.

Ray

[Jon Wehrenberg]

I don't think we need to be so precise we fine tune the pressures based on ambient temperature. That's just my opinion based on the fact that if temperature compensation was important Michelin would tabulate the pressures based on temps.

But also influencing my opinion is that I'm not aware of any fleet owner or operator of trucks putting in any more pressure than the decal on the side of the truck says to put in, and those pressures are likely not taken from the Michelin charts. If anything they are excess pressures so adding more when it gets hot out is overkill.

I have checked tires and added air when it was sub-zero, and within 36 to 48 hours those tires were in 80 degree temps and were overinflated. So far, so good. I add what the Michelin chart says plus 5# and I can't complain about the ride or tread wear or tire longevity.

BTW, my Double Coins are now 4 years old so at half the price and 2/3 the age they were a good investment.

[Jeff Bayley]

I'm so glad I found this thread and didn't have to make a new one on tire pressures. I looked over the post but can't find anything on recommended pressures which I know has been covered before. I believe the correct tire pressure is calculated on axle weight. I remember weighing my bus before and recall that it was pretty near close to 15,000 lbs per axle but I can't find the paper I used to calculate the pressure and this is the first real road trip we've been on in over 3 years. I've been using 100 lbs for the last 7,000 miles but yesterday after driving in cool climate at only 55 MPH the tags were noticeably warmer to the touch but not so warm that I could say it was dangerous. I want to pick up a laser temperature gauge immediately and use that to check for temperature variances on the tires. I backed the tags off to 90 psi after a fellow bus owner said that a Prevost tech had recommended 100 in the drives, 100 in the front, and 90 in the tag. But when I phoned him today he suggested putting more air in the tags and checking the temperature again.

Related, if the tire pressures are different in the tags and drives, I think the level low will adjust the weight automatically such that the load distribution is unchanged as compared to how the weigh would be distributed with 100, 110 or 80. Is that correct ? The reason I THINK that is correct is that I believe if you pull the tag that the bus will droop but also begin to boost up the drives and level itself out again. If I'm going slowly in a turn lane and see that a driveway could make the belly rub, I usually pull the tag to make the turn and also begin to boost both sides up toggling back and forth between left and right. But if time is of the essence and the height is a priority then should probably leave the tag put and raise the coach for a faster raising of the ride height. Further, my tag tires are ready for replacement. I am really in the habit of pulling the tag in all tight turns and when backing up and turning sharp also but they seem to take more than their fair share of premature wear. Or it could be that when I put new tires a few years ago that I rotated the best out of 8 tires to the tags and replaced the other 6 and am not recalling the tread life that was on them. I need to keep better notes.

[Jon Wehrenberg]

Jeff,

This is simple. Don't make it complex.

Weigh your coach fully loaded. Make sure you get each individual axle weight. Then look up the tire pressure required based on the weights your get from this chart.

http://www.michelintruck.com/micheli...ion-tables.jsp

You can keep lifting the tag every time you make a turn, mess around with manually lifting the rear to compensate, but why don't you just try it without lifting the tag? I have had my current coach for more than 6 years and haven't lifted the tag yet.

[pwf252]

Not having anything to add to the tire pressure realm I would like to comment and add an anecdote. I was under the impression that it was appropriate to lift the tags in tight slow turns in either forward or reverse. When we came to the rally in Oklahoma City last year we were in a spot that required making a couple adjustments to back into. Being new to Prevost's and tags I forgot to raise my tags. After getting into the spot and while setting up I noticed the large amount of rubber scrub on the pavement and figured it was because I didn't raise the tags. There was so many rubber marks on the concrete in front of our bus I was somewhat embarrassed thinking some of the other POG members might notice and look at me as the newbee I was. Now that I've become accustomed to both the bus and alot of the other members of POG, I DON"T CARE WHAT ANYONE THINKS. It sure is nice being involved with people that you can feel that comfortable with.