Tire Pressure

Dumb questions I know, but....when getting our tire pressure reading, it's supposed to be a 'cold' reading, correct? If my fronts are supposed to be 110lbs, it's before I go down the road, right?

Now it's hot where I am, and one day before a run I took my pressure readings, one side was a little higher than the other, could the outside temprature and/or sun play that big of a roll in tire pressure? Should I take the readings at first light so daylight doesn't effect them?

Sometimes I realy feel stupid, this is one of those times.

You have the right idea. The sun can affect your readings.

JIM

The ideal is to take the readings when there is no sun on the tires, and before they have been heated up by driving.

Do not take the pressure readings after driving unless the tires have had ample time to cool down. The heat will raise the pressures and provide false readings.

The ambient temperatures will have an impact on your readings so just recognize if you are going to travel from one temperature extreme to another and back again you will see the pressures follow the temperatures. If you are checking pressures on a sub zero morning, and inflate the tires to the recommended pressures, then drive to a warmer climate, when you check pressures again they will be in excess of what you started with. The reverse will occur when you go from warm temps to cold. I don't bother to bleed off air if the pressures are higher because the difference is usually 5 PSI or less. I do add air if I am below the specified pressures however.

FWIW if you ever suspect a loss of tire pressure while traveling you can check pressures, but recognize they are going to be higher than specified. That is OK. The key is to recognize if one tire is significantly lower than its mate. You need to also recognize that the tires on the sunny side of the coach are likely to be a few PSI higher than those not in the sun. To stretch my legs and get a sense of how my tires are doing I aim my infra-red temperature gun at the center of the tread on each tire immediately after stopping. As long as they are all approximately the same I'm OK. I also measure the temps at the wheel near the hub cap. If any shows up hotter than the others I suspect a dragging brake or possible issue with lube on the bearings.

At the tire tread on a warm day it is not unusual to see all readings around 135 degrees. I can't remember how high the pressures get relative to their initial pressure, nor do I remember the temps at the wheels.

Jon
You mention that you have a infra-red temperature gun, I want one, what brand and where can i get one. Thanks George

Ahh, Tires, one of my favorite topics. :)

Your tire's gauge air pressure (as measured with a tire pressure gauge) will increase approximately 2% for every 10 deg F increase in tire temperature. As an example: if you check your tire pressure when the ambient (and the tire) is at 75 deg F and the pressure is 100 psi, then if the tire heats up to 135 deg F the tire's air pressure will rise to approximately 112 psi.

Also, your tires gauge air pressure will increase as you ascend above sea level by approximately 0.5 psi for every 1000 feet of altitude. As an example, if you leave NJ with 100 psi and make a climb to Aspen Co, you'll ascend approximately 8000 feet and your tire pressure will increase to 104 psi.

The dynamic changes in air pressures are well understood by tire engineers. Turns out that a tire's stiffness (spring rate: lb/in) and dampening (lb-s/ft) both decrease with speed and are offset, to a degree, by increases in air pressure due to increases in temperature.

The manual I received with my SmarTire system indicated that the specified tire pressures are defined for 65 degrees. They have a chart which tells you what the pressures should be for higher or lower temperatures. If I remember correctly, David's numbers are just about what SmarTire indicated, i.e. about 2 pounds for 10 degrees.

Ray

Quote:

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Originally Posted by george frudakis

Jon
You mention that you have a infra-red temperature gun, I want one, what brand and where can i get one. Thanks George

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I have a Fluke, but there are many brands and prices for the most part are under $100.

One thing worth mentioning. Tire pressure gauges may not be accurate. Ironically the Milton gauges (a good respected name in the indusrty) are not as accurate as a cheap one with a dial I got at Wal-mart.

I took all my gauges to a large tire place and had them checked against their master gauge. I also exceed the Michelin recommendations on pressure by 5 PSI for each of my tires. I do that because I theorize it give me a little additional margin if I develop a slow leak. Both Michelin and Goodyear agree if you run a tire 20% (or more) below the recommended pressure you have effectively ruined the tire and it should be removed from service. That's a strong reason for being very careful about maintaining tire pressures.

Quote:

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Originally Posted by Jon Wehrenberg

I have a Fluke, but there are many brands and prices for the most part are under $100.

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I just ordered a Raytek MT6 for around $50. It appears to be substantially the same as the Fluke 62 mini.

Jon, thank you for the tire temprature info, I'll take temp readings of my tires from now on to use as a guide, and expect them to average around 135 degrees.

So just to summarize and clarrify, if my tires recommended pressure is supposed to be 110, that is 'cold' and at an ambient temprature of 65? So giving for 2lbs per 10 degrees, If it's 85 when I check (before getting on the road), I should make sure my tires have 114lbs?

And thank you guys for the information and presenting in such a way so I don't feel dumb. :)

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

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

Quote:

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Originally Posted by 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

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Hi Ray,

I took the following from Smartire's website:

<font color="navy">"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."</font>

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.

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:

Quote:

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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.

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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.

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.

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

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.

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.

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.

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.

We were going to mention something to you about the tire marks...............



But it was more fun talking about Jeep parking.

I have had my current coach for more than 6 years and haven't lifted the tag yet.[/QUOTE]

Jon,
Ed wants to know if your switch for the tag works? :D

When we were at Prevost, they lifted the tag every time a bus was moved (per Ed).

If I were to say yes it would be a guess.

However, the automatic system that dumps air on my coach along with the stabilizators that keep the rear from dropping when my wheels are turned to the extreme, left or right works, and because of that I do not need to manually lift the tag. If I were to manually lift the tag the stabilizators would not function and the rear would drop until the drive axle air bags were filled with air to raise the rear to the normal height.

Most Prevost techs are not familiar with that system that was designed for and used on early 45 foot coaches to minimize tag axle tire scuffing as an alternative to the "steerable" tag axle that was supplied on MCI coaches around that time. I think that feature was discontinued and it is my understanding that some converters even went so far as to disable the system if the coach had it installed.

Aren't you glad you asked?

See you guys in Kerrville, where I hope Ed allows you in the bed instead of the fiddle.

Here's something interesting that I ran across in the September 2010 issue of Heavy Duty Trucking magazine regarding tire air pressure. "Pressure will increase by about 2 psi for each 10 degree change in temperature. A properly inflated tire that is up to operating temperature (one or more hour of operation) will typically run about 60 degrees hotter than ambient temperature - adding about 12 psi to that tire."

So I guess the easiest way to check if my tag locks in position and ignores the request to lift is to turn the wheels tight , pull the tag and observe. ?

Jeff,

When the 45 foot coaches were originally built in 94 the tag axles automatically dumped air when the steering wheels were turned to the extreme in either direction. I don't know when or how the stabilizator system to prevent the rear of the bus from dropping came into being but several buses owned by POG members have that feature also.

Having said that I heard but do not have any way to confirm this that Prevost and some converters were disabling those features so it is my opinion you may or may not have either of the systems and if you do they may or may not work.

While you or Stephanie turn the wheel hard left or right, the other should stand next to the tag axle and see if the rear of the coach drops a little from the driving height, or if air is heard to escape from the tag air bags. That's the easiest way to know how your bus is currently set up.

If the rear does drop and air escapes from the tag air bags then the automatic dumping of air from the tag axle air bags works and you do not have to dump air manually. If air escapes but the rear of the bus does not drop it is probable you have the automatic air dumping feature and the bus is equipped with stabilizators. I doubt if it is, but it is possible due to the vintage.

Over the weekend I had the chance to get some service with someone that was real knowledgeable and competent. Among the things I learned, he finished explained about lifting the tag further to what is in this (not too far off topic) thread creep I started related to tire wear here. I now believe my tire wear on the tag is because of my lifting the tag so regularly though turns in town. I learned that the tag takes far too long to lift off the ground waiting for the drive axles to raise the coach up again. Even 3 minutes was not sufficient and when it finally came to it's end it was still around an inch shy of height with the tag. This was translating into my tires rubbing MORE, not less because there is less weight on them so they scrub easier. I had a driver of an entertainer give my some driving tips a few years ago. He was a 25 year man with Greyhound and was an instructor towards the end and he told me to train myself to use the tag often in town. I've also had at least one Prevost tech say similar. Guess I can't always listen to the "experts". I now believe my tires wore on the tag prematurely as a result of pulling the tag often. It was the latest opinion of this technician that lifting the tag gives negligible tighter turn radius and is only practical in the most necessarily help needed in a turn.

The more I learn the more I realize how much I don't know.

Quote:

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Originally Posted by Jeff Bayley

The more I learn the more I realize how much I don't know.

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Welcome to the club I've belonged to for years! :o:rolleyes:

Lifting the tag does have some impact on turn radius. Lifting the tag even though it does not come off the ground also reduces the tire wear due to scrubbing.

When the tag is not "lifted", the term meaning in this context that air has been dumped from the tag axle air bags, but the rear has not raised enough to lift the tag tires from the ground, it reduces the turn radius slightly by moving the pivot point of the rear axles forward slightly. The simple way to prove this is to find a large empty parking lot and go around in a circle with the tag down. Measure the circle. Then repeat the process with the tag "lifted". There will be a slight decrease in the diameter of the circle.

Any time weight is removed from a tire that is being scuffed sideways the wear on that tire will be reduced. Start a turn with the steering wheel to the extreme and once the turn is extablished step outside the coach and look at how much side deflection the tag tires have. Repeat that with the tag lifted and you will see much less side deflection because the tire tends to slide sideways more readily.

But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

I have seen some Prevosts lift the tag tire a couple of inches OFF OF THE GROUND. Is this a converter option? Or does the 40' Prevost lift off the ground and the 45' Prevost does not?

Tuga,

Generally, the 40 tag tire will come off the ground, but the 45 does not.

I guess the definitive way to tell is maneuvering and testing in a parking lot as suggested prior. I'm not confirmed for the rally and we don't have much time left to decide but I wonder if enough people are curious to do testing of this there. Curious to know what others experience has been on wear of tag vs. drive tires.

I raise my tag on any 90 degree or more turn and tight parking maneuvers. Just stand next to someones tag axel when they are doing tight turns. Watch as the rubber is ripped from the tire and left on the ground. Observe the tire being pulled sideways from the rim. Think of the abuse the sidewall belts of the tire sustain not to mention the additional bearing load placed on the axel bearings. I know for a fact that when a car is involved in a side accident and it is pushed across the road sideways tire damage occurs. Insurance companies try not to pay for the tires but when the vehicle wiggles going down the road the problem is obvious. I suspect the ride of a Bus is compromised if tire belts are pulled and stretched sideways.

(this from Jon's post)
But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

Just curious - is tire rotation ever considered?

Quote:

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Originally Posted by Woody

(this from Jon's post)
But lifting the tag when cornering is really not required because over the life of the tires, if they are inflated properly tag axle tires are not being scuffed sideways any more than the steer tires are during turns. I don't know this for sure, but I would suspect steer tires on average have a shorter lifespan than tag axle. When we turn the front wheels to alter the path of the bus those steer tires are resisting the forward motion of the bus and are being scrubbed sideways as the bus turns.

Just curious - is tire rotation ever considered?

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The steer tires are rotating along the direction of the turn and they define the turning circle and have the easier time of it insofar as scrub, the drive axle wheels are the privoting point that describes the other point on the chord of the described turning circle. The tag axle wheels are outside the turning circle and are have a (sideways) component more at an angle to the tangent and 'dragged' (or forced to distort) more sideways than the wheels on the steer and drive axles. This is why they are 'scrubbed' significantly more. They are at the opposite end from the fulcrumm (the drive axle). It is sufficient enough to cause widening of the turning circle and the reason for 'lifting' or removing weight from the tag. It is also the reason for steerable tags that tighten the turn circle by removing this geometric issue, as on the recent Newells, and at various times M.A.N., Daimler-Benzes, Setras, Tatras, and others on some of their commercial bus offerings over the years. Interesting to some may be the reasoning for, and physical constraints against, that invalidated Ackerman steering (where the steer wheels were articulated to describle the two different turning radii, but failed to take into account the greater slip angle(s) of weighted outside tire (and is related to tuning based on specific tire characteristics). Other more distantly related topics we could discuss informally at POG9 might include the geometry behind what is/was known as zero (and plus or minus) point steering, the geometry behind unequal length wishbone suspension systems, camber, caster, toe-in etc.. Or, I can keep my big mouth shut (probably best).

Fulcrum, fulcrum, dammit.

I did rotate a set of tires every 30,000 and at 130,000 with 1/4" of tread remaining replaced them. At the time I was putting 20,000 to 30,000 miles per year on the bus. Because our wheels are polished on the inside for drives and outside for steer and tag it meant either breaking down the tire and wheel (which is how I did it) or polishing the dirty side of the wheel which was a lot more work.

Today I wouldn't think about rotating because I think I will end up with tires aged out instead of worn out. Rotating a tire on 45 foot coaches would require breaking them from the wheels because the inner wheel on the drive axle are steel, unlike the inner wheel being aluminum on the older 40 foot coaches with 12.00 tires.

As to tag tire scuffing, without a doubt they do scuff, but I think everyone is trying to over compensate. Yes there is considerable sideways force on a tag axle tire when it is not lifted, but the tires are built for that. If they are not every 10 wheeler truck or every tandem axle trailer on the highway today would be tearing up tires. Yes they are duals, but they carry loads per tire similar to our tag axle tire loads. I am sure we have all seen a truck in a truck stop parking lot make an extreme turn where the trailer wheels just sit in one spot while the tractor pulls the front of the trailer around in a circle.

As to our steer tires not scuffing as much, I agree totally with what Ken said, however he paints a picture with unrealistic conditions. As we travel down the interstate we drive on crowned roads and often have strong crosswinds. To keep the bus from tracking to the low point or to counteract the crosswinds we turn the steering wheel which keeps the steer tires in a small but ever present side skid. The same side skid occurs when making a turn. The tires are always pointed in the general direction of the turn, but are always pointed in such a way to indicate a smaller diameter turn than the actual path. That side slippage in the steer tires is to compensate for the resistance of the bus to a turn due to the drag of the tag, and because the drive direction is always straight ahead.

Jon, Do you think anyone in the " Prouds " would ever exhibit this much passion about Tires ?

Ken, I would attend one of your seminars.

Jon is correct when he points out that I did not, in my post, discuss crown, which does indeed influence tire wear, and (straight) tracking, and was not attempting to. And, further, I believe Jon will agree that side to side load imbalance, which he has mentioned on more than one occasion, is also a factor that can arise to criticality if not adequately addressed. I don't attempt to discuss crosswinds; another matter. But, in fact, there is compensation related to wheel alignment/adjustments on all heavy vehicles to help alleviate the tendency to pull to the right (of course, the opposite condition occurs in countries with left side driving) for general/standard crown(ed road) conditions, particularly as it can be factor in accidents where inattention leads to sideswipes and wandering off road. Some may be familiar with the compensation device, whose name escapes me at the moment, that is/was somewhat often seen where the adjustment was made to track straight by means a knob accessible to the driver. This can be a big deal. My Bluebird, that I loved dearly, pulled enough that it tended to wear the power steering pump and related equipment, and often could be heard to whine for extended periods, especially on 2 lane country roads.

My office overlooks the Staples Center and Nokia Theater and better yet, the lot where the entertainer buses park for concerts. I don't spend all day staring out the window, but when I do look I have noticed that the pros almost always lift their tags when maneuvering in that lot, even if they have plenty of room to make a wide turn.