Front end allignment

[Jon Wehrenberg]

Hey guys, in case none of you have noticed you are driving a bus.

This ain't an S500 or even a Chevy Malibu. It has the suspension of a truck. It rides on tires made for commercial use. You are likely to feel the bumps.

I think 365s look way cool. I drive a Corvette and it has fat wide tires too. I love the look. But even with my 315s I have a serious issue if I get a blowout. But at least I can stick an alternate size on my rims. My last tire failure was in St. Louis. I could not get a 315 tire there for 2 days. I went with an odd size that I put on the tag. Forget about getting one from a bus company. They lease their tires, so even if they wanted to help you they cannot because they don't own their tires. The closest tire was Prevost in Nashville.

I guess everything in life is a compromise. To get the 365 ride and look you risk sitting wherever you are until a tire shows up if you have a blowout.

[JIM CHALOUPKA]

Sid, I don't like to see your question ignored, so instead of incurring Lew's ire over not knowing what I am talking about here is a link describing the Pro Driver.

http://www.busconversions.com/bbs/in...e;topic=9854.0

http://www.heavytruckedr.org/diesel.html

http://www.analysis-inc.com/electron...ol_modules.php

These links are not complete in the explanation of the Pro Driver, but I think it is enough to get others to post about it.

The last link is for a cartridge for the Pro Link scanning tool. Read the menu of functions to get an idea of the Pro Driver capabilities.
By reading about what it does, you can form your own opinion about, "What is a Pro Driver?".

JIM

[0533]

Uh...Ray....Aren't we supposed to put the required pressure in the tires before we drive? If it is cold in the morning it sounds like you allow low pressure in the hopes your tires warm up and reach the required pressure.

Jon, what you have just stated above is more important info than tire size and tire availability and raises some interesting questions.

OAT has always been an interesting factor in the decision to add/reduce air pressure or wait until the temps rise.

I use the following theory when considering adding air. I call it the "HomePort" approach. Example: In So. Florida the average temps are about 75 to 80º in the winter months by 2 pm.. I check my air around this time (before driving) and allow some difference for the sunny side of the bus.

Here is the big question: It has been colder here now with the addition of cold fronts coming down from CA, (relative terms for you Northern Boys) 50's in the morning. Rising to 80's by 2 pm.

I Had to take the bus to the Fort lauderdale Boatshow last week, before heading out I checked the air pressure, the OAT was around 65º and the tire pressure was about 5lbs low all around. I had checked them the week before and the air pressure was perfect all around, and the OAT was 80º (baseline OAT for So. Florida) so I decided that by the time I got up to speed on the road and by 12 noon I would see proper air pressure again. ( Here is an added question, The tire gauge was also very cold, metal was ice cold can this affect the result)

Yesterday around 2 PM I took the bus for a ride to the Fort Pierce Pilot Fuel plaza at exit 129 off 95. (fuel price $2.98 P/G) Checked the tires before leaving, all pressure is where it should be based on Michelin specs and bus weight, OAT was 82º. I have not added air for a very long time to any position on these tires since I have been back in So. Florida.

What I am hearing from you is that if one sees lower air pressure no matter what the outside temps, no matter where you are you should add/reduce air pressure to get the air pressure back to specs before driving off ???

I did however readjust the pressure for each region of the country, New England in the summer has cool nights and warmer days and average temps are 70's in the summer about 10º lower than Florida winter, I add air while in these climates. I add air if I see a difference over several days of checking, (subject to a huge change) check air everyday before driving) On the road for the summer 8K miles of driving across the East coast.

I have always been careful about checking air (comes from flying I guess) but I do not want to be a slave to the changing OAT on a daily basis and adding and reducing air pressure every few days unless I am spending some period time in the changing environment, lower temps etc.

What is the answer Jon???

[Jon Wehrenberg]

I am making a huge assumption here, so take my opinion with a grain of salt.

The tire companies know that heavy vehicles like ours travel north to south, south to north, they start driving when it is cold and getting warmer and they start driving when it is warm and getting colder.

If they felt that the cold tire pressure (meaning a tire that has not been recently driven) needed to reflect the outside air temperature they would have given us temperature correction values, or the charts would reflect the pressures at specific temperatures.

But they did not. They took a lot of time to develop these charts. Anyone that has looked at the charts can see clearly that they are set up in 5 PSI increments and 400 to 500 pound weight differences. If you extrapolate the weights you can get the correct pressure to the exact PSI. If they thought temperature was important there would be a note about the correction factor or like on our plane POH we would see columns for pressures based on weights and outside temperature.

So you are making a huge leap of faith by figuring when it warms up outside that the underinflated tires you started with will be at the correct pressure when it gets warm. If you want to be consistent then it follows that if you are departing FL in January, heading to the north, you should overinflate your tires. My guess is you do not do that.

I don't know the exact change in pressure per degree of temperature rise, but I would always feel more comfortable bringing my tires up to the proper pressure while cold. I know when I start out at 32 degrees with tire pressures set per the chart that when I am at 75 degrees the next day my pressures will read high, but unless I am remaining in warmer temps for a long period I will leave the tires "overinflated". I will not bleed them down if I am going back to the cold because I know they will be below the recommended pressures.

[0533]

I am making a huge assumption here, so take my opinion with a grain of salt.

The tire companies know that heavy vehicles like ours travel north to south, south to north, they start driving when it is cold and getting warmer and they start driving when it is warm and getting colder.

If they felt that the cold tire pressure (meaning a tire that has not been recently driven) needed to reflect the outside air temperature they would have given us temperature correction values, or the charts would reflect the pressures at specific temperatures.

But they did not. They took a lot of time to develop these charts. Anyone that has looked at the charts can see clearly that they are set up in 5 PSI increments and 400 to 500 pound weight differences. If you extrapolate the weights you can get the correct pressure to the exact PSI. If they thought temperature was important there would be a note about the correction factor or like on our plane POH we would see columns for pressures based on weights and outside temperature.

So you are making a huge leap of faith by figuring when it warms up outside that the underinflated tires you started with will be at the correct pressure when it gets warm. If you want to be consistent then it follows that if you are departing FL in January, heading to the north, you should overinflate your tires. My guess is you do not do that.

I don't know the exact change in pressure per degree of temperature rise, but I would always feel more comfortable bringing my tires up to the proper pressure while cold. I know when I start out at 32 degrees with tire pressures set per the chart that when I am at 75 degrees the next day my pressures will read high, but unless I am remaining in warmer temps for a long period I will leave the tires "overinflated". I will not bleed them down if I am going back to the cold because I know they will be below the recommended pressures.

I would not read into the Michelin (or any tire chart) tire chart the lack of info about the need to compensate for OAT. My guess is they just do not know how to accurately articulate this info in their chart to reflect changing temps and driving distances across changing temperatures. I would be very interested in hearing how over the road truckers handle this question on a daily basis. Maybe this contributes to the many blowouts we see on our highways ??

If I understand you correctly, you feel that over inflation is a better bet than risking driving away with under inflation and hoping for the tires to heat up while underway. This is a safe way to handle the question I guess, but lets examine a sample trip for a moment and draw some agreed conclusions. Do you agree that it is a certainty that the tires will increase pressure as you build speed and heat over time at highway speeds ???

Trip "A". Leaving So. Florida for No. Dakota Dec 1 2008: Temps are 80º in So. Florida (why am I leaving So. Fl>) Fargo No. Dakota is 13º I leave with proper air pressure, check the pressure daily, by the time I get to Kansas City I am 5 lbs low, this is when I would start adding air, and would continue adding every other day until I arrive, and depending on my stay I would adjust the pressure for Fargo. I would reverse the trend on the return trip.

Is it safe to use a 5 lb margin up/dn of normal for this trip planning verses running with over inflated tires all the time. What is the margin up or down?

[merle&louise]

On my previous coach the tire size on all axles was 11R24.5 and they were availabile at most truck stops. Things sure were simple in those days.

As times passes on, will 315 and 365s become more availabile since more and more vehicles will be using them? Hope so!

[BrianE]

Bruce, Over the road truckers are seldom in one place long enough to fool with tire pressures. Most of them simply thump the tires and press on. I personally only air up when in home average temp conditions unless planning on extended travel in a hot or cold extreme. I think tires are a lot more adaptable than we give them credit for.

[Jon Wehrenberg]

I would not read into the Michelin (or any tire chart) tire chart the lack of info about the need to compensate for OAT. My guess is they just do not know how to accurately articulate this info in their chart to reflect changing temps and driving distances across changing temperatures. I would be very interested in hearing how over the road truckers handle this question on a daily basis. Maybe this contributes to the many blowouts we see on our highways ??

If I understand you correctly, you feel that over inflation is a better bet than risking driving away with under inflation and hoping for the tires to heat up while underway. This is a safe way to handle the question I guess, but lets examine a sample trip for a moment and draw some agreed conclusions. Do you agree that it is a certainty that the tires will increase pressure as you build speed and heat over time at highway speeds ???

Trip "A". Leaving So. Florida for No. Dakota Dec 1 2008: Temps are 80º in So. Florida (why am I leaving So. Fl>) Fargo No. Dakota is 13º I leave with proper air pressure, check the pressure daily, by the time I get to Kansas City I am 5 lbs low, this is when I would start adding air, and would continue adding every other day until I arrive, and depending on my stay I would adjust the pressure for Fargo. I would reverse the trend on the return trip.

Is it safe to use a 5 lb margin up/dn of normal for this trip planning verses running with over inflated tires all the time. What is the margin up or down?

Bruce,

Lets enter this discussion with some things I think we agree upon.

The tire pressures listed in the charts based on weights and tire size are those pressures necessary to maintain a column of air of sufficient pressure to keep the tire at a given height (in other words the less pressure, the closer the rim comes to the ground, and vice versa) and to maintain the tire foot print (too much air and the center bulges, too little air and the tire cups).

The engineers make the pressure determinations so the tire tread contact is optimized, and so the amount of flex the side wall experiences when rolling is not so great that the sidewall heats and fails. They also want some flex in the sidewalls as a compromise between a harsh ride and a likely blowout.

If we can agree on that (assuming my understanding is correct,) then it follows that regardless of outside temperatures the cold inflation pressure is a constant. It has to be a constant because the weight of the bus does not decrease when it is cold outside.

It still requires that imaginary column of air at a given pressure to maintain the sidewall height and to apply even contact pressure with the ground all across the tread.

Truckers may not be the best sources for technical data. I would rather get my tire information from the guys that design the tires. I also don't believe OWTs from pilots. I would rather rely on my POH.

Now let's consider the 5 PSI you are discussing. Just so everyone knows my bias I run 5 PSI over the Michelin charts. Here are my reasons. Tread wear is not likely to be one of my considerations. I expect sidewall cracking before I have worn out my tread. I realize I will get less optimal tire contact with the ground, but I consider the impact to be marginal because I hope to never have to drive at my limits of adhesion. We had the refrigerator door open once. That was enough. When that happened we weren't even close to losing traction.

But on a more serious note, over the years I have had the slow leaks every one else has encountered. I have had nails in the tire, a bad Schrader valve and a leaky valve stem. These subtle leaks are the ones that surprise you. If I were to accept starting my day with tires underinflated by 5 PSI, then according to the 20% loss of inflation statement made by the tire experts as being the point at which you convert a good tire into junk, you have just given up 25% of your margin on your steers and almost 30% of your margin on your rear tires.

By running 5 PSI over inflated I have some room for a leak to occur and not adversely affect my tires or get me into the range where a blowout could occur.

Before trips, even if I know I am going from Knoxville in the winter to the warmer temperatures of the Keys, I still top my tires off to my regular 5 PSI in excess of the Michelin charts.

I do know from the experts that runing a tire at 20% less than the pressure in the charts has ruined that tire. I do know that underinflation is the cause of blowouts. I do know that there is universal agreement that if the pressures are too high it is far better than if they are too low.

If there is any doubt about that look at the Spearfish photos of Terry's inner dual. He had a leaky valve stem, and apparently it was a slow leak, right up until the tire started to come apart.

I agree with you that tire pressures increase as we drive due to the heating of the casing. That is normal and the cold tire pressure charts probably allow for heating of the tires over the full range of ambient conditions. I am just as sure that the engineers contemplated that you put air in your tires at the elevation of Sacramento, and then you went over Donner pass at a much lower barometric pressure.

As to your hypothetical trip, yes I would add air heading north, and no I would not release it heading south until I got where I was going because I am less concerned about over inflation than underinflation.

What we need is someone with a heated shop to cold soak the tires at some low temperature such as zero F, and then bring the bus in the shop and let tire temperatures stabilize at 70 F to see what the actual pressure change will be. I bet it will be within a few PSI. Any engineers capable of calculating it? Peter?

[Darl-Wilson]

Maybe this will put this subject to rest:

Effects of Time and Temperature

Tire inflation pressures change due to time and temperature. Tires lose about 1 psi per month due to air escaping through the rubber as it stretches. If you were to check your tires only every six months, it would not be uncommon to find them under inflated by as much as 6 psi. Tire inflation pressures also fluctuate with changes in the outside air temperature. This occurs at a rate of about 1 psi for every 10°F (plus or minus). So the tires you set correctly with an 80°F ambient temperature will be under inflated by 6 psi at 20°F.

Here is another site that deals with racing tires.

[Joe Cannarozzi]

When we leave Chicago for Florida in January I used to panic about the tires in our old Class-C, that was what the dorian system was originally installed on and when I first realized just how much pressures fluctuate under different conditions.

Now I'm with Brian. If it is 0 to 20 outside leaving and my pressures are 90% of where they should be we are out-a-here.

IMO most of you guys over inflate the damn things anyway, little bit a cold air do um a world a good.