Front end allignment

[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?