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Tire Inflation w/ Bus Air
One of the things that I thought was cool about the CC set-up was supposedly being able to top-off the coach tires with the bus air using the fitting inside the driver’s side small door. Even though the bus produces 120-psi, I can’t get the tires up to the 115-psi, all I get is about 110. I am guessing that the 5-psi difference just isn’t enough to overcome line-loss?? In any case, advice appreciated.
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38 Replies
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I question the inflation pressure you are using. At 115 PSI you are pressurizing what I assume is the steer axle for a weight in excess of the Prevost axle limits. Have you weighed the coach axles to determine the proper inflation pressures?
Based on the Michelin chart http://www.michelintruck.com/michelintruck/tires-retreads/load-inflation-tables.jsp that pressure suggest you have 18,000 on the axle.
A more reasonable pressure might be 105.
But to answer the question, our bus air systems are set for a cut out pressure of around 120 – 125, with a cut in pressure of 20 PSI less. So the only way to get a high pressure in the tires would be to add air just after the compressor cycles off (listen for the air dryer to spit) and then after you add a few pounds bleed it down to the cut in pressure to get it to cycle up again and repeat the process.
For the lower pressures our tire size and axle weights call for the bus air system works pretty good, but nowhere near as good as a shop air supply with a 150 to 160 PSI pressure.
FWIW, my axle weight air pressures on the last 2 coaches were front to rear 105, 85, 90 and 105, 85, 80 and in each series of pressures I am 5 PSI greater than the chart to allow a slight cushion.
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I keep a small tank of nitrogen ( available from welding supply shop) in my bus stge area for tire pressure checks before departure.
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I guess I am used to the rule that you inflate your car tires to whatever it says on the door jamb sticker, so I just followed the CC sticker when I bought the coach a year ago. It is a cruddy picture because it is a foil sticker and reflects, but you can see the CC tire data. I would appreciate an education on the plusses and minuses of running at max rated pressure vs. weight rated – fuel savings, comfort, safety, etc..
Oh, on the Nitrogen – did you have your tires completely filled with Nitrogen? If so, have you seen a benefit? (like less leakage since the molecules are larger).
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12R’s vs 315’s, there is a difference boys… I would still go by the Michelin chart after weighing the bus.
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Cool. I have year old XZA2 Energy’s. do I just weigh the axle and divide by tires or do you set each side individually?
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I’m surprised you can still get 120 tires. I thought everyone was going to 315 tires due to availability.
Weigh each axle. Divide by two (or four) to get the weight the tires carry. Use the chart to determine pressures. I have seen charts with axle weights and with tire weights so just be sure the weight reflects what the chart calls for.
120 will take higher pressures than 315 tires.
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That is probably EXACTLY where I went wrong out of ignorance. The new tires are 315/80R22.5 XZA2 Energy. Was “120” some obsolete metric size? In any case, I will weight the bus and follow the tables. Really glad I got the right answer to the wrong question 😉 Thanks all!
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no, I have not had my tires purged of air; simply use the nitrogen to top off. Got in the habit of using nitrogen when we needed it for go-kart racing. Don’t know if I could actually purchase a tank of plain old compressed air at a lower cost than nitrogen.
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All,
Michelin’s site has a great brochure http://www.michelinrvtires.com/michelinrv/toolbox/reference-material.jsp on how to determine wheel position weight. It IS NOT likely the weight of your axle divided by 2! This is a common error and not surprising given truck scales are axle based scales.
For the purpose of tire pressure, inflate your tires on a given axle to the weight of the heaviest wheel position (single or dual). There can be over 1,000 pounds difference between left and right wheel positions on a given axle. On the typical plastic coach it can be over 2,000 lbs difference! This makes a difference in tire pressure.
Measurements must be taken with the coach at it’s heaviest weight that it will have with your use. As an example, if you never put more than a 1/2 a tank of water in the tank, then it’s okay to use the weight measured with only a 1/2 a tank of water. Don’t forget the stuff that gets loaded for your big trip, don’t forget the maximum number of passengers that may be on board. Remember, under inflated tires get hotter and are likely a major contributor to blow outs. When in doubt, inflate the tites to 5 psi above what the chart says, as long as the pressure doesn’t exceed the tire’s maximum pressure.
If you have 8.25″ inner dual wheels, like I do, determine the pressure of all tires on that axle based on what is required for the 8.25″ wheel.
Liam, your pressures aren’t far from mine. At 115psi the steer tires can support up to 7,920 lbs. The steer tires are so critical that I inflate mine to 120 psi, even though my heavist front wheel position weights less than 8,000 lbs. I also inflate all rear tires to 100 psi. I never raise my tag and know the side pressure must be significant, so I avoid pressures at Michelin’s lowest recommended pressure. If I notice more tire wear in the center of the thread I’ll change, but all tires have even thread wear so far.
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Gill,
Assuming you have a 14K front axle, why would you choose a harsh ride by inflating to 120 lbs? The chart shows the following:
7190 95
7440 100
7610 105
7920 110
On the wheel size, an 8.25″ vs 90″ using the Michelin chart shows a difference of 50 lbs in weight using the 12,000 dual weight and a pressure of 85 lbs for both wheel sizes. 50 lbs would be insignificant on a 24,000 lb axle.
I use the chart and inflate 5 lbs over like most. I have weighed separate wheel positions and there is a difference, but not enough to require a higher air pressure on one side vs the other. I just use the higher weight as the base and inflate both sides the same. I did have a plastic CC one time that was 1,300 lbs different side to side on the drive.
At 120 psi on the steer, once they’re hot in the summer, you could be riding on 140 psi and it must feel like a stage coach. I use Pressure Pro and you would be amazed at the temperatures when the tires get hot.
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Can i put 315 on my 1994 xl were 12R were and do i need to change rims ?
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Tom,
The chart I’m looking at shows 7920 lbs @115 psi for RV applications. My axle weight is over 15.5K. The point of stating the reference to 8.25″ wheels was to raise awareness. Yes hot tires have higher pressure. The maximum tire pressure rating is for cold tires. What’s important is that under inflated tires may get too hot at speed.
FWIW, I can’t feel the ride difference between 110 and 120 psi. My 315 XA2 tires are rated for a maximum of 130 psi. I’ll stay with 120 psi on my steer axle and monitor thread wear.
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