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Tire Pressure

Tires and inflation. The great mystery for many motorcoach owners. I found it a bit confusing when we first took delivery of our coach. Our dealer had inflated our front tires to 120 PSI and our rear tires to 95 PSI. The factory had installed a sticker which showed the steer at 120 PSI, the drive at 90 PSI and the tag at 85 PSI. After taking delivery, we took our coach to KAL tire before a major road trip — at that time I did not carry an air compressor — and they inflated all the tires to 120 PSI.  I had the coach weighed on a CAT scale and I made a guess that the tires should be 110 PSI on the front and 85 PSI on the rear. When we were at the factory earlier this year, Newmar weighed the corners of the coach and inflated the tires to 110 PSI at the front and 75 PSI to the rear.

What tire pressure should I use? Unfortunately there isn’t a one-size-fits-all answer to that question.

The tire pressure you should use depends on the type of tire and the weight of the coach. There is no substitute for getting the coach weighed, preferably on all corners.

Assuming that you are running a coach with a tag, here is the process that Michelin recommends to obtain axle and wheel position weights. Not necessarily easy to do on a CAT scale. Before we had our corners weighed, we did a rough calculation based on axle weights only.

We’ll use our coach as an example.

We run on Michelin XRV 305/70R 22.5 LRL tires.

Our coach was weighed with 3/4 tank of fuel, 2/3 fresh water, empty grey tank, 1/3 black tank and no passengers.

Our corner weights:

7,290 lbs. left front, 7,115 lbs. right front
8,270 lbs. left drive, 8,645 lbs. right drive (duals)
4,275 lbs. left tag, 4,345 lbs. right tag

Tire pressures need to be the same across an axle which means that we use the heaviest tire weight on an axle to determine the load/inflation for each side of that axle.

Michelin provides the following table for our tires:

A cold tire pressure of 110 PSI will support a weight of 7,300 lbs. on a single front tire. Our heaviest single front is 7,290 lbs. therefore 110 PSI would be the target tire pressure for our coach.

A cold tire pressure of 75 PSI will support a weight of 9,530 lbs. on the one pair of duals on the drive axle. Our heaviest weight is 8,645 lbs. on our drive duals therefore 75 PSI would be the target pressure for our coach.

A cold tire pressure of 75 PSI will support a weight of 5,375 lbs. on the single tag tire. Our heaviest single tag is 4,345 lbs. therefore 75 PSI would be the target pressure for our coach.

How do you get the required tire pressure for your coach?

You need the tire model. You need the load/inflation tables from the tire manufacturer. Michelin provides an online PDF which you can download here.

And you must know the actual weight of your motorcoach. You cannot determine the correct pressure for your tires unless you know the actual weight of the coach. Preferably the corner weights.

The consequence of running tires underinflated or overinflated can be deadly.

We carry a precision tire pressure gauge suitable for our type of tire which means that it is accurate to at least 120 PSI which is our maximum load. We carry a portable air compressor that can inflate tires to at least 120 PSI. We have found through experience that we cannot count on the tire pressure services at fuel stops to provide enough pressure to inflate our tires to 110 PSI.

Before any trip on our coach we check our cold tire pressure and we use an air compressor to inflate the tires as required. We visually inspect our tires for any signs of wear or distress.

Spring

The Castaway is still there. Still in storage.

We checked on her several times during the winter. And, apart from filling up the batteries with distilled water, everything on board the coach was fine.

We will be taking our coach out of storage next Wednesday. Hopefully, the engine will start without any issues. After a full inspection of the coach, I’ll need to do the following:

  • Inflate tires to nominal levels, front tires to 110 PSI cold, rear tires to 95 PSI cold
  • Turn on the inverter and reset the charger
  • Turn on the engine and idle
  • Turn on 120V AC breakers
  • Close and secure fridge doors
  • Confirm interior cabin is set and ready
  • Bring up jacks and head out of storage

We will be taking the coach to the Hitch House on Saturday for service and warranty work. I am also getting them to detail the coach. I did it myself last year and it took several days for me to complete the work. As we are downsizing and moving in a few months, I will not have time to detail the Castaway this spring. The dealer assures me that they will do a good job (I tend to be a bit fussy about detailing).

Lorraine and I are very excited to be bringing out the coach. With the house now sold, we are much closer to our retirement dream. We will be in Canada for another winter and after that we should be snowbirds by November of 2018.

I can hardly wait.

Not Enough Air

NotEnoughAir

We had to replace a bad tire on our travels to the Upper Peninsula of Michigan last month. That tire, located on the front driver’s side, now checks in at 101 PSI cold. The tire on the other side of the steer axle checks in at 110 PSI cold.

My sense of balance requires both tires to be at the same pressure: 110 PSI cold.

No problem. On our way to the Flying J a few nights back, we planned to check the pressures running hot and level the driver’s side to match the passenger’s side.

Only I did not have enough air from the air pump.

Frustrating really.

I took the air hose, connected it to the tire valve, and waited. Not long, probably 10 – 20 seconds. I had no idea how quickly the tire pressure would change but when I checked, it had not changed at all.

I spent a bit longer, perhaps a minute or so. Checked the tire pressure. And still no change.

I tried 5 minutes. No change in tire pressure.

I then went to the Flying J counter to settle the fuel and dumping charges and to ask them about the air pump. It was working except that 115 PSI was the max. And, as the heat had increased the tire pressure on the driver’s side from 101 PSI to 111 PSI, I was trying to get the tire up to 120 PSI to match the level on the passenger’s side.

With a 115 PSI air pump, that was not going to happen in my lifetime.

They told us to go into the trucker area and use those air pumps.

We made our way over to the trucker area. We are basically the same size as a big diesel bus so we were not entirely out of place. Just mostly out of place. There were at least a dozen lanes and every lane was full. We queued behind one tractor trailer. He pulled out of the lane and stopped about 50 feet or so in front of the pumps.

We pulled in and got to work on the front tire.

Same exact experience as before. Could not move the tire pressure north of 115 PSI.

Time to leave. Except for one little problem, the tractor trailer still stopped about 50 feet or so in front of us. No way out.

I had to do something that I did not really want to do, namely, back the coach out of the pump lane. Lorraine stepped out to spot and we figured out a way to retreat without impacting a truck.

I had no idea as to how to exit the trucker area. It took us another 5-10 minutes of roaming around to finally break free of the Flying J trucker area. I am very sure that I entertained a few truckers as we drove in random patterns around the parking area looking for a way out.

Getting our own air compressor has suddenly jumped to the top of the must have list for our motorcoach.