Lazy Daze Owners' Group

Tire Pressure

March 21, 2022, 06:50:45 pm

I  travel with the fresh water tank almost empty. The contents of the coach is typically just cloths and food. The storage compartments contain more light stuff like tents and fishing poles. The heaviest items would be 3 bikes on the the hitch rack and 2 more bikes on the ladder rack. Also a family of 5 (or 6) in the coach.  I have been inflating rear tires at 80 PSI and 65 PSI on the front (cold). I have never bothered to adjust my tire pressure if the bikes are off and more or less people are in the coach. Wondering what others think.

Thank you,

Matt

Re: Tire Pressure

Reply #2 – March 22, 2022, 07:44:06 pm

65 psi front and 80 psi rear are the pressures I use consistently for our tires, which are of the correct size and load range as specified by Ford and LD. Those pressures correspond with the Gross Axle Weight Ratings of our motorhome, so there is no point to any more tire pressure, as the axles then become the limiting factor. So those are safe pressures provided we haven't loaded the motorhome so as to exceed the weight ratings of the axles. As Larry says, you must weigh the motorhome to determine if you're at or below those weights, as you should be. Total weight and the weight on each axle is sufficient. This is at maximum traveling weight, in our case full cargo, fuel, water and propane, empty gray and black tanks, my wife and I in the front seats.

There can be improved ride quality with lower pressures, but determining those pressures is more complicated. You need a chart for your tires showing the weight limit of the tire at various pressures. Again, weigh the motorhome in maximum traveling configuration. Divide each axle weight by two in front, four in the rear, to determine tire loading. Then go into your chart to determine the appropriate cold tire pressure for that loading. Reducing pressure will also compromise handling a bit.

In our case multiple weighings have shown that we travel close to the axle weight ratings. We wouldn't gain much comfort from the slight reduction in air pressure that might result from careful weighing and pressure calculation. And handling would be slightly compromised. 65/80 is easy to remember and sets the tire at the same safety point as the axles. I also prefer better handling to more comfort.

FWIW I try to check tire pressures on the morning of each traveling day. That assures they are well matched to the current climate and altitude. That also allows me to check for any discrepancies between tires (the rears should all read the same, as should the fronts). A discrepancy not accounted for by temperature (such as one tire in the sun, the other in shade) could indicate a leak in the tire that is low relative to the others.

Re: Tire Pressure

Reply #4 – March 23, 2022, 04:36:45 pm

Eric,

I was referring to what would happen if Matt reduced his pressures from those he uses now to something lower based on a lower load. So he's now using 65/80, the pressures for a LD at the GVWR. But if his actual weight is, say, 1000 lbs less, he could reduce tire pressures accordingly. In that case I think he would experience slightly compromised handling vs. what he experiences now as the lower tire pressure would mean less "stiff" tires that are less responsive to, for example, steering inputs.

On road performance tires are generally stiffer than, say, "touring" tires. This helps steering and other inputs have a more immediate and direct effect on the vehicle's direction, etc. A less stiff tire will respond to control inputs by itself first deflecting a bit before the inputs have an effect. Obviously you can overdo this, but in this case we're talking about just a few PSI one way or the other.

There is always a tradeoff in tire construction and inflation between comfort on the one hand and performance on the other. I personally prefer to have the motorhome be a bit more responsive (it's mushy enough already) vs. a bit more comfortable, so I'm happy with tires a few pounds over the recommended setting. Just my preference.

One thing you don't want to do is have your tires under inflated for your vehicle weight. That's another reason to set them a few pounds high, provided you don't go over the maximum tire inflation pressure (for most of our tires 80psi).

Here is a table from Tire Rack:

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

In my experience the primary "feature" of our motorhome's handling is understeer. Adding some pressure up front counteracts that a bit. Yes, reducing in the rear might help too but I think probably not so much with the dually set-up plus I don't want to risk under-inflation back there. I notice front tire pressure changes a lot more than those in the rear and find a few extra pounds there improves things.

Re: Tire Pressure

Reply #6 – March 24, 2022, 10:46:05 am

In the end I think this gets down to cases. The Interstate is one thing, a mountain road another. On the former you might seek out stability and comfort, on the latter more responsiveness. Less tire pressure means a more flexible tire and reduces the communication to the road of whatever forces are being applied, so on the Interstate a side wind force will deflect the tire a bit before deflecting the path down the road.

More pressure makes the communication more direct. So on a mountain road your steering input will have a more immediate effect on the path of the vehicle and result in less understeer (the tendency of the front end to push or plow ahead instead of making a crisp turn-in).

Everything has its limits though. I remember picking up our LD at the factory when new. In those days I think LD set all the tires to 80psi, the maximum on the tire label, out an abundance of caution I assume, and of course the motorhome was MOL empty. We drove the freeway to our first campsite. Harsh ride, squirrelly handling as Greg pointed out. The label on the driver's door says 65/80psi front/rear, so that was my first adjustment the next morning. Much better.

Further adjustment to reflect actual weight had to await weighing and made things even better. But then over time our traveling weight increased to near the maximum so going back to 65/80 made sense. We tend to travel back roads with more turns, so that plus a safety factor makes me prefer a few extra psi over what we could probably use according to actual weight and the charts.

As Joan always says, YMMV. For some, good handling may mean steady as she goes on the Interstate, for others it may mean more responsiveness on a mountain road. Off roaders and road racers are constantly fiddling with tire pressure to get the performance they want.

I'm sorry to have dragged the initial question off topic. I'd say the bottom line is use 65/80 psi if you don't want to bother with weighing and charts and are sure your gross traveling weight is below the GVWR and axle ratings (i.e. a lightly loaded LD). Otherwise, weigh your rig, get out the charts for your tires, set the pressures accordingly and check them regularly. And always avoid under-inflation, the source of many tire mishaps.

Re: Tire Pressure

Reply #8 – March 25, 2022, 11:57:59 am

Wow! That was a lot of good discussion!! Heading to Yosemite Sunday. So going from sea level at 80 deg.  to 6000 ft at 40 deg. I'm going to continue to travel with the tanks near empty as water is  available at the campground and continue to inflate the tires COLD at 65 and 80 PSI.

Much thanks to all for that information. Really helpful.

Matt