Determining best tire pressure

[@Michael]

I also have a Transit 350HD dually (Coachmen Crossfit/Beyond) and here's what I came up with.

The OEM tire ratings are 107/105 at 70psi. The nameplate GAWR and tire pressures are 4130lb @67psi front, 6720lbs @57psi rear.

Actual measured weights when lightly loaded are 3200lbs Front, 5600lbs rear. Using those weights and 10% safety factor, I get 57 & 52psi:



The axle weights will obviously go up when I pack the van for longer trips, but I don't think I'll ever hit the GAWR for either axle.

@jadatis Thanks for all the work on this.

[parkgt]

Weighed my 2007 Great West Van (2006 Sprinter) which is a full RV. With water tank at 88%, propane and gas full and me on board along with most permanent trip items.

The total weight was 8240, split 3480 F and 4760 R. With wife, clothes, food and beverages we will be right at if not over the 8550 lbs GVW.

Running Michelin Defender 225/75R16 Es and have decided after looking at the chart to try 57 psi front and 75 psi rear. Probably won't go down much it any but may go up depending on how it drives and feels.

[booster]

Originally Posted by parkgt

Weighed my 2007 Great West Van (2006 Sprinter) which is a full RV. With water tank at 88%, propane and gas full and me on board along with most permanent trip items.

The total weight was 8240, split 3480 F and 4760 R. With wife, clothes, food and beverages we will be right at if not over the 8550 lbs GVW.

Running Michelin Defender 225/75R16 Es and have decided after looking at the chart to try 57 psi front and 75 psi rear. Probably won't go down much it any but may go up depending on how it drives and feels.

As was mentioned earlier, if you are referring to the Michelin chart, those are the minimums that will carry the weight and I assume a bit of safety factor. Most have found them too low for tall heavy vans for handling reasons. When going lower on pressure checking the tire temps is a good idea to make sure they don't go very high on your. A Harbor Freight cheapo infrared temp gun works fine for tires.


Also be sure to check them at the temp they will run at if possible, or correct for difference. I use 2 psi per 10*F for the rears and 1.5 psi per 10*F in the front. The old school rule of 1 psi for 10*F is for passenger car tires that run in the 35 psi range.

[parkgt]

Booster,

Thanks for the reply. Without weighing I had been running 60 F and 75 R. Not sure how to interpret your temp commentary?

[booster]

Originally Posted by parkgt

Booster,

Thanks for the reply. Without weighing I had been running 60 F and 75 R. Not sure how to interpret your temp commentary?

More normally we see 80 in the rear and a lot variation in the front, so my assumption was that you would seeing a reduction as the charts are usually lower than what the door sticker say.


Checking the tire temps is a very good way to tell if the tires are working to hard, particularly in a tall van where minimum pressures may not be enough to keep temps reasonable.



If you are referring to the tire pressure initial setting part of the temp, that would be to allow for ambient temp changes from the ambient when you fill the tires. For instance, we wound up with tire pressures that were too low because I filled them in the heated garage at 60*F and it was predicted to be that warm where we were headed. As it turned out, the forecast was bad and it was really 35*. We weren't into dangerous area but were 5 psi low in the rear and about 3.5 psi low in the front. I could feel a bit of handling difference. If I had known it was going to be that cool, I would have set the tires 5/2.5 psi higher so they would have been correct for where we were headed. The biggest risk is for those that have the rear of the van loaded to max tire capacity which requires max tire pressure setting. If you lose 5+ psi you are certainly underinflated. Even though it would be 5 psi over max at the start, you would be right as soon as you got to cooler area. If not on max it is not much of safety issue, IMO, but may affect driving feel. Many people don't worry about it until it gets to be a 50* temp swing continuously. Almost all of us have gone from 80* at the bottom of a mountain to 30* at the top going over and I don't know anyone who adjusts for that scenario. If I were going to stay on top for days, I would likely split the difference and correct for half of the temp change.

[parkgt]

Thanks, I was well aware of the pressure swing going into colder temps. I have also seen pressure in tires go up when climbing from 600' msl to 11,000' in spite of a temps getting colder.

Tires inflated correctly at high altitude can be low when back down.

[booster]

Originally Posted by parkgt

Thanks, I was well aware of the pressure swing going into colder temps. I have also seen pressure in tires go up when climbing from 600' msl to 11,000' in spite of a temps getting colder.

Tires inflated correctly at high altitude can be low when back down.

My understanding, and if we have any physics majors here they would know better, is that the tire internal pressure doesn't really change except for a tiny bit from casing growth when you go to higher altitude. You read a different pressure on the gauge because gauges measure pressure inside the tire minus the atmospheric pressure so you see the reading be high by the amount atmospheric pressure dropped, which I think is around 5 psi at 10-12K feet IIRC. That would mean the contact area of the tread would be same at higher altitude even if the pressure checked higher, which is counter intuitive for sure, as gravity doesn't change very much at these altitudes. That would also mean that if you reduced pressure to normal setting while at altitude the tire would likely run a bit hotter because of more sidewall flex.

[Yoshimura]

Originally Posted by booster

For instance, we wound up with tire pressures that were too low because I filled them in the heated garage at 60*F and it was predicted to be that warm where we were headed. As it turned out, the forecast was bad and it was really 35*. We weren't into dangerous area but were 5 psi low in the rear and about 3.5 psi low in the front. I could feel a bit of handling difference.

But I'm now wondering... If a tire starts to heat up because the pressure is too low, the pressure shouldn't start to increase because of the temperature increase?

[booster]

Originally Posted by Yoshimura

But I'm now wondering... If a tire starts to heat up because the pressure is too low, the pressure shouldn't start to increase because of the temperature increase?

Yes, absolutely the pressure would go up with the temp. I suppose it could even raise the pressure to near what they should have been when you started out cold. The problem is that the higher temperature in the tires is what causes failures much more than pressure does. From all I have heard, the higher the tire temperature, the more likely they are to fail.