Determining best tire pressure

[booster]

Originally Posted by Phoebe3

Please see my comments after each point:

In your screenshot there are two big ovals where you have your front axle weights entered the same as you did up higher right at the beginning.


Where you have those weights in the far right upper corner of the small box inside the oval should be the tire load range per tire capacity, which for your 107 load range would be 2150#. You can either enter it directly of just click on what is there, in your case the 4300# in the left oval, and it will put a pull down arrow next to number. Click the arrow and chose the proper load range for your tires, which for you would be 107 per your sticker. Do the same on the right side oval for the rear tires.


I think you will then get the correct numbers, I did for ours.

[Phoebe3]

OK. here's what I have now.

I got the 70 lbs max pressure from the tire itself.

Changed the numbers in the oval to match the Load Index from the tire and deleted axle override.

Still find it a little disconcerting to run 51 lbs in a tire rated for 70 lbs since blowouts are a concern at low pressures. After the Firestone incident, my spouse spent a lot of time trying unsuccessfully to get tire manufacturers to give us a definitive answer on the correct tire pressure given ambient temperatures.

Really like the idea of being able to use an Excel sheet to know the appropriate pressure.

Do I have this correct now?

[booster]

Originally Posted by Phoebe3

OK. here's what I have now.

I got the 70 lbs max pressure from the tire itself.

Changed the numbers in the oval to match the Load Index from the tire and deleted axle override.

Still find it a little disconcerting to run 51 lbs in a tire rated for 70 lbs since blowouts are a concern at low pressures. After the Firestone incident, my spouse spent a lot of time trying unsuccessfully to get tire manufacturers to give us a definitive answer on the correct tire pressure given ambient temperatures.

Really like the idea of being able to use an Excel sheet to know the appropriate pressure.

Do I have this correct now?

I understand, and personally I would never run right at the minimum pressure for the load if I was not at max on the tires.


One of the big issues I have with the minimum pressure charts and calculators is that they are only for whatever numbers you put in for axle or tire weights. Too often, I fear, people will go to the scales and get the axles weighed when they are not loaded to maximum weight they would ever be. In a B van that would be all water, waste, and gas tanks full, as well as propane if you have it. It should also include the max amount of people loads and in the place they will be riding, as well as pets. Full supplies, heaviest clothes, you name it. All of this can make for a surprising actual driving weight than was measured and overload for the pressure set if it is at minimum. It is also a good idea to allow for side to side weight differences because the tires are rated individually. Our Chevy is a couple hundred pounds heavier on the left rear, for instance, than on the right rear and sometimes more is we have a lot of supplies with us.


This is likely why your rating sticker gives higher pressures, which are likely set for the max load on the front for the tires and probably also on the rear as there are 4 tires so need less pressure per pound. Have you run the calculator with the max rated axle weights used to see if the pressures come out at what you have on the sticker? I think they should be pretty close to each other.


I think the 67 or even 70 in the front is probably very appropriate for the 70 psi rated tires as they are at or near max load. The extra pressure should also improve steering response a bit which can help with duallies which have a lot of built in understeer in them and that slows down the response. Based on what they called out in the front compared to max load, the rears are probably done similarly and be pretty close, but a few more psi for assurance won't hurt unless they start to get to harsh riding for your comfort and sanity.


It will be interesting to see over time how other Transit owners like the tire pressures from a driveability stand point, as there will almost certainly be quite a few that will play with the pressures to see what combo they like best. If it follows like we have seen on the Dodges, Fords, and Chevies (rear drive models of all) in the past most of the people will prefer pressures on the higher end of scale which also gives more margin for slow leaks or inflation errors. It would also allow wider temp swings without pressure adjustment needed.

[jadatis]

Did a long story but dissapeared when submitting. So will now only give the picture I made of filled in spreadsheet for Phobe3.

In short , used AT 69 psi because 67 psi advice on plate proves these tires to be exeption to the rule of 65 psi for 8PR/D-load. And that my calculation is not exact what advice on plate gives, only proves that mine is saver.

If driving max 130kmph/81mph ( and wont go over for even a minute) .
Pressure can dropp to F64/R61 , before it comes in the danger-sone( overheating ) , but still the reserves in weight.

Forgot to remove the 10% reserve rear , removed it after the picture, and then spreadsheet determines 16% reserve for possible and most likely overloading.
If you want to fill in part 2 the real weights, then you have to weigh the motorhome fully loaded ( so also persons) , second best per axle, part 3 is for best weighing per axle-end.
Then the pressure advice is given for those weights and 10% preset reserve.
Above advice is given for wich it is given.
But only do this after weighing, estimating it yourselfes is dangerous, if yudged to low, still tiredamage possible.

And look at the tires sidewall of your motorhome, if you still have the same specifications. What is now under the motorhome is what counts for pressure advice. If that is different , give new data and I will recalculate, but now you seen the example, you can do it yourselfes.

[jadatis]

Originally Posted by Phoebe3

OK. here's what I have now.

I got the 70 lbs max pressure from the tire itself.

Changed the numbers in the oval to match the Load Index from the tire and deleted axle override.

Still find it a little disconcerting to run 51 lbs in a tire rated for 70 lbs since blowouts are a concern at low pressures. After the Firestone incident, my spouse spent a lot of time trying unsuccessfully to get tire manufacturers to give us a definitive answer on the correct tire pressure given ambient temperatures.

Really like the idea of being able to use an Excel sheet to know the appropriate pressure.

Do I have this correct now?

Pretty right now, but some remarks.
You used the 107 also for rear, but dualload needs the second number 105.

Then the 70 psi you read from sidewall?
Is also a value they converse from 475 or 480 kPa, to be on the safe side, and not 69 psi as I used.
But if read from sidewall, was the rest also the same as on VIN-plate from your picture given in earlyer post? So 195/75R16 107/105R .
And what brand of tires used?

How did you determine the given weights in part 2?
Again estimating is dangerous.

The 10% reserve you automatically get when you leave the dark blue cell for it empty, but not wrong as you did.

[Roadtrek Adventuous RS1]

Originally Posted by jadatis

Pretty right now, but some remarks.
You used the 107 also for rear, but dualload needs the second number 105.

Then the 70 psi you read from sidewall?
Is also a value they converse from 475 or 480 kPa, to be on the safe side, and not 69 psi as I used.
But if read from sidewall, was the rest also the same as on VIN-plate from your picture given in earlyer post? So 195/75R16 107/105R .
And what brand of tires used?

How did you determine the given weights in part 2?
Again estimating is dangerous.

The 10% reserve you automatically get when you leave the dark blue cell for it empty, but not wrong as you did.

Please let me know....mine say 80 pounds... cold tire inflation....I guess everything is different?

Phoebe, what does your owners manual say for the correct information for the tires?

[@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.

[parkgt]

I have seen the pressure/volume change many times (25+ years of riding in Colorado) on ATV tires taken from 600 feet to 11,000 feet. When I say "seen it" I mean it could be seen and felt when you are used to the ride.

I have seen those same tires that were adjusted by letting pressure out at high altitude be almost flat when coming down from elevation. Had to reinflate every year.

While this may not be as significant with high pressure truck tires it is not just a a gauge issue. All others in the group noticed the same thing every year. So the differential pressure change is real but may not be a factor for my Sprinter tires.

[booster]

Originally Posted by parkgt

I have seen the pressure/volume change many times (25+ years of riding in Colorado) on ATV tires taken from 600 feet to 11,000 feet. When I say "seen it" I mean it could be seen and felt when you are used to the ride.

I have seen those same tires that were adjusted by letting pressure out at high altitude be almost flat when coming down from elevation. Had to reinflate every year.

While this may not be as significant with high pressure truck tires it is not just a a gauge issue. All others in the group noticed the same thing every year. So the differential pressure change is real but may not be a factor for my Sprinter tires.

That makes perfect sense because the pressure gauge reading is off by the same amount in psi no matter what the tire pressure is. It is not a percent change like it is with temperature at different pressures. If you are running your ATV tires at 10 psi and the gauge reads 5 psi high, you would let out 1/2 your pressure at altitude. If you were running only 8 psi you would let most of the air out. You would be very low at half pressure or less when you come back down. On a tire with 75 psi in it you would only be over by 1/15 of the pressure so barely noticeable when you come down compared to losing 1/2 or more. If you don't let air out when you are at altitude, they will come back right where they were.


If you had a gauge that read out in absolute pressure (referenced against a spring or other load instead of ambient atmosphere) the gauge would read the same at both high and low elevation, minus a touch for casing stretch up high.


Zero absolute pressure is essentially a total vacuum no matter what the atmospheric pressure is and is the base point for all pressure readings used in most calculating formulae because it is always the same point.

[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?

[avanti]

I've sort of been only half-following this thread, so forgive me if I have missed this:

There are two kinds of pressure: So called "gauge pressure" and "absolute pressure". The former is referenced to ambient, and the latter to vacuum. I'm sure that typical tire pressure gauges are measuring the former. But, I wonder if this is always true. For example, a TPMS whose sensor is installed inside the tire may well be measuring absolute pressure, since gauge pressure sensors have to have a vent, which would be difficult in that case. Such a system would behave differently with changes of elevation or barometric pressure.

Anybody know how the tire world treats this distinction?

[booster]

Originally Posted by avanti

I've sort of been only half-following this thread, so forgive me if I have missed this:

There are two kinds of pressure: So called "gauge pressure" and "absolute pressure". The former is referenced to ambient, and the latter to vacuum. I'm sure that typical tire pressure gauges are measuring the former. But, I wonder if this is always true. For example, a TPMS whose sensor is installed inside the tire may well be measuring absolute pressure, since gauge pressure sensors have to have a vent, which would be difficult in that case. Such a system would behave differently with changes of elevation or barometric pressure.

Anybody know how the tire world treats this distinction?

Interesting point and expands the question related to above about using absolute pressure gauges and if you would see a change at altitude, and mentioned you shouldn't see a change.


I did a bit of digging and it appears to be pretty much what would be expected. They measure absolute pressure and apply a fixed atmospheric modifier to give a reading that mimics what people are used to see on an atmospheric referenced gauge. From a tech presentation I found.



"Pressure Accuracy
• Effects of absolute vs. gauge pressure at altitude
− In tire sensors measure absolute pressure
− Typical tire gages measure differential (gauge) pressure
relative to the atmosphere
• CIP is defined as the pressure of the tires after
the has been stopped for at least 1 hour
• The “corrected” pressure using the Ideal Gas Law
is not used
TM
11
− It is not the mass of air present setting tire performance
− It is pressure of the air that defines the load carrying capability and
performance of the tire
• Generally accepted to use absolute pressure with a fixed
atmospheric offset (approx 100 kPa = 14.5 psi)"


This should indicate they will read the same at all altitudes unless there is a lot of casing stretch with the tires. I can't speak for how stretchy the ATV tires are that were the basis of the altitude question.


The quote from the article also seems to back up the point that load carrying capacity of the tire should be based on the absolute pressure so if you reduce you pressure at altitude to read right on an atmospheric referenced gauge, you would also reduce you load carrying capacity.


Very interesting, handn't even thought about it before.

[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.