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Old 02-24-2021, 12:11 PM   #1
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2021 Interstate 19
Hattiesburg , Mississippi
Join Date: Dec 2020
Posts: 65
2021 Interstate Nineteen and Michelin Agilis Tire Pressure

I have the 2021 AI Nineteen on the 2020 Mercedes Sprinter with OEM Michelin Agilis Cross Climate tires LT245/75R16 120/116R E. It also has the Mercedes OEM tire and temperature monitors which I have determined to be extremely accurate and precisely matched to one another in calibration.

Two weeks ago, before the arctic vortex visited its wrath, I inflated my tires at 50 degrees F using the accurate and precise digital Milton Digital Tire Pressure Gauge S-586e to front tires 60.00 psi and rear tires 75.0 psi. I know fom experience that my tires don’t leak down any over a two week period.

Yesterday the early morning ambient temperature was 34 degrees F. My gross vehicle weight calculated to 8,300 lbs at most. I left on a 300 mile trip at 11:30 AM with an ambient temperature of 68 degrees F and a cloudless sky. After five minutes I checked that the front and rear tire pressures were exactly the same side to side. Two hours later, driving on a smooth black asphalt state 4-lane highway at a steady 70 mph with an ambient temperature of 72 degrees with the sun head on, both front tires measured 65 with a temperature of 91 degrees F. Both rear tires measured 80 psi with a temperature of 97 degrees F.

Theses readings are consistent with several other runs that I have made. I am wondering if the Agilis tires tend to run warmer? From what I read these results seem beyond expected.
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Old 02-25-2021, 09:27 AM   #2
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I think your TMPS gives in steps of 5 psi.
When I put in my made spreadsheet next.
60 psi at 50 degrF gives 66 psi at 91 degrF, you read 65
75psi at 50 degrF gives 83.3psi at 97 degrF,you read 80 psi. To 80 psi belongs then a temperature of 78 degr F.

Can also because of little inacuracy, that psi dont mach exactly with temperature, and it all is within acceptable limits.

But average is when ambiënt temp is 65 degrF, and driving 50mph, the temp in tire rises to about 110/115 degr F. So even your 97 degr indicates, that you can do with lower cold pressure, calculated back to 65 degrF, for wich advicepressure is calculated.

I know, a dangerous statement I make.
So first give me the needed data to calculate advice-pressure for you.
Tires data you already gave, but need axleweights, so how is the estimated 8300 lbs devided.
Also estimating weight is a dangerous thing.
Your 120 loadindex can provide a 6000 lbs axleweigt at 80 psi, and I think your GAWR rear is no more then 4400 lbs.

I assume you have internal sensors for your TMPS. External are unreliable in the temperature, because give mayby exact the temperature in between in- and out-side tire air( or whatever gascompound)
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Old 02-25-2021, 09:46 AM   #3
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Did some prelimanary calculation with my made motorhome-tirepressure calculator.

Front GAWR of 4070 lbs with 5% reserve added, gives 54 psi.
Rear 4400 lbs with 17% reserve added, gives 66 psi .
17% because rear in motorhome-config often overloaded. Front stays under gawr mostly.
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Old 02-25-2021, 04:42 PM   #4
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2021 Interstate 19
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Thanks for the analyses.

The Mercedes OEM internal TPS seems to be very accurate and consistent for pressure and temperature in 1 lb and 1 degree steps.

My chassis is a 2020 Mercedes Sprinter 19’ 4x4:

GVWR 9050 lbs
UBW 7705 lbs
NCC 1345 lbs
People 385 lbs
Stuff 50 lbs
Diesel 175 lbs (25 gal)
Black Empty
Gray Emty
Fresh Empty

That is why I say no more than 8300 lbs total.

I did make another 300 mile trip today. I inflated the tires before I left to 50 front and 70 rear (door sticker says 47 front and 70 rear).

My interest is that the rear tires cosistently run 7 degrees higher than the front tires no matter whether the inflation difference front-rear is 15 psi, 20 psi or 23 psi. It seems to me that the running temperature is mostly a function of the weight carried and not so much the inflation pressure - as long as the inflation pressure is close to the recommended pressure.
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Old 02-26-2021, 04:17 AM   #5
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In Europe ( I live in the Netherlands) many motorhomes of this weight, and often to much weight on the rear axle, so the 7 degr difference can also simply be because of that.

Besides tirepressure, I also an interested in weight and weight-division.
Made spreadsheet to estimate the axle-weights if only total weight is known.
For that I have to know total length, and wheelbase, and total weight. Nice would be distance front to front-axle but standard set on 90cm= 3 foot. And is it a campervan wit iron all around.
So if you give that all, I will use my spreadsheet.
Basic idea is that average motorhome has its total-gravity- point in use between 42% and 45% of totallength behind the front. Campervan in Dutch version I also added part for that, and gives rougly between 46 and 48 %

Then I also made spreadsheet to estimate the weightdivision over 4 tires, bij measuring the deflection of the tires.
If you use right pressure, all the tires have the same deflection. Not totally, because right/ left on the axle can be different.
Will search the English version and topic about it, but same deflection is easy to measure for you.
Measure from upper rimm-edge to the hard flat underground.
Deflection is difference between that and measured from bottom rimmedge to top of tire. Then between 15% and 20% of free flexible part of sidewall is the goal.

But can you also give GAWR's
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Old 02-26-2021, 08:52 AM   #6
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Thanks jadatis,

Sidewall deflection is what I am trying to nail down here, I just did not know it. I am trying to understand what I will have to do to get all tires to heat up evenly which would seem to be desirable to me; however, this may be impossible to attain if heat is more a function of load carried than of proper inflation pressures.

I am going to make more runs to gather data and try to get accurate axle weights.

Thanks very much for your interest.

I understand I am reinventing the wheel. I just want to do it for my particular wheels.
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Old 02-26-2021, 10:29 AM   #7
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Mij idea is that when the same deflection, also the same heatproduction at the same speed
All 4 tires make the same speed.
Cooling down of tires rubber is only dependent on temperaturedifferences between rubber and in and outside tire air.
Outside tire air does not change, but inside tire gets hotter.....héé, now writing this, I realise that inside tire is allowed a bit hotter, because cooling down of rubber to inside tire, only makes smaller part of the total cooling down.

Have to overthink this , takes a few nights of sleep.
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Old 02-26-2021, 10:47 AM   #8
1986 32' Excella
KNOXVILLE , Tennessee
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Rear temperature will rise more than front. Rear are drive tires, front freewheels. Engine and exhaust temps play a huge roll in tire temp. Typically, the side with exhaust is higher, relative to opposite side. Just my .02.
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Old 02-26-2021, 10:49 AM   #9
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The famous load/inflation chart for trailer tires is essentially a load/deflection curve and probably applies to LT tires also. Pretty much pressure dictates deflection. So if you check that chart you should be able to work out pressures that give you the same deflection at the given loads.
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Old 02-26-2021, 05:22 PM   #10
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2021 Interstate Nineteen Axle Weights

I weighed the AI 19 4x4 van today at a state-certified weigh station with no people, a full tank of diesel, a full tank of DEF, 6 gallons of antifreeze in the tanks, 60 lbs of items under the rear seat and twelve 12 oz plastic bottles of water in the pull-out pantry at midships.

Steer Axle: 3740 lbs

Drive Axle: 4220 lbs

Total: 7960 lbs

Today prior to this, early in the morning, when the van had not been in sunlight or cloudlight, all four tire sidewalls measured 74.5 degrees using a fluke infrared thermometer gun. At this time I precisely inflated the tires to the recommended 47 psi front and 70 psi rear. I then drove to scales a 15 min drive at up to 60 mph. The tires all uniformly gained temperature to 97 F as measured by the internal TPS sensors.

I have attached the pressure chart for the Agilis LT tires.

It looks as though I could get by running 47 psi front and 55 psi rear at this measured weight.
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Old 02-27-2021, 12:49 AM   #11
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For those axleweights with 10% for reserve added, my spreadsheet gives front 52 psi and rear 59 psi.
You cleverly put some extra load in it to compensate for the missing persons, but still in real use possible a bit higher axleweights.
Mij calculation for the weights given , gives highest reserve with still acceptable comfort and gripp.

Realise that even with accurate pressure device
You calc for max reserve, but by all the inacuracy's and unequall load R/L, you can end up with yust enaugh to prevent overheating at your used maximum speed.

Did you give the 91 and 97 degr first , also measured with that gun( measures treath), or what tmps gave( measures air in tire.
Those are different tem
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Old 02-27-2021, 01:46 AM   #12
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I am throwing out all measurements prior to yesterday’s run to the weigh station. I am going to make another run today using your calculated inflation pressures of 52 psi front and 59 psi rear. I plan to collect both pressure and temperature data.
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Old 02-27-2021, 12:33 PM   #13
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Another Trip To The Scales

I reweighed the van and made another run today.

Weigh station:

For the weight today everything was the same as yesterday except that my wife and I were sitting in passenger and driver seat respectively. I won’t say who weighed what.

Steering axle: 3880 lbs

Drive axle: 4300 lbs

Total: 8180 lbs

(For typical use I would have to add about 400 lbs which would include about 350 lbs for full fresh, gray and black tanks ( a rare occurrence).

GVWR: 9050 lbs

(So for our typical usage we would be running about 500 lbs under the 9050 lbs GVWR limit.

Yesterday’s total weight figure of 7960 lbs without my wife and I in the van should be my van’s UBW (actually 7800 lbs after subtracting winterization antifreeze, water and stuff)

Airstream’s stated UBW: 7826 lbs

Mercedes stated maximum allowed UBW: 7400 lbs ( Must be for 2x2 van?)

UBW is supposed to include all chassis fluids including a full tank of fuel. I would say my measured UBW is 7800 lbs (7960 minus 160 lbs of fluid and stuff).

Airstream stated NCC: 1224 lbs (Net Carrying Capacity)

NCC is GVWR minus UBW. The Airstream Stated NCC is very accurate.

Today’s Run:

Very early this morning my Fluke infrared thermometer read all tires at 64.5 F. I then inflated the front tires to 52 psi and the rear tires to 59 psi as suggested by jadatis.

I left the house very slowly and after less than a minute the Mercedes internal TPS was reading the exact same pressures and temperatures: 52 front, 59 rear and 64.5 F front and rear with readings even side to side.

After driving for 15 minutes at up to 60 mph:

Front tires: 55 psi and 86 F
Rear Tires: 65 psi at 86 F

The rear tire pressure increase started outpacing the front increase as soon as I applied significant throttle.

So the pressure increased 3 psi front and 6 psi rear while keeping the temp increase in all four wheels the same at 86 F.

I have not tried to measure sidewall deflection as all my surfaces are uneven aggregate concrete.

Some questions for myself:

So would this mean sidewall deflection is equal in all wheels? Why is the even increase in heat across all wheels not commensurate with the differential pressure increase? I would guess the TPMS sensor is reading air temperature inside the tire?
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Old 02-28-2021, 12:55 AM   #14
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For those new axleweights I calculated F54psi and R60 psi.
But the 51/59 can do, because calc is with 10% reserve, and for 99mph , while your max speed was 60mph, in general use max 80mph.
This also gives some extra reserve.

F51psi at 64.5degrF gives at 54psi, 88degrF.
So the 86degr is reasonably accurate.

But R 59 at 64.5degrF gives at 65psi, 107 degrF.
And that is to much difference with the 86 degr You read.

There can be a little flaw in my temp calculation, because I assume no vollume-change bij the pressure change, but this can not give the 107-88= 19 degr difference. So it must be in your tmps system.

Mayby the sensors are accurate enaugh, but the little computers software in tge front, can have a liw refreshrate, or dies not want to worry you with temperature-swings.
That is what BMW does with watertemp , within limits gives always 90degrC on clock.

Explanation for the 107 degrR can be external heatproduction, by the drivetrain.
Its a 4x4 so 4wheel drive, but rear is often different drive-train in a stiff axle, then oil can heat up rimms and those the air inside tire.

You can also measure the rimms temp with your gun,( or compare by hand).

Last I want to worn you, dont let looking at and handeling the tmps, keep you from watching the road.

Once we determined the best presdures for your use, I can make 2 pressure/ temperature lists for them. Also made spreadsheet for that.

Some people have nitting as hobby, this calculating is mine.

Greatings from the Netherlands
( ja dat is ) Peter ( from an old Dutch namesong, translates as " yeah thats Peter" written in my year of birth 1959)
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Old 02-28-2021, 04:10 AM   #15
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Hello, Peter. I appreciate your efforts for me. I like to do the the testing part of this. You are not an amateur radio operator too are you? My license is WA5KPE/N5MED. Got my license age 13 in 1963. Not currently active. I am an anesthesiologist so my life revolves around increments and precision.

I will make another run using your new pressures of 54 and 60. The 2nd run was under overcast skies whereas the first was sunny. I try to control everything else. I will also additionally get infrared readings at the end from now on.

My first infrared readings are always early AM under a shed before any exposure to the sky. I have noticed that a tire exposed to an overcast sky for just a few minutes, with everything else being equal, will increase in temperature a degree or two.

I am also going to try different scale sets. There are several in the area.

I guess I will have to make my motto: “reinventing the wheel”.
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Old 03-01-2021, 05:39 AM   #16
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Just an FYI:

Using a IR thermometer is good (as in more data), but be aware of its limitations.

First, it is measuring the outside of things. With tires, the heat is generated internally and not only flows towards the outside, but also flows inwards towards the air chamber. What you are measuring on the outside is affected by the ambient temperature. Yes, so is the air chamber, but the air chamber is more protected and therefore more stable and more reliable.

An IR thermometer will measure the outside of a tire lower than what the internal temperature of the tire is.

There is a rule of thumb that says that a tire shouldn't experience any more than a 10% pressure buildup. This is not a target - it is a maximum.

Why? Because pressure buildup is a measure of the heat being generated. If you apply the Ideal Gas Law, a 10% pressure build up is about a 40°F temperature increase. The Arrhenius equation says that chemical reactions double for every 10°C (Yes, Centigrade! About 18°F). A tire ages faster the higher the pressure buildup! And that's over 5 times faster with a 10% pressure build up!

Want to learn about tires? Please visit my website: Barry's Tire Tech New content every month!
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Old 03-01-2021, 09:25 AM   #17
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I agree with Busted......very common....I also think OP’s psi is quite normal.....10 months/year I’m in 70+ temps......always start with 47/70 “cold” (in shade) ‼️
‘20 IS 19’ TB, 2wd
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Old 03-01-2021, 08:04 PM   #18
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2nd Run

Hi, CapriRacer. Thank you very much for your comment. I have looked thru your website in the past and enjoyed it very much. Part of the reason for this thread actually.

I made a 2nd run today 3/1 (the 1st being 2/28).

Evening 2/28 ai 2100 hrs inflated tires Front 54 and rear 60 sidewalls all 65.5

Morning 3/1 measured Front 54 and Rear 60 sidewalls all 65.5 F

Van readings 54 Front and 60 Rear and ambient 66 F (No photo) less than a minute after leaving house.

Photo 1 shows driving 45 min at 65 mph in overcast at 1032 hrs

Photo 2 shows driving 45 min at 65 mph in overcast cold front at 1651 hrs cool wind from right.

Photo 3 shows weight certificate 8400 lbs at 0941 hrs

Photo 4 shows compressor and Milton digital gauge

Photo 5 shows the van in its house. I have to be careful of sunlight on tires.

1. I have found that the Mercedes ambient temp sensor and the Mercedes internal TPMS sensors are very accurate.

2. The front right consistently runs cooler than the front left but at the same pressure as the front left. The effect of sun, cool wind etc are superimposed on this.

3. The rears lose pressure and temperature on long downhills and immediately regain pressure and temperature as soon as the throttle is applied going up the next long rise.

4. An almost immediate rise in pressure and temperature on the sunny side if the sun appears briefly on an overcast day.

5. I think I was able to observe that the side toward the cool north wind ran a little cooler overall.

6. The 8400 lbs includes two people, full fuel tank and full DEF tank, 6 gallons of antifreeze and about 150 lbs of gear. In planning for a trip I would have to add 350 lbs for full tanks, making 8750 lbs. With a GVRW of 9050 lbs that leaves an additional carrying capacity of only 300 lbs.

FWIW: These are runs I have to make to the Mississippi Coast for other unrelated reasons; so this is not a totally senseless waste of good fuel.
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Old 03-01-2021, 08:57 PM   #19
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Peter, after looking at these figures, can you tell me what you think I should run for 8400 lbs and for 9050 lbs? The last readings were ambient temperature of 64 F which is essentially the temperature at inflation.

Also I was not dealing with sun and scorching hot asphalt in these runs.
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Old 03-02-2021, 04:31 AM   #20
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For the curious

Since I can’’t delete them, please ignore all data points (my measurements) prior to 2/28.

For the curious, the van driver display photos are made by my wife placing the iPhone with a selfie stick resting on the steering column, and I then tap the onscreen shutter release.

For the center display I use a tripod-mounted DJI Osmo with bluetooth remote shutter release. The Ozmo is the most fun that can be had for about $120.
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