Ideal tire inflation practices on our cars - Page 4 - PeachParts Mercedes-Benz Forum

zu! · #46 10-01-2015, 11:18 AM

Quote:

Originally Posted by babymog

Zu:

I like your postings, but your references in this posting are IMO flawed.

In the first example, it is easy enough to weigh the vehicle at a CAT scale (truck stop), get front and rear axle weights (with normal load, can do it with a trailer also if you tow often), divide each axle weight by 2 to get approximate load on each tire (should be fairly even side/side), then look at the tire manufacturer's pressure recommendation for that load. Always round up.
How many pounds each PSI will hold? That one is simple: Each square inch of a tire holds 32lbs at 32psi, ... as someone posted earlier it is the air holding the car up (vertical loading), not the tire, the sidewalls hold the air in and transfer lateral & longitudinal loads.

The second reference is even worse, no matter what your tire pressure you will generate heat driving the car, with his method you will continue to increase pressure looking for the equilibrium. I'm guessing that big-wig was an accountant.

Sorry, just my take.

Haha! No worries Jeff. That anecdote I posted was from a 4x4 forum and I think it becomes more critical when people are upsizing tires for their off-road rigs. It seemed like a good way to measure what your tire pressure is, when the tire no longer conforms to what the manufacturer ever envisioned for its vehicle (eg. 33 inchers on a vehicle designed for 15 inch rims).

As for the big-wig, I finally found his article (hiding in the deep recesses of my iPhone). Here it is, I think it makes sense.

I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.

The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).

The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.

If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.

To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressure in the tire. It should be higher than when cold but no more than 10% higher.

Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR.

In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.

On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.

So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.

babymog · #47 10-01-2015, 11:42 AM

That makes more sense zu!, except where the tire pressure will "not change when hot", the standard formula for calculating pressure-volume-temperature tells us that if the temperature of a gas increases, either the volume or the pressure will also increase. The tire does not get larger so your pressure will rise with temperature, period.

I've probably met this guy at a rally somewhere, ...

zu! · #48 10-01-2015, 12:05 PM

He probably means if the tire pressure does not change after being driven for awhile...which I would equate to "hot".

Sent from my iPhone using Tapatalk

leathermang · #49 10-01-2015, 12:11 PM

That goes along with what I was saying on the front end alignment ..
""
The tires are a certain size at a certain psi.... the temperature makes no difference..... he should have put the tire at whatever psi he wanted to set the front end at.... and set the alignment...... then told the owners they needed to set the tire psi the next morning when the tires were cold..""

If the tire is sitting still ....like for that alignment in question.... you can have cold air at X pressure .... or hot air at the same X pressure and both are the same for working with that tire at that time...
This is just a physics law..
and is different from the discussion relative to dynamic effects of one pressure...added to being driven... changing the pressure inside the tire due to increase of temperature...
That is why that old alignment guy was practicing voodoo... not on purpose... I am sure he believed in his method... but it just did not comply with physics...

Graham · #50 10-01-2015, 01:55 PM

Glad to see some real good tire knowledge posted. Thanks babymog.

I have had my 300D for over 25 years. One thing I have learned over years, is that the car feels like there is something wrong with the steering when pressures all round are set to say 30psig (local shops tend to set them like that). It always surprised me that the recommended front pressures were so low. But when I reset them to MB's spec, car steers fine again.

leathermang, the fuel tank door gives different pressures for front and back. And those are the pressure I use with my economy Kumho tires (probably not much different from 80's tires? (checked them - steel/polyester in tread and polyester sidewalls. Max pressure 44psig))

Not sure what temperature is cold (presumably ambient, 20deg C/68F?). The figures after slow or high speed driving are given on the flap. So if you set to spec cold, you can be as much as 8 psig higher after some high speed driving. I doubt many 240/300Ds can get to 100mph so the 100mph figures are likely irrelevant

From my flap, pressures could be set (front/back) to:

cold (ambient) 28/32
slow speed 32/36
hot 36/40

So it depends a lot on just what car has been doing before you check the pressure. Quite a wide range. maybe in northern winters we should drive the car a bit before checking?

Seems to me that one of main changes in tire technology has been to reduce side wall height. If we install larger diameter wheel, then the manufacturer's recommended pressures could be wrong?

babymog · #51 10-01-2015, 04:03 PM

Quote:

Originally Posted by Graham

<>Not sure what temperature is cold (presumably ambient, 20deg C/68F?).<>

Cold, for the purpose of setting tire pressure, is the temperature of the outside air. Temperature rise while driving will be roughly the same whether you're starting at 10F in the morning or 80F.

For those of us who's seasons can vary temperatures greatly (90F or more in the summer's hottest, -15F in the winter's coldest) we need to check and air up as the weather gets colder to maintain the proper tire pressure.

Mxfrank · #52 10-01-2015, 08:36 PM

What I learned from this thread:

1) In the 1980's, highways were much worse than the potholed crapfests they are today. They were more like pioneer wagon ruts.
2) In the 1980's, tires were made from tree sap and recycled polyester leisure suits. If nothing else, our taste has improved.
3) To increase pressure, release air. To decrease pressure, add air.
4) In the 1980's, tire technology was really primitive compared to today. If you were lucky, you had bias ply radials. More likely, your tires were octagonal, as the round standard took a long time to catch on.
5) Wide tires have better traction. I'm going to forgo snows this year, and get a set of ultra wide dubs. I may omit the valve cores completely to boost pressure, as I like a firm ride.
6) Manufacturer tire recommendations are the suggested starting point in a long life of experiment and adventure. As long as your antifreeze is yellow, you're good to go.

Graham · #53 10-01-2015, 09:05 PM

MxFrank - I think you got it

Re my statement: "maybe in northern winters we should drive the car a bit before checking?"

According to Tirerack (and babymog), I got that wrong.

Takes quite a bit of reading to get to the relevant part!

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73

What I got from that, was that when it is -20 outside, leave the 240D/300D outdoors so you can properly check tire pressure (it won't start anyway, so no problem if you get it wrong)

leathermang · #54 10-01-2015, 09:26 PM

-20 degrees.. just make sure you have the yellow antifreeze up to snuff....

mannys9130 · #55 10-02-2015, 12:40 AM

Quote:

Originally Posted by Mxfrank

What I learned from this thread:

1) In the 1980's, highways were much worse than the potholed crapfests they are today. They were more like pioneer wagon ruts.
2) In the 1980's, tires were made from tree sap and recycled polyester leisure suits. If nothing else, our taste has improved.
3) To increase pressure, release air. To decrease pressure, add air.
4) In the 1980's, tire technology was really primitive compared to today. If you were lucky, you had bias ply radials. More likely, your tires were octagonal, as the round standard took a long time to catch on.
5) Wide tires have better traction. I'm going to forgo snows this year, and get a set of ultra wide dubs. I may omit the valve cores completely to boost pressure, as I like a firm ride.
6) Manufacturer tire recommendations are the suggested starting point in a long life of experiment and adventure. As long as your antifreeze is yellow, you're good to go.

Right? Geez...

I guess tire pressure is like oil. One person out of 10 has a thorough understanding, 2 have a very good inderstanding, and the rest have a mix of good info and misinformation. :/ I don't consider that I have a thorough understanding, but some people just don't get it.

t walgamuth · #56 10-02-2015, 06:26 AM

Quote:

Originally Posted by mannys9130

Right? Geez...

I guess tire pressure is like oil. One person out of 10 has a thorough understanding, 2 have a very good inderstanding, and the rest have a mix of good info and misinformation. :/ I don't consider that I have a thorough understanding, but some people just don't get it.

Its pretty hard to go wrong following the mb recommendations....simple.

mannys9130 · #57 10-02-2015, 06:33 AM

Quote:

Originally Posted by t walgamuth

Its pretty hard to go wrong following the mb recommendations....simple.

They wouldn't have tested them and glued them to the car if they were not good pressures to use.

MB did extensive track testing with cars as far back as the W116 and similar. The 201 alone had an outrageous amount of R&D spending to test the design and prove that it was worthy. If MB didn't think the pressures were the best overall choice all things considered, they would not have chose them.

Skippy · #58 10-02-2015, 07:00 AM

Babymog, I finally got to read your post #44 in depth. That is an excellent and thorough explanation of the factors involved. I salute you.

Graham · #59 10-02-2015, 09:45 AM

Quote:

Originally Posted by mannys9130

They wouldn't have tested them and glued them to the car if they were not good pressures to use.

This is true and I agree with it.

One thing this discussion has brought out though, is that we need to know HOW to use those numbers and especially the effect of temperature.

Seems from the Tirerack link that even if we drive 20 min to fill up and check our tires at gas station, we should be using the posted cold numbers plus 4psi (32/36 for my 300D). At home cold, 28/32.

Temperature does make a big difference in tire pressure and affects what pressure we should set to, especially when day night temperatures vary significantly.

cooljjay · #60 10-02-2015, 01:12 PM

I just lost the tread on a tire yesterday, trying to pass a car at 80mph in the 78......Truth be told the tires were old and separating but this thread has made me blame this, all life and relationship events on my tire pressure....who would have known

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