# Tire pressure PSA, something I just learned

Discussion in 'Equipment' started by AZQKR, Aug 30, 2018.

1. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
There is no such thing as the "right number of molecules" in the tire. All that matters is the pressure difference between the inside and outside of the tire, which is what a simple tire pressure gauge measures. The pressure difference determines the contact patch area, which is what determines the performance of the tire. The physics of this is fairly simple. The only reason I can think of to adjust the pressure to a reference of 68°F is so that the TPMS won't give a false indication of too high or too low pressure.
smbjm and RoundTrip like this.
2. ### BungholioLong timer

Joined:
Oct 9, 2008
Oddometer:
2,566
Location:
Aberdeen, WA
Not if it's the difference of 1-3 psi for the average joe on just about any bike. GP racer yes, a couple psi is important, to maintain consistent tire profile while at 10/10ths at 150 mph.
3. ### AZQKRLong timer

Joined:
May 8, 2018
Oddometer:
1,185
Location:
arizona
Explain why bmw would then set their pressure recommendations at 68 degrees if it didn't matter what temperature it was outside the tire?
There's a reason bmw recommends their tire pressures based on 68 degrees on an unridden cold tire. I don't know the reason, nor do I care why the engineers recommendations are set at 68 degrees. All I do know is that if I set the tires at 68 degrees to 38/42 as recommended, when it's considerably colder than that later, the tires will be under inflated and when it's considerably hotter than that later, the tires will have gained pressure.

As I'll never set tires cold at 68 degrees unless I'm very lucky on just the right day, I'm going to adjust the pressures at 1# per 10 degrees up and down. When I set the tires 3#'s over at 98 degrees all summer, they are properly set for the recommended tire pressures at 68 degrees.

At least two other people here "get it" and understand why the formula is used based on replies. Simply because no one else gets it doesn't suggest it's incorrect, just that people haven't any concern about 3# underinflated or over inflated. 3#'s underinflated contributes to cupping, I don't buy cheap tires, nor do I want to waste money simply believing 3#' makes no difference. If 3#'s didn't make a difference, bmw would simply recommend a range of pressures that were acceptable, and they nor any other maker does that that I'm away of. Tires pressures are written in exact numbers, not a range of numbers suggesting 3#'s variance DOES make a difference to those who know more than I do about proper tire inflations.
4. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
Maybe an illustration about altitude will help.

Have you ever traveled to high altitude with a sealed bag of chips purchased at low altitude? The bag was loose at low altitude, but at high altitude it is very tight. This is due to the pressure difference between inside the bag and outside ambient pressure. At sea level the absolute pressure is 14.7 psi. At 10,000 feet the absolute pressure is 10.1 psi. If the bag was sealed at sea level, it will be 14.7 psi both inside and outside with a 0 psi difference. At 10,000 feet there will be 14.7 - 10.1 = 4.6 psi difference (less actually because the bag was probably not fully inflated, but let's ignore that for this discussion). This 4.6 psi is what causes the bag to be tight. The bag has the same number of air molecules in it, but the pressure difference between the inside and outside of the bag changed.

Tires basically work the same except they are stiffer than a bag of chips. If you set your tire pressure at sea level and then travel to 10,000 feet, the tire pressure will measure 4.6 psi higher than at sea level when measured with a simple tire gauge and at the same ambient temperature. Conversely if you set your tire pressure at 10,000 ft and travel to sea level, the tire pressure will be 4.6 psi lower and your tire will be noticeably flatter. You wouldn't want to drive around on this tire for long--air it back up!

The affects of temperature on tire pressure are similar. The tire's absolute internal pressure is proportional to the absolute temperature of the tire even though the number of air molecules is the same (ignoring leakage). In physics, this is called the Ideal Gas Law: PV = nRT. Assume the tire's volume V is essentially unchanged. n and R are constants. So that means P is directly proportional to T, where P is the absolute pressure and T is the absolute temperature. A TPMS has devices that measure the absolute temperature and pressure inside the tire. Unfortunately they do not have a way to measure the pressure outside the tire, so instead they make a calculation using the Ideal Gas Law to determine what the pressure would be if the temperature were 68°F (293.15 Kelvin in absolute terms). It's the best the TPMS can do since it does not have a way to measure the differential pressure. Fortunately it's close enough for the main purpose of the TPMS, which is to alert you to an extreme pressure loss.

Another helpful formula is W = PA, where W is the weight of the vehicle on the tire, P is the differential tire pressure, and A is the area of the contact patch. Notice that this P is differential pressure, which means the difference in pressure between the inside and outside of the tire, which is what a simple tire gauge measures. The most important goal when setting tire pressure is to achieve the ideal contact patch area, which is determined by tire engineers. To get the ideal contact patch area for a given weight, you adjust the differential pressure. That is why the motorcycle manufacturer will often provide 2 different tire pressures for riding solo vs. with passenger--you need a higher pressure to achieve the correct tire contact patch for the higher weight. Notice the formula does not say anything about temperature! It does not matter when determining the ideal contact patch. Only differential pressure matters, which is the pressure measured by a simple tire gauge.

When setting your tire pressure you should use a simple tire gauge and use the recommended pressure without any correction for ambient temperature or altitude. The +/- 1 psi per 10° change from 68°F is going to give you an idea how far off your actual tire pressure is from the TPMS indicated pressure. The tire pressure gauge will be the correct pressure, not the TPMS.
RoundTrip likes this.
5. ### CharlieInStLouisBeen here awhile

Joined:
Feb 16, 2014
Oddometer:
401
Location:
You're kidding, right?
I think even Robert Boyle would think that you guys need to get out and ride.
MarkM likes this.
6. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
I went riding earlier today. Now I'm trying to mow the lawn but as you may know, it's still warm and muggy in Missouri. I've been taking a break to cool off.

I used to be a measurement/instrumentation engineer for a certain St. Louis aerospace company. I spent a career designing precision measurement systems for pressure, temperature, liquid and gas flow meters, etc. So you can imagine this is an interesting topic to me.
TonyKZ1, RoundTrip and rodentface like this.
7. ### vagueoutLong timer

Joined:
Mar 3, 2010
Oddometer:
4,220
Location:
sydney, east
I really do have an abiding love for the American people but your inclination to over anal-ize anything is amusing. Tyre pressure ! how hard can it be?? (no pun intended).
CharlieInStLouis likes this.
8. ### fleicycletherapist

Joined:
Apr 1, 2013
Oddometer:
10,104
Location:
Western Mass.
How difficult can air pressure and it's relationship to temp be? You obviously do not follow NFL Football. The Wells Report on the Patriots "Deflatgate" incident was 243 pages long. I will spare you a copy and paste, but it's some "fun" reading on this topic. https://en.wikipedia.org/wiki/Deflategate#Wells_Report
9. ### vagueoutLong timer

Joined:
Mar 3, 2010
Oddometer:
4,220
Location:
sydney, east
It's a sunday morning here, i was about to jump on my r 9 turban for a spirited ride into the country but now i really want to cancel that to embed here and read the said 243 page report on air press./temp relationship in a fucking football!!??
MarkM, flei and CharlieInStLouis like this.
10. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
I live in Missouri, the Show Me state. If you are correct, it should be easy to find documentation from a manufacturer or industry group to back it up. Let's see the BMW documentation. Where do they say to correct to 68°F?
11. ### AZQKRLong timer

Joined:
May 8, 2018
Oddometer:
1,185
Location:
arizona
make your own call, people have already made their call and reported what they were told. You set your tires your way, and I'll set mine my way based on the knowledge of others who've talked to the engineers.

But here, you can read this, I excerpted the pertinent part for you

https://www.arden.org/misc/pressure.html

"I inflated my C4's tires to Porsche's standard recommendations for the street (225/40-18 -- 36 PSI at 68 degrees F, 265/35-18 -- 44 PSI at 68 degrees F)"
12. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
Alrighty then, glad we could resolve it. You can take the second- or third-hand advice of friends, who may or may not have understood it correctly from their unidentified sources. May your tires never cup, even though you are probably under-inflating them in the cooler months. I'll stick with published specifications and instructions, backed up by science.
Seriously though, enjoy the ride. Maybe we'll meet one day around a campfire and we can drink beers and argue in person.
13. ### AZQKRLong timer

Joined:
May 8, 2018
Oddometer:
1,185
Location:
arizona
Reread post 71, the bolded RED portion.

Now, you can continue to hold your line or you can accept that recommendations are based on 68F ambient and adjust accordingly. I'd have a beer with you anytime sir.
14. ### MarkMBeen here awhileSupporter

Joined:
Mar 23, 2009
Oddometer:
981
Location:
St. Louis
Looks like you added stuff after I quoted your post. I read that article. More hearsay from a guy who claims to know Porsche's recommendations but without actually providing the source. If it's not directly from the vehicle or tire manufacturer, it's not bona fide.
15. ### AZQKRLong timer

Joined:
May 8, 2018
Oddometer:
1,185
Location:
arizona
Okay then, do you happen to know what the worlds average temperature is?
It's 57 and change degrees F. Detract the arctic and antarctic, take a guess what number it results in?

Do you honestly believe the author pulled that number [ which happens to be the same in this discussion ] out of his butt and just got lucky it was the same number being discussed here?

Some horses can be led to the water, but you can't make them drink it.
16. ### foxtrapperLong timer

Joined:
Sep 6, 2011
Oddometer:
4,157
Because 68 degrees F is the base standard temperature for pressure measure.
https://en.m.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure
I use it all the time in my work as an engineer.

I also know tech writers and editors. Fascinating, as rarely do they have any technology background, though that is what they write, in their creation and editing of technical manuals, owners manuals, repair manuals and such.

Now I’m just an engineer myself. But I’d say a technical writer thought they understood something, and sagely put it in an owners manual. Creating needless confusion and hand wringing.

You won’t harm your bike or tires by over or under inflating them a bit, so have at it. As an engineer, I can tell you that 30 Psig is 30 psig. It is not 32 psig because it’s hot, for that is 32 psig.
TonyKZ1, MarkM and AZQKR like this.
17. ### RoundTripUnintentional deerslayerSupporter

Joined:
Sep 11, 2006
Oddometer:
3,628
Location:
Sunny Tucson AZ
So...the temp referenced in the manual is a SWAG developed to let BMW pick an artificial point in the space/time/temp/pressure continuum that could be aligned with what the display said. From there, watch for deviations.
18. ### foxtrapperLong timer

Joined:
Sep 6, 2011
Oddometer:
4,157
Not at all. It’s a very old standard, in use for centuries, from before humans had refrigeration and such. It’s a sea level average barometric condition for European geeks of old. Carried over today, so geeks world wide can compare data.

BMW seems to have simply cited it incorrectly in an owners manual. Probably a mistake by a tech writer, who are almost always not technical people.
AZQKR likes this.
19. ### RoundTripUnintentional deerslayerSupporter

Joined:
Sep 11, 2006
Oddometer:
3,628
Location:
Sunny Tucson AZ
Standards, we have lots of them.

As a pilot, standard sea level density altitude is 29.92 in/Hg at 15 °C (59 °F), with a temperature lapse rate of roughly −2 °C (-3.6 °F) per 1,000 ft.

ETA: I looked into it here: https://emtoolbox.nist.gov/Publications/NISTJResJan-Feb2007-112-1.pdf and apparently 20*C/68*F for measuring devices has only been in use since 1931. I like learning new things.
20. ### lhendrikPutins Puppet

Joined:
Dec 10, 2008
Oddometer:
2,894
Location:
New York
I'm actually afraid to ride the GSA now, my tires are probably mis-inflated. Bicycle is looking scary too. I may never inflate anything again.