silly but economic theory prevails?

 

That time we got our tire patched at Costco, for whatever reason, the tech replaced the green valve stem cap (part of a kit when they replaced the tires), which is long and round, with a normal one, which is shorter and more angular, i.e. normal.

The long round or oval one is specific for GM TPMS. They only have this when they use that kit to do the TPMS service at the time the tires are replaced. I remember asking years ago for a new one when one was cracked, and the tech said we don't have those, we just have these, and he handed me a normal one.

The normal one leaves like 10-15% of the valve stem exposed at the rim end, so I thought let me just buy a set.

Look on eBay and my choices are 10 for $5.69, or 100 for $7.99.

Since I applied marginal benefit vs. marginal cost, I went for the qty 100!

Meaning, I got 90 more for $2.30 (ever try this at Starbucks where the large is 40 cents more than the medium, and already $6+--we don't go there but my wife got endless gift cards from students for Starbucks).

On the other hand, right now, I only need 1 (they eventually crack at the end and need replacement, so I try not to overtighten them).

So you can see what I mean by the GM caps

https://www.ebay.com/itm/121233279151?hash=item1c3a1108af:g:...

first world problems I know

Green means nitrogen fill

Green caps signify the tire at one point in its life was filled with nitrogen. Air of course is about 78% nitrogen. As you likely are aware there are many fallacies about the benefits of nitrogen in automotive. Basically, if the nitrogen is free, go for it, but do not pay extra.

Also, a good idea to seek out valve caps that incorporate an o-ring. Something like https://www.amazon.com/BEADNOVA-Covers-Motorbike-Bicycle-Pie... Al.

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John from PA

copy that

It's totally free self-serve nitrogen at Costco, so since they fill it as such when tires are purchased, I use it. What I had not expected was they still change by 1 psi per 10F.

Anyway the long round caps fit the General Motors TPMS specifically. A normal cap shows some brass, so that's why I want to get the right cap. But I'll have 99 extra lol. Won't have to worry if they crack or get lost though.

Put up the other 90 for sale

Put up the other 90 for sale in lots of 10 for a buck less than the other seller is selling. You might recoup your costs smile

Everything you needed to know...

johnnatash4 wrote:

What I had not expected was they still change by 1 psi per 10F.

The molecular weight of air is 28.97 grams per mole. The molecular weight of nitrogen is about 14.0 grams per mole, but since the element is diatomic the mole weight is 28.00grams per mole. Thus the change in pressure due temperature change, is virtually identical.

A good resource is at https://www.getnitrogen.org/sub.php?view=nascar but be careful in its use and making sure you are doing the comparison correctly. For example there are columns in deg F and deg K.

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John from PA

just basic physics

John from PA wrote:
johnnatash4 wrote:

What I had not expected was they still change by 1 psi per 10F.

The molecular weight of air is 28.97 grams per mole. The molecular weight of nitrogen is about 14.0 grams per mole, but since the element is diatomic the mole weight is 28.00grams per mole. Thus the change in pressure due temperature change, is virtually identical.

A good resource is at https://www.getnitrogen.org/sub.php?view=nascar but be careful in its use and making sure you are doing the comparison correctly. For example there are columns in deg F and deg K.

Right, as long as we're talking about pressure/temperature relationships, and not diffusivity through rubber, it makes pretty much no measurable difference if it's air or N2 or any reasonably inert gas. It's just basic physics. It's been decades since I learned all this in engineering school, but there are multiple physical laws that describe gas behavior, with Gay-Lussac's law being most applicable to this as I recall.

Moisture Content

rocket_scientist wrote:
John from PA wrote:
johnnatash4 wrote:

What I had not expected was they still change by 1 psi per 10F.

The molecular weight of air is 28.97 grams per mole. The molecular weight of nitrogen is about 14.0 grams per mole, but since the element is diatomic the mole weight is 28.00grams per mole. Thus the change in pressure due temperature change, is virtually identical.

A good resource is at https://www.getnitrogen.org/sub.php?view=nascar but be careful in its use and making sure you are doing the comparison correctly. For example there are columns in deg F and deg K.

Right, as long as we're talking about pressure/temperature relationships, and not diffusivity through rubber, it makes pretty much no measurable difference if it's air or N2 or any reasonably inert gas. It's just basic physics. It's been decades since I learned all this in engineering school, but there are multiple physical laws that describe gas behavior, with Gay-Lussac's law being most applicable to this as I recall.

The primary difference between "compressed air" and "nitrogen" in automobile tires is the moisture content, "compressed air" contains moisture while "nitrogen" typically doesn't. Moisture content will cause larger fluctuations in pressure as a function of temperature than dry "air/nitrogen".
Mark

I received

the 100 caps and they are IDENTICAL to the ones Costco used when they replaced the tires and did the TPMS service.

My buddy said not only would he have chosen 10 over 100 (exact opposite), he wouldn't even bother with caps, and in his lifetime, he doesn't recall ever replacing any.

He even said he hasn't checked his tire pressure in 3 years, and so what if they are low or drop by 5+ psi in the winter.

This is a close friend, and a clear example of how divisive we are today!!

He's not lying, hasn't touched his tires since new 3 years ago, what's the difference? I guess wear? He'd have excessive wear on the outer edges being underinflated when the weather is cold. I told him you can't eyeball tires like our grandparents did, you have to measure, he says no if they're low he'd notice by looking.

I told him that from 2002-2019, I never once serviced the furnace or AC, and it was installed by the previous owner in 1999. He is insistent because of my lack of maintenance, the system broke in 17 years, and had I had maintained it, it'd still be good today. I don't agree, the refrigerant leaked out. My point is even with close friends seems nowadays we argue! I said I'm bringing my gauge to the BBQ and we'll measure your tires. How much you bet 17 psi or something lol

What is TPMS service?

What is TPMS service? I don't believe that their batteries are replaceable. What am I missing?

assumption

minke wrote:

What is TPMS service? I don't believe that their batteries are replaceable. What am I missing?

I've assumed over 2 tire purchases that they serviced the schraeder valve.

What is a bit nuts about GM is the OE TPMS are only like $10-$15 each, why not just replace the unit? I replaced the BMW and I think they were $110 for 4 or something. Not only that, multiple part numbers for the same year car, need to specify. Maybe I didn't need them but I said better do it while the tires are replaced, then to pay to mount/dismount 4.

It seems that's how they have the green caps, as part of a kit. Because I have asked before and was told we don't have that specific cap we have these...and then they hand me a normal one which is not as long as the GM ones.

This is a copy/paste

Do you need TPMS service kit?
TPMS Repair: This service kit is recommended each time a tire is repaired or replaced. It includes installing a new TPMS Service (repair) Kit, including grommet nut, grommet, valve cap and valve core. We're required to replace the TPMS service kit if the TPMS sensor is removed during a tire replacement or repair.

That sounds like make-Work

That sounds like make-work on the stem. I haven't bought tires in a while and the last time Discount Tires just sold me tires.

it could be

minke wrote:

That sounds like make-work on the stem. I haven't bought tires in a while and the last time Discount Tires just sold me tires.

I truly believe that we learn what we do when kids, then those are what we think until something practically tells us otherwise.

For example, on my car forum more than 13 years ago, I asked "how do you guys mark your tires?" Meaning when swapping the winter tires on. Today I use tire crayons, I think a paint marker is too permanent but I have them.

Someone said, why do you have to? The rears are wider than the fronts, so you know which are front, which are rear. I said what about left and right, I don't want to reverse the direction of the rotation. Response? Why not? The tires are not directional.

I thought back to a Corvette where each tire had a distinct part number because they were staggered, and directional, and asymmetrical.

Turns out? This is a 1970's concept pre radial tire, where you should not reverse the rotation. And here it was 2008 and I'm still doing it (and still do to this day can't break the habit hahahahahahaha). I only rotate front to rear, not across. And with the German car I can't do front to rear, so no rotation whatsoever.

So when it comes to the valve stem? I thought that was done for every tire job, the valve stems are replaced, the wheels are computer balanced, and it's a good idea to get an alignment afterwards. But again, this is probably a 1960's or 1970's concept. Today there's an concept of not throwing things away that are in working order.

When I got a bead seal leak fixed, the first thing the person did was to replace the valve stem, then clean the mounting service, then apply some sort of sealant....

But I do believe that the only time Costco would have a GM valve cap, is when they have the tpms kit, and that's why when I've asked for one, they say they don't have those and gave me a standard one...

I even believe that tpms sensors have gone away, it's only a car from maybe 2000-2010 that would have them. But then again, wouldn't it make sense that if a tire had a leak, when it's going to affect a driver, is when the car is parked and they leave for work, etc.? Don't systems that rely on abs sensors need time to rotate to determine the disposition of the tires?

Or you can be like my buddy, who cares less, unless it's flat. He was laughing when I said I'll bring my air compressor too to the BBQ on Sunday, so we can get your tires up from 17 psi to the recommended 35....lol I truly believe you cannot eyeball it and not check for 3 years. But he's likely proving I'm wrong. My buddy is smart. He said he gets that the tires will be 5-6 psi lower in the winter, stating so what it's better for snow.

I can't drive without tire rotation

When I rotated tires myself I followed (IIRC) my Volvo manual and moved the rears straight to the front and swapped (right to left) my fronts to the rear. I may have done five wheel rotations long ago. When I ceased doing it myself I had the tire store do it my way. Left to their own deviced they’d do the reverse. After a while I caught on that theirs was just another right way and I shut up.

I can’t remember why, but they replaced a stem on my wife’s Volvo but used one which was too long. They did say that they didn't have the right length. I got a flat when the stem hit a curb while parking.

My ‘17 F-350 has a TPMS. From the Owners Manual: “… Note: Each road tire is equipped with a tire pressure sensor located inside the wheel and tire assembly cavity. The pressure sensor is attached to the valve stem. ...” When my wheels are rotated I ask them to re-educate the system so it knows which transmitter is on which wheel. I’ve been told that the system re-learns by itself but I don’t understand that.

The phrase "tire rotation" annoys me. I can't drive without tire rotation.

so I guess

some cars do still have sensors.

On my wife's 2011 GM, I have the relearn tool which I think I said I paid almost $20. But in 2011, it was over $90 and only one vendor sold it.

It actually works. Because the unit had poor instructions (as do many products today), I was not doing it correctly at first. I was positioning it as if I were adding air. The correct way is to basically touch the wand part to the tire sidewall, below the valve stem.

Primitive, yes, but the car honks to tell you it detected the tool. I always forget how to put the car into learn mode, I think it's press lock and unlock on the fob simultaneously.

The first time was a nightmare--the dash said service tpms or something--at that time I said I should have accepted the fronts were now rears, and rears fronts...

When doing my own winter tire changeovers, I wish I had a quick jack. I do each wheel one at a time.