tire pressure valve stem caps

[Lawman]

Are they any good? If so, where can I get some?

[ESokoloff]

AFAIK they are all no good/not to be trusted.

[RTs4me]

Don't, Don't, Don't! IMO, one of the worst possible add-ons to any vehicle. Check pressures yourself, before every ride, with a known accurate guage.

 

I have heard reports of these devices suddenly blowing out their 'core', with predictable results.............

[Joe Frickin' Friday]

Are they any good? If so, where can I get some?

 

You can usually find them at ordinary auto parts stores, but as others have stated, they're bad juju. The valve cap is supposed to be the main sealing device; the Schrader valve inside the stem is just the filling valve. These pressure sensing caps hold the Schrader valve open and replace the simple sealing action of the original valve cap with a complicated pressure-indicating mechanism that incorporates a seal with questionable reliability; if that seal craps out, you will lose all the air in your tire.

 

Get 'em if you want, but I'd recommend against it. Stick with a standard tire gauge, or (if you've got some play money lying around) get a tire pressure monitoring system for real-time feedback on tire pressures and temperatures.

[Jerry_75_Guy]

As everyone else has said; 'avoid these things!'

 

My wife wanted a set for her car and received some from my brother for Xmas a couple of years ago: they all failed and nearly left her with flat tires.

 

Again, they're junk. Do not use them.

[Lawman]

As everyone else has said; 'avoid these things!'

 

My wife wanted a set for her car and received some from my brother for Xmas a couple of years ago: they all failed and nearly left her with flat tires.

 

Again, they're junk. Do not use them.

 

What a great resource this place is...I would have certainly bought some if not for this DB..

Thanks...

[Fugu]

The valve cap is supposed to be the main sealing device; the Schrader valve inside the stem is just the filling valve.

 

Is this fact or just your thoughts on the matter?

 

If the cap is supposed to be the real seal, why do we see so many made of cheap plastic that splits if tightened too much?

[ESokoloff]

My believe is that the valve is the primary seal & the cap is a backup.

Some around here think it's the opposite, but I've yet to see any proof/documentation to support either version.

[ShovelStrokeEd]

Yes, it is a fact. The valve stem cap is the primary seal and the cheap, plastic ones should never be used on a motorcycle. They are OK on a car as there is much less likelihood of the valve within the stem opening due to the spinning forces in the tire. All racing organizations, that I am aware of, require metal valve stem caps on both wheels. In addition, when you install the metal valve stem caps, make sure they are gasketed within to provide the necessary seal.

[Ben_Ricci]

...the cheap, plastic ones should never be used on a motorcycle. They are OK on a car as there is much less likelihood of the valve within the stem opening due to the spinning forces in the tire.

 

I just put metal caps on my valve stems. But I have a couple questions concerning this (due to my ignorance). On the ST the front tire has a metal stem and plastic cap (stock) and a rubber stem and metal cap (stock) on the rear. I just bought a second "rear" metal cap and put it on the front stem. When I change my tires this weekend, I'm putting a metal stem on the rear wheel as well. Is there any technical logic behind BMW's decision to put plastic on metal and metal on rubber (front versus rear as described above)?

 

Secondly, after contemplating all this, I thought, "Gee, my car doesn't have metal stems or caps. Those tires rotate too." Is it because car tire size or weight puts less stress on the stems/caps, or something else I'm obviously unaware of? I'm naively curious.

[Fugu]

So a motorcycle wheel spins faster than a car wheel?

 

Significantly faster, so that the valve core is in danger of being pulled off its seat by centrifugal force and allowing air to escape?

 

I've heard tales of land speed record vehicles using Presta style valves that can't allow this to happen, but I don't know if it's folk lore or not.

 

Given that the diameter of a motorcycle wheel and tire is not that different from many automobile wheel and tire assemblies, and that many automobiles are way faster than my poor old RT, I'm not sure this is making sense.

 

I do have metal caps w/ rubber seals in them on my bike, though.

[Mike05]

Have FireFlys on my bike.

 

I still check pressure religiously but on at least two occasions, came out in the morning to see them "flashing" (supposedly do so when the pressure drops 4 lbs. or more)

 

I wasn't aware of any after-market tire pressure monitoring systems when I bought these; will probably go that route soon.

 

That being said, the "FireFlys" have worked just fine.

[Bob Palin]

I'm amazed to find they are the primary seal. The only bike I've owned that had metal caps is the BMW, I often ran without them on older bikes as I'm really good at losing/forgetting them at gas stations when checking the pressure.

[UberXY]

My believe is that the valve is the primary seal & the cap is a backup.

Some around here think it's the opposite, but I've yet to see any proof/documentation to support either version.

 

I'm with you. I thought the only real value of the cap is to keep crap out of the shraeder valve.

[ShovelStrokeEd]

It is a matter of orientation. Bike wheels actually spin slower than do car wheels (they are taller). On a car, however, the stem is not pointing straight towards the axle but rather a good deal more parallel to the ground. On most bikes, the stem is centered or nearly so in the wheel and if the stem is on the bottom, it will be pointing straight up.

 

The kids in my neighborhood like to steal my BMW valve stem caps. PITA to keep replacing them. I also failed to notice that the rear one had disappeared again on day during an impromptu investigation of some of the capabilities of my Blackbird and can state that that particular valve, on that particular day, managed to keep all the air in the tire at a tire circumference of 75.5" and a wheel RPM of 2493.

[Joe Frickin' Friday]

The valve cap is supposed to be the main sealing device; the Schrader valve inside the stem is just the filling valve.

 

Is this fact or just your thoughts on the matter?

 

It is a fact. Click here, go to page 24 (left column). Goodyear is pretty specific about using valve caps, going so far as to specify metal caps for high pressure applications (e.g. commercial aircraft tires @200+ psi).

 

If the cap is supposed to be the real seal, why do we see so many made of cheap plastic that splits if tightened too much?

 

Um, because they were tightened too much? The generic form of your question is "why do parts fail when overstressed?"

 

• It's a small-diameter (5/16") thread, so it doesn't take much torque to produce a lot of axial force.
  
• It's a smooth plastic cap face being drawn up against a smooth brass stem face, so it doesn't take much axial force to produce a good seal.

For plastic caps, these guys (see page 2) recommend a tightening torque of 0.17-0.4 N*m. That's only 1.5-3.5 lb-inches. Basically, snug the cap modestly using tip of thumb and tip of index finger; do not twist and twist between tip of thumb and side of index finger to the limit of your gripping ability.

 

I've never had a problem with plastic schrader valve caps splitting on me. If this is an ongoing issue, I'd recommend metal cap #7612, along with a torque wrench set to to (the required maximum) 30 inch-pounds.

[ESokoloff]

Sorry Mitch, still not convinced

 

Too much interpretation of the reader to be conclusive evidence IMHO.

 

 

For plastic caps, these guys (see page 2)

My interpretation of this is that the cap backs up the valve & keeps it clean.

 

 

metal cap #7612,

If you look at plastic cap #660V it has no seal so in this example the cap merely keeps the valve clean.

As far as I'm concerned, I'm not going to trust cap #1155 to serve as a primary seal either.

[Joe Frickin' Friday]

For plastic caps, these guys (see page 2)

My interpretation of this is that the cap backs up the valve & keeps it clean.

 

It is true, it performs both of these functions. But I'll note that they've called it a sealing cap in that document.

 

metal cap #7612,

If you look at plastic cap #660V it has no seal so in this example the cap merely keeps the valve clean.

 

True, the #660V cap shown there has no separate sealing sealing washer. But because it's made of soft plastic and is working with low pressures, it still is able to seal against the brass stem. Note that they've listed a working pressure of 0-100 psi, which would be irrelevant if it were not expected to provide a sealing function.

[ltljohn]

I found metal caps with a rubber insert at my local small bike shop for about a buck each. I have noticed since getting rid of the plastic ones that I do not have to add air as often. Just my observation

I call that cheap insurance.

[ESokoloff]

Lack of

Sealing Washer Material ( + ) has no sealing unit

=

ummmmmm let me think......

[Joe Frickin' Friday]

Lack of

Sealing Washer Material ( + ) has no sealing unit

=

ummmmmm let me think......

 

Yes, I see that there is is no separate sealing washer for that cap. But (again...) you'll note they've listed a working pressure of up to 100 psi, which would be pointless unless the cap is intended to provide a seal. That seal is provided by the plastic material of the cap itself, obviating the need for a sealing "unit."

 

Take a plastic cap off of your car tire, and look inside. The exterior of the cap's end may be rounded or sloped, but the interior will have a smooth flat face, intended to seal directly against the valve stem.

[Fugu]

That pdf file is pretty thin "evidence" and obviously aircraft applications have to deal with issues bike tires never encounter.

 

As for gorilla tightening plastic caps, I have no idea how much torque two of my piggies can apply, but I have serious doubts about the ability of plastic to seal to a brass stem well enough at the super low torques specified adequately to hold back 50 psi, let alone the 75 I used to run in the 10 ply tires on my truck.

 

There's a reason soda bottle caps have gaskets and they only hold 12 psi.

 

Although I do have a cap for soda bottles that allows me to put beer in a bottle and put CO2 pressure on it (for hauling around kegged home brew) and those caps are gasket less- but they have a wholly different contour on the inside, not just a flat spot.

 

I'm still a bit skeptical although SHovelstrokeD makes a good point re: valve orientation while forgetting that 20s are more and more common. The valve stems on my SUV/ car thingy are going roundy round faster than those on my RT because my car has 20s, but the orientation being key makes sense.

[ESokoloff]

I have e-mailed Schrader (Michigan ) in regard to the primary/secondary issue.

 

Lets hope we can finally but this one to bed

[ESokoloff]

No word from Schrader

 

For what it's worth, Perrier water now comes in a plastic bottle with a plastic cap with out any additional seal.

Unlike the schrader cap, this one uses a wedge seal.

After the cap seal makes contact with the bottle, it takes another 180º of course thread travel until the cap is seated.

Not so with the caps on my truck. The fine threaded cap seats nearly as soon as it makes contact providing not much of a seal as the plastic is too hard to compress into the voids of the brass stem.