Custom tire pressure indicator caps
 

I noticed that Radio Shack has these 34 psi indicator caps on closeout for 97 cents per packet of four, so I checked them out.

http://rsk.imageg.net/graphics/produ...265941w345.jpg
4-Pack 34 PSI Tire Pressure Indicator Caps
Catalog #: 63-1214
(These are available in stores only, if still in stock. Not available for order on the website)

The instructions say they will pop out at the indicated pressure and pop back in when it falls by 20%. That would be around 27.2 psi.

I pumped up one of my I-167 tires to 30 lbs and tested a couple. At 30 psi, the two I tested both popped out. I let out a little air and continued to test. One popped back down at around 27.5 - 28.0 psi and another popped back down at 27.0 - 27.5. Readings taken with a digital tire gauge which indicates to the nearest half pound.

The construction of these devices doesn't seem to lend itself to adjustment. I was able to partially disassemble one, but not to the point where anything useful could be done.

Apparently there is a small bladder inside which gets squeezed by the air pressure of the tire. The bladder is constructed so that it "pops" out the indicator when squeezed hard enough. There is a very small hole in the "window", I'm guessing to allow the "popper" to expand into the clear area without compressing any air there.

The lowest psi rating I've seen these for sale online is 26 psi. To get into a useful range for an i-series tire, it would have to be rated at 19 psi or a few pounds less (80% of 19 = 15.2)

The Radio Shack I visited also has some 32 psi indicator caps, but they are only marked down to 2.97 for the four-pack. They seem to be on closeout too. The 32 psi caps do not appear on the RS website.

I'm thinking these indicator caps may be tweaked to trigger at a lower pressure by introducing a small amount of large-grain objects between the bladder and air orifice, in effect 'pre-squeezing' the bladder. If the bladder can be squeezed by the equivalent amount of 19 psi, it will only take 15 more to hit 34. The objects need to be blunt, non-compressible and larger then the opening in the air orifice itself to avoid leaking out. I've got something in mind for the large-grain objects, but will have to try it to see if it works out.

The plan is to "pop" one of these by putting it on a tire with over 30 lbs of pressure. Next put a piece of plastic tape over the vent hole. Then remove the indicator cap from the tire. Hopefully it will stay "popped". If it does, the chamber between the bladder and the air orifice will be enlarged temporarily.

Then the gasket and brass air orifice are removed with needlenose pliers. Next, these "large-grain" objects are placed into the area between the bladder and where the air orifice goes. The orifice and gasket are replaced. The plastic tape is removed from the vent hole.

The indicator cap is then tested on a tire to see what pressure it "pops" at. Hopefully it will be less than what it was initially, (which was 20% less than 34 psi). If the new threshold is only a little less than 27 psi, it doesn't look like the idea will pan out. If the new threshold is less than 15 lbs, then a few of the grains can be removed, and checked again, until the "pop" occurs at the point desired.

If I am able to make progress on this, I post the results.

Probably more trouble than it's worth, as if I were to leave the caps on the tires, they would no doubt be stolen eventually. But it would be interesting to have custom indicator caps that tripped at 13-15 psi.

Well, got a chance to try it out.

Didn't go so well.

I was able to get the blunt, large-grain material between the air orifice and the bladder, but it didn't change the trip point by much. The tape sealing the vent didn't hold back the popper and I had to tamp the grains in, up to a point where the popper was just starting to pop (without any air pressure applied).

"Before" trip point about 27.5 psi, "After" trip point still above 26 psi. And the "pop" action was more subdued, not as dramatic a change. There was a small improvement, but not nearly enough to get it down to the range an i-series would use.

So it seems impractical to try to adapt these inexpensive "automobile tire" ranged indicators to PT use. At least by the method I attempted.

The automobile indicators rated at 26 psi might be practical for use on the p-series (as the trip point would be around 21 psi), but the i-series need a "natural" rating around 16-19 psi. And the XT would need ones rated around 4-6 psi.

Oh well, all it cost me was 97 cents plus tax, and around an hour of leisure time.

There is a writeup of building a wireless tire pressure monitoring system if anyone is really game. Maybe one of the segrious pimpers will dive right in the next time they have the tires apart.
http://www.atmel.com/journal/documen...aisestheBa.pdf

Thanks for the link.

The microcontroller and battery is overkill for the PT tire, IMO. Temperature and an exact pressure are not needed. But I do like the RF ID tag combined with the pressure sensor. The RF ID tag could be real simple, either answering in two different ways at a cutoff pressure, or just failing to respond when the pressure is below a cut-off point.

The good thing about the RF ID tag is that it is powered by the RF interrogating pulse and doesn't need a battery inside the tire.

I've seen hobbyist-type RF ID tags and readers from Parallax, Inc., but I haven't seen something off-the-shelf that reads pressure that would be appropriate for this application yet. Also some tags are bulky and might be challenging to fit into a PT tire. The range is about 4 inches, so the interrogation would probably have to activated when the tag spins next to the RF ID reader. This can be done with a magnet and hall effect sensor mounted to trigger when the tag is in range.

A good part of the bulk of a passive RF tag is it's antenna. The antenna has to be resonant on a particular frequency to absorb enough energy to transmit back the ACK. If the tag could be fabricated to detune the antenna at a particular amount of pressure, the interrogating system could determine whether the tire was above or below the threshold pressure by detecting or missing the ACK responses from the tag.

IMO, those are not that accurate anyway. Secondly, The ones that I have been familiar have red collar and a yellow collar that is hidden by pressure in that bladder. "Full" pressure keeps the collars hidden, the slight loss of pressure allows the yellow to be exposed, and the continued loss of pressure allows the red to be exposed.

It is possible that a multi-tier warning may be identifiable by the amount of collar that is exposed. (Possibly a half red flag is good, while a full red collar is bad...etc.)

I have not had a much problem as most, apparently. I just look at the tire, or squeeze it with my thumb, or... use a pressure guage upon occation.

I have had those caps on cars in the past, and they typically work when you install them and fail long before the tires need air...

Good luck...

Yeah, the earlier models attempted to have a red, yellow, and green indicator. But these latest models just have a single indicator, triggering at 20% below their specified rating. Having a single indicator makes it a bit more compact. And the four in the four pack I tested all triggered within 1/2 pound of each other, which is plenty accurate for a PT tire.

On an automobile, being a harsher environment, I can see the indicators failing / wearing out before the tire does.

But on a PT, I think they could be effective, if only they were available in a suitable pressure range. It would be nice to tell pressure at a glance before going out on a long glide instead of being "reminded" (by noticing the veering tendency) when you're several miles out.

I guess there aren't enough 4, 15, & 22 psi tires out there to make it a viable option for the manufacturers of such items to make them available.

Anyway, it's not that big of a deal now that the PT mainstream is going tubed, as leakage is a bit less. Anyhow, you wouldn't see the pop-up indicator (if it was on a suitable pressure range), due to it's new location on the i2 - LOL.

I dunno. I regularly lay down with my head on the floor and look under the seg. Don't you?

Well, I got so used to doing that before I flipped my charge port LEDs, maybe it's a habit I'll have to get back into. - heheh

Segrious pimpers!
 

I don't have much interest in tire-pressure indicators, but the term "segrious pimpers" made the whole thread worthwhile. Thank you for the hearty chortle! :-D

Very Cool
 

Forget little caps that can't be seen when your stem is on the inside of the rim (as on the i2)... Check this out:

Bridgestone Wireless Sensor

It may very well turn out that mere mortals won't be able to afford this, but if that's not the case, I think this would be the ideal solution! Well, ideal would be if INC would incorporate this into the i2's InfoKey... :)

-Fernando