bystander
08-31-2006, 05:20 AM
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/product_images/pRS1C-2265941w345.jpg
4-Pack 34 PSI Tire Pressure Indicator Caps (http://www.radioshack.com/product/index.jsp?productId=2103413)
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.
http://rsk.imageg.net/graphics/product_images/pRS1C-2265941w345.jpg
4-Pack 34 PSI Tire Pressure Indicator Caps (http://www.radioshack.com/product/index.jsp?productId=2103413)
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.