Possibly the most important and most often overlooked safety equipment in a vehicle is the tires. They undergo some incredible stresses, and we hardly even think about it even when slamming over bumpy roads. Proper tire inflation is vital to safe, sustained performance and reliability, as well as better fuel economy. As someone interested in all of the above I check my tires often and keep them right up to the design pressure printed on the sidewalls. About a year ago I upgraded from the stock Prius tires to Michelin Hydroedge, a good quality tire with rave reviews at many of the tire sites on the net. I've been running these at a *minimum* of 44 psi cold, meaning that under normal operation they can get a little bit warmer and head towards 50 PSI -- still well within their safe operating pressure, of course. And the more pressure inside the tire the less flexing and heat-generation it undergoes, giving better long-term wear characteristics. The debates over sidewall vs. door- placard tire pressure continue to rage on forever unabated in the forums, both professional and otherwise, and I'm not going to get into that aspect here.
Really, I'm not too worried about the tires, because they're made to take the stress. But what keeps all that nice air *in*? The inflation valve. In a tubeless tire, the valve just sits through a hole in the rim and holds air in the volume formed by the tire and the rim together. Obviously the valve must have an airtight seal against the inside of the rim too. There are many brands of valve stems, some shown here, all made to a specific standard of sizes but right away we can see minor design differences coming from different suppliers. The stem is a simple enough thing, and you wouldn't think of looking for problems there after considering how the tires themselves get pounded against the road in all weathers and these little guys are just along for the ride.
This is why I was really disturbed to discover, only a year after getting the new tires, that the valve stems on my wheels were starting to crack around their own bases and appeared to be well on the way to leaking or possibly even separating completely from the rims. Now, *that* would suck at highway speed, wouldn't it?? This was on all four tires, so it wasn't like I had brushed up against a curb. A little research turned up a couple of articles about poor-quality offshore valve stems making it into tire installations, but then failing as little as six months later. I posted about it into one of the forums: Subject: Check your valve stems! After changing the oil today I went around with the pressure gauge; everything still right where it should be. But I was mildly horrified to notice that my valve stems appear to be cracking around their bases. That makes me almost afraid to even put a pump on them, because given past experience with bicycle valves a crack like this can quickly grow and start leaking. I think what happened was that when I got the Hydroedges put on just about a year ago, the tire place [Hogan in Woburn, for the record] replaced all the perfectly good Toyota-sourced valve stems with some crappola offshore aftermarket ones simply because they have this policy of always replacing valve stems along with tires. They shouldn't have bothered. There's a growing body of evidence that all valve stems are NOT alike and the ones that tire places tend to buy in bulk may well be absolute shit. The guy on the phone at Hogan was like "well, you've got that lifetime valve warranty..." to which I retorted "that's not the POINT! I'm not going to be coming back for an annual VALVE STEM replacement, that's just silly." And they have no upgraded options to give me, i.e. they put this garbage on every tire they mount. Which means I have to go shopping for better stems that fit these rims [and how to determine the specs, I have no idea yet] and then take those to Hogan so they can swap them in. I *knew* something was wrong back then, when I immediately observed that the new stems seemed a lot more flimsy/wiggly than the old ones after the new shoes went on. Dammit, I should have raised high holy hell right there, but I gave them the benefit of the doubt that they knew what they were doing. Of course these were the same people who left the car powered on the whole time they were doing the wheels and then turned it *off* when they went to drive it off the lift. Right up the hill from a Toyota place, too, and claimed that they "do Priuses" all the time and know all the subtleties. Hogan corporate is in for a jolly ol' earful, lemme tell ya. I don't want to start a 3000-odd mile road-trip on these things. Oh, and no, I don't think it's a pressure issue. Most stems are rated for 60 psi [and thus could likely withstand more] and I think I'm safely under that. But maybe long-term effects on cheap stems ... hmmmm. Something we should all think about. ... Toyota is right down the hill from there, and I'm going to swing by tomorrow so one of Hogan's Honchos can take a look at what's going on. I'll compare the two brands of valve stems between Hogan and the Toyota parts dept to see if I can at least pick the lesser of two evils.
After having the entire set replaced the next day, I finished the story:
Okay, I think I've learned more about valve stems today than I ever expected. There are many varieties of the standard "TR413" snap-in type (left) designed to fit in a 0.453" rim hole, and rated for 65 PSI. For light trucks and the like, there's a higher pressure "TR600HP" type (right) with a heavier base inside the rim, rated to 100 PSI. The interesting thing is that these type numbers and pressure ratings are across all the manufacturers -- TR means Tire and Rim Association that sets these standards, and the real issue comes down to what manufacturer produced a given batch of stems. Long story shortish, I have four new TR413 valvestems. The guys at Hogan were baffled as to why mine were starting to crack but offered that possibly running at sidewall instead of 32 PSI might have helped the process along. The funny part is I did wind up getting stems from Toyota -- the Hogan guy gave me one of his and I walked down the hill to the Toyota counter to see what they had, and the parts guy and I stared at the two and while they did look different, we couldn't determine any real quality/flexibility/etc difference. I thanked him and started to walk out and he stopped me and handed me the whole handful of four, saying "think of us next time", so there I was with four of whatever the Toyota place would use to do tires. They did seem just a tiny bit shorter and stiffer than the ones from Hogan, so I trudged back up the hill to show them all this and we decided to go ahead with the swap and use the stems from Toyota. They were cool with it either way, and I even got to hang out in the bay and natter with the tech and puzzle over what might have happened. He said he's been using these stems for a long time and has never seen this sort of issue.
I managed to keep a couple of the old stems for an autopsy, since the tech was otherwise simply throwing them out. Their standard supply of snap-in valves appears to come from Dill Air Controls, evidently one of the elder "big three" in the tire valve business -- apparently where they are in North Carolina used to be the glorious hub of tire valve manufacture, but recently they and everyone else such as Eaton and Schrader have been forced to outsource and most of their product is actually made in China like everything else. A guy I talked to later at Dill let me in on a little of this history and in a somewhat oblique way expressed the challenges of quality control in that scenario, specifically citing that the most likely cause was lack of good anti-ozone protection in the rubber compound. He was quick, nay, almost emphatic to reassure me that running pressure is completely NOT the issue. He hinted that making things out of good long-lasting EPDM elastomers has almost become a lost art. Well, as far as I'm concerned that solves the big mystery. This is a shining example of how outsourcing is KILLING US. Literally, when a tire valve failure could easily lead to a vehicle crash. Ironically, the stems from the Toyota parts counter said "Schrader China" on the bottom, making me feel even more hopeless about finding a good, long- lasting stem that day. I couldn't tell if this means they were actually sourced via Schrader the company or just that it was a Schrader-type valve core, but the stems from Toyota felt just a wee bit more robust so we used them for the replacment. Now I'm considering this sort of an experiment, and will keep an eye on things to see if anything has improved since a year ago.
At one point we tried to rescue one of my stems that got tossed in the trash, and instead I pulled out a different one that had been on some other tire. On its base is emblazoned "Germany", and who knows, might have come off a BMW or Audi or VW that someone brought in. But this yields an interesting comparison: note the much thicker rubber and different taper near the stem base of the German unit, which would probably sit as rigidly in the rim as my original long-lost Toyota-sourced ones. Where could we guess that they were made, and where could we find some? The big slash through the Dill ball came from when the tire tech started cutting the old valves away from inside and carefully extracting them out the front, so we could preserve the outer part of the stems as intact as possible, and handing them to me instead of winging them at the trashcan. He was very accomodating, even if he didn't bother using a torque stick on his air wrench later when putting the wheels back on. He also needed a refresher on Prius startup after the job was done.
Anyway, time for a little analysis. I needed a cross-section of the cracked areas, so I carefully carved a couple of sections away from the brass core tube over about a quarter circle. Note that the brass piece doesn't extend all the way down into where the ball of the valve is, so there's only a slightly stretchy web of rubber between that and the flange. The rubber that's bonded to the brass is *very* tightly so, making it impossible to cut that cleanly away, but I recovered enough to grab.
Pulling gently on the strips clearly shows the extent of the problem. Leaving these on for not a whole lot longer would have probably paved the way for disaster. One can easily see how the road dirt was working itself into the cracks, too.
Now, how much long-term strain are we really talking about here? Should be easy to do the math. The hole diameter up near the brass insert measures just about a quarter inch across. (.25 / 2) ^ 2 is .0156, times 3.1416 is 0.049 square inch, giving a factor to multiply against tire pressure. So a tire at 44 PSI will be pushing outward on the area around the brass piece with a force slightly over 2 pounds, held back only by the tensile strength of the rubber around the base of the stem. And it isn't a piston -- the air pushes in all directions so it's maybe a little more.
Going back to the bigger picture, here's the rest of the stem with no stress on it and the cracks closed and almost invisible, and then tugging on it to try and simulate tire pressure. The cracks open right up, of course, and one can see where separation might occur if they worked themselves in deep enough. I can't believe that this is all I had between me and deflationary doom -- and for how long prior to my discovery, I wonder?? It is interesting that the cracking seems to have started right on either side of that little retaining ridge, also implying that there may have been a molding issue on top of everything else. Now, some hypermilers like running their tires above 50 PSI, which would have the valve stems withstanding 2.5 or more pounds of force inside that little hole and more like 8 pounds on the whole valve body through a .453 rim opening. This might lead to a conclusion that higher pressure would lead to more rapid valve stem failure, but let's remember that these things are supposed to all be rated at 60 or 65 PSI *cold* depending on whose marketing literature you read, and it's supposedly all set down in the Tire and Rim Association standards. Many newer car tires are sidewall rated at 51 PSI, and some light truck tires even higher. What sort of peak pressure might we think is generated by taking a 50-something PSI tire over a nasty bump on a hot day?? Valve stems obviously need to withstand worst-case thermal pressure rise combined with transient shock loads, with a healthy design margin on top of that. Like tires, well-made ones are probably good for 100 PSI or more against them before blowing out of the rim -- and think about it: all the stories we see about tire inflation overpressure injuries come from the *tire* failing explosively, not the valve stem giving up [which, in fact, might be a safer failure mode]. And how about the valve stem in that 60-psi donut spare in the trunk?? Bottom line is, we can't just go round putting 32 PSI in everything and ignoring the many benefits of higher pressure tires in general. But that needs support from durable, high-spec products.
If/when these new ones show any sign of degrading, my next step will likely be a set of these:
They're called clamp-in metal valves, and anchor firmly into the rim using a retaining nut and a shaped O-ring to seal in the back. The really short ones like this are used by offroaders and construction vehicles as protection against a rubber stem being ripped off by something banging against the rim. Here's another article [PDF] discussing them among alternatives for use with today's higher pressure tires, especially in trucks. Maybe I should have ordered some this time around, but I really wanted to get the problem dealt with pronto and with new stems in now, I suppose an upgrade can wait a while. And eventually, I suppose all of this will become academic as mandatory tire pressure monitoring systems become commonplace, and plain old valve stems slowly give way to the TPMS sender units which have their own valve assemblies and are generally a bolt-in style of unit.
Update: I eventually *did* install a set of bolt-in valves in late 2009! Read about it here.
Return to main Prius page