I have some experience in this area, with two external valve-stem sensor systems for about four years now. Basically, I agree with @RICZ
, i.e., overthinking it.
On my XCT (sold last year, as many of you know), I was running the Doran TPMS, for about three years.
nice system, in that it included its own display module (e.g., no phone or pocket fob required), the display uses bike power, and the display is waterproof. Also, the sensors are sealed units (which Doran touts as a sort of safety feature). Last, unlike a few systems on the market, no wheel rotation is necessary to check the current pressure, e.g., you can check things before starting to ride.
OTOH: the non-replaceable batteries lasted way less than Doran claimed they would, and replacing the sensors is expensive; when stationary -- never when moving -- the sensors sometimes lost connection with the display. (They used an older RF communication protocol, not Bluetooth.)
I wrote a review of this system for webBikeWorld in 2014, which you may find a little useful:
Given the hassles and costs of the Doran, at the start of the 2017 I added the FOBO sensors (and stems -- more of that in a minute) to my current bike (a Burgman 650 scooter, which @PaiN
did a minor dirt-throwing, burnout-like, spin-up, when starting a brief test ride).
I like the FOBO system a lot more, although I've given up the real-time-riding monitoring (because I don't have any helmet speakers, let alone Bluetooth ones, and I only mount my phone visibly on the bike for occasional GPS purposes).
That is, I use the system mostly for a pre-ride check of pressures. And we all know what a PITA it is to check, say, the rear wheel of a bagger the old-fashioned way, i.e., on the ground, with very little clearance.
There's a nice review of the FOBO system on webBikeWorld:
I did NOT write that one, but I do have two lengthy comments ("B.P.") at the end of that review. I first noted some limitations, and then, later, I basically ate crow, noting that I switched over to FOBO.
So, in four or five years, and, I dunno, 30,000 - 40,000 miles, two systems, a bunch of sensors, no leaks. As you note, you would get alerted to a slow leak. If the entire sensor just fell off, or fell apart, the original Schrader valve would take over again, just as it does in a stock situation. (Those valve-stem caps are really there just to keep dirt out, not as air sealers.)
Keep in mind with the FOBOs that there's an O-ring around the outside, as the sensor cap meets the bottom of the unit. This is just there to keep the dirt and water out of the guts of the unit. That is, this O-ring is NOT there for keeping air in. You can unscrew this big cap -- for replacing a battery, for instance, and no air is lost. But I guess the O-ring/cover combination could probably serve as a sort of backup system.
Beads and Cores:
Back in 2012 or 2013, when I had my first tire changes on the XCT, at an actual Victory dealer, I had balance beads put in the tire. The service manager told me he used them in his own XC. He also told me that they replaced the valve-stem cores with some special ones that have some mechanism to prevent beads' clogging up the core. I never really looked at one of these cores on the inside, so I don't know whether that's a smaller opening, or some sort of screen, or complete BS.
In any case, if you have beads now, and can add air normally, and check the pressure normally, I don't see why putting sensors on, externally, would alter this situation any.
Me, I switched to Centramatics ( Wheel Balancer MV100-106 from Centramatic
) for the last two years on the XCT.
They're a little costly (but less than, say, new pipes). And they might theoretically affect the suspension, adding a bit of unsprung weight (but the Vic wheels are so heavy to begin with that I'd call that negligible). The good news is that they work great, and this permanently dispenses with beads (and any special valve cores).
For the Burgman (no beads or Centramatics, and smaller and lighter wheels), I waited until a tire change in the spring of 2017. At that time, I had the shop not only replace the tires, but balance the wheels, with the tires and new valves stems (more in a second) and sensors on them.
For anyone going with external sensors, I recommend getting FOBO's T-valves (or equivalent, somewhere else, I suppose). See https://my-fobo.com/Product/FOBOACCS
This allows you to leave the sensor on when you do need to add air. That in turn means you don't have to mess with the little security-ish locknut (if you choose to use it -- I do). Also, the batteries will last a bit longer (as FOBO notes that unscrewing a sensor, and then screwing it back on, uses a little more juice than just leaving it alone).
Internal Battery Life?:
If you do find the right curvature or otherwise get internal sensors mounted, I'd worry about the battery life. I know that sensors in cars have batteries that seem to last forever, but my experience with the (external) Doran sensors has left a bad taste in my mouth.
My point here is that unless you do your own tire changing, including dismounting and mounting, then this might be a negative side of internal sensors. I suppose you could just change the batteries whenever you change tires, in a proactive manner, and not have to worry about it.
Those are my thoughts. If you have any questions that I haven't covered and think I might be able to answer, let me know.