After a year spent fighting with my Powertap SL+, I've made the leap.
That's a snarky allusion to SRAM's ad copy from a few years ago, when they first introduced their road bike componentry. But it's true: I've traded in my Powertap for a crank-based powermeter built by SRAM.
Powermeters (for the three of you who don't know but are inexplicably still reading) measure the effort a cyclist expends. Unlike speed or heart rate, which can vary dramatically from day to day, power provides a constant frame of reference. A 15 mile-an-hour ride into a wind is much harder than 20 mph on a calm day, and the powermeter's ability to keep track of how many watts a rider has expended will reflect that increased effort.
I use one because I'm always trying to cut just a few hundred calories a day from my diet, and watts are much more accurate than heart rate when I try to calculate how much energy I've expended on the bike. If I eat too much (always a probability) I don't lose fat, but if I don't eat enough, I can't recover from training and I get sick. But my hart rate varies A LOT with hydration, fatigue, temperature, etc, yielding wildly inaccurate measurements of how many calories I burn on a ride. Watts, again, provide a constant measure of energy expenditure.
Training with power also helps keep me honest. My ego often tries to convince me to chase my training partners up the hills, but since I'm supposed to be riding relatively easy right now, the powermeter's wattage display give me an excuse to ease off a bit and go slower. Luckily, my skinny friends are usually kind enough to wait.
Powermeters can be built into the hub of a bike's rear wheel or into its crank. Strain gauges measure how much force is being inflicted on the the chain. I have no idea how they work; my sense of physics is even worse than Bryan's. But as I pedal along, a computer on my handlebars tells me that I'm producing 200 watts as I roll along a flat trail, 300 as the road tilts upward, 450 as I climb a steep hill, or 1200 when I sprint as hard as I can without puking.
When I bought my Powertap sl+ last year, what arrived was only a rear hub, to which my local mechanic laced a DT Swiss 415 rim, using 28 Sapim CX-Ray bladed spokes. I broke 4 spokes within the first few weeks of riding, and the wheel continually developed lateral wobbles. Then we rebuilt the wheel using stouter, straight-gauge spokes, but those eventually pulled out of the holes in the rim, causing it to crack.
I tried replacing the rim with a Stan's Alpha 340 tubeless rim, but that build was simply too flimsy for my sizeable, uhh, ego. It flexed from side to side under hard pedaling, so my friend Ryan and I made a Christmas present out of the wheel. The recipient is a 120-pound climber who loves it. The technology is great, but it's probably not the best choice for a 195-pound guy who's hard on wheels.
One of the reasons I originally bought a Powertap was its portability; I thought I would be able to easily move the wheel from my road to my cyclocross to my time-trial bike--but the wheel was too heavy to race on the road and too light to ride on cyclocross courses, so it ended up staying on my road bike only for training. I never used it in a road race, a time trial, or any gravel rides, so I robbed myself of the most valuable power data.
Ironically, my new Quarq Powermeter crankset is freakishly easy to move from bike to bike to bike. Because my Trek Madone uses Trek's BB90 press-fit bearings--and my 'cross and TT bikes use standard English outboard bottom bracket shells--I can simply loosen one bolt, remove the two-piece crank, slide it into the other bike, and re-tighten that same one bolt with a torque wrench. It literally takes less time than moving the old Powertap wheel from bike to bike. Since I run SRAM components on the road and 'cross bikes, the GXP bottom bracket on the 'cross bike is already compatible with the crank. I had to change the bottom bracket on my TT bike, but that was a $40.00 upgrade. Easy. So, rather than taking off the wheel and/or changing the tire for road training, road racing, or gravel riding, I just move the crank.
My old powertap wheel build would cost about $1100 to build today; they're discounted at many retail sites. The cost of a new Quarq is about $1700, but the crank's greater utility and (hopefully) reliability make the extra $600 easier to justify.