Junior Velo’s first product review. It may or may not be the last…
My son has been training with a Power Tap Pro rear hub on the road for the last half-year or so. With track season (still in Summer for juniors in the USA) on the way, I was curious to see what power he’s producing on the track.
There were several possible solutions. Ergomo, SRM, and Polar all work on track bikes with less modification than the Power tap, but each has its own drawbacks. Admittedly, the main drawback of SRM is the price, but that was a factor. No free products for review at a small operation like Junior Velo.
Installation is fairly simple. If you can remove and replace an axle on any hub, you can handle this one. You just need cone wrenches, a 10 mm Allen wrench, and a regular wrench to fit the locknuts. I won’t give instructions, because they come with the kit. One thing I wondered about, and can now answer, is how much re-dishing the wheel will need. Our wheel (Mavic Rim, set up for Shimano 10-speed) only needed about 1.5 turns on each spoke nipple to re-center it. That means you won’t need to buy new spokes in most cases. Of course you can’t do this blindly - have the work done (or at least checked) by a pro unless you have built wheels before.
Another issue for power measurement on track bikes is what happens when you put back pressure on the pedals. With no coasting, the power measurement device is going to detect some kind of stress. Will that register as positive power, zero, or negative? Will it break something? The answer for Power Tap Pro and SL hubs (or at least the computers they come with) is that you have to read the manual and use the manual calibration option. This prevents the automatic calibration from being confused by back pressure. I’ll add to this article when we see what the data actually look like.
Here are the pros and cons I have collected so far:
- As easy a hub modification as you could ask for.
- A lot cheaper ($100) than a new system, if you already have a PowerTap system.
- Spacers are included in case you need to make this work on a road-spaced frame.
- The axle is too long. They try to cover everyone’s needs with one axle, rather than offering a standard track length. Even on a bike with fat Aluminum dropouts, we will need to remove about 1.5 cm of axle. To do it right, you need a threading tool to repair the damage you do with the saw. For $100, they could offer a choice of sizes.
- It’s just a little wide. I had to discard the thin washers which go inside the locknuts.
- The axle nuts provided are the cheap type which turns all in one piece. In my opinion only the type where the wide part which contacts the frame turns separately are worth having, due to reduced damage to the dropouts.
I’ll add notes if we learn more on the track. By the way, the picture shows the untrimmed axle, but I have already replaced the new axle nuts with used ones of a better design.
The promised notes:
We tried out the PowerTap in a workout yesterday. Pursuit was on the menu, so the screen shot below is a pursuit-like effort. It’s not a true standing start - just a slow roll into a standing acceleration.
The system seemed to worked well overall. In the screen shot (click for a more legible size), you can see that power is shown as zero when the rider is slowing down before the effort - he was definitely putting back pressure on the pedals at that time.
There were a couple of oddities. The two spikes in the cadence (upper, green) line don’t make sense. Either the system switched between hub-based and crank-based cadence detection or it just plain produced some wrong numbers. The computer also reported impossibly high speeds at first when I forgot to turn the power feature on at the beginning of the workout (the PowerTap had recently been used as a bike computer/HRM without the hub). That was resolved when the settings were correct.
This graph doesn’t show the regular turn-straight-turn oscillation of speed shown in some widely circulated power meter graphs. I think that can be attributed to a windy day and a lack of formal pursuit work since last year.