Because try as you might, your cranks just don’t overlap your wheel?
When it comes to performance metrics, I’m a big fan of the Ant+ standard – it’s supported by a wide range of dedicated devices like the Garmin Edge series GPS, as well as Android phones like the Xperia Active. It’s the native comms protocol for heart rate monitors, speed/cadence sensors and the more expensive category of cycling power meters (SRM, Quark, Vector, PowerTap…)
Unfortunately, your typical Ant+ speed/cadence sensor just isn’t compatible with laid-back cycling. Devices like the highly recommended Garmin GSC-10 rely on the fact that you can stick a sensor on the chainstay of a diamond-frame bike and it can ‘see’ a spoke magnet (for wheel speed) and a crank magnet (for cadence) from the same location. Try that on a bike where the pedals are several feet from the nearest frame member overlapping a wheel!
Enter Bontrager’s Interchange range of Ant+ digital sensors, which includes two of particular note for recumbent riders: discrete speed (part #424633) and cadence (part #426479) sensors!
At £25 or more apiece these sensors aren’t cheap, but however unconventional your ride you can still generate quality speed and cadence data into your chosen Ant+ computer.
I’m very happy with the quality of the data from these sensors. To illustrate, here’s a sample downloaded from my mobile phone showing first the raw cadence/speed data:
Now, take a look at the gearing (derived from speed and cadence using Excel). Can you tell when I down-shifted on the way uphill?
This is pretty useful information for me, especially looking at optimisation of gearing when rebuilding/specifying a drivetrain. For instance, with a couple of minutes work you can plot a chart like this:
Clearly, for this bike on this route, optimisation of the drivetrain in the narrow band between 60 and 85 gear inches is going to pay dividends.
In contrast, worrying about whether the bike is geared higher than 100″ is neither here nor there, and if it’s not overly efficient at < 40″ it’s not such a big deal either (if you need to draft, YMMV).
Note that because the chart is plotted against time in gear, not distance, it controls for the distorting effect of slow climbs (or screaming descents) nicely.
Bontrager sensor downsides
There are a couple of niggles which are disappointing at the price: one for each sensor in fact!
The speed sensor mounts easily using an o-ring, so it can be repositioned or moved between bikes in moments. However, the spoke magnet is a relatively poor design which is difficult to undo (mine looks like the plastic hinge is fatiguing, the overall lifespan remains to be seen). You can buy the far superior Cateye spoke magnet for a few pounds, but hey! you just spent the best part of £30 on one from Bontrager
In constrast, the magnet which fits to the cranks is a sensible slip-on rubber ring design, a big improvement over the Garmin crank magnet which you need to hold on with a zip tie (not much good if the inside face of your cranks isn’t convex, or at least flat).
Top marks… but then the cadence sensor itself requires cable ties to mount, instead of a rubber o-ring. To make matters worse it only accepts thinner cable ties than the standard ones I stockpile – cue some sweating with pliers every time I want to fit it to a bike (and then knifework when it needs to be removed).
These are relatively minor quibbles, especially if swapping between different bikes isn’t a big use case for you. Actually having reliable speed and cadence data on a recumbent is worth the cost and hassle in my opinion (assuming you want to capture this data in the first place, of course).
I once ‘solved’ this issue for myself by dismantling a GSG-10 and extending the wheel sensor with bell wire, lashing it to the fork of my RaptoBike while the main body of the sensor sat on the boom. That worked fine for winter training sessions on the turbo, but it didn’t survive more than a couple of trips in the real world.
The Bontrager sensors are much better.