Last summer I discovered the joy of using bluetooth health sensors with an Apple Watch when I started using a Wahoo Tickr heart rate monitor while cycling. Having constantly updating and accurate heart rate information helped me to exercise better, and I’m happy to say I still use the Tickr each and every time I jump on the bike. But, it turns out it was not the complete solution to the problem to be solved.
The Problem to be Solved
Since this is the NosillaCast, let’s start there — what is the problem the Tickr partially solved, and my newest fitness accessory, the Cadence RPM from Wahoo, finishes solving?
Something I didn’t understand before I started my fitness journey is that when you exercise well, you finish more energised and feeling better than when you started. But, that only happens when you exercise well, and well absolute does not mean hard. It’s very difficult to explain what it is really means other than to say you’ll know it when you feel it. I call it being in the zone, and with the help of the Wahoo Tickr I’ve spent more time in the zone in the past year than before. But, every now and then, I’d still have a day where it just would not happen. I’d set out, and half an hour in I’d notice my calorie burn was lower than expected and so was my heart-rate. I’d try to get my heart rate up by pushing harder on those pedals, and it just wouldn’t work. My pulse would stay below my usual exercising level, and I’d feel crappier and crappier until I got home feeling drained rather than energised, and on the whole, quite cranky.
Thanks to the Cadence RPM I now understand how that was happening, and, more importantly, how to nip what I now understand as a negative feedback loop, in the bud. The fundamental problem was that I was missing a critical piece of data — how quickly I was turning those pedals. In cycling jargon, I didn’t have a measure of my cadence.
Those of you who’ve not learned to drive, or who’ve learned in the days of automatic transmissions and EVs will need to use your imaginations and your understanding of basic physics and history here, but those of you who can drive with a stick will grok this straight away.
When you use an internal combustion engine to power a vehicle, what do you get to control in terms of power? You choose how hard you want the engine to work with the throttle, and how fast you want it to spin with the gears. You quickly learn that engines are only able to efficiently produce power within a narrow band of spin speeds, which we count in revolutions per minute, or RPM. For a typical diesel engine that’s about 1,500 to 2,000 RPM, while a petrol (gasoline) engine tends to be quite content in the much broader range from about 2,500 RPM to 4,000RPM. The key point is that to drive efficiently you need to watch that rev counter (or get to know your engine so well you can intuit the RPM from the sound it’s making).
What has any of that got to do with cycling? Well, when you cycle, your legs are your engine, and just like an internal combustion engine, your legs only work efficiently within a finite range of muscle expansion and contraction rates. When you’re cycling you can literally measure that rate of muscle expansions and contractions in RPMs! Obviously my legs don’t work anywhere near even the low end of a diesel engine’s power band at 1,500RPM, but they can get up to about 110RPM when conditions are right!
So, getting back to the problem to be solved, what was really happening on those days I was just not getting into the zone? By trying to cycle harder I was actually opening the throttle while shifting into a higher gear, causing my RPM to fall too low. So low that if my legs were an internal combustion engine they’d have stalled!
The reason for this mistake is that cycling badly feels like cycling harder, and cycling better feels like working out less, but those impressions are totally wrong! I now know is that when you keep your RPM high you feel like you’re cycling less hard, but you’re actually putting down more power, and your heart rate will go up. What I now know is that when I’m not in the zone I need to check my RPM, and what I’ll probably find is that it’s too low, so I need to shift to a lower gear, pedal faster, and it will feel like I am working less hard, but what I’ll actually see is my heart rate climbing back up into the right range, and I’ll be back in the zone within minutes.
The Generic Solution
OK, so that’s the why of this review, now let’s get into the how.
When you track a cycling workout on an Apple Watch you can use the Digital Crown to scroll between multiple screens, each showing different sets of data. In modern versions of WatchOS this even works when the screen is locked BTW. The second screen down has a placeholder for cadence in RPM, but it’s usually blanked out with a - symbol because the watch has no way of knowing how many times you’re turning the pedals per minute. It needs some help, and that’s where bluetooth cadence sensors like the Cadence RPM from Wahoo come in.
My Solution — the Cadence RPM from Wahoo
This little device attaches to either your pedal crank or your cycling shoe, and uses accelerometers to count the turns which it relays to your watch over Bluetooth LE. When I say little, I do mean little, it’s a small black pill that Wahoo’s website tells me is 1¼ by ¾ by 1 inch in length, width, and height, and weighs just seven grams!
The reason it can be so small and light is that it’s not powered by a relatively inefficient rechargeable battery and doesn’t have a charge port. Instead, it’s powered by a the same kind of CR2032 button cell that powers Apple AirTags. Like AirTags it uses low energy Bluetooth, and like the AirTag, the expected battery life is about a year.
Because you only need to replace the battery once a year, two of the three mounting options are a lot more permanent than I was expecting. If you choose to attach this sensor to your pedal crank, you’re going to need some new consumables once a year!
OK, let’s back up a little. When you open the box you’ll get one sensor, and three different mounting options. Two of those three will attach the sensor to the bike-side of the pedal crank attached to the front gears. (Why so specific? Well if you were to attach it on the other side of either crank it would rub against your cycling shoes as you pedal, and you were to attach it to the inside of the other crank it would probably clip the part of the bike’s frame that connects the centre axel to the rear axel.) The third option attaches it to a cycling shoe, and in this case, you can choose left or right.
If you have one bike and one pair of shoes then I think it probably makes most sense to attach the sensor to one of your shoes (if you can), but if you have two pairs of cycling shoes and one bike, you should probably attach your sensor to the bike. Remember, you’re going to be attaching this thing for a year at a time!
But wait, what if you have, say, two pairs of cycling shoes because it often rains where you live, so you often need to give one pair a day to dry while you use the other, or, what if you have a hybrid bike for cycling on dry roads, and a mountain bike for wet roads and trails? Well, the answer is simple, you buy two! Thankfully, they’re not expensive 🙂(Just €39 each)
OK, so let’s look at the three mounting options:
- The shoe mount is a little plastic caddy that’s designed for cycling shoes with velcro straps. The caddy slides over one of the strap and then you snap the sensor into the caddy. I don’t know how well this works because my mountain biking shoes have laces, and my traditional cycling shoes have straps that are too wide for the caddy to slide over 🙁
- The second option is literally an M3 adhesive pad to stick the sensor to your pedal crank. This is a little too permanent for my liking, so I didn’t go with that option either.
- The third and final option is a little neoprene cover with holes for small zip-ties that you slide the sensor into and then zip-tie to your pedal crank. This is easier to mount because you can tie it loosely, adjust it until it’s perfect, and then yank the ties tight and snip off their ends. I will of course need to cut those zip ties off once a year to replace the battery, and I’ll need new zip ties to re-attach it. But, I have lots and lots of zip ties of all sizes in the house at all time, and I can’t say that about M3 adhesive pads!
So, how does it work?
Well, you wake it up by moving it, then, the first time you use it, you open the Bluetooth settings on your Apple Watch and pair it. After that, you just start cycling, and it will wake up, connect, and just work! There are no buttons, all you have is a little teeny tiny blue LED that flashes when the sensor wakes up. The thing ships with a battery in place, so as soon as you take it out of the box you’re likely to see that LED flashing at you.
I should also mention that for those of you who track your cycling with a traditional cycling computer, you can probably use the Cadence RPM too, because as well as supporting Bluetooth LE, it also supports the ANT+ standard.
In the past sensors were connected to cycle computers by delicate little wires you had to zip-tie to your frame. (And no matter how diligent you were, you still always seemed to somehow snag them on something and snap them!) It’s no surprise that cycle computer makers switched to wireless alternatives as soon as they became viable, and after the inevitable wild-west phase of competing standards, ANT+ won out. So, the fact that this tiny little sensor can work over Bluetooth or ANT+ really does make it extremely versatile.
What Have I Learned?
Finally, what have I learned about my cycling RPM?
I decided to start my exploration of cadence by doing absolutely no prior reading what so ever about what kinds of ranges are good or bad, but to simply measure what I do naturally to see where my intuitive baseline is. After a few days it soon became clear that my default behaviour was to vary my cadence a lot! My low-end was about 65 RPM, and I went up to about 100 RPM. I also notice that when I was feeling at my very best, like I was perfectly in the zone, my cadence was usually at the higher end of that range, between about 85 and 95 RPM.
Then, once I knew what I was doing, I searched for advice online, and soon found that the consensus view goes something like this:
- Beginners tend to pedal at lower cadences, sometimes as low a 50 RPM, and rarely above 65 RPM. This beginners tendency is not good, because the human power band only really starts around 65 RPM!
- Experienced amateurs tend to pedal a little faster, between about 65 and 85 RPM, which is entirely within the human power band, so much more efficient.
- Athletes try to stay at the very top of the human power band between about 85 and 110 RPM. This is actually the most efficient part of our power band, and it falls off dramatically if you cross about 115 RPM.
OK, so at least I was never letting my cadence drop below the start of the power band at 65 RPM, but what would happen if I proactively tried to stay more tightly within the top of the band? That’s exactly what I’ve been doing for the past month or so, and the result is that I now get reliably satisfying exercise in all conditions!
What changes with the weather, the terrain, and my energy levels is how far and how fast I go, and what stays remarkably steady is my heart rate. I now shift gears a lot more often, and in so doing, keep my cadence nice and tight between 85 and 95 RPM on level roads, while allowing myself to drop into the high 70s briefly on short hills, and choosing to pedal a little faster, up to about 105 RPM, when there’s a strong tail wind or I’m racing down a big hill.
The Bottom Line
I’ve found these little devices work reliably and well, and the only quibble I have is that it feels wrong to zip-tie something battery powered into place. I don’t know if the batteries will last me a year given how much I cycle, but I can say they definitely last more than two months 🙂
In short — I’m really happy I chose to spend €78 buying two €39 Cadence RPM sensors from Wahoo!