Power Meters on Mountain Bikes

Should you run a power meter on your mountain bike?

I'll start this with a bit of troll-bait and say that if your primary training app is Strava, then no: you shouldn't. Strava is a social platform that, on its own, isn't super useful for structured training, even when you pay for the premium features. Strava's main features that support structured training are segments, laps, power curve, and "fitness". Forget about relative effort or "suffer score"--it's a useful metric only in hindsight.

Many of us run power meters on road bikes, and pretty much everyone who uses Zwift for any serious training is using some form of power-measurement, whether it's on the bike or build into the trainer. Power meters allow us to gauge our performance improvements (or losses) over time, train to specific targets like having a powerful sprint or being able to crush a time trial.

The data they provide can also be extrapolated to determine other datapoints like VO2Max, which is very basically a measure of how much oxygen your body can process, and is seen as a kind of performance cap.

They're also fantastic for interval training, which is generally easier for most folks to do on the road because the smoother surfaces tend to make it easier to hold a given power. Need a 5-minute interval? Go find a decently tall hill. Easy-peasy. Easier-peasier? Get on Zwift and do an interval session. Hard as hell and not worth the effort? Hold 5-minutes of steady power on a trail. We'll come back to that.

Zwift dangled a very silly carrot for me late last year that made me do interval training (which I HATE HATE HATE), and around the same time I started exploring yet another extrapolated datapoint from power meters: work (or W').

W' is, similar to VO2Max, a kind of functional upper limit on your body's capability. While VO2Max can change a little over time with fatigue and health, it generally isn't directly affected by your training load. W', however, is a measurement of your anaerobic capacity, expressed in kilojoules (kJ). It's like a reading of how much battery you have for work above critical power, which is roughly your threshold power. An amateur may have a W' value of 10 kJ, where a pro may come in closer to 25 kJ.

You only use this capacity when you're operating above your critical power, which is a computed value (similar to FTP) based on a complicated formula that uses your peak 2-minute and 10(ish)-minute power numbers to determine the specific rate of power drop-off in your power curve. Unlike most other metrics, though, you can recharge your W' battery by operating below critical power.

So say, for instance, your critical power (CP) is 290W, and you have a W' of 20kJ (wow--go you!). You're riding a tough, but manageable road race hiding in the group pedaling away at 250W for an hour. It's time for the big sprint, and when you give it the beans, you deplete that W' at ~1kJ/sec. Math will tell you that you have about 20 seconds to do your worst to the group, but after that you're at complete exhaustion.

...except you aren't completely exhausted, because just dialing it back to that steady 250W and not exceeding 290W will recharge your battery. After a few minutes you may be able to tackle another 15s sprint, and if you can sit-in long enough, maybe a full 20. But every time you rotate to the front and take a pull at 350W, you're taking a few joules.

Having the ability to measure and observe W' lets you see exactly what you have in the tank. It tells you when it's time to stop attacking and start saving for the big sprint, or when it's time to push the attack because you're sitting on TONS of kJ and nobody's making any moves.

Also similar to VO2Max, your W' can be trained, but changes are much slower than, say, FTP.

W' is complicated. It isn't sexy, it's not well explained on the internet, and isn't surfaced directly--to my knowledge--in any cyclocomputer screen or performance metrics. It can be inferred, though, with maths, and those maths can be added to a Garmin device thru a Connect IQ datafield.

And that means: if you have a power meter, you can monitor your real-time anaerobic capacity.

So while it may be nigh impossible to get direct useful insights into average power on a mountain bike ride, you can use that data to determine how hard you're going and when it's time to back off, which is dark magic when your power outputs are wildly variable.

You can also use that to gauge a race effort, which I covered earlier this year in my MonsterCross race report. And it's exactly why I threw hard-earned money at a mountain bike power meter.

My choice for the Lux wasn't anything crazy expensive: I went for the cheapest spider-based power meter I could find--even one that has a known propensity for being off by a couple %. Because I'm not interested in average power for that bike or even really in knowing specific peak values: I'm only after the amount of time I'm spending over critical power, and roughly how far over CP, to determine W' bal.

So should you run a power meter on your mountain bike? If you're willing to do enough testing to determine your CP values AND you're using a head-unit that can display W' AND you're using a training platform that supports additional datafields AND you're tired of blowing up in the middle of mountain bike races, then yes. Yes you should.

Dude what's wrong with your mountain bike??

 Ever let a buddy hop on your mtb just to check it out? It's a rare treat to find someone dimensionally similar and who has the same cleat system, and we're all a little peevish about letting others touch our equipment. But you owe it to yourself to swap rides once in a while.

I had the pleasure of riding with a bunch of high school shredders this spring as Alastair was competing in his last-ever season of varsity high school racing. These kids met every Tuesday and Thursday, and the range of equipment varied from Walmart+ to S-Works Epics. This sport is WILD. But once in a while the kids would want to check out each others' bikes, to hilarious effect. "Dude--what's wrong with your suspension? It's like you're on a jackhammer!" "Bro why is your seat so LOW?" "OMG how do you even ride this?"

I laughed. Everyone did. Occasionally a coach would take a quick look, but usually the bikes were left to the riders' preferences, and it was just assumed that they knew what they were doing.

I also assumed that I knew what I was doing. When I got my Canyon Lux WC CF 6 in early 2023, I had come from years of struggling to make the old 27.5 Anthem fast. It wasn't fast. It was never gonna be fast. But the Lux was. Right out of the box, zero config: fast. No pedal strikes, less fatigue, whip-sharp handling.

Mountain biking was fun! For the first time since I dropped dollars on a suspension bike in 2016, I was actually enjoying myself! I put as many miles on the Lux in 1 year of riding as I did on the Anthem in 6.

And over the years Alastair had become quite the adept mountain bike rider, such that he was competing for top-10 positions in the crowded JV fields at NICA & VAHS races. So we got him a really fast top-flight Trek ProCaliber 9.7, and suddenly his improvement just stalled. He could ride faster and push harder, but wasn't progressing in races and struggled mightily with confidence at speed.

He tried riding my Lux in a race and came out feeling even worse, with no improvement in position and back pain that lingered for days.

Fast forward to June 2 of this year. One day after graduation. One month after his final chance to race for high school glory. We took our bikes to Leakes Mill Park, which has a great trail system that takes about an hour to ride. Our goal was 2 laps, but riding behind him on the first lap I could not believe how twitchy his bike looked. His handling was this wild combination of pitching his body weight and catching himself by yanking the bike up under him. It looked very uncomfortable, and he was avoiding every root like it was a venomous snake.

After the lap I asked if we could swap bikes.

Within 1 minute we were doing the familiar "dude what is wrong with this thing??" Both of us! His suspension felt locked when it was open, and he insisted my saddle was too low. We rode the lap on each other's bikes with no changes, identitcal heart rates, and amazingly the lap was only a few seconds different from the first. 58 minutes of discomfort, but *different* discomfort.

I had a shock pump in the car and took 20psi out of his fork, then jumped back into the trail. It was like a brand new bike, but we didn't have time to play any more.

Then this past weekend we went out again, this time with him on a fork that works, and me still too low on the saddle. I fought to hold his wheel at the upper end of Z2 for almost 90 minutes before pulling out a tape measure and a hex wrench. 0.75" higher and suddenly the same watts were ~15bpm lower. I felt instantly fresher. Maneuvering was weird for a bit because the saddle felt like it was in outer space.

So with *both* of us on bikes that worked, we went to Pocahontas State Park and did a vibes-only ride, and we PR'd almost every single trail we touched...in Zones 2 & 3. Uphill, downhill, flat, and technical: everything was improved. Alastair was no longer ducking and weaving to avoid roots, and my power:hr was considerably higher than...ever on a mountain bike.

Now I can't wait to see what we can both do at a race-pace!

So if you know someone who's about your same size and can stomach the idea of letting someone else ride your bike, swap. Not for a day, but mid-ride. Because I'd ridden Alastair's bike before and hadn't noticed the problems. What I needed was the immediate context of having been on mine. And be honest with your feedback. Platitudes won't make you faster.