Cycling power meter types, explained

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A power meter is a strain gauge — a small sensor that measures how much a piece of metal flexes — bonded somewhere in the path that runs from your foot to the rear wheel. That is the whole device. Everything a buying guide calls a “type” is really just an answer to one question: where in that path did they put the gauge?Pedal, crank arm, spider, or hub. Each spot flexes under your effort, so each spot can be turned into a wattage reading. But each spot can only see the force that reaches it — and that is the difference the marketing buries under a percentage.

Here is the percentage, gathered from the manufacturers’ own pages. Favero rates the Assioma DUO pedals at ±1%. Garmin says its Rally pedals capture data “within 1% accuracy”. 4iiii rates the Precision 3+ crank arm at ±1.0% from 0 to 4,000 watts. Stages rates its Power LR at ±1.5%, and Quarq rates the DZero spider at ±1.5%. Notice the spread: it is half a percentage point across the entire market. If you were choosing on the accuracy number, you would be choosing on nothing.

Where the gauge sits, and what that buys you

Walk the force from the outside in. Every placement is a real product category; each has a mechanical consequence you can reason about without ever testing one.

Pedal-based.The gauge lives in the pedal spindle, the very first metal your foot loads. Because there is a pedal on each side, a dual pedal set can measure the two legs genuinely independently — each spindle only ever sees its own leg. It is also the only power meter you can move between bikes with a pedal wrench in about two minutes, which is why Garmin’s own headline for the Rally is that the pedals are “rechargeable and easy to transfer.” The catch is that a pedal meter marries you to that pedal’s cleat system, and a spindle hanging out in the open is the part of the bike most likely to meet a rock or a curb.

Crank-arm.The gauge is bonded to the flat of a crank arm — almost always the left, non-drive arm, because it is cheap to reach and does nothing else. 4iiii describes exactly this: the Precision is a left-arm meter that reads your left leg. The arm flexes under that leg’s effort and the gauge reads the flex. It is the least expensive way into power, it is completely hidden and protected, and it moves between bikes only if you are willing to swap the crank arm onto a compatible crankset. The thing it cannot do is see your right leg at all, which is the whole of the next section.

Spider / chainring. The spider is the star-shaped piece that connects the crank axle to the chainrings, and it is the last place the force from bothlegs passes through before it becomes chain tension. A gauge here therefore reads the combined torque of both legs. Quarq’s DZero sits here and its page states it is accurate “within ±1.5% measuring both legs.” A spider meter travels with the crankset, so moving it between bikes means moving the cranks — more involved than pedals, less involved than rebuilding a wheel.

Hub.Furthest downstream of all: the gauge lives in the rear hub, reading the torque in the wheel after your legs, chain and cassette have all had their say. In principle this is the tidiest place to measure — it sees the power that actually reaches the road. In practice the category has all but vanished. SRAM, which had bought PowerTap from Saris in April 2019, discontinued the PowerTap G3 hub (and the P2 pedals) on 15 February 2021, per road.cc. A hub meter is also the least portable design there is: it is welded to one specific wheel, and that wheel has to fit the bike.

Single-sided vs dual-sided: the assumption hiding under the ±1%

This is the part that matters, and it has nothing to do with the accuracy spec. A left-arm meter, or a single left pedal, physically measures one leg. To hand your computer a total, it doubles the leg it can see.4iiii states the mechanism plainly: “to calculate the total power output of a rider, the single-sided data is doubled,” and gives the example that a left arm reading 125 W becomes a displayed 250 W. Read that again. The number on your screen is your left leg, times two.

That doubling is only correct if your legs are perfectly matched — a clean 50/50 split. Most people’s legs are not. If your left leg is quietly doing 48% of the work and the meter assumes 50%, your total is wrong, and it is wrong by a margin that has nothing to do with the ±1% on the box. The gauge can be a flawless ±1% at reading the left leg and still hand you a total that is off, because the error is not in the measurement — it is in the assumption stapled to it. 4iiii is fair about this: it argues most imbalances are “fairly minor” and that power is mostly a reference to yourself over time. That is true. It is also exactly why a single-sided meter is fine for tracking your own trend and a poor choice for anyone who wants a real left/right balance number.

A dual-sidedsetup — two pedals, or two instrumented crank arms like the Stages Power LR — removes the assumption by putting a gauge on each side. Garmin draws the same line across its own range: the single-sensing Rally 110 measures power from one side, while the dual-sensing Rally 210 adds left-versus-right balance and the rest of the pedalling-dynamics metrics. You pay more for the second gauge, and what you are buying is not more accuracy in the ±1% sense — you are buying the deletion of a guess.

One honest nuance on the spider and hub meters, because it cuts the other way. A spider sits downstream of both legs on a single shared part, so it reads their combined torque directly, but it has no separate gauge per leg — any left/right split it reports is inferred from wherein the pedal stroke the force appears, not measured on each side in isolation. A hub is further removed still. So “measures both legs” means something stronger for dual pedals and dual arms (two real gauges) than it does for a spider (one gauge, inferring the balance). Worth knowing before you pay for a balance figure you assume is measured when it is partly calculated.

What the accuracy number is actually for

Since every meter here claims ±1% to ±1.5%, it is worth being clear about what that claim even does for you. Power training runs on two different things, and they are not the same. Consistency— the meter reading the same watts for the same effort today, tomorrow, and after a cold start — is what lets you compare Tuesday to Tuesday, chase a personal threshold, and trust an interval. Absolute accuracy — the meter agreeing with some external truth, or with your friend’s meter, or with the trainer at the gym — is a different property, and it is the one that goes visibly wrong when two devices with two different ±1% offsets disagree by a few watts.

For almost everyone the first property is the one that matters, and it is largely independent of the headline percentage. A meter that reads 3% high but reads it the same way every single timewill train you perfectly well, because your zones are built on its numbers, not on anyone else’s. This is why we are not going to rank these tools by accuracy. We have measured none of them; the manufacturers’ figures are all within half a point of each other; and the property that actually governs a good training experience — repeatability across temperature and time — is not the number printed on the box. Where the absolute figure earns its keep is the day you switch meters, ride a friend’s bike, or race on power against other people — then two honest ±1% devices can still show you different watts, and neither is lying.

Moving it, powering it, and reading it

Three practical things separate these placements once accuracy is off the table.

Transferability. Pedals move in minutes; a crank arm moves if the crankset is compatible; a spider moves with the whole crankset; a hub moves only if you move that wheel onto a bike it fits. If you own one bike, none of this matters. If you have a road bike and a trainer bike, or a race bike and a winter bike, the pedal meter is the one you can carry between them, and that portability is often worth more than any spec on the page.

Battery.Placements split into two camps. Some meters carry a built-in rechargeable cell — Garmin made “rechargeable” the literal headline of the Rally 110/210 launch — and you top them up like a phone. Others run on a replaceable coin cell you swap when it dies. Neither is better in the abstract; a rechargeable never needs a spare battery in your saddlebag, and a coin cell never leaves you unable to ride because you forgot to charge. Just know which one you are buying so it is a choice and not a surprise.

Reading the number.A power meter is deaf and mute on its own — it does nothing but broadcast watts over ANT+ or Bluetooth. Something has to catch that signal, show it, and record it, which is a bike computer’s entire job. You do not strictly need a dedicated unit; a phone can pair to the same broadcast, and we lay out the real trade-off in bike computer vs phone. But the meter is only half the purchase. Budget for the head unit that turns its signal into a number you can act on, and while you are budgeting, it is worth seeing where a power meter sits against groupset spend— on many bikes a spider or crank meter costs about what a full groupset tier upgrade does, and it will change your training far more than a shinier derailleur.

What we haven’t done

We have not tested a power meter, compared two on the same crank, or measured anyone’s left/right balance. Every percentage on this page is a manufacturer’s own published claim, quoted from the page linked beside it and read on 8 August 2026 — Favero, Garmin, Stages, 4iiii and Quarq. Everything else is mechanism you can verify by following the force from your foot to the road and asking, at each gauge, what can this one actually see?