Wondering whether a crank based power meter is right for you? It measures the flex in your crank arm and turns it into watts. Left-only versions are cheaper; dual-sided ones measure both legs and cost more.
This page is one part of our wider guide to cycling power meters.
| Item | Value |
|---|---|
| Left-only example: left leg 125W | Computer shows 250W |
| Dual-sided example: left 150W, right 145W | Computer shows 295W |
| 4iiii claimed accuracy | +/- 1% |
| Stages Dura-Ace G3 L-R claimed accuracy | ±1.5 per cent |
| Stages Dura-Ace G3 L-R power range | Up to 2,500 watts |
| Cost of two crank-based meters on a Dura-Ace chainset (BikeRadar, UK) | £700 added |
The figures come from 4iiii’s single versus dual blog and BikeRadar’s Stages review. Accuracy claims come from the makers and prices change, so check the sources before you rely on them.
How a crank based power meter works
Small strain gauges are bonded to the crank arm or the spider. When you push on the pedal, the metal flexes by a tiny amount and the gauge’s electrical resistance changes. The meter’s electronics convert that signal into torque.
Power is torque multiplied by how fast the crank turns. An accelerometer or gyroscope supplies the cadence, so the unit can report watts without any extra sensor. For the wider principles, see our explainer on how do cycling power meters work.
Because the sensor sits in the crank, you measure force before it reaches the chain. That means drivetrain wear and chain lubrication do not change the reading.
Left-only, right-only and dual-sided designs
Crank-based meters come in three common layouts:
- Left-only: a sensor on the non-drive crank arm measures one leg.
- Dual-sided: one sensor on each side measures both legs separately.
- Spider-based: gauges sit in the chainring spider and measure both legs’ combined output.
With a single-sided meter, the data is doubled to estimate total output. 4iiii gives an example: if your left leg produces 125W, your computer displays 250W as your total output.
A dual-sided meter measures each leg independently. In 4iiii’s example, a left leg at 150W and a right leg at 145W gives a total of 295W, the exact output rather than one side doubled. It also lets you see left/right balance.
How accurate are they?
Makers publish tight figures. 4iiii states that both its single and dual models deliver within +/- 1%, while BikeRadar reports that the Stages Dura-Ace G3 L-R has a claimed accuracy of ±1.5 per cent.
Treat these as manufacturer claims, not independent lab results. Reviewers such as BikeRadar compare units against a second meter, and in that Stages review it tracked a Quarq system in static and mostly outdoor tests.
The main weakness of a left-only meter is the assumption that both legs are equal. If your legs produce noticeably different power, doubling one side will misstate your total, and the error can shift as you fatigue or change position.
Cost, reliability and swapping bikes
BikeRadar’s reviewer says that for most riders a single-sided, left crank option provides sufficient data with enough accuracy and can be bought for considerably less. If every data metric matters, a left-right model is the answer.
Dual-sided measurement is not cheap. In BikeRadar’s review published November 21, 2019, two crank-based meters added £700 to the standard Dura-Ace chainset’s cost.
BikeRadar adds that crank-based meters are less easily swapped than power pedals, but they are more reliable and less prone to damage. If you move a meter between several bikes, that trade-off matters.
Swapping a crank means removing the arm and refitting the chainset, which takes tools and some care. A sensor mounted elsewhere avoids that; our page on the hub based power meter covers that alternative.
Practical points before you buy
A crank-based meter ties you to a crank. Before buying, check these details, because the wrong match is an expensive mistake:
- Compatibility: chainset model, spindle standard and arm length must suit your bike and drivetrain.
- Battery: some units use a replaceable coin cell, others a rechargeable battery. Replaceable cells are simple on a long trip; rechargeable ones need a cable.
- Protocols: make sure the meter speaks the same wireless language as your head unit or app.
- Calibration: most units need a zero offset before rides. Do it with the bike still and the cranks unloaded.
- Installation: some brands offer factory fitting on a crank; others sell a ready-to-ride chainset.
Fitting matters for accuracy. A loose or poorly seated crank arm can distort the flex the gauges measure, so follow the maker’s torque instructions for the crank bolt.
When left-only readings can mislead
Most riders are not perfectly symmetrical, and that is fine as long as the difference stays steady. A steady offset still gives you consistent numbers for tracking fitness, since you compare like with like.
Trouble starts when the gap changes. Injury, fatigue, a different bike fit or a switch from seated to standing climbing can all shift the share each leg contributes. Your total then moves in a way that has nothing to do with your real output.
Two habits help:
- Stay with the same meter for the training you compare. Changing units mid-block adds noise.
- If you ever ride a dual-sided meter, look at your balance across easy rides and hard efforts to see how far a doubled left leg would drift.
This is also why accuracy claims need context. A figure such as +/- 1% describes the claimed accuracy of measured power; it does not account for the error from assuming both legs contribute equally.
Crank arm versus spider: where the gauges sit
Where the strain gauges sit changes what the meter can see. A gauge on the left arm sees only that leg’s force. A gauge in the spider measures the combined contribution of both legs at the chainrings.
BikeRadar describes the Stages dual-sided design as polycarbonate measuring pods that fit on the inside of the left crank and alongside the drive-side crank within the spider. Other makers choose different layouts, so read the product page for the model you are considering.
Spider-based systems can be a good match if you change chainrings often, since the sensing element stays in place. Arm-based systems are usually lighter on the budget and simpler to fit, but they are tied to the particular crank arm.
Who gets the most from each type
Different riders need different things from the same number. Think about what you will actually do with the data.
- Pacing and zones: a steady, repeatable number matters more than perfect symmetry, so left-only often does the job.
- Racing and time trials: riders chasing small gains may value the full picture from both legs.
- Returning from injury: balance data can show whether the weaker leg is catching up.
- Bike fit and technique work: separate left and right data helps spot asymmetry in the pedal stroke.
Whatever you pick, use the same meter consistently. A trusted, repeatable number is worth more than a slightly better one you cannot compare with last month.
Which one should you choose?
- Decide whether you need left/right balance data. If not, a left-only unit is usually enough.
- Check bottom-bracket and drivetrain compatibility, frame clearance, and the spindle standard and arm length.
- Look at the claimed accuracy, battery type and whether the unit sends both ANT+ and Bluetooth.
- Compare the price of left-only against dual-sided in your market.
If you have a known leg imbalance, an injury history or want to study pedalling in detail, dual-sided is the safer choice. If you mostly want pacing and training zones on one bike, left-only is a sensible starting point.
Frequently asked questions
Does a left-only power meter just double my left leg?
Show answer
Yes. The single-sided reading is doubled to estimate total power, so 125W on the left shows as 250W on your computer, according to 4iiii.
Do I get left/right balance from a left-only meter?
Show answer
No. Independently measured balance requires separate measurement of each leg; some spider meters report an estimated balance.
Is a crank based power meter more reliable than pedals?
Show answer
BikeRadar’s Stages review says crank- or spider-based meters are more reliable and less prone to damage than power pedals, though harder to swap between bikes.
Are the accuracy figures guaranteed?
Show answer
No. They are manufacturer claims, such as +/- 1% from 4iiii and ±1.5 per cent for the Stages model. Real results depend on installation, calibration and your riding.
