Single vs Dual-Sided Power Meters

Almost every power meter family sells the same product twice: once measuring one leg, once measuring both, with a meaningful price gap between them. Favero does it with the Assioma Uno and Duo. Garmin does it with the Rally 100 and 200 series. 4iiii does it with the Precision and the Precision Pro. The marketing implies that the dual version is the accurate one and the single version is a compromise, and that is not what is going on.

This page is about what actually differs, what balance data is really telling you, and the small number of situations where paying up is the right call.

What single-sided actually does

A single-sided meter measures the force applied by your left leg, then multiplies by two. That is the whole mechanism. It does not estimate your right leg from anything clever — it simply assumes the two are equal.

So if your legs split 50/50, the number is right. If they split 52/48 in favor of the measured leg, the reported total is roughly 4% high. If they split 48/52 the other way, it reads roughly 4% low. Notice that this error is a bias, not noise: it does not wander from ride to ride, it sits in the same place every time.

That distinction is why single-sided works. A training plan is built on repeatability. Do an FTP test on a single-sided meter and every zone derived from it inherits the same bias, so your threshold intervals are still at threshold, your endurance rides are still endurance, and your month-on-month progression is still real. The bias cancels out of every comparison you actually make.

Is your asymmetry actually big?

The honest answer for most riders is no, and it is worth knowing that most people who assume they are lopsided are not. Small differences are normal and are not something to fix. Balance also shifts within a ride — most riders drift toward their stronger leg as they fatigue, and again during very hard efforts — so a single reading tells you very little.

There is also a way to find out before you spend: if you ride a smart trainer and you can borrow a dual-sided meter for a week, you will know. Failing that, the practical signal is symptomatic rather than numeric — one-sided knee pain, a saddle you keep sliding to one side of, or a history of a one-leg injury are all better reasons to want balance data than curiosity is.

The four cases where dual-sided is worth it

1. Single-leg injury rehabilitation.This is the strongest case by a distance. After a knee, hip or ankle injury, riders guard the affected leg without noticing. Balance data is the only way to see it, and it turns a vague "does it feel okay?" into a measurable return-to-load progression. If this is you, buy dual-sided.

2. Active bike-fit work. A persistent asymmetry can come from a leg-length discrepancy, a cleat position, or a saddle that is fractionally off center — all fixable. You cannot chase what you cannot see, and a fitter with balance data is working with evidence.

3. Cross-device consistency. If you need your outdoor power meter to agree closely with your indoor trainer — because you race online, because a coach is prescribing from one and you are executing on the other, or because you are simply the sort of person who will be annoyed forever — dual-sided removes one whole source of disagreement. Some of the remaining gap is drivetrain loss, which our accuracy guide explains.

4. You already know you are asymmetric. If a fit study or a previous meter has shown you a genuine, persistent imbalance beyond a few percent, a single-sided meter will report a total that is meaningfully wrong, and you will know it is wrong every time you look at it.

What dual-sided will not do

It will not make you faster. It will not correct an imbalance — knowing about one does not fix it, and most attempts to consciously even out a pedal stroke reduce efficiency rather than improve it. And it will not make your training more precise in any way you can act on, because the interval you are prescribed is a total wattage regardless of which leg produced it.

The most common outcome of buying dual-sided without one of the four reasons above is that you watch the balance field for three weeks, conclude that you are 49/51, and never open it again. That is a lot of money for a fact.

The upgrade path matters more than the decision

Here is the argument that resolves most of this. Several platforms let you start single and add the second sensing side later — Garmin's Rally range is built that way, and Favero's Assioma has the same structure. That converts a permanent decision into a reversible one.

So: buy single-sided now, ride it for a season, and if you find yourself genuinely wanting the balance figure — or if you pick up an injury and suddenly need it — add the second side then. You will have had a power meter for the whole intervening period, which is worth vastly more than having a better power meter three months later. The specific Favero version of this decision is in Assioma Duo vs Uno.

Where the saved money should go

If you take the single-sided route, the difference is not small, and it buys things that will change your riding more than a balance field ever will. A proper fan is the highest-return purchase in an indoor setup, because heat ends more sessions than anything else. A heart-rate strap adds a second, independent view of your effort for a fraction of the cost. And a mat solves noise, sweat and floor protection at once.

The full field, single and dual, is ranked in best power meters, with the pedal-specific options in best power meter pedals and the cheaper crank route in best crank power meters.