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Crank-based vs spindle power meters: frame clearance and compatibility

By Crankworthy · Published · Updated

A left-arm power meter adds a pod to the inside of your non-drive crank arm, so the question is whether your chainstay leaves room for it. 4iiii’s pod is 5.5 mm tall and they ask for roughly 8 mm of gap — about the thickness of an 8 mm hex key slid between arm and stay — and they will not sell you one without a photo of that check 1. Spindle-based meters avoid the clearance question entirely by putting the electronics inside the axle, but then the bottom bracket shell decides what you can fit.

Two architectures, two different constraints

Both types measure the same thing — how much the metal flexes under your pedalling — but where they put the sensor decides what limits them.

A strain gauge is a fine metal grid bonded to a part that flexes. Load it and the grid deforms, its electrical resistance changes, and the electronics convert that into torque. Multiply torque by how fast the crank is turning and you have power, which is why every one of these also carries an accelerometer to measure cadence.

Crank-based meters bond the gauges to a crank arm or to the spider behind the chainring. A left-arm-only unit measures one leg and doubles it; a dual-sided system measures both and can report a genuine left-right split. Spindle-based meters put the gauges inside the bottom bracket axle, where nothing external can catch on the frame and the weather cannot reach.

So the crank-based question is geometric: does the pod clear the frame. The spindle-based question is one of standards: does a spindle carrying electronics suit your bottom bracket shell.

The clearance numbers

Figures from 4iiii’s own buying guide [1]. Other manufacturers publish their own — check the one you are buying rather than assuming these transfer.

Pod height (4iiii Precision 3)
5.5 mm on the inside face of the non-drive crank arm [1].
Gap needed
Roughly 8 mm between the inner face of the left crank arm and the chainstay [1].
How to test it
Slide an 8 mm hex key through the gap as the crank passes the chainstay. If it passes freely at every point of rotation, you have the clearance [1].
Proof before purchase
4iiii ask for a photo of a successful clearance check before they will ship a factory-installed unit [1].
Where it goes wrong
Gravel and mountain frames with flared chainstays built around wide tyres. The tyre clearance that makes the frame good is exactly what closes the pod gap.

Spindle meters and what your shell allows

Putting the electronics in the axle trades a clearance problem for a standards problem: the spindle has to fit your bottom bracket, and the bearing has to fit around the spindle.

The tension is simple geometry. A press-fit BB86 or BB92 shell has a nominal 41 mm bore 2. The larger the spindle running through it, the less radial room is left for the bearing between spindle and shell — which is why a 24 mm steel spindle is the comfortable fit in those shells and the larger 28.99 mm and 30 mm spindles are tighter propositions.

That does not make a large-spindle meter impossible in a narrow shell, but it does mean the bearing is working harder, and it is worth knowing before you buy a spindle-based meter for a press-fit frame. On a threaded BSA or a T47 shell there is more room to play with 2.

The other practical point: a spindle meter usually arrives as a crank-and-spindle assembly, so you are replacing the crankset rather than adding to it. A left-arm meter is the cheaper upgrade precisely because you keep everything else.

Before you buy

  1. 1

    Measure the gap with a hex key

    Rotate the cranks slowly and try to pass an 8 mm hex key between the inside of the non-drive arm and the chainstay at every point 1. The tightest point is the one that counts.

  2. 2

    Check the manufacturer’s own requirement

    Pod heights and required gaps differ between makers and models. Use the number from whoever you are buying from, and follow their clearance-check procedure if they publish one 1.

  3. 3

    Match the crank family

    A left-arm meter is sold for a specific crank. The arm must match your crank’s spindle interface, and it has to be the same length as the arm on the other side.

  4. 4

    Set the crank length in the app

    Crank length is part of the power calculation. If your new arm is a different length from the old one, set it before the first ride or every number will be wrong.

  5. 5

    For a spindle meter, check the shell first

    Identify the shell standard and its bore, then confirm the meter is offered for it 2. This is a crankset purchase, not an add-on.

Cranksets and bottom brackets in the catalog

We do not stock power meters. These are the host cranks and shells the question is usually asked about — crank length is recorded on each.

What this job needs that we don’t stock

Named so you can source them elsewhere — we would rather say this than point you at a product that is not the right one for the job.

  • Power meters of any kind — crank arm, spider or spindle. None are in our catalog, so this guide names no product to buy.
  • Press-fit bottom brackets, which is where the spindle-diameter question actually bites.

References

  1. 1.The Complete 4iiii Power Meter Buying Guide — 4iiii Innovations (5.5 mm pod, ~8 mm gap, 8 mm hex key check, clearance photo required)
  2. 2.Bottom Bracket Standards and Terminology — Park Tool (shell bores and widths)

Related

Part of the bike upgrade and compatibility guide.

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