Pedal power-meter crank length: physical arm, device setting and readback are separate
Identify the installed arm and exact pedal system, find which controller supplies its crank-length value, and preserve a readback record without confusing calibration, scale factors or a changed bicycle with a verified setting.
Published 15 October 2026 · OpenBikeFit
One arm. Two independent records.
- Physical reference
- 165 mm
- Displayed setting
- 170 mm
Independent fictional length ledger: 170 − 165 = +5 mm; 5 ÷ 165 × 100 ≈ +3.03% relative to 165 mm. Using 170 mm as denominator gives 2.94% instead. These are LENGTH comparisons, not measured watt errors or ride-file correction factors.
Authority comes from the exact manual.
- Rally 110/210, scoped manual rule
- The connected watch/computer entry takes precedence over a later Connect-app length entry—only for this documented model.
- Assioma, FAQ A010
- Where the computer supports length entry, its setting overwrites the app value. Preserve this model-scoped statement separately.
Original schematics, not to scale or a real device screen. No sensor is connected, setting changed, acknowledgment checked or power accuracy verified.
On this page
- 01Identify the installed arm, side and sensor family
- 02Treat the displayed number as a setting, not a measurement
- 03Find which controller owns this model’s length
- 04Separate a selected screen from a confirmed readback
- 05Keep length, zero offset and scale factor separate
- 06Use ratios as length arithmetic, not a power correction
- 07Recheck the record when pedals move between bikes
- 08Preserve the source of indoor and outdoor power records
- 09Make a small hand-off record without inventing fields
- 10Know what this free guide and the crank workspace do not do
Keep an honest equipment record
Read the blank reference spaces
General paper references plus supplementary notes, not dedicated sensor/readback fields or a device verification. No saved bike or electronic setting is created.
Open toolA crank replacement or a move between bicycles can leave two different questions hidden behind one familiar number. What is the length of the arm actually installed, and what length is the pedal power-meter system using? Reading a marking answers a hardware-identification question. Reading a menu answers a configuration question. Neither action, by itself, proves the other was completed or that a recorded power value is accurate.
This guide is a reading and reference journey for adults. You can follow it without connecting a device, loosening a pedal, entering a measurement or starting a camera. It supplies no installation recipe, calibration procedure, power correction or recommended crank size. The fictional records below help expose missing information; they are not observations of your bicycle or a test of any named product.
Identify the installed arm, side and sensor family
Begin with the exact bicycle configuration, installed arm model and side. Preserve the identified centre-to-centre crank length, its source and any unresolved endpoint. The physical measurement owner remains our separate crank-length guide: the arm outline, pedal body and shoe stack are not interchangeable endpoints. If a marking is obscured or contradictory, leave the length unresolved until a suitable part specification or physical measurement establishes it. A familiar-looking arm is not a substitute.
Then identify the actual measuring system and generation. Pedal, crank-arm, spider and trainer systems do not become one configuration interface merely because each reports watts. This article concerns pedal-system crank-length references, not a rule that every power meter has the same field. Keep the product manual with the model it describes. The Rally RS/RK/XC entry page, for example, scopes its illustrated controller procedure to Edge 830 or 1030 and directs other devices to their own instructions.
Treat the displayed number as a setting, not a measurement
Write the physical length and the observed electronic value on separate lines, each with its unit. A screen showing 172.5 does not establish that someone measured a 172.5 mm arm. The Rally 110/210 entry manual explicitly documents that value as its default when no length is entered and says the correct length is needed for its power calculation. Do not generalise that default to another generation, brand or measuring architecture.
Record how the value was obtained: a newly opened relevant sensor field, a remembered earlier screen, a catalogue specification or an unconfirmed draft typed into an app. These are different evidence states. An empty field is unknown, not zero and not permission to copy a common size. A length unit also belongs to the value; do not silently treat a number copied from an unrelated display as millimetres.
Find which controller owns this model’s length
Identify the connected watch, cycling computer or app that can supply the value, not just the device on which it is easiest to find a menu. The Rally 110/210 System Settings page states that, after length is entered using the connected watch or computer, a later Garmin Connect app entry does not take effect. That is a documented rule for that system, not a universal rule about every Bluetooth or ANT+ device.
Favero’s Assioma FAQ A010 describes its own precedence: where the computer supports setting crank length, its value overwrites the app value. Keep the two manufacturer statements scoped rather than merging them into a made-up common protocol. Your reference record should name the documented controlling device and the source used to identify it. If support depends on an unverified controller or software version, retain that uncertainty rather than claiming that the last number typed must win.
Separate a selected screen from a confirmed readback
A device can be listed, selected or connected without your record showing which particular configuration was checked. Use the applicable instructions to establish the intended sensor and controlling device. Preserve a technical model or locally meaningful equipment label; a public hand-off note does not need a rider’s name, full serial number, location, account identifier or screenshots containing personal information. Do not expose an entire paired-device list merely to document one length.
For the reading record, distinguish intended value, observed readback and time or condition of observation. A fictional example says: bicycle A has an identified 165 mm arm; the relevant controller field was observed at 170 mm; confirmation of an updated effective value is not recorded. That is a mismatch to resolve, not a finished update. A proposed correction written on paper remains proposed. Record a later confirmed observation separately instead of rewriting the earlier evidence as though it had always matched.
Keep length, zero offset and scale factor separate
Crank-length entry, an operation labelled calibration or zero offset, and an optional power scale factor are not interchangeable confirmations. The Rally 110/210 settings page lists crank length and Auto Zero Offset separately. The Assioma FAQ likewise discusses length entry and calibration in separate entries. These distinctions do not provide a universal sequence, interval or mathematical equivalence between the operations. Use the exact current instructions for the actual configuration.
A message saying calibration succeeded is not, by itself, a readback of the physical arm length or the effective length parameter. Nor does a length field prove a calibration operation happened. Do not copy a calibration frequency from one product into another product’s record. Keep each required action, observation and unresolved status on its own line. This guide does not perform those actions, verify a load-free condition or authorise a static-weight test or scale-factor adjustment.
Use ratios as length arithmetic, not a power correction
Take an independent fictional pair: physical reference 165 mm and displayed setting 170 mm. The signed setting-minus-physical difference is 170 − 165 = +5 mm. Dividing that difference by the explicitly chosen physical reference gives 5 ÷ 165 × 100 ≈ +3.03%. Reversing the denominator is a different calculation: 5 ÷ 170 × 100 ≈ 2.94%. A percentage without its reference hides which comparison was made.
Those are length differences only. This page has not measured pedal force, angular velocity, left/right contribution, a device algorithm, calibration state, transmission behaviour or the origin of an existing ride file. It therefore does not establish a +3.03% watt error or supply a factor for rewriting historical power. Do not adjust a scale factor until two unrelated displays agree simply because this length ratio looks plausible. Keep the mismatch and the relevant manufacturer question instead of manufacturing a corrected result.
Recheck the record when pedals move between bikes
A physical move and an electronic configuration update are separate events. In a fictional equipment log, bicycle A uses 165 mm arms and bicycle B uses 170 mm arms. The pedal model is unchanged. A previous readback associated with A does not document B’s current configuration. Preserve which bicycle, arm side, pedal system and controller the observation belongs to. Do not average the two arm lengths to produce a convenient shared setting.
The same distinction matters when only an arm changes on one bicycle, when a different controller is used, or when the observed value comes from an earlier session. Consult the applicable manual for what must be checked again after the actual change, including its separate installation and calibration requirements. This guide supplies no shortcut around them. If you cannot identify which configuration the recorded screen describes, mark the readback unconfirmed rather than treating a successful connection as proof of correct setup.
Preserve the source of indoor and outdoor power records
A trainer value and a pedal-system value can come from different measuring sources. Before interpreting an apparent change, retain which source produced each record and whether the bicycle, arm, controller and relevant settings changed between observations. A trainer used only to support a bicycle during a camera check is another role again: its presence does not mean its power data was recorded or compared. Our separate trainer-reference guide owns support and bicycle-pose questions, not electronic power validation.
A difference between two displayed watt values is not automatically explained by crank length. Nor does agreement prove that both systems are accurate. An average from a different period or configuration cannot isolate the effect of one changed setting. Keep original records and label the comparison conditions instead of editing old values to resemble a later display. This reading journey does not analyse training files, validate a power meter, measure fitness or attribute a performance improvement to a crank change.
Make a small hand-off record without inventing fields
A useful technical note can keep six independent items together: the installed arm identity and physical length source; the pedal-system model and manual revision; the controlling device identified by that source; the intended length; the actual observed readback with its observation condition; and separate unresolved installation or calibration questions. Missing items remain unknown. A second person should be able to tell which statements were observed, which came from documentation and which are only proposed next actions.
The free blank bicycle worksheet offers a physical crank-length field and general reference or procedure spaces. It has no dedicated sensor, controller or electronic-readback fields. Use your own supplementary paper notes for this additional equipment record; do not relabel a clamp dimension or a saved digital measurement row as a sensor setting. Opening the preview creates no bike profile or completed customer record. Keep personal details out of the technical note and manage any completed paper through your own controlled workflow.
Know what this free guide and the crank workspace do not do
The existing OpenBikeFit crank workspace compares entered crank dimensions and retains its separate saved-work, mechanical-evidence and repeated-measurement boundaries. It does not read a paired pedal, discover which controller owns a parameter or confirm an external screen. A saved crank-length value in a local bike passport is not an electronic write to a power meter. The guide link transfers no entered values, changes no field and checks no acknowledgment.
Keep the next action proportional to the unresolved question: identify the arm through the physical-length guide, find the exact device instructions for controller authority, or retain a clearly unconfirmed readback for qualified technical support. None of these reading steps certifies compatibility, installation, riding safety, physical fit or power accuracy. All public tools remain free. Ordinary page navigation still makes web requests, but this static learning flow adds no sensor connection, device command, persistence, upload, automatic clipboard or recipient.
Practical questions
Frequently asked questions
My display says 172.5 mm. Does that identify the installed crank?
No. It is an observed setting. The Rally 110/210 manual documents 172.5 mm as its default when no length is entered; other systems need their own documentation. Identify the physical arm separately.
The app and cycling computer show different lengths. Which one counts?
Do not assume the last edit wins. Read the exact model’s controller-authority rules and preserve an observed readback from the relevant configuration. A screen in one app is not universal proof.
Does successful calibration confirm the crank-length setting?
Not by itself. Length entry and calibration or zero offset are different evidence items. Keep the relevant readback and each required operation separate; this guide does not execute or verify either.
Can I correct old power files with 170 divided by 165?
No universal correction is supplied here. The fictional +5 mm and +3.03% example describes a length comparison with 165 mm as denominator, not a measured power error, device algorithm or validated historical-data adjustment.
Can I average different left and right crank lengths into one value?
This guide supplies no averaging rule or scale-factor workaround. Preserve the actual side-specific identities and unresolved controller capability, then use the exact manufacturer guidance or technical support for that configuration.
Will this guide pair a pedal or update my saved bike?
No. Its fixed links are reading actions without entered values or device commands. The blank paper preview creates no record; any deliberate saved-bike work remains a separate existing workflow with its own gates.