Aerobar pad stack and reach: BB coordinates are not frame or bracket offsets
Name the pad point, surface, load state and origin before comparing an aero cockpit. Centre, rear edge, holder, basebar and frame stack/reach do not describe the same endpoint.
Published 11 October 2026 · OpenBikeFit
Aero contact reference · not to scale
Frame ≠ clamp ≠ named pad.
Fictional BB-relative F=(390,560), B=(480,605), P=(460,650) mm. X forward, Y upward.
Local B→P=(−20,+45) mm. A flat 100 mm pad line: rear R=(410,650), centre P=(460,650).
Different origins and endpoints; not frame sizing, pad selection or a physical installation.
On this page
- 01Set the origin and coordinate directions
- 02Frame, basebar and pad are three endpoints
- 03Preserve the manufacturer’s local convention
- 04Pad centre and rear edge need a named relationship
- 05Cushion top is not the holder surface
- 06Keep left and right records independently
- 07Record the reference without substituting an app input
Free blank worksheet
Keep a technical paper reference
Use reference and question spaces for extra armrest fields. Fixed digital slots do not replace these distinct endpoints.
Open toolA frame chart, a stem calculation and an armrest record can all contain stack or reach while ending at different points. For an aero cockpit the armrest is another endpoint beyond the frame and clamp. Before comparing values, identify whether the note concerns a pad centre, rear boundary, holder, cushion surface or another explicitly marked point. The origin and observation condition belong beside that identity.
This article describes reference geometry on an existing assembly, not an ideal position, bicycle size or an instruction to raise or slide a pad. It supplies no relationship between a body measurement and a pad coordinate. A well-labelled number can still be physically inaccurate, and matching coordinates does not establish load support, riding suitability or assembly permission.
Set the origin and coordinate directions
For the fictional side-view construction use BB centre as the origin, X forward and Y upward, with the bicycle upright and the declared axle alignment level. Pad reach is the horizontal component to the named pad point; pad stack is its vertical component. Neither is the direct diagonal distance from BB to the point, nor a tape length following the frame or the bracket.
A real note must state how the bicycle was supported, how alignment and zero were established and how projections were observed. Do not balance or alter hardware to reproduce the illustration. A trainer, tyre condition or support can change orientation relative to gravity. Record the condition and retained uncertainty rather than silently importing the axes from a catalogue picture.
Frame, basebar and pad are three endpoints
Imagine fictional BB-relative frame point F=(390,560) mm, basebar clamp B=(480,605) mm and named pad centre P=(460,650) mm. Frame reach/stack are 390/560; clamp coordinates are 480/605; the pad is 460/650. None is an alternative name for either of the others. The same bicycle can legitimately carry all three pairs without contradiction.
Relative to B, this P has local horizontal offset −20 mm and vertical contribution +45 mm: (480−20,605+45)=(460,650). The local 45 is not pad stack 650, and the local −20 is not BB-relative pad reach 460. Addition works only because these hypothetical offsets are already declared in the same global horizontal/vertical directions; a distance along a tilted bracket is not automatically such a vector.
Preserve the manufacturer’s local convention
The linked SRAM Vuka Aero Rev A manual uses a Pad X dimension between the pad rear and the stem clamp, and separately defines armrest width. That is a local component convention rather than a universal BB-relative coordinate. Its source-specific centre correction must stay with its named pad and reference, not become a constant for every manufacturer or pad shape.
Profile Design likewise lists separate local armrest and extension fields for a named assembly. If a product page does not establish the exact origin, endpoint or state needed for your comparison, retain the field as a source label with its unresolved definition. A catalogue drawing, frame stack number or receiver height cannot supply the missing pad vector by itself.
Pad centre and rear edge need a named relationship
For a deliberately flat fictional pad reference line 100 mm long, lying horizontally with rear R=(410,650) mm and front=(510,650) mm, its geometric midpoint is P=(460,650) mm. Centre reach exceeds rear reach by 50 mm under those assumptions. This is an endpoint example, not a universal half-pad conversion: a curved cushion or differently defined centre needs its own reference.
If a separate straight 100 mm line is tilted +10° with its front upward, its rear-to-midpoint vector is (50 cos10°,50 sin10°)≈(49.2,8.7) mm. Adding 50 only horizontally would miss both components. This second example is not the level pad in the first drawing, a suggested tilt, a loaded surface or a rule for an actual armrest.
Cushion top is not the holder surface
Name whether the observation concerns a holder, an unloaded top surface or a loaded contact point. In a fictional level construction, holder height 640 mm plus an explicitly vertical 10 mm layer gives a top reference at 650 mm. That addition requires the stated direction and surface; a nominal foam thickness on a tilted or shaped support is not automatically a vertical height contribution.
A loaded cushion can present a different observed surface. This guide gives no compression formula or correction from rider mass, material or pressure. Keep unloaded and loaded observations separate with their own method and repeatability conditions. Neither the pad outline nor the product layer label establishes where a person’s arm contacts it.
Keep left and right records independently
An average pad coordinate can hide a left/right difference. Identify each side and retain its raw readings, reference point and condition. A matched Y does not prove matched X, lateral station, yaw, surface or support. Do not replace missing right-side data with the left value or assume mirror symmetry from a shared component name.
The pad-width guide adds a declared lateral coordinate without changing this origin. Extension and grip positions add another point rather than redefining P. Keeping these columns separate lets a later reader see what was actually recorded, what came from a manufacturer and what is only fictional arithmetic. Unknown is not zero and is not a symmetrical default.
Record the reference without substituting an app input
Keep BB datum, bicycle alignment, side, exact pad, named station and surface, loaded state, raw readings, instrument and method on a technical sheet. Use the blank paper worksheet’s reference and open-question areas for extra armrest fields. The fixed digital whole sheet has no dedicated pad-stack or pad-reach slot; frame and bar-clamp entries must keep their own endpoints.
The existing stem comparator models a clamp point, not the entire armrest support chain. It cannot infer an unseen pad offset, approve a riser or select an aero position from a frame number. Continue to a separate width and grip record rather than declaring a complete aero setup from two coordinates. No hardware, body, race-rule or aerodynamic verdict follows from this illustration.
Practical questions
Frequently asked questions
Is frame stack the height of the armrest?
No. The frame endpoint and named pad endpoint are different. Preserve BB-relative pad coordinates or a fully declared separate offset.
Can I add half the pad length to rear-edge reach?
Only in an explicitly resolved geometry with the same definition and orientation. The fictional flat 100 mm line gives +50 mm; a tilted or shaped pad needs its own relationship.
Is a 45 mm local stack the same as 45 mm above BB?
No. A local vertical contribution is measured from its named local origin. The fictional clamp Y 605 plus 45 gives pad Y 650, not Y 45.
Can the stem calculator calculate an unknown pad offset?
No. Its clamp model does not supply an armrest, surface, load or hardware vector. Keep those fields separately unresolved.