Dropper post travel, total length and stack height: name the endpoints and state
Separate moving travel, extended total length, collapsed residual height and the frame collar reference. A familiar size or stack label does not identify every dimension.
Published 14 October 2026 · OpenBikeFit
Three endpoints. Two declared states.
- Original fictional common axis: actuator A→seal S = 230 mm; extended seal S→rail R = 185 mm.
- Declared axial travel T = 150 mm; collapsed S→R = 35 mm. Extended A→R = 415 mm; collapsed A→R = 265 mm.
- Separate hypothetical T = 140 with the same fixed lower body and residual gives extended 405 mm, still collapsed 265 mm. No actual adjustment or part permission is implied.
Original schematic; not to scale. No physical measurement, preferred travel, clearance, control function, compatibility or installation is established.
On this page
- 01Start with the exact post and configuration
- 02Travel describes a moving interval
- 03Stack height is incomplete without a legend
- 04A fictional interval ledger makes the difference visible
- 05Travel reduction and body shortening are different claims
- 06Total length still needs a bottom and a top
- 07Hand off the complete definition
Free blank paper worksheet
Keep separate supplementary dropper notes
General reference and question spaces, not dedicated dropper travel, actuator or state fields; no installation or fit approval.
Open toolStart with the exact post and configuration
A dropper is not one fixed rod with a length label. Build an identity record before comparing dimensions: manufacturer, model, generation, diameter variant and the applicable travel configuration. Keep the frame identity alongside it, but do not let the frame name fill a missing post field. An apparently identical photograph can hide a different lower body, actuator or travel arrangement. Unknown identity remains unknown.
The PNW Loam Gen 2 page lists travel, collar-to-rail, total length and insertion dimensions as separate specification fields. That separation, not its numerical catalogue or sizing verdict, is the lesson here. Do not transfer one generation’s geometry to another. Save the exact source date and its drawing legend so another reader can identify what the recorded dimension actually describes.
Travel describes a moving interval
Define T as an applicable axial movement between named full-extension and full-lowering states of a declared straight telescoping construction. The word axial matters: T follows the motion axis, not a vertical plumb line beside an angled bicycle. T does not locate the frame seat collar, bottom of the post, saddle surface or a cable outlet. Those are independent references even when a drawing shows them together.
An intermediate position is another state, not necessarily a new maximum travel specification. Preserve whether a record is a published capacity, a source-defined configuration or an observation on an assembled bicycle. This guide does not ask you to actuate, remove, disassemble or alter a post. If the state cannot be established from an applicable document or existing controlled record, do not turn a visible interval into a travel value.
Stack height is incomplete without a legend
Park Tool uses a rail-to-seal-collar stack reference in its dropper discussion. Other product descriptions may emphasise the residual height when lowered. Keep the exact endpoints and state rather than assuming every use of stack means the same thing. The frame seat collar and the post seal collar are not one interchangeable datum; their relative position can change with insertion.
Write a stack entry as a sentence: from this named seal reference to this rail reference, along this axis, in this state. A bare 35 mm or 190 mm is not enough. Do not compare an extended reference from one source with a collapsed reference from another and label their difference an error. Their numbers can legitimately describe different intervals. A missing definition prevents conversion even if the units match.
A fictional interval ledger makes the difference visible
For an original teaching construction only, choose a fixed seal datum S, a rail datum R, and an actuator endpoint A on one declared straight axis. At full extension S→R is H = 185 mm. The declared telescoping travel is T = 150 mm, with a fixed rail head and no rotation or suspension motion. The collapsed residual S→R is therefore K = H − T = 35 mm.
Independently declare A→S as B = 230 mm. The extended A→R interval is L = B + H = 415 mm; the collapsed interval is B + K = 265 mm. Neither B nor T is automatically the frame insertion depth. These are invented dimensions with common endpoints, not a real product table. If the total excludes A or uses a different upper point, this sum no longer represents that source’s total.
Travel reduction and body shortening are different claims
In a separate hypothetical configuration of that same teaching construction, suppose an authorised source declares T = 140 mm while K and B remain exactly fixed. Then H becomes 175 mm and extended L becomes 405 mm. The collapsed total remains 265 mm. This conditional arithmetic does not tell you whether any real mechanism supports such a change or whether its endpoint geometry stays fixed.
Do not infer that a travel adjustment cuts 10 mm from the lower body, removes an actuator, changes minimum insertion or makes a frame obstruction disappear. Preserve each source-defined configuration separately. The applicable manufacturer instructions, parts and permissions are required before any mechanical change. No modification method, pin arrangement, pressure instruction, torque or universal adjustment allowance is supplied here.
Total length still needs a bottom and a top
A total can include an actuator projecting below the lower tube, or terminate at a different named base reference. At the top, rail centre, clamp structure and saddle surface are different stations. Trace both endpoints on the exact drawing before combining total with insertion. A numerical difference can describe an exposed interval only when the axes, state and lower endpoint really match.
Keep diameter identification and rail/clamp permission as separate questions. The existing owners cover receiving bore, adapters and rail sections without duplicating them here. Matching travel and total cannot prove either interface. Nor does the smallest residual stack establish preferred travel, clearance, control function, riding suitability or a comfortable saddle height. This guide supplies dimensional identification, not a shopping ranking.
Hand off the complete definition
A useful paper record has exact identities and sources; travel configuration; extended and collapsed states; each named interval with its included endpoints; and the questions still unresolved. Mark an absent reading Not measured rather than zero. Keep published dimensions separate from physical observations and repeat the same definition when recording a second state. Preserve contradictory records instead of averaging unlike quantities.
Continue to the insertion and actuator guide before applying a frame depth to this ledger, then to the saddle-state guide before calling a rail interval saddle height. The free blank worksheet can hold general references, with separately labelled supplementary notes for the missing dropper fields. Do not attach a private completed customer sheet to a general inquiry. No app record, sizing verdict or installed status is created by reading these pages.
Practical questions
Frequently asked questions
Is 150 mm travel the total length of the post?
No. Travel is an applicable moving interval. The total needs independent top and bottom endpoints and a declared state.
Does every stack height mean the collapsed height?
No. Preserve the exact drawing’s endpoints and state. Extended rail-to-seal and collapsed residual intervals are different quantities.
Does 10 mm less travel remove 10 mm of insertion body?
Not automatically. Fixed lower body, actuator, collapsed reference and applicable insertion constraints need separate evidence.
Do the fictional 415 and 265 mm totals identify a product?
No. They are original common-endpoint arithmetic, not manufacturer specifications or part-selection permission.