Hub spacing and OLD versus axle length: name the two contact faces
Distinguish hub over-locknut or built-in contact span from internal and retaining axle lengths, keep front/rear and end-cap configurations explicit, and avoid inferring a wheel conversion.
Published 11 October 2026 · OpenBikeFit
Contact span is not shaft length
- Fictional centred contacts: −71 to +71 mm → W = 142 mm; −74 to +74 mm → W = 148 mm.
- Span difference 6 mm is not a two-3-mm-washer recipe. Rotor/cassette planes and conversion permission remain separate.
- Separately declared shaft L = 170 mm minus W = 142 mm gives 28 mm, NOT engagement or required replacement length.
Original schematic; not to scale. Not a real part drawing, tolerance, conversion, installation or physical inspection.
On this page
Free blank worksheet
Keep supplementary paper references
The worksheet provides general question spaces, not dedicated axle or brake fields or installation approval.
Open toolName the contact-span endpoints
OLD is commonly expanded as over-locknut dimension: the separation of the specified hub end faces that contact the receiving assembly in that configuration. On other designs the relevant surfaces belong to end caps rather than exposed locknuts, and a manufacturer may call the quantity built-in dimension or hub spacing. Retain the source term and its endpoints instead of assuming that every width label is measured across identical components.
Write the hub identity, front or rear position and installed end configuration next to the span. Distinguish a published nominal dimension from a physical observation. Neither is an outside flange distance, spoke-centre span, overall retaining-shaft length or bearing separation unless the document explicitly says so. A named datum makes a number checkable; a bare width does not.
Hub width is not retaining axle length
The Robert Axle Project explicitly distinguishes a named hub standard from the longer retaining axle in its linked product description. The receiver and head/thread arrangement occupy their own geometry outside or around the hub contact span. The number in a hub label therefore cannot be copied into an axle-length field as if both ended at the same two surfaces.
In an invented declaration, hub contact span W is 142 mm and a separately named shaft length L is 170 mm. L minus W equals 28 mm. That subtraction is only the difference between two declared lengths: it does not allocate 28 mm to the thread, engagement, receiver walls or head contact. It cannot tell you the required replacement length or permissible tightening.
Keep internal hub parts distinct
A hub can contain an internal axle with its own shoulder, bearing and end-cap relationships. It is not automatically the removable retaining axle that passes through the fork or frame. An internal spare-part length can differ from the complete built-in dimension without contradiction. Name which part and which endpoints a drawing describes before comparing it with the wheel or receiver.
The DT Swiss support page separates the identified hub specification from individual parts and configurations. An associated part list is not an instruction to substitute everything shown in it. A conversion item appearing beneath a broad support entry needs exact applicability evidence; its presence does not authorize changing a 148-labelled assembly to 142 or any other span.
A wider label does not supply a conversion
Fictional centred contact planes at lateral coordinates −71 and +71 mm have a span of 142 mm. Another pair at −74 and +74 mm spans 148 mm. Their total span difference is 6 mm. The assumed centre origin lets each abstract plane move outward by 3 mm in this drawing; it does not establish that a real hub, frame, rotor or cassette can be adapted that way.
Do not turn those 3 mm values into washer thicknesses or a centring procedure. Real configurations can have different end contacts, internal parts, flange positions, rotor mounting planes, driver relationships and manufacturer restrictions. A width difference alone does not describe those changes or prove that an approved kit exists. Boost is a system label to resolve against exact parts, not a universal arithmetic conversion.
Rotor and cassette planes are separate references
Two hubs can share a contact span while differing in the brake interface or rear cassette/driver arrangement. The plane of a rotor contact and the cassette interface should be named separately from the hub end faces. Do not infer either by halving the hub width. Nor does the centreline assumption in the fictional diagram reveal a real wheel dish, spoke tension or flange geometry.
The rotor guide covers mount, diameter and thickness as separate questions. The existing cassette guide covers cassette, driver body and receiving hub identity. Those are connected references, not duplicate width fields. A complete discussion can preserve all of them without claiming that an apparent numerical match has established a physically permitted wheel.
Read front and rear charts without merging them
Front and rear configurations serve different receiving assemblies and can use different diameters, spans and retention systems. Put the position at the start of each record. Do not transfer a number from a rear axle listing into a front-fork question because the shaft diameter or wheel family matches. Model years and variants can also matter; preserve the exact source revision when it is available.
A tyre bead-seat diameter such as 622 is a radial rim reference, not this lateral contact span. Wheelbase is a horizontal distance between wheel centres, not a hub width. Keeping lateral, radial and whole-bike quantities separately named prevents the word wheel size from silently joining dimensions that have no shared endpoints or units-based conversion.
Take a reference record to the exact source
Collect the receiving frame or fork identity, wheel position, hub and end-cap identities, published span with its definition, axle/skewer reference and the two brake and drivetrain interface records where applicable. If a physical dimension is available, record the method, surfaces and uncertainty separately. Do not force a match by rounding or changing the contact-face convention.
Use supplementary paper notes beside the free blank worksheet rather than relabelling an existing fit dimension as hub spacing. Continue to the axle-length and thread-pitch guide to identify the separate retaining part. This page does not ask you to remove a wheel, dismantle a hub, add spacers, re-dish a wheel or trial-fit anything. Unknown applicability remains a question for the exact manufacturer or mechanic.
Practical questions
Frequently asked questions
Is a 142 mm hub supplied with a 142 mm retaining axle?
Not by that label alone. Hub contact span and retaining-shaft length have different endpoints; obtain the exact axle specification and its length convention.
Does 148 minus 142 mean two 3 mm washers?
No. Six millimetres is a numerical span difference. An actual conversion needs exact component and receiving-system permission; the centred drawing is not a spacer recipe.
Does equal OLD establish rotor or cassette position?
No. Hub end contacts, rotor mounting reference, driver interface and wheel dish are separate relationships. Do not infer them by halving the span.
Can I copy rear hub spacing into a front-fork record?
No. Front and rear assemblies need separately identified parts, receiving systems, configurations and sources, even when another label happens to match.