Guide 03
Chainline & chain tension
Medium / fiddly
Iterative — set, check, adjust
The single chain wants two things: dead straight (chainline) and
drum‑tight (tension). The hub gives you the first by design; the frame’s horizontal dropout
gives you the second. This is the build playing to its strengths.
Why this is even possible
All your gears live inside the Alfine hub, so there’s one chainring, one
sprocket — nothing shifting the chain sideways. That means it runs in one straight line and
you can use a fat, strong, long‑lasting single‑speed chain a derailleur bike could never
run. No derailleur also means nothing to take up slack — so you tension it, by sliding
the wheel.
Part A — chainline (getting it straight)
The one rule: front chainline = rear chainline. Both are the distance from the bike’s
centreline to the middle of the sprocket. Match them → the chain runs straight.
Step 1 — measure the REAR (don’t trust the spec sheet)
The Alfine’s locknut‑to‑locknut width (OLD) is 135 mm, so the centreline is 67.5 mm in from
either locknut face.
Rear chainline = (OLD ÷ 2) − (drive‑side locknut face → centre of sprocket)
e.g. measure 21.5 mm from the drive‑side locknut face to the middle of the sprocket teeth →
67.5 − 21.5 = 46 mm. (Alfine nominal is ~45–47 mm, so 46 is a healthy sanity check —
but measure your hub.)
Step 2 — that number is now your front target
Step 3 — hit it from the FRONT via the BB spindle
This is why square‑taper BBs are the friendly choice — they come in many spindle lengths, so you
dial the chainring in or out.
Lengthen the spindle by 2 mm → chainline moves outboard ~1 mm
(the spindle grows symmetrically, so the drive side gets half)
Worked example
Rear target = 46 mm. You mock up your crank on a 113 mm spindle and measure the chainring
at 44 mm — 2 mm too far in. Need +2 mm → add ~4 mm of spindle → a ~116–118 mm BB.
A 118 lands ~46.5 mm. Match.
Step 4 — fine‑tune from either end
- Rear nudge: Alfine sprockets are dished — flip the sprocket to shift the rear
~±2.5 mm with no other change.
- Front nudge: thin chainring spacers, or step to the next spindle length.
The relief — tolerance
No derailleur means no cross‑chaining, so this is forgiving. Within ~1 mm is great; ~2 mm is
fine. You’re aiming for “visually straight so the chain has no urge to climb off,” not
laboratory precision.
Part B — chain tension (the horizontal dropout)
On a normal modern frame the wheel sits in a vertical slot — you couldn’t tension a
chain if you tried (that’s why so many frames got rejected). Your Wizard’s $44 bolt‑on Single
Speed dropout has a horizontal slot: you slide the wheel back to pull the chain
drum‑tight, then clamp it. Slide back = tension.
- Set the chain length (heavy‑duty single‑speed chain) so the wheel sits roughly mid‑slot when
tight — leaves room to adjust as it wears.
- Slide the wheel back until the chain has just a little up‑down play (not banjo‑tight, not
flopping). Internal‑hub chains like a touch more slack than a fixie.
- Keep the wheel centred and square in the frame as you tension — easy to accidentally
cock it to one side in a horizontal slot. Check rim‑to‑stay gap both sides.
- Clamp to spec.
The non‑turn washers (real fiddle)
The Alfine axle mustn’t rotate in the slot, so Shimano non‑turn washers seat in it — the
likely pair for a rear‑facing horizontal dropout is 5R (yellow, drive) + 5L (brown).
Choose empirically: offer the washers to the actual dropout and pick the pair that sets
the hub’s cassette joint parallel to the chainstay. Filing the washer tab or the slot to seat
it is normal, accepted practice — don’t panic when you have to.
(Refs: sheldonbrown.com/nexus‑mech.html · Shimano DM‑CASG004.)
See also: Bottom bracket (spindle length) ·
Alfine hub setup ·
Wheel building.