2x to 1x conversion: chainline, spacers and narrow-wide setup
By Crankworthy · Published · Updated
Bolting a single ring to a 2x crank’s outer position leaves the chainring sitting too far outboard, which is what causes the grinding and the chain dropping when you backpedal in the biggest cog. Fix the chainline first — shims or bottom bracket spacers — then handle retention with a narrow-wide ring and a clutch derailleur, and only then worry about gearing.
Why chainline is the whole job
A 1x chainring has to serve every cog on the cassette, so it wants to sit level with the middle of the cassette. On a 2x crank, the outer ring position is nowhere near that.
With two rings, the pair straddles the middle of the cassette and the front derailleur hands the chain between them. Remove the inner ring and bolt a single ring to the outer tabs and the chain now runs from a position meant for the fast half of the gear range across the whole cassette. In the biggest cog the chain is at its most angled, which is where the noise, the wear and the backpedal derailment come from.
Chainline is measured from the centreline of the frame to the centre of the chainring teeth. Sheldon Brown’s method is still the simplest: hold a ruler against the seat tube and measure across to the middle of the ring 1. The number you are aiming for is the rear cassette’s own chainline, which follows from hub spacing.
Target chainline by hub spacing
Rear chainline figures as Sheldon Brown gives them [1]. Match the front ring to the number for your hub, and remember a crank has its own designed chainline too — SRAM road cranks are built for 45 mm, their Wide versions for 47.5 mm [2].
- 130 mm road
- 43.5–45 mm rear chainline [1].
- 142 mm mountain
- 47.5–50 mm [1].
- 148 mm Boost
- 52 mm [1].
- SRAM road and gravel cranks
- 45 mm standard, 47.5 mm on the Wide spindle — the crank moves, not the ring [2].
- Measuring the front
- Ruler against the seat tube, measure to the middle of the chainring teeth. On a double, measure to the halfway point between the rings [1].
Correcting an outboard chainring: two ways, one trade-off
You can move the whole crank inboard with bottom bracket spacers, or move just the ring with shims behind it. The second is usually better, because the first moves your feet too.
Shifting a spacer from the drive side to the non-drive side on a threaded external bottom bracket pulls the entire crankset inboard. It works, it costs nothing, and it changes your stance: both pedals move left, which can bring the non-drive crank arm or your heel closer to the chainstay.
Shims between the spider tabs and the chainring move only the ring, leaving pedal stance alone. The catch is bolt length — single-ring chainring bolts are short, and a shimmed stack needs longer bolts to keep proper thread engagement. Do not reuse the short ones and hope.
A direct-mount crank sidesteps the whole question: pick the ring offset that produces the chainline you need, which is the cleaner fix where the crank allows it.
Keeping the chain on
Two parts do the retention work: a narrow-wide chainring at the top, and a clutch rear derailleur at the bottom. Neither substitutes for the other.
Narrow-wide teeth alternate thick and thin to match the chain’s alternating inner and outer links, filling the lateral space that lets a chain wander off a conventional ring. Conventional rings are deliberately thinner and ramped to help a front derailleur lift the chain — exactly the behaviour you do not want once the derailleur is gone.
A clutch derailleur resists the cage swinging forward, which keeps tension in the lower run and stops the chain slapping and bouncing off over rough ground. Shimano brands it Shadow RD+, SRAM as Type-2 or Type-3 — our catalog records the clutch on each derailleur.
One compatibility trap worth knowing: 12-speed chains are not all alike at the tooth interface. Shimano’s 12-speed HG+ chain and SRAM’s Flattop have their own profiles, and manufacturers sell rings cut to match. If a new 1x setup runs rough on an otherwise correct chainline, check the ring is cut for the chain you are running.
The order to do it in
- 1
Work out the target chainline
Take it from your hub spacing 1, then measure what your crank currently gives you with a ruler against the seat tube.
- 2
Choose the ring and its position
Direct mount: pick the offset that lands on the target. Spider-mounted: plan for shims and longer bolts if the outer position is too far out.
- 3
Check the derailleur can take the cassette
A 1x conversion usually pairs with a wider cassette, and a short-cage road derailleur will not clear a big cog. Check the largest-cog rating before buying anything.
- 4
Size the chain to the big cog
Wrap the chain around the chainring and the largest cog without routing it through the derailleur, bring the ends to the point where they just meet, then add links per the derailleur maker’s instruction. On full suspension, do it at the point of maximum chainstay growth.
- 5
Set the B-gap last
The gap between the upper jockey wheel and the cassette is what makes the big cog shift cleanly. Manufacturers give either a gauge or a stamped alignment line on the cage — use theirs, not a guess.
Narrow-wide rings and clutch derailleurs in the catalog
Match the ring to your crank’s mounting standard and the derailleur to the cassette you plan to run.
- Wolf Tooth Drop-Stop Chainring 104 BCD (32T)104 BCD 4-bolt, 32T Drop-Stop B narrow-wide, 7075 alloy$64.95Amazon
- Race Face Narrow Wide Chainring 104 BCD (32T)104 BCD 4-bolt, 32T narrow-wide, 7075-T6 alloy, 9/10/11/12-speed
- Wolf Tooth Cinch Direct Mount Chainring (32T)Cinch direct-mount for Race Face/Easton cranks, 30/32/34T narrow-wide
- Shimano SM-CRM75 Direct Mount Ring (32T)Direct-mount 12-speed ring for FC-M7100/M8100 cranks, 30/32/34T
- Shimano Deore RD-M5100-SGS11-speed Shadow RD+ clutch, long cage, 51T max cog$52.49Amazon
- SRAM NX Rear Derailleur (11-speed)11-speed, Type-2.1 roller bearing clutch, X-Actuation$82.65Amazon
What this job needs that we don’t stock
Named so you can source them elsewhere — we would rather say this than point you at a product that is not the right one for the job.
- Chainring shims and long stack bolts — the hardware this job actually needs to correct a spider-mounted chainline is not in our catalog.
- Derailleur hanger extenders, for keeping a short-cage road derailleur on a wider cassette.
References
Related
Part of the bike upgrade and compatibility guide.