Choosing the right chainring offset for your bike's chainline
The geometry between your front chainring and rear cassette quietly decides how smoothly your drivetrain runs. Get the offset wrong and you will hear chain rub on the front derailleur, feel rough shifts on the back, and watch the chain wear prematurely on a crooked angle. Riders across Australia - from inner Sydney commuters to weekend gravel explorers in Tasmania - often swap cranks, chainrings or wheels without checking whether the chainline still matches the frame. The result is usually a noisy ride and a chain that asks to be replaced far too soon.
A well-tuned chainline keeps the chain running as straight as possible between the crank and the rear sprockets. Chainring offset is the tool that lets you dial that straightness in, whether you ride a flat-bar commuter down Lygon Street, a hardtail through the trails at Mount Coot-tha, or a touring bike loaded up for the long way to Cairns.
What chainline and offset actually mean
Chainline is measured in millimetres from the centreline of the frame to the centre of the chainring (or the middle of the chainring cluster). On a single-ring setup, the chain should sit roughly above the middle cog of the cassette. On a two-ring crankset, the chainline is usually taken from the midpoint between the two rings, and the rear derailleur and its spring take care of small misalignments.
Offset is different. It describes how far the chainring sits inward or outward from the crank arm's centre axis. Standard road and MTB cranks have a chainring bolt circle, but the spider or direct-mount interface shifts the ring left or right. A larger offset moves the chainring outward toward the drive side; a smaller offset (or a negative offset on some narrow-wide rings) pulls it inward. The frame's chainstay length, bottom bracket width, and rear hub spacing all influence which offset keeps the chain straight.
If the chain runs at an angle, the load on each tooth changes. The chain stretches unevenly, and the rear cassette wears into a shark-fin shape over time. Mechanics at local workshops in Adelaide and Brisbane see this every week on bikes that came in for a "new chain please" and left with a chainring recommendation instead.
Measuring your frame and current setup
Before buying a new chainring or crank, take three measurements. They will tell you what offset your frame actually needs rather than what the catalogue claims.
First, measure the rear half of the chainline by lining up the rear wheel in the dropouts and measuring from the seat tube centre to the middle of the cassette. Second, measure the bottom bracket shell width - older MTBs use 68 mm, modern Boost frames use 73 mm with a 148 mm rear hub. Third, check the chainstay length, either from the spec sheet or with a tape measure from the rear axle to the centre of the bottom bracket.
| Drivetrain type | Typical rear spacing | Suggested chainline | Common offset (mm) |
|---|---|---|---|
| Road double, rim brake | 130 mm | 43.5 mm | 4–6 |
| MTB 2x, non-Boost | 142 mm | 50 mm | 3–5 |
| MTB 1x or 2x, Boost | 148 mm | 52 mm | 3 (Boost 1x) or 6 (2x) |
| Gravel 1x wide-range | 142 or 148 mm | 49–52 mm | 3–5 |
| Touring, Rohloff | various | 50–55 mm | 5–7 |
| Fat bike | 170–197 mm | 55–65 mm | 8–12 |
Readings from a tape measure will not match these numbers perfectly, but they let you know which bracket of chainline you live in. A commuter hardtail with a 142 mm rear end that wants a 50 mm chainline does not need the same chainring offset as a Boost trail bike chasing 52 mm.
Single ring versus multiple chainrings
A 1x drivetrain makes the chainline question simpler because there is only one ring to align. Most 1x-specific narrow-wide chainrings for Boost frames use a 3 mm offset so the ring sits slightly outboard of the crank. That position lines the ring up with the middle of an 11- or 12-speed cassette. If you convert an older bike to 1x and the rear spacing is 135 or 142 mm, you often need a different spider or a chainring with a different offset - sometimes 5 mm or even 6 mm - to land the chainline near 50 mm.
Two-ring cranks add another layer. A 2x setup typically targets a chainline between the two rings, and the front derailleur is shaped to cope with a few millimetres of sideways drift as the chain climbs the cassette. Cyclists in Brisbane who commute on 2x gravel bikes with 46/30 rings often run a 6 mm offset chainring on the inside to keep the small ring closer to the frame. The trade-off is that the big ring ends up further outboard, which can bring the chain close to the chainstay on some frames.
Three-ring cranks are rare on new bikes but still appear on older touring rigs and some entry-level hybrids sold through Australian retailers. They generally want a 50 mm chainline and a chainring stack that is centred on the spider. Replacements for these are getting harder to find, so many riders swap the triple for a compact double and accept the new chainline maths.
Boost, super boost and older standards
Hub spacing has crept wider over the past decade, and chainline numbers have moved with it. A traditional 100/135 mm quick-release setup sat the rear end about 7 mm narrower than modern Boost 110/148 mm, which is why old-school cranks have a different chainring offset than current ones. Super Boost 157 mm rear ends push the chainline out to roughly 56 mm, and the latest downhill bikes with 83 mm bottom brackets run closer to 59 mm.
If you replace the wheels on a 2010-era hardtail with a new Boost wheelset, the chainline shifts and the old chainring will sit too far inward. Mechanics at shops in Perth and Hobart regularly rebuild bikes for riders who upgraded wheels but never checked the offset. The fix is usually a new chainring with a different offset, or a spacer kit between the chainring and the crank arm.
For riders running internal gear hubs, such as the Rohloff or Alfine, chainline can be set with the hub itself, but it usually wants to sit near 50–55 mm depending on the spline pattern. Touring cyclists heading up the Stuart Highway often run these hubs because they handle heat and dust better than derailleurs, and a correctly offset chainring is what keeps the chain from walking off the front ring under load.
Matching offset to common cranksets
Not all cranks accept every offset. Shimano, SRAM and Campagnolo spiders are machined for a specific offset range, and aftermarket narrow-wide rings for them are sold in matching variants. Race Face and SRAM DUB cranks have their own direct-mount standards with 3 mm offsets for 1x. Older Shimano 4-arm cranks want a 4 mm or 5 mm offset on the inner chainring when running 2x, while their 5-arm predecessors wanted different spacings entirely.
When in doubt, the spider's part number tells the story. A 104/64 spider on a Shimano 2x crank pairs with chainrings that have specific offsets designed for that bolt pattern and crank family. Mixing a road inner ring with a mountain outer ring usually fails because the offsets are designed for different chainline targets.
Aftermarket ring makers publish their offsets for each standard. The most common figures Australian riders will see when browsing shop shelves:
- 3 mm for Boost 1x narrow-wide rings on Race Face, SRAM DUB and similar cranks
- 4 mm for inner chainrings on Shimano 4-arm 2x road cranks
- 5 mm for older MTB 2x cranks and 130 mm-spaced road doubles
- 6 mm for some gravel 2x cranks with 46/30 chainring pairs
- 8 to 12 mm for fat bike and downhill specific chainrings
Reading the spec sheet before clicking "buy" saves a frustrating return trip to the shop.
Getting it dialled and avoiding common mistakes
Installing the wrong chainring offset shows up in three places: the sound of the chain against the front derailleur cage on the big ring, the position of the chain on the smallest cassette cog (it will look angled rather than straight), and accelerated wear on one side of the cassette and one side of the chainring. Checking the chainline before tightening the chainring bolts saves all three headaches.
A few habits keep the chainline honest over the life of the bike:
- Re-measure after any wheel, frame or crank swap
- Use a straightedge or a long ruler across the chainstay and chainring to confirm alignment
- Replace the chain at the same time as the chainring so wear patterns match
- Keep the front derailleur cage parallel to the chainrings after any offset change
- Check chainring bolts every few months on bikes that see dust, such as those ridden on the Blue Mountains trails
A good local workshop will measure chainline on the stand during a service. Dropping the bike in for a quick check the next time you are at your nearest shop, whether that is a small store in Fremantle or a busy inner-city workshop in Collingwood, usually costs less than a new chain. Riders who do their own work should keep a 4 mm hex, a ruler, and a small torque wrench within reach before touching the chainring.
Globe Cycle's workshop guides walk through chainline checks for specific frame types, and the maintenance section covers everything from cassette wear to derailleur adjustment. Subscribe to the newsletter for new build guides, seasonal maintenance checklists, and the occasional deep dive into the parts that decide how your bike actually feels on the road or trail.