Short answer: a chainring fits by bolt circle diameter and bolt count, and a crankset fits by the bottom bracket spindle standard. The two most common road patterns are 110mm five-bolt and Shimano's asymmetric 110mm four-arm, and they are not interchangeable despite sharing a number. Crank length is a fit choice rather than a compatibility one, and shorter cranks are becoming the default for good reasons.
Contents
- Crank length
- Crank to bottom bracket
- Chainring bolt patterns
- Minimum ring size, and why it matters
- Chainring speed compatibility
- Direct mount and 1x rings
- What the ring combinations mean
- Chainline and Q-factor
- Frequently asked questions
Crank length
Crank length is measured from the centre of the bottom bracket spindle to the centre of the pedal thread. Road cranks are commonly 165, 170, 172.5 and 175mm, with 160mm and 167.5mm now widely available too.
The traditional method was to pick by height, roughly 170mm for riders under 175cm and 172.5mm above that. That approach is increasingly regarded as outdated, and the industry has moved noticeably towards shorter cranks across all rider heights.
What shorter cranks actually do
- They open your hip angle at the top of the pedal stroke. This is the main benefit. A closed hip angle is what limits many riders from holding a low position comfortably, and shortening the crank reduces how far the knee comes up.
- They let you run a lower front end without the same discomfort, which matters for anyone chasing an aero position.
- They tend to raise cadence slightly, because the circle is smaller.
- They do not meaningfully cost you power. This is the finding that surprises people. Studies comparing crank lengths across a sensible range find power output largely unaffected, which is why the trade is mostly free.
Riders with hip or lower back discomfort, restricted flexibility, or a history of knee pain at the top of the stroke are the ones who benefit most. Shorter riders on 175mm cranks fitted by an old height chart are frequently the most obvious candidates.
Cranksets in cranksets, with lengths from 160mm upward.
Crank to bottom bracket
The crank must match the bottom bracket, and the bottom bracket must match the frame. Get the middle one wrong and nothing fits.
| Crank system | Spindle | Used by |
|---|---|---|
| Shimano Hollowtech II | 24mm | Shimano road and MTB |
| SRAM DUB | 28.99mm | Current SRAM road and MTB |
| SRAM GXP | 24mm / 22mm stepped | Older SRAM |
| 30mm | 30mm | Rotor, Race Face, BB30 and 386EVO era cranks |
| Square taper | Tapered square | Entry level and older bikes |
The frame's shell decides which bottom brackets are available to you, and the bottom bracket then decides which cranks fit. Our bottom bracket standards guide covers the shell side in full, and it is the place to start if you are changing cranks.
A crank of one standard can sometimes be fitted to a frame built for another using the correct bottom bracket and spacers, which is why the bottom bracket is where the compatibility question actually lives.
Chainring bolt patterns
Chainrings attach by bolt circle diameter, the diameter of the circle passing through the centres of the mounting bolts, together with the number of bolts.
| BCD | Bolts | Found on |
|---|---|---|
| 110mm | 5 | Traditional compact road |
| 110mm asymmetric | 4 | Shimano 105 R7000, Ultegra R8000 and R8100, Dura-Ace |
| 130mm | 5 | Traditional standard road |
| 135mm | 5 | Campagnolo |
| 107mm | 4 | SRAM AXS road |
| 104mm | 4 | Older MTB |
The trap is the two 110mm entries. A traditional 110mm five-bolt ring and a Shimano 110mm four-arm ring share a number and fit nothing in common. Shimano's four-arm pattern is deliberately asymmetric, with the arms unevenly spaced, so rings are specific to it. Count the arms on your crank before ordering anything.
Minimum ring size, and why it matters
The bolt circle sets a floor on how small a chainring can physically be, because the ring has to reach outward past the bolts.
| BCD | Smallest inner ring |
|---|---|
| 130mm | 38 or 39 teeth |
| 110mm | 33 or 34 teeth |
| 107mm (SRAM AXS) | 33 teeth |
This is the practical consequence people meet most often: if your crank is 130mm BCD, you cannot fit a compact 34 tooth inner ring to it. Getting lower gearing means a new crankset, not new rings. If you are on a 110mm crank, a smaller inner ring is a cheap and effective change.
Chainring speed compatibility
Chainrings are designed around a chain width, and the ramps and pins that lift the chain are cut for a specific speed count.
In practice, 10 and 11-speed road rings are broadly interchangeable and most riders notice nothing. Moving between 11 and 12-speed is less forgiving, because 12-speed chains are narrower and the ring profiles are cut for them. A mismatched ring usually still works, and shifts slightly worse under load.
Where it matters most is at the extremes: an 8 or 9-speed ring on a 12-speed chain is a genuinely poor combination. Match the speed where you can, and see our chain compatibility guide for the chain side.
Direct mount and 1x rings
Single ring setups increasingly skip bolts entirely. A direct mount ring attaches straight to the crank spider interface, which is lighter and stiffer but ties you to that manufacturer's fitting.
Two things to check on any 1x ring. It should be narrow-wide, meaning alternating tooth thicknesses that fill the chain's inner and outer links to stop it dropping without a front derailleur. And the offset must match your frame, quoted as a figure such as 3mm or 6mm, which sets the chainline.
Bolted 1x rings exist too, such as 110 BCD road rings in 40 to 44 teeth, which let you convert a 110mm crank to single ring without a new crankset.
What the ring combinations mean
| Rings | Name | Suits |
|---|---|---|
| 53-39 | Standard | Racing, flat terrain, strong riders |
| 52-36 | Mid-compact | Fast riding with some hills |
| 50-34 | Compact | Most riders, most terrain |
| 48-35 / 50-37 | SRAM AXS | Paired with a 10-tooth cassette sprocket |
| 46-30 / 48-32 | Sub-compact | Loaded touring, steep climbs, gravel |
SRAM's smaller-looking rings are not lower gearing. Because AXS cassettes start at a 10 tooth sprocket rather than 11, a 48 tooth ring gives a comparable top gear to a 50. Compare the ratio rather than the tooth count, as covered in our cassette and gear ratio guide.
Chainline and Q-factor
Two measurements that rarely cause problems until they do.
Chainline is the distance from the frame centreline to the chainring, or to the midpoint between two rings. Road doubles sit near 43.5mm and modern 1x road setups slightly wider. Get it wrong and the chain runs at a permanent angle, which is noisy and wears everything faster.
Q-factor is the distance between the outer faces of the two crank arms, so how far apart your feet sit. Road cranks are typically 145 to 150mm, mountain cranks 168 to 178mm. A large change in Q-factor alters your knee tracking, so if you move between bikes with very different figures and develop knee discomfort, this is worth investigating alongside cleat position.
Frequently asked questions
Should I switch to shorter cranks?
Consider it if you have hip or lower back discomfort in your riding position, if you struggle to hold the drops, if you have limited flexibility, or if you are a shorter rider on 172.5mm or 175mm cranks fitted from an old height chart. The evidence suggests power output is largely unaffected across a sensible range of lengths, so the trade is mostly free, and the gain in comfort and position can be substantial. It is not a change to make on a whim, though, since it costs a crankset and possibly a power meter. Discuss it as part of a bike fit rather than in isolation.
How do I measure my chainring bolt circle diameter?
Count the bolts first, then measure. On a five-bolt ring, measure from the centre of one bolt to the centre of the bolt two positions away, then multiply by 1.051. On a four-bolt symmetric ring, measure between two opposite bolts directly, which gives the BCD. Shimano's four-arm cranks are asymmetric and cannot be measured this way, so identify those by the crank model instead. If in doubt, photograph the crank arm from the drive side and send it to us.
Can I change crank length without changing my saddle height?
No, and forgetting this is the most common mistake when swapping cranks. Crank length affects how far your leg extends at the bottom of the stroke, so shortening cranks by 5mm means raising the saddle by the same 5mm to keep your leg extension unchanged. Your saddle fore and aft position may also want a small adjustment, since raising the saddle on an angled seat tube moves it rearward slightly. Mark your original position before you start.
Are oval chainrings worth fitting?
The evidence is mixed and the experience is personal. The theory is that an oval ring varies the effective gear through the pedal stroke, easing the dead spot at the top and bottom. Some riders report noticeably smoother pedalling and less knee strain, and others feel nothing at all or actively dislike the sensation. There is no strong evidence of a performance gain for most cyclists. If you are curious, treat it as an experiment rather than an upgrade, and give yourself several rides to adapt before judging.
Do chainrings wear out, and how would I know?
Yes, though far more slowly than chains, typically outlasting several of them. The clearest sign is a chain that skips on the chainring under hard effort, particularly out of the saddle, after you have already fitted a new chain and cassette. Visually, worn teeth develop an asymmetric hooked profile, sharpened on the drive side like a shark fin rather than symmetrical. Riders who leave a chain on too long wear chainrings much faster, which is the strongest argument for measuring chain wear on schedule.
Send us a photo
Bolt patterns are far easier to identify from a picture than a description. Give your crank model and groupset name to ask TBA Guide, and we will tell you which chainrings fit and what the smallest ring you can run is.
