Bike Chain Slack Guide
Set correct chain slack, measure chain stretch across 20 links against the 2% service limit and time your next lube.
About this tool
Bike Chain Slack Guide
Set correct chain slack, measure chain stretch across 20 links against the 2% service limit and time your next lube.
A chain slack guide checks the three things that decide how long a motorcycle's final drive lasts: free play, wear and lubrication. Free play is compared against the range your suspension layout needs, wear is measured the workshop way — the length across 20 links against a nominal of 20 × chain pitch, with a service limit at 2% elongation — and the lube interval is scaled for sealed chains and for wet or dusty riding. Chain pitch comes from the size code itself: the first digit is the pitch in eighths of an inch, so a 520 chain has a 15.875 mm pitch.
How to use Bike Chain Slack Guide
- 1
Open Bike Chain Slack Guide on AltFTool — it loads instantly in your browser.
- 2
Pick your Chain size — 520 carries a 15.875 mm pitch — and your Suspension layout, such as Commuter, twin rear shocks at 20-30 mm.
- 3
Enter Length across 20 links (mm) with the chain taut, Free play you measured (mm) at the lower run, and the odometer readings, ticking Sealed O-ring / X-ring chain or Rain, dust or coastal riding.
- 4
Read Chain wear used against the 2% service limit, with Target free play, Adjustment needed and Next lube due at.
Why use Bike Chain Slack Guide
Slack matched to the bike
A linkage monoshock needs more free play at rest than a twin-shock commuter — the target reflects that.
Wear measured, not eyeballed
Turns a 20-link measurement into an elongation percentage against the 2% limit.
Interval that adapts
Halves the lube interval for rain, dust and coastal salt instead of quoting one number.
Common use cases
- Setting free play correctly after a rear wheel refit, without guessing between 20 mm and 40 mm
- Deciding whether a chain at 1.4% elongation has life left or should be replaced with the sprockets
- Timing the next clean-and-lube on a sealed chain ridden through the monsoon
Frequently asked questions
How much slack should a motorcycle chain have?
Typically 20–30 mm on a twin-shock commuter, 25–35 mm on a monoshock without linkage and 30–40 mm on a sport bike with linkage, measured at the midpoint of the lower run with the bike unladen. Always check the swingarm sticker or owner's manual first — the figure varies with swingarm length and suspension geometry, and it is the only number that matters for your model.
How do I know when a bike chain needs replacing?
Measure across 20 links with the chain taut: a new 520 chain spans 317.5 mm, and the usual service limit is 2% longer, about 323.9 mm. Replace sooner if you find a tight spot when rotating the wheel, if links are stiff or kinked, or if you can pull the chain more than half a tooth clear of the rear sprocket at the 3 o'clock position.
How often should I lubricate my bike chain?
Every 500 km for an unsealed chain and around every 800 km for an O-ring or X-ring chain, halved in rain, dust or coastal conditions — and after any ride through heavy rain regardless of distance. Clean the chain first with paraffin or a dedicated cleaner and a soft brush; a wire brush or petrol will destroy the O-rings on a sealed chain.
Is a tight chain worse than a loose one?
Yes, considerably. A chain set too tight is loaded continuously as the suspension compresses and the swingarm passes the point of maximum chain run, which wears the gearbox output shaft bearing, both sprockets and the chain itself, and can break the chain over a large bump. Slightly loose costs you a little driveline slop; too tight costs you components.