Single-Speed & Dirt Jump Drivetrains: Tension, Dropouts & Chain Width

No derailleur means no automatic slack take-up

A geared drivetrain's rear derailleur does double duty — it shifts between cogs, but it also constantly takes up chain slack as you shift, using its spring-loaded cage. A single-speed drivetrain has neither a derailleur nor that slack take-up, which means chain tension has to be set some other way, entirely at the frame's dropout.

Four ways a frame can set chain tension

Single-speed and dirt jump frames set tension through one of a small number of dropout designs: horizontal or track-style dropouts, where the wheel physically slides forward or backward in a slot to take up or release slack; sliding dropouts or an eccentric bottom bracket, which move the wheel or the BB shell itself along a track or eccentric mount to the same effect; and vertical dropouts, which — because the wheel can't slide anywhere — need either a separate chain tensioner (a small spring-loaded pulley that takes up the slack a derailleur would otherwise handle) or a precisely chosen gear ratio paired with a half-link chain that lands the tension right without any adjustment device at all. A vertical-dropout single-speed frame with no tensioner and no half-link plan is the one combination that genuinely needs an extra part to work.

The rear cog usually rides a completely standard freehub

A single-speed or dirt jump rear cog doesn't necessarily need a dedicated single-speed hub. The common setup is a standard cassette driver — HG, XD, or MicroSpline, any of them — fitted with a single-speed conversion spacer kit that takes up the width a full cassette would otherwise occupy, leaving room for just the one cog. Dedicated thread-on single-speed hubs exist too, but they're not the only path; if you're converting a geared wheel to single-speed, a spacer kit on the freehub you already have is usually the simpler option.

Chain, cog and ring width: two families, and they should match

Single-speed drivetrains run one of two chain widths: a tougher 1/8-inch chain (borrowed from track and BMX use), or a narrower 3/32-inch chain (the same width used on geared derailleur drivetrains, common on converted MTB setups running narrow-wide rings). The chainring, rear cog, and chain should all share the same width class. A mismatch doesn't cause an instant failure the way a genuinely wrong-sized part would — it runs, but with side-play, extra noise, and faster wear, and in the worse of the two mismatch directions, an increased chance of the chain dropping. That worse direction, based on real rider reports rather than a manufacturer bulletin (no chain or sprocket maker publishes an official width-tolerance chart for this), tends to be a narrower chain run on wider-spaced sprockets; a wider chain on narrower sprockets is more commonly tolerated with just some side-play as the downside. Treat this as a real but soft distinction, not a hard rule — component-specific results vary, and the honest state of the evidence is "generally true, not universally guaranteed."

How BuildMyMTB checks this

A frame tagged as single-speed drops the geared drivetrain slots — shifter, derailleur, cassette — from what's required for a complete build, and asks for a single rear cog and a rigid seatpost instead of a dropper. By deliberate decision, all four brake slots are dropped from completeness too, since a rear-only or fully brakeless build is how these bikes are genuinely ridden. The rigid seatpost slot gets the same direction-aware diameter check as a dropper post always has — too big is an error, too small is a shim warning. Chain, cog, and ring width class mismatches raise a warning rather than an error, reflecting that this is a "runs but not ideal" outcome rather than a physical impossibility. A vertical-dropout single-speed frame with no tensioner provision surfaces an informational note about needing a tensioner or half-link — every dirt jump frame in the catalog today uses sliding dropouts, so this note is currently dormant, but the check exists for when a vertical-dropout frame is added.

Practical takeaways

• Check your frame's dropout type before assuming chain tension will "just work" — a vertical-dropout frame needs a tensioner or a carefully chosen half-link setup, the other three dropout styles don't.
• You usually don't need a dedicated single-speed hub — a standard cassette driver plus a spacer kit is the common, simpler conversion path.
• Try to keep chain, cog, and ring on the same width class (1/8" or 3/32"); if you can't, a wider chain on narrower sprockets tends to be the more forgiving mismatch of the two directions.
• Remember brakeless and rear-only builds are treated as genuinely complete on a single-speed frame — that's not a gap in the checker, it's how these bikes are actually ridden.