Repair guides › Frame & Fork
Cassette or freewheel? Tell them apart
The backbone. Everything bolts to it, and its geometry decides how the bike fits, steers and how wide a tire it clears. 6061 aluminum tubes TIG-welded into a front triangle (top, down, seat and head tube) and a rear triangle (seat stays and chainstays). Threaded inserts (nutserts) take racks, fenders and bottle cages. Aluminum does not rust, but it can crack, especially near welds, and it does not bend back.
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Steps
- Take the rear wheel off (see Remove and refit a wheel) and look at the center of the smallest cog on the drive side.
- Cassette: a separate ring sits in the center with 12 notches, often marked LOCK or with an arrow. The cogs slide onto a splined body (the freehub) that is part of the hub.
- Freewheel: the tool notches are cut into the cog block itself, and the whole block threads onto the hub. Common on 6- and 7-speed and budget bikes.
- Spin the cogs backward: on a cassette the freehub body turns with them; on a freewheel the whole block spins on its own threaded body.
- The smallest cog helps too: 11 teeth almost always means a cassette; 13–14 teeth often means a freewheel.
- Note which you have in My Bike → Bike specs. It decides which removal tool to buy (lockring tool and chain whip, or a freewheel tool) and which gears fit.
Diagram
Cut in half to show the insideRear hub and cassette
Signs it needs doing
- Creaks that follow you even after the BB, pedals and seatpost are greased
- Hairline crack or paint flaking in a line at a weld
- Rear wheel no longer sits centered (bent dropout)
- Dent in the down tube or top tube
Articles about this job
- The best first upgrades for a cheap bike (and what to skip): what I found as a beginner
- Walmart bike vs bike shop bike: what I found you really get for the money