A freewheel is a self-contained block: sprockets and ratchet in one unit that screws onto a threaded hub. A cassette is only a set of sprockets, with no ratchet inside. A freehub is the hub-side ratchet body that a cassette slides onto. Those three words describe two different systems, and parts from one system do not swap into the other.
The quick comparison
| Term | What it is | Where the ratchet lives | How it attaches |
|---|---|---|---|
| Freewheel | Sprockets and ratchet as one unit | Inside the freewheel | Screws onto external threads on the hub |
| Freehub | Splined, ratcheting body that is part of the hub | In the hub | Fixed to the hub shell |
| Cassette | A set of sprockets only | Not in the cassette | Slides onto the freehub splines, held by a lockring |
What a freewheel does
A freewheel is the older design. Its sprockets and its ratcheting mechanism come as a single unit, separate from the hub. It attaches by screwing onto external threads that are part of the hub shell.
The ratchet is what lets you coast. The outer body, which carries the sprockets, can spin freely one way relative to the inner body that threads onto the hub, but not the other way. Most freewheels do this with spring-loaded pawls that engage a ratchet. When you stop pedaling, the pawls click over the ratchet teeth, which is the familiar ticking sound. Pedaling makes them lock and the freewheel goes quiet.
Because you pedal in the direction that tightens the threads, a freewheel needs no tool to install. The flip side shows up at removal. A freewheel that has been ridden hard for a long time can be very difficult to remove, especially with strong pedaling in low gears. Removal also needs a dedicated puller (an extractor) that engages the core of the freewheel, and different brands have used different spline patterns, so the tool has to match.
What a freehub and a cassette do
A freehub moves the ratchet out of the sprocket cluster and into the hub. The cogs no longer carry any ratcheting parts. Instead they slide onto a splined cylindrical shell, and a threaded lockring (or in some older systems a threaded small sprocket) holds them on. The set of sprockets is the cassette.
That split matters in three practical ways:
- Wear parts are cheap to replace. Sprockets wear faster than the ratchet. On a cassette system you replace the sprockets and keep the ratchet body, which is still replaceable on most hubs.
- Stronger axle geometry. In a multi-sprocket freewheel, the drive-side bearing sits inboard of the freewheel. Moving the ratchet into the hub lets that bearing sit out near the frame, which greatly reduces stress on the rear axle. The Wikipedia Cogset article describes bent or broken axles as extremely rare on cassette systems.
- Clearer inspection. Off the bike, a cassette looks like a freewheel but is plainly different: it contains no internal ratchet.
Why the parts are not interchangeable
A freewheel mates to hub threads. A cassette mates to freehub splines. A threaded hub has no splined body, and a freehub hub has no thread for a freewheel, so the two cannot be swapped without changing the hub or the wheel.
Even within threaded freewheels, there are standards to respect. The standard ISO thread is 1.375 x 24 TPI (about 34.92 x 1.058 mm). A French-thread freewheel can begin to thread onto an ISO, British or Italian hub and then bind. The advice is blunt: never force a freewheel, because you will ruin the hub.
Within freehubs, the body type must match the cassette. Wikipedia lists these fitments:
- Shimano Hyperglide (HG): 7-, 8-, 9- and 10-speed cassettes with a smallest sprocket down to 11 teeth.
- Shimano Hyperglide 11 (HG-11): 11- and 12-speed cassettes with an 11-tooth smallest sprocket, on a body wider than the original Hyperglide.
- SRAM XD and XDR: certain 11- and 12-speed SRAM cassettes with a 10-tooth smallest sprocket.
- Campagnolo N3W: the Campagnolo 13-speed EKAR cassette, plus older 9- to 12-speed Campagnolo cassettes.
A cassette that does not match your freehub body will not mount correctly, so check the body type before you order. For a deeper look at how sprocket sizes translate into riding, see Understanding Bike Gearing.
Which system do you have?
- Look at how the sprockets sit. A freewheel is one chunky unit screwed onto the hub. A cassette is a stack of sprockets held on by a lockring at the outer end.
- Check the bike’s age and class. By 2010 the vast majority of derailleur bikes had moved to cassettes, while freewheels remain on some new single-speed and cheaper derailleur bikes. Shimano introduced its first freehub in 1978, so a vintage bike is far more likely to be threaded.
- When unsure, ask a shop. A shop can identify the type in seconds and tell you which tool and replacement part fit.
What this means when you buy
- Replacing worn sprockets on a cassette wheel: buy a cassette that matches your freehub body type and speed count, not a freewheel. Worn sprockets usually mean a worn chain too, so see How to Maintain Your Bike Chain.
- Replacing a freewheel on a threaded hub: buy a freewheel with the matching thread standard and use the correct extractor for the old one. If it will not budge, stop and let a shop deal with it rather than risk the hub.
- Buying a new rear wheel: confirm the freehub body (or thread) matches the drivetrain you plan to use before you commit.
- Seeing play or wobble at the rear hub or axle: have the wheel checked by a mechanic before riding, since the axle and hub carry your load. How to True a Bicycle Wheel covers the wheel side of the rear end.
Sources
Last reviewed: October 2026.