More returns start with freehub and axle mismatches than with bad power accuracy. The bike either does not mount, the cassette will not fit, or shifting is garbage from day one.
Read this before checkout, not after the box is open and the race starts in an hour.
Freehub body types (the important ones)
| Body | Common with | Notes |
|---|---|---|
| Shimano HG | Many road 8-11s cassettes | Default on a lot of trainers; confirm speed range |
| SRAM XDR | SRAM 12s road | Different from older XD mountain standards |
| Micro Spline | Shimano 12s Micro Spline | Not the same as HG |
| Campagnolo | Campy freehubs | Brand-specific; do not force HG cassettes |
| Zwift Cog style | Virtual shifting kits | Single cog; software does the 'gears' |
Cassette vs Zwift Cog
Traditional cassette: you keep mechanical or electronic shifting like outdoors. Great if you want road feel and multi-app freedom without virtual shifters.
Zwift Cog (and similar single-cog systems): simpler chain line, less cassette wear drama, but you rely on in-app virtual shifting hardware or controls. Best when Zwift (or compatible virtual shifting) is home base.
- Match cassette speed to your rear derailleur capacity and chain.
- Do not mix random used cassettes with worn chains without checking wear.
- Torque the lockring. Hand-tight is how ticks are born.
If you buy a trainer 'with cassette included,' still verify it matches your bike. Included often means a common HG cassette, not your XDR reality.
Axles and adapters
Quick-release and thru-axle bikes need different hardware. Thru-axles also vary by length, thread pitch, and diameter. Forcing the wrong axle is how frames and trainers get damaged.
- 1
Measure what is on your outdoor bike
Note QR vs thru-axle, diameter (often 12mm), and overall length if you can.
- 2
Check the trainer adapter kit list
Many units include common adapters; some require a separate purchase.
- 3
Mount carefully the first time
Threads should start cleanly by hand. Stop if anything binds.
Pre-purchase checklist
- Freehub body matches your cassette plan (or you budget a body swap).
- Cassette or Cog included vs separate line item.
- Thru-axle adapters for your frame.
- Front wheel block or level setup if needed.
- Return policy if the wrong body ships (it happens).
Key takeaways
- Freehub body is not optional trivia; it is binary compatibility.
- Cassette and Zwift Cog paths are different ownership models.
- Thru-axle adapters must match length and pitch.
- Verify the exact SKU bundle, not marketing photos.
FAQ
Often yes on modern direct-drive units, sometimes as a paid part. Check the brand parts list before you assume the swap is cheap or user-serviceable.
