Drivetrain compatibility

Start with the question you actually have.

Use a known groupset if you recognise it, or start cassette-first. BecSpec fills what it safely knows and keeps asking only for the details that genuinely change compatibility.

What are you trying to do?

Choose a question or skip straight to your setup.

Know the groupset?

This is a shortcut, not a guess. A preset fills only stable family information; cassette range, cage, chainrings and freehub stay blank when the groupset had multiple valid versions.

Known groupset / generation

1. What cassette is on the bike?

The cassette is a discovery anchor, not proof of the entire drivetrain. Same speed count does not automatically mean compatible.

How many cogs?

Smallest sprocket

Largest sprocket

Visible brand

Bike style

Family if recognised

Cassette model code (optional)

2. Checking a wheel/freehub?

Target freehub / driver

3. Checking the rear derailleur and capacity?

Read the model from the derailleur body if you can. Cage, 1x/2x and chainring sizes can all affect whether a wider cassette is actually supported.

Rear derailleur model

Cage

Front setup

Large chainring (2x)

Small chainring (2x)

4. Checking the shifter/indexing?

Enter the rear shifter/control model. Equal speed count does not prove the shifter and derailleur use the same indexing family.

Rear shifter / control model

Manufacturer-supported/conditional rules only. No forum hacks promoted to compatibility.

Current boundary

This checker resolves cassette/freehub fit, selected rear-derailleur ranges and indexing relationships, plus a Shimano-first front-difference and total-capacity gate. Known-groupset presets reduce identification friction but never override model-specific exceptions. The climbing planner carries wheel/freehub and front-capacity consequences into the recommendation. Exact crank and front-derailleur model compatibility remain separate.

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