Compact crank
A compact crank is a special variant of a pedal crank that is mainly used on racing bikes . The compact crank is designed to combine the advantages of derailleur gears with two and three chainrings.
Classic double cranks on racing bikes have a bolt circle diameter of 130 or 135 mm depending on the manufacturer (a historical standard was also 144 mm) for screwing the chainrings to the crank spider, which means that the small chainring is no less than 38 or 39 due to the specified diameter of the bolt circle Teeth (or 42 teeth for older bolt circle standards). This means that no very small unfolding is possible. Cyclists must therefore drive at least 15 km / h on steep climbs if they want to avoid an unergonomic, low cadence .
One possible way out is a crank with three chainrings, as used in typical mountain bike gear groups. A separate screw connection with a pitch circle of 74 mm is common for the small chainring, which enables chainrings from 24 teeth. A disadvantage of triple cranks is the higher weight, which is particularly noticeable uphill. In addition, many racing cyclists find triple cranks unaesthetic.
The compromise is a double crank with a smaller bolt circle diameter, usually 110 mm, which has long been the norm in cyclocross . This enables chainrings from 33 teeth, which significantly improves the mountain suitability compared to the classic double crank, even if the small developments are not possible that would be possible with triple cranks. The large chainring of compact cranks usually has 48 or 50 teeth. A disadvantage of the compact crank compared to the classic double crank is that one or two large gears (for high speeds) are missing. In addition, the gear ratio jump between the large and small chainring is comparatively large, which reduces the comfort when shifting.
literature
- Frank Lewerenz, Martin Kaindl, Tom Linthaler: Das Rennrad Technikbuch. 1st edition. Pietsch Verlag, Stuttgart 2005, ISBN 3-613-50486-3 .