Hammerschmidt gear crank
The Hammerschmidt system is a bottom bracket gear system for bicycles that was developed by the US company SRAM and presented in 2008. It is based on the classic principle of a planetary gear , as it has been installed in hub gears for decades, for example .
The lower gear has a 1: 1 ratio, and the crank is coupled directly to the chainring with 22 or 24 teeth. The transmission is only used in the so-called " overdrive " mode. The "sun gear" is now firmly attached to the frame. The gear ratio is determined by the dimensioning of the sun gear and the small "planet gears".
The planet gears drive the chainring via the " ring gear ", which now rotates 1.6 times as fast as the crank (or 0.6 times faster than the crank). The circuit is activated with the first pawl system, which is moved by a Bowden cable and connects the sun gear with the frame. (In 1: 1 mode, the sun gear rotates with it). The second pawl system on the outside of the sun gear unlocks when you step backwards and allows the chain to turn with you. During assembly, the gear unit is flanged to the so-called ISCG chain guide mount on the frame with three screws. In overdrive mode, the torque is diverted into the frame via this connection.
The aim of this drive is to do away with the derailleur for all-mountain bikes and freeriders .
A similar bottom bracket circuit has been available from the Swiss company Schlumpf innovations for several years, but it is switched with the foot, but does not require an ISCG chain guide mount on the frame.
literature
- Fritz Winkler, Siegfried Rauch: Bicycle technology repair, construction, production. 10th edition, BVA Bielefelder Verlagsanstalt GmbH & Co. KG, Bielefeld, 1999, ISBN 3-87073-131-1
Web links
HammerSchmidt on www.sram.com
swell
- ↑ Bike magazine No. 9/08