Outside spring diameter: Preferably max. 77mm Spring force: In operation For the calculation I have stated the maximal possible force in chain, calculating this from the maximal drive torque. Maximal motor torque [Nm] is given in suppliers datasheet for each motor. There are two of the springs, so maximal force is split on those two. Anyway, the springs shall have some compression force “in reserve” to withstand dynamic load (for example stone blocked suddently between wheel and chain). Therefore I choose reserve factor to be around 2. I have also chosen that the compression between minimal and maximal load should be about 10mm (10 precompression, another 10mm as operation area). Longer operation area will produce cheaper spring (lower force, smaller wire diameter), but the possibility of chain derailing rises. With this, the following calculation results are obtained: