appreciate u or any ching here could advise how u r able to use 78/20 ratio, any mod ? I have just finished my s100, facing some problems.....using 48 pitch 78 spur, the biggest pinion I could use is 16, photo attached.
Maybe Joeechiu c-hing is using 64 pitch gears when he refers to 78/20.
Anyway, in your situcation, you can do 2 things:
1) modify the motor mount by filing the motor hole to allow your motor to slide more outwards
or
2) using a smaller spur gear to achieve your rollout (ie 20/78 ~ 16/64)