problem with chain wheel set and belt

I have a strida LT and change the chain wheel set to aluminium. The problem is when I’m driving the chain wheel while made aturn the belt will slipped from the chain wheel. How is that happen? and how to fit it.

pls. sent the answer to

Thank you and looking forward from your reply

Bob Vong

Hi Bob,

I’m afraid there are several thinkable causes for the mentioned issue;
loose belt, incorrect mounting of the belt wheel and defective part 338 to name a few…
Some pics might help, best from above and sideways.