finally I found 2 nylon washers of internal diameter 15mm, seems the problem is fixed.
however I still not going to ride for a few weeks...... quite bad situation there in Hong Kong......
however I still not going to ride for a few weeks...... quite bad situation there in Hong Kong......




, the bike is very comforntable and efficient to ride, front wheel rarely raise up and rear traction is good. (well, at least for my setting.)