hey everyone, thanks for all ur feedbacks! i really appreciate it and your suggestions will be taken into consideration, really! anyway, Sir Gohan: yea, i didn't notice that wheel part not being stable, thanks for pointing that out.
sir blomkaal: yea, i believe you are right about moving the big and small tires back a little so the gravity will be more stable, i'll do that! thanks!
Sir Xander: i really created that chair with my own imagination and about ergonomics, i don't know that stuffs and no one has ever thought me that, probably something similar but not really ergonomics.