Fitting bike while standing on pedals
 

I work for Jimmy Johns as a delivery cyclist. I always move fast so I never sit down to pedal. My Specialized Rockhopper is an 18" frame all-stock from about 1990 with a rigid front fork and an Exage rear derailleur.

This bike fits when I sit to pedal. I'm 5'9". However, when I stand and pedal I feel like I am on the edge of a cliff precariously leaning over the handlebars. Any emergency braking would throw me over the handlebars. So I ride leaning backward with my butt behind the seat so that I am not so far forward.

I want to fix this either by buying a larger frame bike or buying a longer stem. My current stem is 135mm which is the longest I can find. It needs to be at least 160-180 mm to do any good. I think a 20" frame may fix my problem.

What do you think?

I think you should sit down.

Getting a longer stem will will make you even more endo prone.