Bike too big or just fine ? modification required ? Need pro advice Hi I accidentally refer to wrong bike fit and purchase online a 20" Frame Mountain bike (XC hardtail)!
While the bike is about 20" (51 cm)  Frame , I'm about 175 cm (5 foot 9 inches tall and go 79.x cm (31 inch) inseams . 

My arm quite lengthy so i don't have problem to reach to handlebar and no need to adjust my saddle . just nice clearance about 0.xx-1 CM from my crotch to the top tube .. it might hit my crotch (and might ruin my future!) so I will need to very cautious when ride off road

Considering all the cost .. I feel that it will be nice if just keep it and do at road and slightly hard terrain .. but it is safe if I go to XC, rough terrain ? what are the risk ? and any suggestion on how to make it safer and just fit to me ?



After moving Spacer
The odds of your "parts" hitting the top tube is directly proportional to your riding skill and difficulty of the terrain. Off road or trail riding can mean different things to people of different skill levels. A very competent rider can ride a too large bike over terrain a lesser skilled rider may find impassable. So it is difficult to say if a particular frame is too large for you without fully understanding the terrain and your level of comfort riding in that terrain. When I introduce someone to off trail riding I generally try to borrow a bike that is a little on the small side until they get comfortable. Novice riders seem to feel they have more control on a slightly too small frame. It seems to lessen the fear of impacting the top tube or their feet not hitting the ground. My point is that the bike may feel large now but as you gain confidence and skill you may find it doesn't quite feel so large anymore.