Originally Posted by
spd28
Shifts work well in the two smallest chain rings in front but when I move to the largest chain ring the shift is smooth but when I push on the pedal with any force the chain wants to drop of the large chain ring and makes lots of noise. I’m not seeing any indication that the derailleur cage is touching the chain once on the chain ring. Since I’m not able to replicate this issue while on my bike stand I’m having trouble diagnosing the problem.
Check the frame for cracks around the bottom bracket, particularly seat tube and chainstays. With the bike on its wheels, lean it away from you and press on the bottom bracket area with your foot, looking and listening for anything that shouldn't move. Repeat from the other side. It probably isn't cracked, but this could be the cause of your problem and it's a quick and easy way to check.