Vibration on pedaling uphill only
 

Does anybody have an idea why I am getting a strong vibration throughout the bike when I am pressing hard, especially on the left pedal, while going up semi-steep hills. No vibration on moderates or flat. It rumbles as I press, then stops and happens again on the next press on right a littlle, then stops, then hard again on left press. I have tried all gears, worse on the mid-lows, but happens throughout. Thanks for any input.

Flex in the frame and rear wheel would be my guess. The vibration and "rumbling" would be the chain being affected by the flexing-

I had a similar problem, and I just fixed it today.

I was missing a bearing in the rear wheel.

Does your rear wheel wobble side-to-side?

My rear wheel was rubbing the brake pads, but only when pulling uphill.

Now I have eighteen bearings. Yesterday I only had seventeen.

I would also suspect frame flex. I used to experienced it with a 1990 Specialized Epic Carbon frame. First time I honestly thought we were having a quake around here. But it happened every time I was hammering on a steep hill. When I changed frame to an aluminum 2001 Bianchi EV2 the phenomenon was gone even though I had the same wheels and other components.

For me, vibration when pushing real hard has turned out to be problems in the bottom bracket. It's easy to check for. Take the chain of the chain rings and try spinning the pedals, listen for noise and feel for resistance. Check for too much play by pushing pedals from side to side and levering up and down.