John Redcorn

New to bikes as an adult since dec. My db mtb doesn't do this but my ~1 month old crosstrail does. 300ish miles (wish I had time for lots more) on it, I've noticed this for a while.

Spinning along at cruising speed it's not there, but getting up to speed from stopped, going up a hill, powering down a hill; I can feel a 40-50ish hz vibration in my feet. It's not constant, but in 2 separate predictable spots of the pedal revolution, with a tiny break between, each spot of vibration being 90 or a little more or little less degrees of the rev. The two spots of vibration are close to each other in the pedal rev, there's a spot of complete smoothness that's bigger or close to the same as both the vibrating spots combined.

Seems the same no matter the gear but these type of things are hard for me to judge.

I have replaced the pedals, vibe was there with either set. Didn't replace because of this but I broke a plastic one on the DB and put the plastic ones the crosstrail had on the DB, got some aluminum hopefully non-breaking ones for crosstrail.

Something I should look for? I know I can and probably should take it to the shop I bought it from just last month but I'm afraid of being without it!

FBinNY 

This is normal and characteristic with chain and sprocket wear. In a new chain and sprocket, each roller is nicely cradled in the trough between the teeth, and each subsequent roller swings in to place without touching until the bottom. If you live where there are ferrys, imagine the captain guiding the boat into the slip without touching the sides.

With wear the shapes of the teeth, and the pitch of the chain aren't so perfectly matched, and it's like trying to get the ferry into the slip in a crosswind. It does get there, but only by hitting and rubbing down the side of the slip. The noise/vibration is each roller hitting and roiling down the face of the next tooth.

It comes in pulses because people don't pedal applying power through 360 degrees. Instead there's peak power when the pedals are horizontal, and low power as they come vertical, so you have 2 zones of more wear, (you can see it if you look closely) and the vibration pulses twice per revolution to match.

John Redcorn

Maybe I'm reading what you're saying wrong, but this bike is brand new, should it be doing this?

FBinNY 

You might be feeling normal "chain noise" under load, or possibly the effect of the chain running past the shift gates, where engagement will be less smooth. Other than that I can't say without the bike in my hands.

But, without knowing the degree, I can tell you that some engagement vibration is common on chain drives, and it's entirely possible that that's all you're feeling. You also might be able to improve (or worsen it) by changing your chain oil.