I am a newbie to bikes, but like to tinker and maintain them myself as much as I can. I have been riding a 1992 Trek 830 with slicks on it up until a few months ago, and it didn’t do this which is what is leading me to ask.

I have a new Rocky Mountain Oxygen 30 with Tiagra shifters/derailleurs and an FSA Omega Crankset. I only have 300 miles on the bike, so I am still learning every time I go out on it how it handles, etc. In the smallest rear cog and the largest front chain ring, while mashing, I get this odd feeling as if this gear doesn’t ride as smoothly as the rest of the gears. It almost has a skip to it. No real noise difference, and I don’t have a trainer so I can’t video what is going on.

I know mashing isn’t the correct form most of the time, and the bike rides great through all of the rear cogs except for the smallest one, so I ride in any of the others most of the time.

I have had the crank arms loosen up a bit, but that was evident and they have been tightened back down. Is there anything else you guys/gals would recommend I look for/at?

Maybe this is normal for a road bike and the situation, but like I said, my old mountain bike doesn’t do this, although the crankset and cassette are different.

Did any of what I said even make any sense???

Is the chain rubbing the front derailleur in that gear ? I wouldn't expect a skip on a new bike unless you have a tight link. Lean the bike up, pedal backwards very slowly and watch for a tight link as the chain goes over the small sprocket & through the pulleys. Or perhaps the chain is rubbing the inside of the frame in the smallest cog.

I will check all of those after work. I was thinking about the frame rubbing, but I had both wheels off cleaning it this weekend after a messy ride, and I didn't notice any scratches or marks in that area.

Wish some of my fellow riders had some know how and could ride it for me, but they all take it to the shop for the simplest of repairs.

Is your Trek the matte black with purple and orange? That was the color of my 1992 Trek 830.

It
is the Antelope model, so it is black with white(ish) decals

It wasn't any of the meantioned above, and I got home a little too late to take it for a quick spin to try and replicate it. Any other items I should look at?

Stop mashing, then it won't matter until you're above 35 mph

What is your RD cable tension like? If it is slack against the down tube, then you are too loose, which would mean the RD is resting against the stop. To be properly adjusted, tension should pull it slightly off the limit screw. But usually too loose is apparent easily, because it will be marginal in shifting from the smallest to second smallest cog.

Slightly too tight can cause the same problem, and shows up as a sluggish drop from the second to the smallest. This usually can cause intermittent lift of the chain, and is more common than totally too loose. Back the cable adjusters off, a 1/4 turn at a time, and check how it shifts. The best cogs to use for this are 2nd to 3rd smallest, back and forth, and if those are crisp, drop to the smallest and make sure that it is a crisp shift.