Rough shift to smaller cogs only
 

Hi everyone. First time poster. Glad to be on board.

I have a Trek 1500 SLR road bike with an Ultegra groupset, with the exception of the crankset, pedals, and bottom bracket, which are 105. This is my first road bike, but I have done some light work on bikes in the past and I feel as though I have a good fundamental understanding of how they work.

I bought this bike second hand. I fell within the first week (learning to ride clipless...) and knocked the rear derailleur around. It started having some trouble shifting, so I began working on it.

Since then, I have replaced the rear derailleur hanger, adjusted the limit screws, adjusted the cable tension and barrel adjusters, taken the shifters apart and reassembled them, moved the brakes around... pretty much fine tuned it to no end, repeatedly undoing everything and tuning it from scratch in an effort to make sure that I didn't leave anything out.

This could be important: One thing I HAVE NOT done is clean and degrease the moving parts on a fine level and relubed the chain. I've done the obvious scraping of gunk from the pulleys and cogs, and the bike stays relatively clean in general, but I haven't gone "full monty" and used hot water with solvents, toothbrushes, etc. etc.

Ok so that being said, when the bike is on the rack, it's absolutely a thing of beauty to shift. Minimal noise, no jerking, visibly perfect alignment of the derailleur beneath the respective rear cog. The works. With the trimming function I can even crosschain it with no noise (yeah, I know I'm not supposed to, I'm just making a point) and it's all good. Great even.

But... when I get onto the bike and ride, I am experiencing what I consider to be a minor problem with the shifting. When I go to a larger rear cog, it's just as great as on the rack. But when I shift to a smaller rear cog, it jerks into place, similar to what you would expect when making this shift going uphill and under tremendous load (i.e. what you're not supposed to feel ever). It's not as bad in the lower torque gears. Just to clarify: it's not "skipping". When it is in gear, it's solid. It's just a momentary jerk when shifting. Honestly it's not really even that bad as far as making the ride unpleasant, but it's something I want to get sorted in an effort to avoid unwanted wear to my drive train.

Help?

One thing that you have not mentioned is attending to your shift cables. If they do not move smoothly because they need cleaning or lubrication, or possibly replacement, your shifting, especially to the smaller cogs, will suffer.

You're right I did fail to mention that. The cables are good. No sticking or gritty feeling. Totally responsive the entire length of the cable and through all sheathing/bending. No visible rust or fraying.

Even though the cable/housing seems OK it's best to pull, inspect and make sure it's working at its best. There should be no corrosion on the cables where they are in or near the housing. If so replace the cable. It helps as well to spray out any contamination in the housing with WD-40 or similar. Then feed the cable back in, applying a medium oil (not grease!) to the cable as the portion that will be in each section of housing is inserted.

Once that is done check the rear derailleur itself, as you mentioned the problem occurred after a fall. Check especially the upper pulley to make sure it does not have excessive side play. You may have to find a matching model to your derailleur for comparison. If the pulley moves too much you might end up tweaking the adjustment inward slightly (to make the shift to large cogs) which then doubly compromises the shift in the other direction. Also check the pulley and main derailleur pivots to make sure there is no play at all. Finally, although more rare it is possible for the derailleur body or pulley assembly to be bent in a fall. Check the derailleur pulley assembly from the rear as you shift through the cogs and on different chainwheels. It should neither lean in or out no should it orient to the right or left of parallel. Actually, the definitive way to determine if the derailleur is the problem is to mount a different one that is in good condition.

It's unlikely to be the sole problem, but as with the cable it's best to be going for the best operation possible, so also check the chain for wear. As chains develop longitudinal wear they also tend to flex more laterally, which can degrade shifting, and would have a similar effect as a bad top pulley.

Hey fluff, welcome. You replaced the derailleur hanger but you did not mention that the alignment was checked. There is no law saying a new hanger will be perfectly aligned out of the box. Get your bike to a bike shop for them to do a quick check.