westlafadeaway

I replaced the front chainring and something's wrong- the crank seems to have resistance at every 1/2 revolution; it seems to increase in tension, then eases, then increases tension, every half turn. I took off everything and the sealed cassettes (which are making a noise and are a little rough but were ok before this switch- next on my list of replacements) spin ok with the crank off. But with the crank on it and the chain it gives a weird, uneven resistance feeling.

Some back story: I would bring in my bike for light maintenance at my LBS (part of my purchase warranty) and I thought they'd at least look at things to and give me the heads up that something was wrong. I was wrong and now I'm trying to become more self-reliant as far as maintenance and it's been rough so far. I do ok on adjusting the derraillers, but not spot on but I hope with practice it'll get better. A month ago, I realized I had neglected to change the chain and ruined the cassette and the center (triple) chainring. so I bought a new chain and cassette (ultegras). Of course, my wife threw away my previous chain before I could determine the chain length, but I think I got it right or close. The front middle chainring of my triple was worn, too, and a guy at Performance recommended just replacing the inner, but that the other two chainrings were close to going. In fact the smallest ring was pretty shot, too- it would skip and not feel right. So i just stayed in the middle chainring while I waited for my new triple in the mail.

I got the triple and replaced it. now I'm having this problem with the crank having resistance. I'm not sure if the problem is at the crank or from the chain- or I'm doing something else completely wrong.

Any advice? Thanks in advance.

FBinNY 

You're description is characteristic for a single speed or fixed wheel with the chain too tight. In that situation an eccentric chainring (they all are to a degree) will cause the chain to be too short and bind twice per revolution.

This would not normally be an issue with a derailleur bike since the RD cage would take up any difference. However if your chain is wa-a-a-a-y too short, so it just barely loops two sprockets then you could have the same issue.

The correct minimum length is that which loops the two largest sprockets (crossed big/big combination) with an inch to spare. That ensures that there's always slack in the lower loop for the RD to take up.

If you do have slack in the lower loop, then it isn't a chain length issue, and you have to look elsewhere for the problem. I'd start by slipping the chain over the crank arm and off the crankest, pulling it back at the FD, and seeing if the cranks spin free and true without the chain attached.

Retro Grouch 

1. Wrap your chain around the big chainring and biggest rear cog bypassing the derailleur. Figure the closest place you could connect it. Add one inch of chain (2 links) to that length. That's the shortest chain that you can safely use with that gear combination.

2. While the chain is off the crankset, spin the crank with your fingers. If you can feel the resistance you are indicating, you have a problem with your crank or bottom bracket.

Andrew R Stewart 

Not seeing or feeling the bike does make the trouble shooting hard but... The first thing that comes to my mind is a rubbing, maybe the chain ring against the frame or if the crank arms were changed the inside of the arm against the Bb cartridge lip. The mentioned manor of investigation is solid, reduce and separate the various elements and see how each one functions. Andy.

Airburst

What is a sealed cassette? Do you mean the bearing cartridges? The things that go between rotating parts and stationary ones?

westlafadeaway

yes Air- I meant the cartridge... I was a little tired at the time of my post... I finally figured it out- it was that my cassette must have been just a hair loose and one of the rings was rubbing just enough to lock up the back end. I didn't notice it until the chain was off and I traced the problem... duh. Just barely even noticed it.

This forum has helped me tremendously... I just ordered the parts to do my first wheelbuild <fingers crossed> !

Airburst

Good luck with the wheelbuild, if you've gotten as far as ordering parts I assume you've seen Sheldon Brown's article on the subject, but if not, it's extremely helpful.