Throwing chains.
September 19, 2010 2:03 PM   Subscribe

What is up with my front derailleur ?

I have a schwinn Touirist (previously mentioned i think) it's a cheap bike. I bought it at a discount store. I paid more or less what i could afford to spend on a bike for it. I am trying to use it for a 1 mile daily commute, but it is tough because the mile is almost entirely up hill on the way in (it's great on the way back though).

Last week i was riding and i turned from one street on to another which was going from one grade of hill to a greater grade, so i down shifted to the smallest ring on the front, after getting back to a less extreme grade i went back to the second (Middle ring) on the the front, at which point the derailleur shifter or something and was no longer correctly aligned. The chain was rubbing in the middle gear and would not move anywhere else. I spent the last week riding it in middle gear only on the front, with the derailleur housing shifted so there was no rub.
About an hour ago I took it took a bike shop where they realigned it and made sure that all of the gears were accessible. On the repair stand (i saw it with my own eyes) it worked great. I got it back to my apartment, took it off my trunk mount car rack and tried it out, and went from the smallest ring to the middle with some difficulty and from middle back to smallest (2 to 1) it threw the chain. So i get off, bring it up to my apartment, and lift up the back and spin the crank without me on it. It does fine. it shifts through all the gears with no problem and doesn't throw the chain. Any idea what the issue is? Is there anything i can do, or do i need to take it back to the bike shop and tell them this story?
posted by djduckie to Sports, Hobbies, & Recreation (8 answers total)
When you're on the bike, does your weight somehow flex the frame? Maybe the frame's rotten and/or broken?
posted by goblinbox at 2:24 PM on September 19, 2010

Take it to the shop and tell them it's different when you're riding. Take them out to the parking lot and ride circles around them while shifting, or whatever they suggest.
posted by rhizome at 2:31 PM on September 19, 2010

Response by poster: When you're on the bike, does your weight somehow flex the frame? Maybe the frame's rotten and/or broken?

Not that i can tell. The bike was more or less brand new about 6 months ago. all the welds look strong. Maybe my weight is the problem.

The shop closes in 15 minutes, and they won't be open again until the end of the week. it'll have to wait to be returned.
posted by djduckie at 2:42 PM on September 19, 2010

The shop should fix it.

I had similar problems and it turned out to be the cables sticking in the housing. New cables and housing made the problem go away.

The frame can flex when new, especially if it was a cheap frame to begin with.

The problem may also be the way you shift: you shouldn't be using gear combinations that cause the chain to run at an extreme angle. E.g., don't use the big front ring with the big rear ring, or the small front ring with the small rear ring. Only use middle rear rings with the middle front ring. Even the most expensive bikes on the market have issues running the chain that way. It will cause the chain to rub the front derailer cage, and sometimes the chain can bind or fall off in rare cases. The problem is especially bad with a triple crank like you have.
posted by twblalock at 3:05 PM on September 19, 2010

I just had my midrange derailleur adjusted and it started rubbing again. There is also a small amount of flex in your cranks, so when you bear down while riding the chain can move slightly to the right. If you want to spend thousands of dollars you can get a recumbent with an internal front hub that is shifted by hitting the axle with your heel while you ride. [sorry for the minor derail--pun intended]
posted by mecran01 at 5:26 PM on September 19, 2010

Are you shifting under a heavy load? You have to be pedaling while you shift, of course, but it's often helpful to ease up on the force you're applying, especially when you're going to a larger cog or chainwheel. This is why it's sometimes hard to shift when you're going up hill.

On some bikes, the rear shifting is indexed (click, click, click) while the front shifting is sort of semi-indexed--it's one click puts you in the range of each chainring, but you still have to adjust the lever (or whatever you have) slightly to get to the sweet spot. On on of my bikes, I have to adjust my front shifter if I go from a large gear to a small gear in the rear.

But in any case you should take it back to the shop and tell them your story. It shouldn't throw the chain, for starters. They may have neglected to check the limit screw that prevents the derailleur from moving too far to the left. This video by The Bicycle Tutor explains front derailleur adjustment, if you want to tackle it yourself.
posted by hydrophonic at 7:35 PM on September 19, 2010

Previous posters have mentioned all the usual suspects. Bring your bike back to the shop and explain your problem. Don't be afraid to say, "Stop me when you have enough" and then give excruciating detail.

Personally I think you're either shifting under too much load or the frame is flexing. I know you said the welds look good, but I doubt even an experienced framebuilder could diagnose excessive flex just by visually inspecting the welds after the frame is painted. A couple millimeters' displacement under hundreds of pounds of load would be enough.

But really, the mechanics at the shop know much more than I do, so you should just talk to them.
posted by d. z. wang at 8:22 PM on September 19, 2010

Unless you're really heavy, I doubt that the frame is flexing. What can be more likely is that the crank arm isn't torqued correctly and when you pedal the chainrings move - something that's not always evident when the bike is on the workstand at the shop.

The other option is hat twblalock suggested regarding the cables. They could be binding and not moving the derailleur cage over all the way.

Just because the bike's six months old doesn't mean that parts got screwed up since, or were never right in the first place.

Agree that explaining to the shop dudes in excruciating detail is a good idea.
posted by Man with Lantern at 9:59 AM on September 20, 2010

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