New bicycle has shifting problems
November 4, 2017 12:45 PM   Subscribe

I built a bike frame, and I can't get the bike to shift onto the large chainring; it seems like the front derailleur can't quite reach far enough. Can I fix this with a different derailleur? Different crankset? Or something else?

I built the frame of this bike out of bamboo, and since it's my first try at framebuilding I expected a little bit of wonkiness. The only real problem, though, is that I can't get the chain to shift onto the largest chainring.

It appears that the front derailleur doesn't have quite enough reach; when I've moved the shifter as far as it will go, the derailleur is pushing on the chain but it doesn't jump to the large chainring. If I manually move the chain to the largest chainring it stays and doesn't contact the derailleur cage.

Here's what I've ruled out:
  • Worn parts: everything is new, with less than 20 miles of test riding on it.
  • Limit screws: I've taken the high limit screw through its whole range.
  • Shifter range or cable tension: the shifter has plenty of room to keep moving when the derailleur stops. I'm not using indexed shifters.
So:
I have a triple crankset. If I switch to a double, will the large chainring be closer to the center line of the bike (and thus within reach of the derailleur)? I don't think I'll miss the small chainring very much and this would be an easy, if not especially cheap, fix.
Is it better to try to find a different derailleur with a longer reach? I'm using a Shimano Sora 3x9 front derailleur.
This seems like a stretch (ha), but could a too-long chain somehow cause this? The chain as it arrived was about a link and a half long, but out of laziness and a desire to get test-riding ASAP, I put the chain on without shortening it.

Or is there something completely different I'm overlooking?
posted by egregious theorem to Sports, Hobbies, & Recreation (11 answers total) 1 user marked this as a favorite
 
My only thought on what you can change without doing anything hardware related is to make sure that the front mech's low enough, as described in this article.

After that, the next thing that I'd do is put a shim inside where it clamps to the frame, and as it's a bamboo bike, I'd also check that the place where it clamps to the frame is exactly parallel to the chainrings.

Thirdly, before going as far as changing the chainrings, I'd check the bottom bracket spindle length, and check that's not too long.

Right, off to do my own bottom bracket replacement now: I'm putting a motor assist in.
posted by ambrosen at 1:02 PM on November 4, 2017 [2 favorites]


Chain length too long by a link or two won't cause the issue.

Are you using a square taper bottom bracket? What spindle length? You could consider a shorter spindle.

Of course, a 1x would eliminate this problem.
posted by spikeleemajortomdickandharryconnickjrmints at 1:51 PM on November 4, 2017 [1 favorite]


On the "overlooking" part, is the chain on the smallest back sprocket? Mine won't either if it's not but it's ok because it wouldn't make much sense to be configured big to big anyway.
posted by bonobothegreat at 2:11 PM on November 4, 2017 [2 favorites]


Large chainring is the one that you get by pulling the cable, with the bike on a stand or upside down, pull the lever with your hand. Does it shift (make sure you are not crosschained - smallest gear in the rear)? If not, is it hitting the limit screw? If you need just the tiniest bit more, you can open up the clamp that holds the derailleur and put a shim under it on the chainring side (half a piece of tubing that just fits over your tube).
posted by 445supermag at 3:13 PM on November 4, 2017 [1 favorite]


Came here to say the same as bonobothegreat: The front won't go onto the inside-est if the back is on the outside-est.
posted by at at 4:43 PM on November 4, 2017


Check where your shifting cable is clamped into the front derailleur. On some designs it's possible to clamp the cable on either the outboard or the inboard side of the bolt itself. Each position produces a different leverage, meaning you're pulling a different amount of cable when you press the shifter with your hand. Whichever side you're clamped on, try the other one to see if it helps.

Seconding to try placing the derailleur higher or lower on the down tube. This has a surprisingly big impact. Relatedly, one thing to check would be what your seat tube angle is, and whether it matches the seat tube angle the derailleur is designed for (not sure if this information would be easy to find).

Also check your bottom bracket setup. Does your frame BB width work for your spindle width? Are you supposed to be using spacers? What BB width does the front derailleur expect? What width is the crankset?

Either way, the Sora 3x9 front derailleur is an excellent derailleur in terms of reach. It's one of the go-tos for weird configurations. I doubt that that model is the source of your problem.

If you're still having trouble, check Sheldon Brown's article on chainline.
posted by danceswithlight at 4:50 PM on November 4, 2017 [1 favorite]


To get the chain up onto the largest chain wheel, you have to push against the powerful derailleur spring at near maximum extension as well as the chain itself, and with a bamboo frame it's conceivable to me that the push back on the down tube is flexing it enough in the opposite direction to keep the chain from catching on the chain wheel.
posted by jamjam at 5:03 PM on November 4, 2017 [1 favorite]


Thanks, everyone! I probably should have mentioned that the derailleur is mounted to a boss on the seat tube, not using a clamp, because the largest clamps I found were 34.9 mm and my seat tube is over 41 mm in diameter. So I can't change the positioning much without nontrivial modifications to the frame. But I positioned the boss with my specific crankset and derailleur in mind, so I doubt that's the source of the problem--and I have fiddled with the height to the extent I can, without any success.

I've tried the shifting with the rear derailleur in several positions, including the smallest sprocket; same behavior in each case.

All this leads me to think that this:

You could consider a shorter spindle.

is the simplest solution to my problem. (Leave it to me to think about switching out the more complicated or expensive parts and forget about the simple ones!) I must have guessed the size poorly back when I bought the bottom bracket. I needed to tighten the lower limit screw quite a bit to stop the derailleur pulling the chain off the cranks to the inside, so shortening the spindle to move the entire crankset in seems like the way to go.

To get the chain up onto the largest chain wheel, you have to push against the powerful derailleur spring at near maximum extension as well as the chain itself, and with a bamboo frame it's conceivable to me that the push back on the down tube is flexing it enough in the opposite direction to keep the chain from catching on the chain wheel.

This is an interesting point too. I noticed while trying to fix this last night that--despite my efforts to attach it very securely--the derailleur boss moves, albeit almost imperceptibly, at the extreme ends of the shifter travel where the tension is highest and lowest. I don't think it's a big enough effect to cause problems, but it's there (and it's an example of why I didn't even bother trying to use indexed shifting on this bike).
posted by egregious theorem at 5:39 PM on November 4, 2017 [1 favorite]


If its mounted to a boss, it should be easy to shim it out, try a couple washers between the derailleur and boss.
posted by 445supermag at 8:04 PM on November 4, 2017


FWIW, I was having trouble shifting onto the big ring, and after trying a few things, I discovered that my front derailleur was rotated a few degrees off center, so the rear of the inside cage plate couldn't push the chain far enough.
posted by adamrice at 10:24 AM on November 5, 2017


Updating in case you all are curious: I switched from a 122.5 mm spindle to a 115 mm spindle and it now shifts easily to all three chainrings. Thanks, everyone!
posted by egregious theorem at 9:05 PM on November 13, 2017 [2 favorites]


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