Bicycle Mechanics - FD Shifting from Middle to Small Ring Problem
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08-03-12, 07:34 PM
I have a late 90's Kona Lava Dome. Picked it up a few weeks ago. It's a great bike. It needed a good cleaning and the chain, chainrings and cassette were worn out. By the way, it's a 3X7.
I put a new Sram 12-32 7 speed cassette on the bike. A new Sram 8 speed chain followed and I replaced the old crankset with a Shimano Altus unit (it was cheap, cheap, cheap from Nashbar and the bike isn't exactly new). The installation and set-up went well but shifting from the middle to the small ring can't be done under load, or for that matter, when pedaling lightly. Upshifts are no problem and shifting from the big to the small ring also goes well. I've done some tweaking and am able to get the middle to small ring shift if I'm not cross chaining to any great degree.
Shimano specs a 123mm bottom bracket for the Altus crankset and I have to wonder whether that BB might be a little too wide for the Kona. The worn out triple was mated to a 112mm unit. The FD is also showing its age but seemed to work well with the old crankset. There are no issues with the RD.
I'm wondering if a switch to a shorter bottom bracket might make for a more usable shift on the front end.
08-03-12, 08:30 PM
What were the ring sizes between old & new crank?
Ditto on the cassettes.
If you made some major changes on tooth counts, there might be a problem with chain wrap capacity or ????
08-04-12, 05:41 AM
The chainrings are the same size - 42 through 22. I went from an 11 or 12 to 28 cassette to the present 12 - 32. The chain was sized using the big ring to big ring plus one link method (2 links was too slack). The RD is a Deore LX long cage and has more than enough capacity. There were no issues shifting the set up on the stand and the problem only shows itself while on the road.
I'm going to measure up the chain line. I gave it a rough wag when I installed the crankset and it came out to around 48-50 mm. There is some slop in the FD mech. That may be a contributing factor as well. Al
08-04-12, 11:08 AM
When you say-
"shifting from the middle to the small ring can't be done under load, or for that matter, when pedaling lightly"
What is happening? Is it throwing the chain off the small ring, or is the shift not occurring?
IF the latter, I'd try back out the low limit screw slightly.
08-04-12, 11:39 AM
I agree with Bill on backing out the low-limit screw very slightly. You want to give it just enough travel so it can drop the chain into the granny ring without throwing it off the side. Trial and error...
But I'll mention that chainrings don't like to "give up" the chain under load, especially when you're grinding uphill and running out of gears in the cassette, so it helps to plan ahead and get into the granny when the incline is still reasonable. And when it's too late to shift into the granny ring... ;)
(Jobst Brandt pic from pardo.net)
08-04-12, 02:01 PM
Thanks everyone for the advice. First, it goes without saying that you aren't going to get a great chainring shift under a heavy load although current 105 and Ultegra stuff seems to work flawlessly. Secondly, the limit stops were adjusted correctly.
Anyway, I did come up with a solution. First, I checked the front chainline. Shimano wants you to use a 123mm BB with their Altus crankset to give you a 50mm chainline and I suppose that will work perfectly with a Shimano FD. I measured 50mm. My FD is a Sram 7 speed unit and a little online digging suggests that the optimum chainline is 47.5mm. I did the Sheldon Brown calculations for the cassette and found its centreline to be 47.5mm. It would appear that there was a mismatch between what Shimano and Sram wanted for their setups. Go figure.
I scurried down to the LBS and bought a 118mm BB and it would appear that my shift problems are solved. I had to twist the tail end of the FD cage slightly towards the bike to eliminate any chain rub on the cage and the FD is literally bottomed on its low end stops but everything seems to work . I took the Kona for a little spin down the street and I am happy.
Live and learn. Al
08-04-12, 04:58 PM
Seems counterintuitive to me that moving the crankset inward would help it shift toward the granny ring, but props on getting it to work. :thumb: Perhaps the cage being a little lower (assuming you didn't change the mounting of the FD), or twisted a little, now helps it to nudge the chain off the middle ring...
Several of my bikes have the crankset farther outboard than necessary, and it's puzzling. When I replace the bottom brackets, I'm planning to use shorter ones as well.
08-04-12, 07:04 PM
I'm sure it's a combination of things. The original BB was 112mm with the original crankset. I've got the feeling that Shimano really wanted me to use one of it's FD's with its crankset. The centerline was on the third cog from the top before I swapped out the BB instead of the centre cog. Like I said, everything worked on the stand but not while riding. Shifting the chainrings 2.5mm to the left seems to have made all the difference. Al
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