Bicycle Mechanics - Front cranks (35/49) won't allow chain to jump to larger ring unless going DOWNHILL!?
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11-29-11, 09:56 PM
I have a 89 Merlin Ti which I got last summer. From day one, I found the bike reluctant to move from the small ring to large in front crank. Initially, I had trouble with the bike going up hills, so I went to a 'compact' 35/49 crank, still w.in the mfr. specs for the Shimano 8 spd. set up (Dura Ace 8000). I replaced shifters (N.O.S), the rear derallieur (now a Shimano Deore) and nothing improved, but I started getting a lot of cat calls from various shops implying the changes are to blame, even though the slippage was the same.
I am ABSOLUTELY certain the bike likes moving to the large crankwheel when the bike is going down a hill. That, of course, also shows up when they put it on a bike stand at the shop w. the front wheel pointed down. Soon as I get on even a level road or one with even a slight rise, the left shifter doesn't do squat, or occasionally clicks with the chain looking like a snapping summer garden hose, never reaching the wider ring. Very frustrating.
Apart from only using the bike in zero gravity, and assume I pedal as lightly as Spongebob chasing butterflies when trying to shift, what in anyone's guess is a reason why the bike shifts reasonably well in front when I'm going down hill, in any rear gear, but acts like it has no earthly connection when flatlands arise?
11-29-11, 10:51 PM
You never mention doing anything with the front derailer. Have you adjusted it at all? http://www.parktool.com/blog/repair-help/front-derailleur-adjustments
Since it shifts well downhill when there's little or no load, I suspect that your problem might be that you're not easing up on the pedal pressure enough during the shift.
Unlike cars, there's no clutch to eliminate load during a shift, so you have to do this yourself, by easing up on the pedals (while still turning them) especially when shifting the front. The front derailleur is different from the back in that it operates on the loaded side of the chain. When you shift small to large on the front the chain has to be lifted by getting snagged on a tooth, ramp or pick-up pin on the outer chainring. However under tension, the chain is fighting to move to a smaller ring.
If it's still sluggish with little or no load, then it's a matter of properly adjusting the front derailleur, and not an issue with the cranks.
11-30-11, 02:53 AM
Stop mashing the pedals like when you go down hill.
to let the shift happen.
that's an interesting ring combo there, I have to say.
I'm guessing the big ring doesn't have ramps or pins to aid shifting, which is why it is harder to get the chain to derail.
11-30-11, 01:08 PM
Well, I am not mechanically inclined, but the bike shops' last attempt has left the chain scrambling onto (not quite) the large ring. It is really pathetic to watch. I asked them if the f.der. should be replaced, but they went "not yet"...which doesn't tell me a lot.
11-30-11, 01:14 PM
No, actually, it does have the ramps and rings. The only thing left to replace, since the compact cranks weren't a part of the original design, would be the spindle, which is 7/10 of a mm. too long with the new set up. That part, from Phil Wood, is about $110, but the bike has been shifting ok at the rear, so I've been avoiding putting it in=another $200 for what may/may not cure the front der. issues.
I should also add that I had the rear der. aligned-wasn't terribly off, but a little, and it is using a compatible Shimano cassette. The issue of being lighter when I go front rear crank transitioning is doubtful, as I have been light as a feather on it...as I literally see the landscape change to anything downhill-and the bike go into what I call 'normal front shift behavior-it amazes me to think I might have the first Apple accelometer built in someplace that is causing it to be fussy...unless it is warped in the frame or my butt is pushing too far down on it-58cm (220lb)!
11-30-11, 04:09 PM
Maybe your chainrings are directional and need to be flipped facing the other way for the ramps and bumps to catch the chain and lift it. The other issue may be the FD. You might need to lower it a tad, until it peaks just above the teeth on the big ring by 1 - 2 mm max. And look at the pivot angle of the FD as viewed looking from above downward along the seat tube. Most FDs like to have the outer part of the cage parallel to the plane of the frame, which then puts the inside of the cage slightly angled out pointing forward (because the FD is usually narrower up front and wider on the rear). Having sufficient inside cage slant will push the chain outward to the big ring more effectively.
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