Classtime 

I asked in Mechanics sub forum and haven't solved the problem yet. I really want to mod my 77 Grand Record with a compact ten speed. I had the 6700 derailleurs, shifters, chain, and modern Sugino Mighty Tour on a modern bike that worked flawlessly. I moved everything over to the Motobecane and no matter how I adjusted the front derailleur and no matter where the chain was on the cassette, it would randomly drop over the big ring. I got a new chain and the results were the same. THE ONLY DIFFERENCE is the bottom bracket--the crank wants a 110 and that is what I used on the other bike but I don't have Swiss rings for my 110mm Phil Wood so I used a Shimano 113. I've never ever had such difficulty adjusting a FD. I'm too cheap right now to replace the chainring(s) to see if that helps and it worked so well on the other bike, it is hard to believe that is the problem.

repechage

Review the chain line.

base2 

When you moved the chainrings over, did you get the clocking of the inner ring correct? If not, the chain rollers won't mesh with the teeth in the big ring properly. It'll be hard to shift, then when it does, the chain will be inclined to skip over the big ring in a big "s" path on top of the teeth & drop off the other side on account of the effort to get there.

Classtime 

They seem to be clocked properly.

Bianchi84

Do you mean that the chain "over-shifts" off of the big ring? Do you have pictures of front derailleur set up from side and from above, to see clearance and angle?

scarlson 

Do you think clocking matters on a non-ramped/pinned set of chainrings? And if so, how do you determine clocking for these? I suppose it depends on a number of factors and there are only so many ways to try and see if they work worse or better. Asking because I have a front derailleur that always does this. Seems to be chainline dependent also (or maybe chain tension), as it mostly does it in the smaller cogs. But I would love to make it stop!

P!N20

What's the FD limit screw doing while all this is happening?

L134 

I had a similar problem which I solved. Unfortunately, I made two changes at once so can't be sure which change was the fix if not both. I changed out a 110 BB for a 107 BB (and I can't remember why I decided to do that - maybe I didn't like the chainline; maybe because I had the 107, it fit and I was desperate, or ?) and installed a new outer chainring (replacing an old 42t middle chainring I was using as an outer chainring with a new, modern TA 42t outer chainring). I went from very difficult shifts to outer ring with frequent drops to beautifully smooth shifts with zero drops. You went from perfect shifts with a 110 BB to imperfect shifts using a 113 BB (if I'm reading the situation correctly). If possible, I'd install a 110 BB, or shorter, on the new bike.

base2 

I agree. I don't see where the problem would be then.

Classtime 

Never had trouble like this.


One way and then the other.

Random overshift

Another one

Random perfect shift

Too close? Already tried higher.

fender1

Shimano FD's like a small space between themselves and the chainring, a couple of millimeters, so I think you are good there.

I am going to say a shorter bottom bracket would likely solve the issue. I run a Sugino Compact double, 46/34 on a bike with 130 rear spacing, 10 speed 11-32 cassette and am using a 110mm BB. I am using a Shimano CX70 FD and bfirters. It all works well. Very easy to set up and very crisp shifting.

scarlson 

Wait, I am confused. Why would a shorter BB make it better? I would think that putting the rings closer inboard would make the chain more likely to go off the outside, no? Educate me, please!

fender1

I can't! Just a guess on my end. Its the wrong bottom bracket for the crank. That is the is a place to start. I would also check the frame alignment since friction shifting, which the Moto was built for, has a lot more wiggle room when it come to adequate shifting performance vs. modern indexed stuff.

scarlson 

I mean, isn't a double a double, indexed or friction? You're sorta banging it up and down against the upper and lower limits which I assume should be well adjusted in either case.

I've had tandems where this is essential. You gotta hit the small ring right as you start climbing that roller or else in a second you and your stoker are gonna be walking and that's not a good time. But seems like some setups just ain't got it, for whatever reason

Sure, I know someone is going to say you could shift with more finesse and probably keep the worst of it from happening by jockeying the derailleur carefully, but in the heat of the moment, in the dark, in the rain, accidentally on singletrack with a road bike, riding with someone you want to impress - it's not gonna happen. And that's precisely when you would not like your chain to fall off!

Rooney 

I'd screw in the high limit based on this photo. I'll also echo the call to try a 110mm axle since that seems to be the only significant component change.

Classtime 

Chain is on the small ring. Photo shows alignment of cage with rings. It would be nice if I could find a JIS spindle of 110mm so I could se my current Swiss cups.

Rooney 

Hah, so it is. Nevermind me.