Celeste Mike

Hi all. I've got a vintage road bike, a 1986 Cannondale, and I'm trying to hack the gearing to get a wider range out of it as a 2x7 (and still work within the limits of its derailleurs). It would have originally come with 42/52 chainrings and a 13-24 6-speed in the back. I've gone and installed 39/54 rings, and a 12-24 7-speed cassette. Now when the chain is on the 39 ring it rubs the 54 ring when it is on the two smallest rear cogs. I'm thinking I could move Xmm spacers from the non-drive side of the rear hub to the drive side and re-dish the wheel, but this may not be enough without dishing the wheel too far. (It is a freehub and cassette, not a threaded on freewheel.) Can I also move the chainrings outward a little by changing the BB spindle length? (Square taper JIS spindle, Shimano FC-6207 crankset.)

I am inexperienced with resolving chainline issues, I thought these changes would be a lot more straightforward. How would you all approach this?
(Not looking to get into a discussion/critique of my gearing choices, just interested to learn if and how this is fixable.)
Thanks!

ThermionicScott 

That sounds normal to me. The small/small combination is perhaps the least efficient gear combo, and is almost the same gear as your 54/16. So if you find yourself in the small ring and wanting to go faster, just shift up into the big ring and back to the middle of your cassette.

79pmooney

I expect to be able to use all the combos on my bikes but I don't expect the big crossovers to not rub or sound great. If I need to adjust the FD to rub a little to keep the chain off the big chainring teeth, I'm OK with that. I don't ride big miles in the small-small and the big-big only by accident. Small-small is mostly on big climbs with short level patches that I do not want to double shift twice for.

Mike, have you sighted down the cassette (freewheel) to see where the chainrings line up with the cassette? On a proper 7-speed, the gap of the chainrings should line up exactly with the middle cog. Now older "more experienced" cranksets often slide further on to the BB spindle so the chainrings sit further inboard. Take a look at what you've got ans come back and tell us if the crankset is in line or how many mms it is off. (Quote me so I get the email.) This will give us an idea of what needs correction and what the best options are.

Celeste Mike

I've just measured, and the middle cog does line up exactly with the center of the gap between the chainrings.

I'm used to using all gear combos except the large/large on my 42/52 bikes... I could see not using the small/small on this bike, but the fact that it's rubbing on even the second smallest cog is problematic :/ (even though I know it's redundant to the 54/18).

Iride01 

Smaller large ring or larger small ring or longer chain stays. Or move your front chain line out and possibly have issues going to the lower ratio cogs on the back.

79pmooney

I was afraid I hear this. You've found the limits of chainring difference that work without chain rub. Now, small amounts of chain rub can be tolerated IF you have chairings that aren't aggressive in picking up chains. Those rings we all used 40 years ago required real effort to get the chain to climb up. No way was anything happening with a little simple rub. Now with pins and ramps, small contact and you are on your way to a big gear change. As I think I said earlier, I've used my FD to keep the chain off contacting the big ring. That works if you can live with the chain over time wearing a groove in the cage and if you do it long enough, destroying it. But as long as you keep the small-small or next to small use down to a minimum, that will take a long time and the wear is easy to see and monitor. (This is a place where those old fashioned friction shifters have real value. Setting the FD exactly where you want it is easy.)

Or - bite the bullet (both wallet and pride), go back to the 42 and an an inner chainring. A triple! Requires a longer spindle, almost certainly a new RD, probably a new FD, longer chain. I raced the classic 42 (43, 44) X 52 (53, 54) back in a distant millennia with a 13-19. Loved it! I was an out of the saddle animal. I could do most of New England's hills on that 19. When I hung up the race bike, I knew I wanted to keep those gears I loved so much. (The gear choices, the easy shifts, the patterns I knew so well.) But that low gear had to get lower! So my post race bike (the 1979 Peter Mooney of my user name) got a 53-42-28 crankset using the same wheels. Later when I went to 7 speeds, the 12 and 21 got added. I still love that setup and use variations of it on my 9-speed though the chainrings are getting universally small to reflect that I am 40 years older now.

Now my collection is 5 bikes, all with triples or are fix gears. 20 years ago I spent a year riding a race bike with a 53-39. With Portland's hills. I hated it. I have a memory etched in my brain of taking the shortcut home up Laurel Road on the 39. (1000' in not a whole lotta distance.) Last ride on that setup! (Yes, had I been in my 20s, not 50s, I'd have done that on a 42 and much smaller FW, just because I could. Fortunately I lived on the other coast! And instead it was Pac Monadnock on my race bike.)

alcjphil

The biggest culprit here is the size difference between the chainrings, the bigger the difference the more likely you will have chain rub while in the small ring and in the smallest cogs. I have a 2 x 7 setup on my old Limongi using 50-38 rings with almost no chain rub while in the smallest cog. Going back to your 52 tooth big ring might make all the difference

70sSanO

As others have stated, your large chainring is too large for the small ring you want to use.

There are a few things that might help a bit, but because it impacts the "2" smallest cogs on a 6 speed freewheel, it may take a combination of tweaks, more correctly identified as hacks.

You can get shim spacers and move the large ring out a slight amount, or the inner ring in a bit. I have done this with thin shim washers with a slight rub on only the small cog. I think a 10mm ID works, but you need to measure.

https://www.ebay.com/sch/i.html?_fro...asher&_sacat=0

You can try a narrower chain. Maybe run a 9 speed. However, the wider chainring gap might cause problems with the chain falling between the rings.

And as mentioned already, change the chainline with a wider BB spindle. You will sacrifice, maybe eliminate, the big-big combination but will gain some inner ring range.

And you can do what you suggested in moderation by slightly re-spacing, re-dishing, the rear hub. You might be able to use the shim washers, or similar, to slightly move the freewheel inboard. Maybe 2mm max so replicate a 130mm OLD. Or just build the OLD to 128 and slightly spread the dropouts. It is pretty well known that Cannondale dropouts will spread to 130mm, but I'm more cautious at 128mm.

My personal guess is that you will want to use a combination of all of these to be able to dial it in.

Good Luck.

John

70sSanO

One more thing I just remembered, I you have a freewheel and put spacers on the drive side, you will weaken the axle because the locknut is further out from the drive side cone.

You might be able to slightly increase it, but adding a 2mm to the drive side will put the same stress on the axle as running an 8 speed freewheel.

John

Celeste Mike

Thanks for the feedback, everyone! I'll mess around with it but probably just swap out one of the chainrings for the time being.

I'm still confused by this though. I mean I get what's happening, I just thought that a 39/53 wouldn't at all be out of place on this kind of bike. So I'm surprised that a 39/54 is proving so problematic. How do modern compact and sub-compact doubles work, and with much wider cassettes than my little 7 speed? And aren't they like a 16 tooth jump versus the 15 I'm trying to pull off here? Are they only on longer chainstayed bikes? (Mine are definitely short, at 400mm.)

ThermionicScott 

Back when your bike was made, experienced riders "knew" not to use the big-big or small-small gear combinations. It wasn't an expectation that you could use every gear combination without rub or other issues.

I suspect that the newer 10- or 11-speed systems that promise every gear combination, are spacing the chainrings outward so that the small-small can run cleanly. But this increases the chain angle when you use the big-big combo, so there's no free lunch.

Trakhak

This. Having begun racing in the mid-60s, when shifting on a "10-speed" bike meant sharing only one or, at most, two middle sprockets between the chainrings, proper shifting technique for a 7-sprocket-2-chainring combination for me is using the 4 smallest sprockets with the outer ring and the 4 largest with the inner ring.

By the way, spare a thought for Cannondale's frame designer, who ensured that the bike would shift perfectly with the original cluster and crankset---not a given with short chainstays (and with Cannondale's large-diameter seat- and chainstays making the clearances tight).

Road Fan

I think I’ve missed something: What’s rubbing, exactly? If it’s the chain on the front derailleur, that can often be cleared by adjusting the derailleur position either by feathering the lever or by changing the cage travel limits in the front. If it is only recently rubbing, what changed over the years? Decades of little impacts could move parts or even chain stays, actually changing the alignment of the frame. Chain wear or chainring warp age could also have a part in a chain on the small ring rubbing on the inner face of the big ring, while cross-chained. For rubbing in the rear, there could be a derailleur or (and?) hub out of normal position, so one should consider aligning rear triangles, drop outs and derailleur hanger.

Road Fan

This is why I mention that something may have changed since the frame and components were a new, matched set. The original design might not have been very far from having an interference problem of some small sort.

Racing Dan

Im fairly certain the distance between the rings are greater on a 34/50 than a old school 39/53. That would help the chain not rubbing in small/small. Your super short 400mm chain stay doesn't help either.

Celeste Mike

The chain rubs against the large chainring when it is on the small chainring in the front, and it is on either the smallest or the second smallest cog in the rear. Front derailleur is not an issue.

Also, the chainrings (Stronglight Dural), chain (KMC 8 speed), and rear cassette (Sun Race 7 speed) are brand new. I've just replaced the drivetrain. (Still working with original Shimano 600 derailleurs.)

Road Fan

So to me the obvious points are that the chainring spacing might be narrower than with your old set up, and the front chainline might be less than with the old one, even if the old one was "perfect." One remedy might be to go with a cheaper KMC 10 speed chain. It might be a little narrower than the 8 speed. You can review all the specs on-line and see if any of them are a bit narrower.

Chainring warpage or just out of true still could be part of the spectrum, or maybe there's a thin spacer which can move the outer ring out a bit and the inner ring in a bit.

I never really liked the Shimano 600 derailleurs, but for this setup they should work ok, especially the front one. If you don't have any rubbing which involves the derailleurs, I would keep what you have. On general principles a more modern Shimano rear or a Sun-Tour would all be actual upgrades (indexable rear derailleurs work better in friction shifting than do the older ones from the friction days, but I would not expect them to improve your current problem.