vintagerando

What I am trying to do is get a gear combination which will allow me to keep using the 80s Campy Super Record RD and be able to climb the hills in my area. One solution is just to buy a IRD Claissica 13-28. Campy SR is supposed to work with 28t. So, I could leave it at that; live with 28t. A more ambitious route would be to buy the SOMA Fab Nuovo Retro RD cage plate which will allow the Campy RD to go up to 32t.
One thing I don't understand is "wrap". I know the formula, but what are the implications of the "wrap" number. The SOMA Nuovo Retro cage is described as having the "capacity of 38t chain wrap." If I had a IRD classica, 14-32, and my rings are 53-42, the wrap is 30t. So, what does that tell me? The Soma Retro cage setup would work? I guess that last option is to dump the Campy SR and use some other RD. I don't know if any RD from that era, 80s can do 32t.
Your input is appreciated.

oldsmobum

Implications...
Chain capacity, or chain wrap capacity has to do with how much chain length the derailleur can accommodate. since the derailleur acts as a chain tensioner. Wider difference in gearing, means more chain length. It is determined by finding the difference in between high and low cogs and chainrings, and adding them up. Chain wrap refers to the amount of contact the chain makes with the cassette. Literally how much it wraps around the cassette. Having the proper amount of chain wrap will allow for optimal shifting and better cassette life. As long as you stay within the maximum cog size spec (which you are) you shouldn't have a problem with this. It becomes an issue when the b-tension adjustment is brought out too far to clear large cogs.

So, with an 14-32, and a 53-42:
32-14= 18 rear difference
53-42=11 front difference
11+18=29 tooth capacity required

Basically that lets you know that with your setup, your chain won't be way too long, or too short in any gear combination. Which in either circumstance can be bad, but the safe practice would be to run the chain long and avoid a small/small ring combination, which doesn't make sense anyway. Usually these specs are pretty conservative from the manufacturer, and there is a little leeway. but at the same time you should execute caution when you approach or exceed manufacturers spec's. I've seen Setups that were within drivetrain spec but didn't work well because the derailleur hanger was too short. Hope this helps and I didn't ramble too much.

fietsbob

Its a Knock Off of a Campag Rally Cage .. minus the threaded Brass Insert of the Original.

JohnDThompson 

+1 this. Thus it will have the same chain wrap capacity as the Rally: 38T

fietsbob

The <C> 3450* used a spring loaded upper Pivot, (So did Shimano) after that they Just changed The Cage.

* I'm still using the one I got in the 70's on a 50,40,24_ 13~28 freewheel ,,

oldsmobum

Didn't notice that. I was curious why that derailleur/capacity... Forgot all about the Rally, even though I just sold one. Neat concept!

evets11

That's 41t capacity vs. Soma's stated 38t. I've noticed you can usually "push" true capacity a bit from 3-6t depending on RD. You pushed it 3t. You think my setup with a "Soma Cageized" Campy Nuovo Gran Sport RD with a 13-32t 7sp Sachs FW, and "Triplizerized" Campy 144bcd 52/42/28t crank setup (43t capacity) will work without chain sag in sm./sm. + no catasrophic chain pull on RD cage in big/big with a Soma cage? I haven't bought the Soma cage, 144bcd triplizer nor longer BB spindle yet? This is the gearing I'm planning on riding the 90 mile route in L'Eroica Ca. in April 2018. I don't want to spend the $200+ for this conversion until I'm SURE it will work. P.S.- I MAY just drill out #5 74bcd CR bolt holes on each crank spider arm. The arms on the NGS crank are flat, so this idea should work easily.

fietsbob

you can also buy a 144 bcd triplizer with a 2nd 74 bolt circle About Triplizers - Red Clover Components

https://www.redclovercomponents.com/s..._42_Teeth.html





..

Ghrumpy

"Wrap" or Total Gear Capacity (TGC) and maximum rear cog capacity often land on the same number for a given derailleur, but they are not closely related. You need to deal with them separately, for the most part.

The maximum rear cog capacity (for a drop para rear derailleur like the late SR) is a function mostly of the parallelogram length, but also of the cage and pulley geometry. The latter is how the late SR specs a 28t max rear cog instead of the 26t of the Record, Nuovo Record, and early Super Record. All four have the same parallelogram length and TGC. The late SR has a different pulley cage that puts the upper pulley slightly lower relative to the lower pivot. The SOMA/Rally long cage adds teeth to the max rear cog capacity by placing the pulley even lower relative to the pivot than the SR. A longer derailleur hanger will do the same thing, but at the expense of slightly lower shift quality to the smallest cog.

@oldsmobum described wrap or TGC well. Nothing really to add there. The SOMA/Rally long cage increases the TGC by allowing more chain to be run.

So if you had a long cage with the same pulley geometry as the Nuovo Record, it would still have the same max rear cog, but more TCG.

You can often "cheat" the max rear cog in a couple ways. One, if you have long horizontal dropouts, is to move the axle as far rearward as possible. This has the same effect as a longer hanger, but allows the chain to wrap more so the small cog shift quality isn't quite as compromised. For a given chain length, it also causes the pulley cage to rotate "backwards" slightly, which pulls the upper pulley slightly farther away from the largest cog.
You can also run the chain one link short, which also pulls the upper pulley back and away from the cogs, increasing the clearance. However, this makes big-big cross-chaining more likely to overwind the derailleur springs. You'll have to studiously avoid that if using this hack.

speedevil 

You might consider a Roadlink, if you want to handle a larger rear cog than usual. Note that this does not increase the RD capacity, just moves the RD away from the cassette a bit so that the upper guide pulley doesn't hit the larger cog. This also avoids using the B screw to space the derailleur to a different position or moving the axle around in the dropouts.

bikemig 

There are period correct derailleurs that can handle this much wrap. A long cage suntour cyclone will shift better than a campy NR RD. A shimano long cage crane (essentially a first generation dura ace) is a fine looking derailleur that will do the job as well. Or you could go with a "compact" vintage crank like a stronglight 99 and run something like 47/32 rings. Here is one for sale in CV

https://www.bikeforums.net/classic-v...l#post20163199

Personally I'd shrink the crank and pick up something like a stronglight 99 since that lets you run a campy NR RD short cage and is likely to give you better gearing since the freewheel cogs will be more closely spaced with say a 13-26 than a 13-32.