Help please: Tooth capacity Campagnolo Gran Sport

[BalkanExpress]

Can anyone help with the maximum tooth capacity for a 1960's Gran Sport rear derailleur?

i have the min and max cog (14-26) as these are engraved on the back but an looking for maximum difference in teeth: front large-front small added to rear large -rear small.

I have seen a figure of 18 mentioned which would make sense in a world of 52-46 front rings but i was hoping for a bit more than that.

Any guidance is much appreciated.

 

[Ajax Bay]

I have used such an RD successfully with a 54-40 and a 14-27 block (=27t). HTH
Googling suggests a 28t capacity (maybe your "18t" is a typo for that). How much more do you want? Are you trying to run a triple chainset with it?

 

[BalkanExpress]

I have used such an RD successfully with a 54-40 and a 14-27.
HTH

Many thanks I'm thinking of 40-26 and 14-26 (but a 27 would be even better)

I know that's low, but, the plan is to have the bike "eroica ready" even if I'm not

 

[Once a Wheeler]

I have a lot of experience with this gear and its wrap-round limitations. Officially, the limit is 20 teeth. However, if your length of chain-stay and choice of front and rear tooth differences do not hit the sweet spot, it does not quite accommodate 20 teeth. I got round this by leaving out the stop screw on the roller cage. The gear worked perfectly well but when you removed the wheel, the roller cage then wound the chain around itself and it became a bit of a fiddle to get the wheel back in. I was using 42x52 on the front and 14–24 on the rear.

In the end I tired of the Campag wrap-round limitation and tried Huret Allvit gears instead. They were marred by a very imprecise change on the small sprockets, although the underlying concept was good. I then went on to the Simplex parallelogram models and found them fine. Simplex were relatively less well engineered and made from inferior materials; but the added spring at the hanger end of the parallelogram made all the difference. I think it would have gone to a 25 tooth difference easily.

Campagnolo were obviously aware of the problem because the successor gear, the Record, anchored the roller cage at the top roller instead of at the mid-point between the two. The wrap-round was consequently increased by half-a-dozen teeth or so.

See the whole catalogue here: http://classiclightweights.net/united-kingdom/holdsworth/bike-riders-aids-1967-8/

 

[Specialeyes]

This sounds intriguing... a 60s something-or-other Eroica-ready!

*waggles hand over crystal ball* wooooo ooooo, I see Stronglight / Specialites TA in your future *flourishes cape*

 

[BalkanExpress]

I have a lot of experience with this gear and its wrap-round limitations. Officially, the limit is 20 teeth. However, if your length of chain-stay and choice of front and rear tooth differences do not hit the sweet spot, it does not quite accommodate 20 teeth. I got round this by leaving out the stop screw on the roller cage. The gear worked perfectly well but when you removed the wheel, the roller cage then wound the chain around itself and it became a bit of a fiddle to get the wheel back in. I was using 42x52 on the front and 14–24 on the rear.

In the end I tired of the Campag wrap-round limitation and tried Huret Allvit gears instead. They were marred by a very imprecise change on the small sprockets, although the underlying concept was good. I then went on to the Simplex parallelogram models and found them fine. Simplex were relatively less well engineered and made from inferior materials; but the added spring at the hanger end of the parallelogram made all the difference. I think it would have gone to a 25 tooth difference easily.

Campagnolo were obviously aware of the problem because the successor gear, the Record, anchored the roller cage at the top roller instead of at the mid-point between the two. The wrap-round was consequently increased by half-a-dozen teeth or so.
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See the whole catalogue here: http://classiclightweights.net/united-kingdom/holdsworth/bike-riders-aids-1967-8/

Thank you very much, this is all fascinating stuff, I can see a fair amount of experimentation on the horizon

 

[BalkanExpress]

This sounds intriguing... a 60s something-or-other Eroica-ready!

*waggles hand over crystal ball* wooooo ooooo, I see Stronglight / Specialites TA in your future *flourishes cape*

How did you know that...it must be magic

Yep, i’m scanning the sites for a suitable chainset, i’m thinking a very low geared double to avoid chain line issues and allow a period correct front mech.

As for the bike, it will be the Coppi from the other thread

 

[Ajax Bay]

Interested to see what double you manage to find which is both 'period correct' and has a BCD allowing a 26t inner.
But I agree with @Once a Wheeler , this RD worked better when I had the original 50-40 rings on and a 5sp 14-25 block. With the 54t on I could not go large/large with a 27t on the back: the chain would've been too long to wrap.

 

[midlife]

I'd be tempted to use a later Record or Nuovo Record rear mech

 

[BalkanExpress]

I'd be tempted to use a later Record or Nuovo Record rear mech

It has a nuovo on at the moment, and I could stick with that but i’d Like to get a bit older if i can as it would be in keeping. An original record would be good but are not cheap. I have plenty of time, so can look for a well priced one. I’ll also look at Simplex as @Once a Wheeler has suggested.

 

[midlife]

Going older, a Skitter might fit the bill?

 

[BalkanExpress]

Going older, a Skitter might fit the bill?

Had to google that Could do but they seem to be very rare .

 

[BalkanExpress]

Interested to see what double you manage to find which is both 'period correct' and has a BCD allowing a 26t inner.

As @Specialeyes magically foresaw a TA 5 vis will do the job, This runs a 50.4 bcd outer, so i could run a single ring at 22 teeth, but even for me that is excessive. Inner has a BCD of 80 so I can go down to 26. Plenty of similar 50.4 chainsets out there mainly stronglight 49d but you do see the odd magistroni or gnutti