Old School Campagnolo friction rear dérailleur.

[TheCyclingRooster]

Hi to you all out there. During the rebuilding of the recently acquired Benotto - a problem has arisen with the limitation associated with the Short Cage that is apparently 'the norm' on the old school campy kit.
I have found that the existing Nuovo Gran Sport is struggling to accommodate a 25T as part of a 13:15:17:19:21:23:25 screw-on and a triple 52:40:28. I am of the understanding that they have a maximum capacity of a 26T.
I am reluctant to shorten the chain any further in order to pull the cage backwards as this would leave the parallelogram in a perpendicular position and possibly putting excess strain on the chain and the system.
Without reverting to a modern indexed lever control unit and shifting it with the friction lever - which old school campy offers either a medium cage or better still - a long cage.
I am wishing to accommodate a 28T as part of a 7spd 14 - 28.
Would I be better sticking with the screw-on that was on the bike - a 6spd 13:14:16:18:21:26 ? I so what difference does the position of the 26T sprocket as number 6 make against having a 25T as number 7?

 

[Smokin Joe]

No way will a short cage mech run on a triple with those rings. I don't recall Campag ever doing long cage mechs back in the day and if they did you'd probably struggle to find any. You will probably find you need to put up with a modern version.

 

[RecordAceFromNew]

I think your issue is wrap capacity more so than max sprocket size here. There is variation, not least because the allowable max cog size is also dependent on the length of your frame's derailleur hanger, but I think most can accommodate a 28T large sprocket. You can tell by shifting into the largest cog and see if you can still find X mm of daylight between the top jockey wheel and the largest sprocket when the chain is as tensioned as you are prepared to accept. For every 2mm in X you can have an extra T.

Regarding wrap capacity, I think most vintage Campag rear mechs of the Record/Super Record and derivatives (i.e. Nuovo Gran Sport, 980 etc.) have a capacity of around 26T to 28T. In your case your chainset has already taken up 24 of that. I think most vintage Campag triple chainsets had a range of 20T or so, which require a rear mech capacity of 32T in combination with a typical 13T-25T rear cluster.

That is where the Rally rear mechs came in. Some versions are better performers than the others. Well there are worse things one can do than to spend a wet Sunday afternoon digesting this...

 

[TheCyclingRooster]

No way will a short cage mech run on a triple with those rings. I don't recall Campag ever doing long cage mechs back in the day and if they did you'd probably struggle to find any. You will probably find you need to put up with a modern version.

Hi Smokin Joe. The short cage mech' is working fine with the triple chain-ring until it comes to getting the 25T to engage - it is very close to the jockey wheel and is running but protesting,hence my notion that a slight shortening of the chain will create a resolve.
Just as a point-in-passing,the triple ring set-up is working fine on a Double 'Old School Campagnolo front dérailleur using the Campagnolo down-tube friction shifter.

 

[TheCyclingRooster]

I think your issue is wrap capacity more so than max sprocket size here. There is variation, not least because the allowable max cog size is also dependent on the length of your frame's derailleur hanger, but I think most can accommodate a 28T large sprocket. You can tell by shifting into the largest cog and see if you can still find X mm of daylight between the top jockey wheel and the largest sprocket when the chain is as tensioned as you are prepared to accept. For every 2mm in X you can have an extra T.

Regarding wrap capacity, I think most vintage Campag rear mechs of the Record/Super Record and derivatives (i.e. Nuovo Gran Sport, 980 etc.) have a capacity of around 26T to 28T. In your case your chainset has already taken up 24 of that. I think most vintage Campag triple chainsets had a range of 20T or so, which require a rear mech capacity of 32T in combination with a typical 13T-25T rear cluster.

That is where the Rally rear mechs came in. Some versions are better performers than the others. Well there are worse things one can do than to spend a wet Sunday afternoon digesting this...

Hi RAFN. Yes I do understand the term 'wrap capacity' but the rear cage is allegedly capable of a 26T maximum. The dérailleur hanger is a regular Campagnolo/Benotto one and the axle is set at the forward position and set with the Campagnolo adjustment screws/pins so as to avoid the axle going in deeper/closer into the mech'.
The daylight test had already brought about a tight/touching scenario and further considerations. Thank you for the "homework" - extremely interesting.
My rear dérailleur looks like a regular 3500 Nuovo Gran Sport.
I have since earlier today - changed the 28T inner of 3 for a 32T this might change again to a 30T (after tackling Parbold Hill) and shortened the chain by one further link.
What I now comfortably have working is - all but the 25T rear working on the 52T ring. The full 7spd range working on a 42T (previously a 40T) middle ring and the 25T rear working on the 32T ring and with no chain-slap.
The 42T middle of 3 gives me a more comfortable range than did the 40T - baring in mind that I am riding 140mm cranks,this is the equivalent to an increase of 2T on any one ring.
The whole set-up gives me an equivalent of 54:44:34 from a 52:42:32 triple set-up.

 

[RecordAceFromNew]

Hi RAFN. Yes I do understand the term 'wrap capacity' but the rear cage is allegedly capable of a 26T maximum. The dérailleur hanger is a regular Campagnolo/Benotto one and the axle is set at the forward position and set with the Campagnolo adjustment screws/pins so as to avoid the axle going in deeper/closer into the mech'.
The daylight test had already brought about a tight/touching scenario and further considerations. Thank you for the "homework" - extremely interesting.
My rear dérailleur looks like a regular 3500 Nuovo Gran Sport.
I have since earlier today - changed the 28T inner of 3 for a 32T this might change again to a 30T (after tackling Parbold Hill) and shortened the chain by one further link.
What I now comfortably have working is - all but the 25T rear working on the 52T ring. The full 7spd range working on a 42T (previously a 40T) middle ring and the 25T rear working on the 32T ring and with no chain-slap.
The 42T middle of 3 gives me a more comfortable range than did the 40T - baring in mind that I am riding 140mm cranks,this is the equivalent to an increase of 2T on any one ring.
The whole set-up gives me an equivalent of 54:44:34 from a 52:42:32 triple set-up.

If you aren't worried about the derailleur being able to take up chain slack when you are on small front small back and/or a potentially very expensive if not painful/deadly accident going into large front large back , then wrap capacity is obviously irrelevant to you.

Now focusing on just the max sprocket size, have you tried adjusting the B screw (or body tension screw in the case of Campag) on your Gran Sport?

it seems to me another contributor to your having difficulties with larger rear sprocket is the position of the axle. Looking at the photo, I think you will get a little more clearance (or daylight between top jockey and large sprocket if you will) if you move the axle as far back as you can. Not that it is directly relevant, but for frame builders Shimano specifies 24-26mm for road and 28-30mm for mtb for the distance between the centre of the axle and the centre of the boss for rear mech attachment. This does have implication on chain length though (per above health warning).

If you have done all you could so far then chain length is the only remaining adjustment for handling larger rear sprocket. The question then is what does the chain look like on large front large back (assuming it is long enough for it)? If there is still quite a bit of an S wrapping around the jockey wheels then what might it look like if you can simulate its shortening by a pair of links (by gathering it)?

Hope it helps.

 

[TheCyclingRooster]

Hi again RAFN. Thank you for your further assistance on this rather initially frustrating issue. I no longer have any slack when on the smallest ring and the smallest sprocket BUT I have yet to try a 30T ring - I have one on my Bianchi Via Nirone7 Alu Carbon (Avatar) which was destined for removal and replaced with a 28T.
I can comfortably achieve a 52T - 25T and I will look in the morning at moving the axle backwards a little - there is possible a 6mm difference between the present position and that which would be the case by moving the axle to the rearmost position in the drop-out. It is currently 24mm and 30 or 31mm could be the revised measurement by moving it backwards..
The rearward move could result in an link being re-inserted but still with an overall gain.
Before I make the adjustments tomorrow morning I will engage the 52T to 25T and take an image and post it onto the forum.
My suspicion is that the existing chain length will not currently not allow the axle to go backwards BUT the addition of one link will allow this and still produce an easier shift on the 52T to 25T and still maintain a chain-slap free 30T to 13T albeit that would not normally be a selection that would be getting used.
By persevering to achieve good and totally usable shifting/selections at the opposite ends of the extremes - all eventualities are catered for.

 

[TheCyclingRooster]

Hi to you all that are following this topic. As a follow-up to my last input,I have carried out some tweaks but I am still not entirely convinced that I have hit the wall as far as gearing choice goes.
The real issue still remains as the limitation regarding the largest of the triple rings.
I adjusted the Campagnolo drop-out adjusters and also added a link because the move backwards resulted in a chain that was like a guitar string.
I have replaced the the inner of the triple with a 28T and changed the 52T for a 46T. The triple is now 46:38:28.
I now have a range that will fully function on all 7 of the screw-on 13:15:17:19:21:23:25 with the 28T & the 38T rings BUT the 46T ring is limited to using only the 13:15:17:19:21:23 albeit the 25 would not normally be used with the largest ring of a triple.

The chain is almost going straight through the dérailleur

cage with virtually no change of direction around the rollers (as per the image).

I am intending to try a 50:42;30 and insert an additional 2 links into the chain in the hope that I can reach a compromise between not having the chain on the 50 - 25 to tight and when on the 50 - 13 I do not have any slap in the chain either.

 

[RecordAceFromNew]

In case you are unaware, every 4T reduction will "slacken" the chain by a pair* of links. In other words, all else being equal optimum chain length is reduced by a pair of links if you reduce the big ring by 4T.

It might be worth changing things one at a time, systematically.

I think it is the right thing to do to move the axle as far back as possible (effectively to raise the sprockets as far up as physically possible to avoid clash with the top jockey wheel).

The second thing to do is to maximise the spring tension with the B (or body tension) screw, which should also maximise that clearance.

Then pick a chain ring combination that has the smallest (big - small) range and smallest large ring size you are still happy with - this will maximise your chance of being able to achieve no slack at small small while the chain is still long enough at big big.

Then use a chain length calculator to work out what the theoretical optimum length is, and start from there.

* a pair is 1 inch worth, in case there is any misunderstanding

 

[TheCyclingRooster]

In case you are unaware, every 4T reduction will "slacken" the chain by a pair* of links. In other words, all else being equal optimum chain length is reduced by a pair of links if you reduce the big ring by 4T.

It might be worth changing things one at a time, systematically.

I think it is the right thing to do to move the axle as far back as possible (effectively to raise the sprockets as far up as physically possible to avoid clash with the top jockey wheel).

The second thing to do is to maximise the spring tension with the B (or body tension) screw, which should also maximise that clearance.

Then pick a chain ring combination that has the smallest (big - small) range and smallest large ring size you are still happy with - this will maximise your chance of being able to achieve no slack at small small while the chain is still long enough at big big.

Then use a chain length calculator to work out what the theoretical optimum length is, and start from there.

* a pair is 1 inch worth, in case there is any misunderstanding

Hi again RAFN. Thank you for the link to the Chain Length Calculator,another amazing little item of time & frustration saving technology.
I assume that the chain-stay length is from the centre of BB to the centre of the rear axle position.
On your opening point - I am aware that a change of 2T will involve a difference in chain length by 1 full link,so yes Pro-rata that would equate to 4T and a pair of links.
Your point 2 - I had already proceeded at one tweak at a time but until physically taking the bike out I had never ridden a 46T as a BIG ring of 3. I would probably be a little better with a 48T or temporarily cannibalise the 50T off of my Bianchi,I could even just do a direct swap as the Bianchi is on a 50:40:30 and start off by adding in the additional links.

My happy triple combination with the 7spd 13-25 would be a 50:40:26 - remember that I am on 140mm cranks effectively a 52:42:28.
According to the calculator,both the 28T & the 30T would use the same chain length in conjunction with a 50T top-ring.
I cannot actually see a body tension screw on the Nuovo Gran Sport but there are two stop-screws - one for each direction of movement in relation to the selected sprocket size.

 

[RecordAceFromNew]

My happy triple combination with the 7spd 13-25 would be a 50:40:26 - remember that I am on 140mm cranks effectively a 52:42:28.
According to the calculator,both the 28T & the 30T would use the same chain length in conjunction with a 50T top-ring.
I cannot actually see a body tension screw on the Nuovo Gran Sport but there are two stop-screws - one for each direction of movement in relation to the selected sprocket size.

Regarding the body tension screw, borrowing one of the 3500 photos from velobase per below, can you see the little dome shaped screw head right next to the head of the stop screw on the right?

You see 28T and 30T having the same optimum chain length because chain length can only be changed by no less than a pair of links and as I indicated earlier you need to change 4T to make the difference of a pair. Nevertheless the 30T will be less likely to go slack on small small, given I suspect you are likely pushing the Gran Sport's wrap capacity to the limit with a triple.

Sorry I don't get the bit about 140mm cranks making a 50T equivalent to a 52T. If you are talking about gain ratio, I would have thought a 140mm arm requires nearly 20% greater force at the pedal than a "typical" 170mm arm for the same torque at the bb, so the effort to push a conventional 52T ring at 170mm is equivalent to a 43T with a 140mm... Or have I missed something?

 

[TheCyclingRooster]

Hi again RAFN. That little dome screw that you have referred to is sometimes called the stop-screw - I believe. What I cannot see is what exactly it does!!!
I have noticed that on my Nuovo Gran Sport it is all the way in - as is the case in the image that you have provided.
Your reiterated point about 2T and the links in relation to a pair and a 4T difference - I had to some extent sussed that out during my tweaking a couple of days ago.
On that point I am now on a 50:40:30 and still with the 13:15:17:19:21:23:25. The 50 - 25 is usable,the 30 - 13 is also usable albeit with a little bit of fall on the chain but not troublesome. The reality is that either or both of those scenarios are not likely to be used on the road.

My reference to the 140mm cranks being the equivalent to a 2T increase in chain-ring size was based upon some info that I received some years ago - indeed in 2008 and just after I had had my left knee totally replaced at the beginning of November 2007.
The comparison was based upon the effective difference between a regular 170mm crank and the 140mm cranks when it comes to the effort of turning them over. I recall that it was a subject of discussion/research among the members/users of the CTC forum.
I would certainly appreciate any further input on the subject of using shorter than normal cranks. They are actually the ones that are used on a tandem and used by 'stoker'. My - cranks and Stronglight rings came from SJS Cycles,Bridgewater,Somerset. The cranks are branded as Thorn. I actually have three complete triples and a few additional rings.

 

[RecordAceFromNew]

Having looked at that "stop screw" on an early 80's Campag SR not dissimilar in design to your Gran Sport, I think all it does is to stop the cage unwinding and losing body tension completely. It is possible that these vintage mechs don't really have any fine spring tension adjustment, like the more modern designs described here.

With a shorter crank, a corresponding increase in force at the pedal is required to deliver the same power all else being equal (because power = torque x angular velocity while torque = force x arm length), which might not be so good for your knees. Instead of that a better alternative objective may be a physiological one, by maintaining the gain ratio, which is what a correspondingly smaller chainring would accomplish for a shorter crank. I have not seen the ctc article you refer to, so it might be something entirely different again.