IRD 10spd Elite wide range cassette with a compact double crank?



Aero-X

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Jan 12, 2006
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Hi guys,

I'm currently running FSA compact 50/34 cranks with Dura-Ace 7800 10spd drive train on my bike. I use 12/27 cassette in the back. I believe 12/27 is lowest you can go with a shimano short cage rear derailleur, but I still want to go lower on the cassette without going triple(an expensive conversion). So I thought about going with IRD(interloc racing design) Elite road wide range 10spd cassette, either 11/32 or 11/34. In order to do so, I believe I need a long cage rear derailleur. Would a long cage Dura-Ace rear derailleur(RD-7803) work with IRD's wide range 10spd cassette without any compatibility problems? The current XTR cassettes are 9spd only, so IRD seems to be the only way to go.

Any advice would be greatly appreciated. I need to save my knees from petellar tendonitis...

Aero-X
 

dgregory57

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I can't find the specs for the 7803, but in general the Shimano road rear derailleurs only handle a rear cog up to 27 teeth. Even long cage (such as the current Ultegra long cage).

Even though you can usually go a tooth or two larger than spec (I run an 11-28 on my 105 long cage) 32 or 34 is probably too large, and would require a mountain RD, if you can get it to work (which you may be able to).

So, if you want to go with a 34 tooth large cog on your cassette, I would suggest trying it with a high end mountain RD capable of 34 teeth in place of your Dura-Ace... and don't forget to lengthen the chain.

EDIT: Even though mountain RDs are currently rated for only 9 speed... I think the amount of cable pull is the same as 10 speed, so you should actually be able to get it to work... You should ask the IRD people what derailleurs they suggest. :)
 

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Aero-X said:
Hi guys,

I'm currently running FSA compact 50/34 cranks with Dura-Ace 7800 10spd drive train on my bike. I use 12/27 cassette in the back. I believe 12/27 is lowest you can go with a shimano short cage rear derailleur, but I still want to go lower on the cassette without going triple(an expensive conversion). So I thought about going with IRD(interloc racing design) Elite road wide range 10spd cassette, either 11/32 or 11/34. In order to do so, I believe I need a long cage rear derailleur. Would a long cage Dura-Ace rear derailleur(RD-7803) work with IRD's wide range 10spd cassette without any compatibility problems? The current XTR cassettes are 9spd only, so IRD seems to be the only way to go.

Any advice would be greatly appreciated. I need to save my knees from petellar tendonitis...

Aero-X

11-34, to have enough chain for big-big and no droop in small-small, probably need a MTB, 95mm, long cage rear derailleur. I don't think the 7803 will give you enough chain. The cage is not long and gets the extra chain with big pulleys only. See if you can find a 2006/7 XT or XTR..not the current ones w/o a barrell adjuster on the rear derailleur.
 

alfeng

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Jul 23, 2005
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Aero-X said:
I'm currently running FSA compact 50/34 cranks with Dura-Ace 7800 10spd drive train on my bike. I use 12/27 cassette in the back. I believe 12/27 is lowest you can go with a shimano short cage rear derailleur, but I still want to go lower on the cassette without going triple(an expensive conversion). So I thought about going with IRD(interloc racing design) Elite road wide range 10spd cassette, either 11/32 or 11/34. In order to do so, I believe I need a long cage rear derailleur. Would a long cage Dura-Ace rear derailleur(RD-7803) work with IRD's wide range 10spd cassette without any compatibility problems? The current XTR cassettes are 9spd only, so IRD seems to be the only way to go.

Any advice would be greatly appreciated. I need to save my knees from petellar tendonitis...
As others have noted, Shimano's ROAD rear derailleurs are not happy with cassette cogs bigger than 27t/28t rear cogs ...

BUT, several years ago, I discerned that the limitation is because of the 11t pulley wheel ... and, if you change the upper pulley wheel to a 10t, then the largest cog the derailleur can handle is often 32t (depending on the frame's rear derailleur hanger). On one frame, I was able to fit a 12-34 XTR cassette with a Shimano 6503 rear derailleur whose upper pulley wheel was swapped, but that was an exception (i.e., the frame had a long derailleur hanger).

As far as the short cage on the 7800 rear derailleur, I believe that any cage from almost any recent (post-1998) Shimano MTB (there are probably exceptions at the bottom of Shimano's lineup of MTB rear derailleurs) or Ultegra-or-105 pseudo-"touring" (e.g., 6503, 6603, 5503, 5603) rear derailleur can be substituted for the cage in your 7800 rear derailleur ... maybe, not. Maybe only the 6603 & 5603 cages can be subsituted ... OR, you can just buy a 6603 Ultegra rear derailleur (the pulley wheels are much BETTER than the ones on the 105 rear derailleur, yet the Ultegra rear derailleur only cost a few dollars more) because the cost of the cage (if you aren't cannibalizing a rear derailleur that you already have) + labor (if your shop does it) will probably be close to the price of an Ultegra rear derailleur.

OR, as Peter suggests, use a Shimano MTB rear derailleur ... but, AFAIK, you'll need to use the hubbub.com alternate anchor position to make the 9-speed Shimano rear derailleur act like a 10-speed Shimano rear derailleur.
 

artemidorus

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If you can only find a new MTB RD without a barrel adjuster, you can use an in-line barrel adjuster that sits just inboard from your shifter. I'm using 9spd 105 shifter and the current XTR RD with an in-line adjuster, successfully. The barrel slips very slowly, and needs readjustment every few months.
 

rparedes

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Jul 21, 2007
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Aero-X said:
Hi guys,

I'm currently running FSA compact 50/34 cranks with Dura-Ace 7800 10spd drive train on my bike. I use 12/27 cassette in the back. ....So I thought about going with IRD(interloc racing design) Elite road wide range 10spd cassette, either 11/32 or 11/34. ...... so IRD seems to be the only way to go.

Any advice would be greatly appreciated. I need to save my knees from petellar tendonitis...

Aero-X

They were several IRC reviews a few months back. IRD cassette does not work well with shimano drivetrain....
Have you tried a SRAM 11/28 cassette?
 

artemidorus

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Mar 10, 2004
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rparedes said:
They were several IRC reviews a few months back. IRD cassette does not work well with shimano drivetrain....
Have you tried a SRAM 11/28 cassette?
Why not go 9 spd and use an MTB cassette? Few people have a burning need for an 11 t rear sprocket.
 

rparedes

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Jul 21, 2007
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artemidorus said:
Why not go 9 spd and use an MTB cassette? Few people have a burning need for an 11 t rear sprocket.
I think it will require a big re-do: cassette, shifter, RD...
 

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alfeng said:
As others have noted, Shimano's ROAD rear derailleurs are not happy with cassette cogs bigger than 27t/28t rear cogs ...

BUT, several years ago, I discerned that the limitation is because of the 11t pulley wheel ... and, if you change the upper pulley wheel to a 10t, then the largest cog the derailleur can handle is often 32t (depending on the frame's rear derailleur hanger). On one frame, I was able to fit a 12-34 XTR cassette with a Shimano 6503 rear derailleur whose upper pulley wheel was swapped, but that was an exception (i.e., the frame had a long derailleur hanger).

As far as the short cage on the 7800 rear derailleur, I believe that any cage from almost any recent (post-1998) Shimano MTB (there are probably exceptions at the bottom of Shimano's lineup of MTB rear derailleurs) or Ultegra-or-105 pseudo-"touring" (e.g., 6503, 6603, 5503, 5603) rear derailleur can be substituted for the cage in your 7800 rear derailleur ... maybe, not. Maybe only the 6603 & 5603 cages can be subsituted ... OR, you can just buy a 6603 Ultegra rear derailleur (the pulley wheels are much BETTER than the ones on the 105 rear derailleur, yet the Ultegra rear derailleur only cost a few dollars more) because the cost of the cage (if you aren't cannibalizing a rear derailleur that you already have) + labor (if your shop does it) will probably be close to the price of an Ultegra rear derailleur.

OR, as Peter suggests, use a Shimano MTB rear derailleur ... but, AFAIK, you'll need to use the hubbub.com alternate anchor position to make the 9-speed Shimano rear derailleur act like a 10-speed Shimano rear derailleur.

Nope. 8/9/10s rear derailleurs(only exception being DA 8s) all have the same dimension and are cross compatible with different 'speed' shifters. a shimano MTB rear derailleur, even tho '9s' will shift 10s spacing fine and dandy w/o any modification or gizmo.
 

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artemidorus said:
If you can only find a new MTB RD without a barrel adjuster, you can use an in-line barrel adjuster that sits just inboard from your shifter. I'm using 9spd 105 shifter and the current XTR RD with an in-line adjuster, successfully. The barrel slips very slowly, and needs readjustment every few months.

Or one of the jagwire barrel adjuster doohickeys that sick into the shifter or rear derailleur.
 

alfeng

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Jul 23, 2005
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Nope. 8/9/10s rear derailleurs(only exception being DA 8s) all have the same dimension and are cross compatible with different 'speed' shifters. a shimano MTB rear derailleur, even tho '9s' will shift 10s spacing fine and dandy w/o any modification or gizmo.
Well, that isn't an accurate statement ...

The 8-/9-/10-speed Shimano rear derailleurs do not have the same dimensions.

FWIW. The length of the parallelogram of the 8-/9-speed XTR is 45mm (hinge pin to hinge pin). I measure the anchor point for the rear derailleur cable as being 20mm from the rear hinge.

Since my recollection is that the 8-speed 105 rear derailleur looks like the XTR's parallelogram, I would suppose that the 8-speed ROAD rear derailleurs also have 45mm parallelograms.

The 9-speed XTR rear derailleurs (e.g., 952) that I have are same as the 8-speed XTR rear derailleurs (950 & 951/[rapid rise]) except for the number of teeth on the pulley wheels + a cosmetic difference in the derailleur hanger bolt.

A 9-speed Ultegra 6503 rear derailleur's parallelogram is 43mm long, but the anchor point remains at 20mm from the rear hinge. Despite the difference in the length of the 9-speed ROAD parallelogram, the apparent reason that a Shimano MTB rear derailleur can be substituted for one of the 9-speed ROAD rear derailleurs is because the SLANT (the plane in the Z-axis) is at a steeper angle on the 9-speed rear ROAD derailleurs, thus both the 8- and 9-speed Shimano rear derailleurs apparenty have the same horizontal cage movement.

Now, the 10-speed Ultegra rear derailleur has an even shorter parallelogram (38mm) & steeper SLANT, but the anchor point is decidedly further from the rear hinge (22.5mm) -- approximately the same change in distance that using the hubbub.com anchoring would achieve when applied to an 8-/9-speed Shimano rear derailleur.

Now, while one might suppose that the combination of changes might account for continued interchangeability, I will note anecdotally that a 'quick' non-road test of a 10-speed Ultegra rear derailleur with its "normal" rear derailleur cable anchoring + 10-speed Campagnolo Ergo shifter yielded the same shifting rate as with either a 9-speed XTR or Ultegra rear derailleur with the hubbub.com anchoring + 10-speed Campagnolo shifter -- that may only be a fortunate coincidence for me since my preference is to use Shimano rear derailleurs/etc. (I love Shimano EXCEPT for their shifters) mated to Campagnolo shifters!

The 10-speed Shimano rear derailleurs retain the double-bladed "molly" hook/(washer) that was originally introduced with the "regular" 8-speed Shimano rear derailleurs to allow them to be used with the original, 8-speed DA shifters (i.e., Shimano's ALTERNATE ANCHOR POSITION @ 9 o'clock).

To achieve BACKWARD compatibility when using a 10-speed Shimano rear derailleur with a 9-speed cassette & shifter you would more-than-likely use the ALTERNATE REAR DERAILLEUR CABLE ANCHORING POSITION (I don't remember the documentation indicating this, BTW, and I don't have a sheet handy), hence the legacy molly "washer."

a shimano MTB rear derailleur, even tho '9s' will shift 10s spacing fine and dandy w/o any modification or gizmo.
I can neither refute nor confirm this statement through direct application ... but, I would be inclined to believe it is incorrect based on my successful substitution of a 10-speed Shimano rear derailleur for a 9-speed rear derailleur which was hubbub'd -- that suggests to me that a 9-speed Shimano MTB rear derailleur may NOT be directly interchangeable "w/o any modification or gizmo" (e.g., shiftmate) when used with a 10-speed Shimano shifter & cassette.

So, while I was grateful that Shimano's 8-/9-speed MTB & ROAD shifters & rear derailluers are indeed interchangeable despite what Shimano's marketing and/or tech people apparently indicated back at the turn of the Century, when my wife told me that the 12-27 cog wasn't giving her a low enough gear -- since I had a 12-32 & XTR rear derailleur -- it was only a question of a few minutes of parts swapping to determine that Shimano's folk were inaccurate with regard to the mandate to only use components from a given group in a given drivetrain (i.e., only 105 with 105, Ultegra with Ultegra, DA with DA, XTR with XTR, etc.), otherwise who knows if-or-when I would have crossed the threshold of investigating possible component compatibility.

While I think my experience is reproduceable, if someone wants to adhere to whatever-or-whichever "conventional wisdom" they are familiar with with regard to the rear derailleurs & shifters, then that is their prerogative.

But, if anyone ends up with unsatisfactory results when trying to use their 8-/9-speed Shimano rear derailleur with a 10-speed Shimano drivetrain, or vice-versa, then I think it is worth trying the alternate anchoring positions.
 

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alfeng said:
Well, that isn't an accurate statement ...

The 8-/9-/10-speed Shimano rear derailleurs do not have the same dimensions.

FWIW. The length of the parallelogram of the 8-/9-speed XTR is 45mm (hinge pin to hinge pin). I measure the anchor point for the rear derailleur cable as being 20mm from the rear hinge.

Since my recollection is that the 8-speed 105 rear derailleur looks like the XTR's parallelogram, I would suppose that the 8-speed ROAD rear derailleurs also have 45mm parallelograms.

The 9-speed XTR rear derailleurs (e.g., 952) that I have are same as the 8-speed XTR rear derailleurs (950 & 951/[rapid rise]) except for the number of teeth on the pulley wheels + a cosmetic difference in the derailleur hanger bolt.

A 9-speed Ultegra 6503 rear derailleur's parallelogram is 43mm long, but the anchor point remains at 20mm from the rear hinge. Despite the difference in the length of the 9-speed ROAD parallelogram, the apparent reason that a Shimano MTB rear derailleur can be substituted for one of the 9-speed ROAD rear derailleurs is because the SLANT (the plane in the Z-axis) is at a steeper angle on the 9-speed rear ROAD derailleurs, thus both the 8- and 9-speed Shimano rear derailleurs apparenty have the same horizontal cage movement.

Now, the 10-speed Ultegra rear derailleur has an even shorter parallelogram (38mm) & steeper SLANT, but the anchor point is decidedly further from the rear hinge (22.5mm) -- approximately the same change in distance that using the hubbub.com anchoring would achieve when applied to an 8-/9-speed Shimano rear derailleur.

Now, while one might suppose that the combination of changes might account for continued interchangeability, I will note anecdotally that a 'quick' non-road test of a 10-speed Ultegra rear derailleur with its "normal" rear derailleur cable anchoring + 10-speed Campagnolo Ergo shifter yielded the same shifting rate as with either a 9-speed XTR or Ultegra rear derailleur with the hubbub.com anchoring + 10-speed Campagnolo shifter -- that may only be a fortunate coincidence for me since my preference is to use Shimano rear derailleurs/etc. (I love Shimano EXCEPT for their shifters) mated to Campagnolo shifters!

The 10-speed Shimano rear derailleurs retain the double-bladed "molly" hook/(washer) that was originally introduced with the "regular" 8-speed Shimano rear derailleurs to allow them to be used with the original, 8-speed DA shifters (i.e., Shimano's ALTERNATE ANCHOR POSITION @ 9 o'clock).

To achieve BACKWARD compatibility when using a 10-speed Shimano rear derailleur with a 9-speed cassette & shifter you would more-than-likely use the ALTERNATE REAR DERAILLEUR CABLE ANCHORING POSITION (I don't remember the documentation indicating this, BTW, and I don't have a sheet handy), hence the legacy molly "washer."

I can neither refute nor confirm this statement through direct application ... but, I would be inclined to believe it is incorrect based on my successful substitution of a 10-speed Shimano rear derailleur for a 9-speed rear derailleur which was hubbub'd -- that suggests to me that a 9-speed Shimano MTB rear derailleur may NOT be directly interchangeable "w/o any modification or gizmo" (e.g., shiftmate) when used with a 10-speed Shimano shifter & cassette.

So, while I was grateful that Shimano's 8-/9-speed MTB & ROAD shifters & rear derailluers are indeed interchangeable despite what Shimano's marketing and/or tech people apparently indicated back at the turn of the Century, when my wife told me that the 12-27 cog wasn't giving her a low enough gear -- since I had a 12-32 & XTR rear derailleur -- it was only a question of a few minutes of parts swapping to determine that Shimano's folk were inaccurate with regard to the mandate to only use components from a given group in a given drivetrain (i.e., only 105 with 105, Ultegra with Ultegra, DA with DA, XTR with XTR, etc.), otherwise who knows if-or-when I would have crossed the threshold of investigating possible component compatibility.

While I think my experience is reproduceable, if someone wants to adhere to whatever-or-whichever "conventional wisdom" they are familiar with with regard to the rear derailleurs & shifters, then that is their prerogative.

But, if anyone ends up with unsatisfactory results when trying to use their 8-/9-speed Shimano rear derailleur with a 10-speed Shimano drivetrain, or vice-versa, then I think it is worth trying the alternate anchoring positions.

Not sure what you just said in your 'epic' upstairs BUT. I have installed many otherwise '9s' rear derailleurs onto shimano 10s drivetrains as well as using a '10s' rear derailleur as a replacement rear derailleur on many 8s and 9s 'systems. The ONLY rear derailleur requiring the mysterious alternative cable placement was when using a DA 8S rear derailleur in order to use the 'new' at the time 9s STI. OR when using DA 8s STI and needing a new rear derailleur after DA 8s were out of production. All that stuff from more than a decade ago(9s shimano introduced in the 1997 model year).

So the parallelograms aren't identical..all are cross compatible, road to MTB, 8s, 9s, 10s. except for DA 8s and perhaps DA 7900.
 

rparedes

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alfeng said:
...or you'll need to use the hubbub.com alternate anchor position to make the 9-speed Shimano rear derailleur act like a 10-speed Shimano rear derailleur.

What is the hubbub alternate anchor point?
 

alfeng

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Jul 23, 2005
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rparedes said:
What is the hubbub alternate anchor point?


ergolever_xtergo.jpg




http://www.hubbub.com/articles_ergopower.html


 

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rparedes said:
What is the hubbub alternate anchor point?

Once more, no need for a 'hub bub' or any alternative anchor position if you are using a so called '9s' rear derailleur with a set of shimano 10s shifters.

NOT required.

I have instaqlled everything from a 9s Deore MTB rear derailleur to a Sora 8s rear derailleur onto 105, ultegra and DA 10s STI shifter systems and have anchored the cable where you anchor it with a '10s' rear derailleur. There are some alternative type stuff when mixing shimano and Campagnolo shifters and rear derailleurs but not needed if you are using common money.
 

alfeng

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Jul 23, 2005
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Not sure what you just said in your 'epic' upstairs BUT. I have installed many otherwise '9s' rear derailleurs onto shimano 10s drivetrains as well as using a '10s' rear derailleur as a replacement rear derailleur on many 8s and 9s 'systems. The ONLY rear derailleur requiring the mysterious alternative cable placement was when using a DA 8S rear derailleur in order to use the 'new' at the time 9s STI. OR when using DA 8s STI and needing a new rear derailleur after DA 8s were out of production. All that stuff from more than a decade ago(9s shimano introduced in the 1997 model year).

So the parallelograms aren't identical..all are cross compatible, road to MTB, 8s, 9s, 10s. except for DA 8s and perhaps DA 7900.
FWIW. I hope you/(Peter) didn't think that I was saying you couldn't interchange one Shimano rear derailleur for another -- you know, some say that bumblebees should not be able to fly, etc.

I primarily wanted to observe that you made a harmlessly inaccurate statement ... but, if I had simply refuted your remark & made the statement that the various Shimano rear derailleurs did not "have the same dimension" then some of the anal retentives who populate this Forum (we all know who some of them are) would subsequently have wanted 'facts' ...

However, I am still inclined to believe that with the advent of Shimano's 10-speed rear derailleurs, the various rear derailleurs are no longer as compatible as you suggest despite what you say you have observed BECAUSE I believe I have employed a comparable situation whereby I have simply hooked up a 9-speed ERGO shifter to a 9-speed Shimano rear derailleur (normal anchoring) to be used with a 9-speed Shimano cassette ... it definitely works WELL ENOUGH -- the shifting is smooth & the chain is quiet ...

BUT (and, here's the qualifier), there is what I will refer to as a "phantom" cog which is bypassed in the shifting sequence (the 4th from the largest -- aka, the 6th from the smallest ... a less consequential cog in the sequence than some others to me, but perhaps not to someone else) ... so, only 8 of the cogs are "indexed" in a 9x9 Campagnolo-Shimano mismatch vs. a hubbub'd 10x9 Campagnolo-Shimano mismatch which yields all 9 cogs being engaged during the indexing.

Undoubtedly, the reason the casual 9-speed ERGO + 9-speed Shimano rear derailleur & cassette combination "works" is because the space between the chain's plates is much greater than the width of any cog ... in addition to only using 8 of the 9 cogs, it's probably not ideal because the chain is undoubtely not as centered on the upper pulley wheel after the cog is engaged & the pulley wheel may wear out prematurely (or, not ... since it isn't noisey, who knows?). But, unrealized excessive wear on the upper pulley wheel (if there is any ... ) would be a small price to pay to be able to use Campagnolo rather than Shimano shifters for the type roadways (i.e., mountain) that I ride on.

And so, I suspect that if you/(Peter)/anyone were to scrutinize the mis-matched 9-/10-speed Shimano combos that you describe, you may find that EITHER a cog is bypassed OR the chain may "dwell" on one of the cogs in the shifting sequence, depending on what the mis-match happened to be ... maybe, not.

In the grand scheme of things, it doesn't matter all that much because while I know that EITHER a hubbub'd 9-speed Shimano rear derailleur OR a 10-speed Shimano rear derailleur works better with a 10-speed Campagnolo shifter + 9-speed Shimano cassette than the less perfect combination of a 9-speed Campagnolo shifter + 9-speed Shimano derailleur/cassette, they all still result in indexing that certainly works effectively; so, the same should/could be true with Shimano shifters ...

Presuming the same occurs with Shimano-Shimano mismatches, the various Shimano rear derailleurs could indeed be declared to be interchangeable ...

NB. I haven't tried it, but I suspect that if you were to take an 8-speed Campagnolo rear derailleur whose anchor point is more mid-parallelogram than on the 9-/10-/11-speed rear derailleurs, that the same, acceptable compatibility could be achieved despite Campagnolo spec'ing one rear derailleur design for their 8-speed ERGO "stuff" and another for the rest ...

I think an important (?) thing to note is that more components are compatible than manufacturers would like the end user to believe.
 

alfeng

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Jul 23, 2005
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... There are some alternative type stuff when mixing shimano and Campagnolo shifters and rear derailleurs but not needed if you are using common money.
FWIW. I think the JTEK SHIFTMATE, as clever as it is, is only for those who don't know about the hubbub anchoring when using a 10-speed ERGO shifter, or who don't want to lose the use of one-cog if they have 9-speed ERGO shifters (either the 1998-99 pointy-hooded type, or the subsequent versions [I don't know if there is any difference in how the two 9-speed versions theoretically index]) ...
 

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alfeng said:
FWIW. I hope you/(Peter) didn't think that I was saying you couldn't interchange one Shimano rear derailleur for another -- you know, some say that bumblebees should not be able to fly, etc.

I primarily wanted to observe that you made a harmlessly inaccurate statement ... but, if I had simply refuted your remark & made the statement that the various Shimano rear derailleurs did not "have the same dimension" then some of the anal retentives who populate this Forum (we all know who some of them are) would subsequently have wanted 'facts' ...

However, I am still inclined to believe that with the advent of Shimano's 10-speed rear derailleurs, the various rear derailleurs are no longer as compatible as you suggest despite what you say you have observed BECAUSE I believe I have employed a comparable situation whereby I have simply hooked up a 9-speed ERGO shifter to a 9-speed Shimano rear derailleur (normal anchoring) to be used with a 9-speed Shimano cassette ... it definitely works WELL ENOUGH -- the shifting is smooth & the chain is quiet ...

BUT (and, here's the qualifier), there is what I will refer to as a "phantom" cog which is bypassed in the shifting sequence (the 4th from the largest -- aka, the 6th from the smallest ... a less consequential cog in the sequence than some others to me, but perhaps not to someone else) ... so, only 8 of the cogs are "indexed" in a 9x9 Campagnolo-Shimano mismatch vs. a hubbub'd 10x9 Campagnolo-Shimano mismatch which yields all 9 cogs being engaged during the indexing.

Undoubtedly, the reason the casual 9-speed ERGO + 9-speed Shimano rear derailleur & cassette combination "works" is because the space between the chain's plates is much greater than the width of any cog ... in addition to only using 8 of the 9 cogs, it's probably not ideal because the chain is undoubtely not as centered on the upper pulley wheel after the cog is engaged & the pulley wheel may wear out prematurely (or, not ... since it isn't noisey, who knows?). But, unrealized excessive wear on the upper pulley wheel (if there is any ... ) would be a small price to pay to be able to use Campagnolo rather than Shimano shifters for the type roadways (i.e., mountain) that I ride on.

And so, I suspect that if you/(Peter)/anyone were to scrutinize the mis-matched 9-/10-speed Shimano combos that you describe, you may find that EITHER a cog is bypassed OR the chain may "dwell" on one of the cogs in the shifting sequence, depending on what the mis-match happened to be ... maybe, not.

In the grand scheme of things, it doesn't matter all that much because while I know that EITHER a hubbub'd 9-speed Shimano rear derailleur OR a 10-speed Shimano rear derailleur works better with a 10-speed Campagnolo shifter + 9-speed Shimano cassette than the less perfect combination of a 9-speed Campagnolo shifter + 9-speed Shimano derailleur/cassette, they all still result in indexing that certainly works effectively; so, the same should/could be true with Shimano shifters ...

Presuming the same occurs with Shimano-Shimano mismatches, the various Shimano rear derailleurs could indeed be declared to be interchangeable ...

NB. I haven't tried it, but I suspect that if you were to take an 8-speed Campagnolo rear derailleur whose anchor point is more mid-parallelogram than on the 9-/10-/11-speed rear derailleurs, that the same, acceptable compatibility could be achieved despite Campagnolo spec'ing one rear derailleur design for their 8-speed ERGO "stuff" and another for the rest ...

I think an important (?) thing to note is that more components are compatible than manufacturers would like the end user to believe.

AND Campagnolo ERGO work great with Sram for shimano/Sram 10s cog spacing BUT the OP was talking about a MTB RD for 10s STI. And my point(last time) was a 9s shimano MTB RD works great with shimano 10s STI w/o any alternative cable anchoring. I don't prescribe mixed shifters/RD because each maker has lots of choices. No need to mix a set of ERGOs and sully it up with a shimano RD.
 

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alfeng said:
FWIW. I think the JTEK SHIFTMATE, as clever as it is, is only for those who don't know about the hubbub anchoring when using a 10-speed ERGO shifter, or who don't want to lose the use of one-cog if they have 9-speed ERGO shifters (either the 1998-99 pointy-hooded type, or the subsequent versions [I don't know if there is any difference in how the two 9-speed versions theoretically index]) ...

Last pointy top was 1998 Athena and below. Record and Chorus were rounded top altho the shift innards were the same thruout the line. Last pointy top rhuout was 1997, first year of 9s.
1999 the innards changed again to incorporate ERGOBrain but the shift disc dimensions stayed the same.