Pre 2001 Mirage capacity question

[magirus]

Hi all, my first post in anger after my intro post. I need to (hopefully) gear down my trusty old Bianchi hack due to increasing age and decreasing fitness. The current set up is Campag Mirage 8speed Ergos and mechs with a Shimano 7speed hub and cassette, works flawlessly. What I want to do is fit a compact 50/34 chainset, and change the cassette from the 11-24 I cobbled together quite a while ago. The problem is the Mirage rear mech, which I'm pretty sure wont handle a cassette down to 32 or even 28, as the distance between the pulley wheel centres is only 45/46mm, a very short cage indeed. Obviously I can fit a Shimano 8speed mech which will take any cassette I like, but then the difference in cable pull becomes a factor as it adds up at one end of the cassette. So does anyone know the capacity of these older mechs, pre 2001 I assume due to the position of the B screw? Also, maybe I have it wrong and a Shimano 8speed mech will work without issue, even given the difference in the amount of cable pulled, so does anyone run this combo or has tried it and failed? Here's a pic of the mech, and thanks in advance for any help.....

 

[hamster]

Definitely it's pre-01, you are right.

I have got them to work (OK Chorus but the design is identical) with a 12-23 and 30/39/48. It's rattly but engages big-big safely. So that's a total tooth difference of 29T.

Your combo is 16+13T, again 29T, so I reckon it will just about work. My bike has fairly short stays (can't quite get a fingertip between wheel with 23c tyre and seat tube) which definitely makes thing worse.

 

[magirus]

Thanks for that hamster.I probably have longer stays that you, so may get away with a 28 low gear if I use a longer B screw and keep off the extremes of the cassette in either chainring.

 

[hamster]

Just make sure there is enough chain for it to engage big-big (wrap the chain round between the two, plus two links). Otherwise an accidental engagement of that gear combo will result in a wrecked rear mech and wheel.

Trust me. I have seen it happen.

 

[magirus]

Just make sure there is enough chain for it to engage big-big (wrap the chain round between the two, plus two links). Otherwise an accidental engagement of that gear combo will result in a wrecked rear mech and wheel.

Trust me. I have seen it happen.

Thanks again. I found an old Shimano Exage 300EX rear mech in my parts drawer, and the pulley wheel centres are @ 10mm further apart, so that will give me a little more leeway hopefully. Also I've found a screw to replace the B tension screw and give it a little more length. This mech is rated to take a sprocket up to 28t, so hopefully it will all work, the only downside is that it will only (allegedly) handle a difference of 13t between the front rings, and clearly the compact difference is 16t. Worth an hours tinkering I think. We shall see!
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