Need new front derailleur

[Prof Young]

Bikers:
I won't bore you with the details, but my newbie attempt to adjust the front derailleur on my Schwin Varsity resulted in it getting hung up on the chain and having the snot bent out of it. It's not repairable and close inspecting suggests it's pretty cheaply made anyway. So, I need a new front derailed that won't be so expensive as to be silly to put on a Schwin Varsity, and also simple enough to do my self. Any suggestions will be appreciated. Thanks.
Prof Young

[nfmisso]

I like the Shimano M413 - 48T rated max, triple.
http://www.amazon.com/shimano-alivio-fd-m413-front-derailleur/dp/B003I7F9WG/

Your choice of FD depends on:
* number of teeth in largest chain ring
* double or triple - double FD do not have enough travel for triple chain rings, but triple FD have no problem with doubles.
* distance from the seat tube to the chain rings. On my SR, I had use a much shorter BB axle with the road FD I used (purchased for a very very good price on CL )
* outside diameter of your seat tube - i cut a piece of 6061 T6 tube to act as a shim to mount the 34.9mm (1 3/8") FD on bikes with a 28.6mm (1 1/8") seat tube. I find the 34.9mm mount FD are cheaper (or higher level FD for the same price).

[Prof Young]

(08-08-2012, 01:32 PM)nfmisso Wrote:  I like the Shimano M413 - 48T rated max, triple.
http://www.amazon.com/shimano-alivio-fd-m413-front-derailleur/dp/B003I7F9WG/

Your choice of FD depends on:
* number of teeth in largest chain ring
* double or triple - double FD do not have enough travel for triple chain rings, but triple FD have no problem with doubles.
* distance from the seat tube to the chain rings. On my SR, I had use a much shorter BB axle with the road FD I used (purchased for a very very good price on CL )
* outside diameter of your seat tube - i cut a piece of 6061 T6 tube to act as a shim to mount the 34.9mm (1 3/8") FD on bikes with a 28.6mm (1 1/8") seat tube. I find the 34.9mm mount FD are cheaper (or higher level FD for the same price).

Nigel:
I understand all those factors save for the number of teeth on the largest ring. What does that have to do with derailer selection and fit? Thanks for the help.

[nfmisso]

If you look at the lower surface of the derailleur it is curved to match the curve of the largest chain ring. Too small a radius, and the middle will end up too far from the teeth, and the rivets of the chain will get trapped in there, ripping the derailleur apart.

Google "Shimano tech docs" for diagrams on installing derailleurs.

[Prof Young]

(08-10-2012, 01:06 AM)nfmisso Wrote:  If you look at the lower surface of the derailleur it is curved to match the curve of the largest chain ring. Too small a radius, and the middle will end up too far from the teeth, and the rivets of the chain will get trapped in there, ripping the derailleur apart.

Google "Shimano tech docs" for diagrams on installing derailleurs.

Nigel:
Yes, that makes sense. So I need to count the teeth on my biggest ring before I shop for a derailer.
Thanks much.

[dochoot]

What did you end up getting? I think I need a similar part (1973 Varisty).