Converting to Compact Double?

[melissam]

Hi,

I have an old (1984) Bianchi Limited that I've recently reclaimed from the garage. It's currently on bike trainer duty, but I'm considering greater things for my fine, trusty steed.

My bike currently has a 52/39 in the front on a Takagi crankset (I think), with a Suntour derailleur. It's also got a 6 speed freewheel (remember those?) in the back. Downtube mounted friction shifters, thank you very much!

What's involved in converting to a compact double? Is it as simple as changing out the chainrings?

Thanks!

-- Melissa

PS I know that some of you are probably thinking that this is the *perfect* bike to convert to a fixie. Don't think that the thought hasn't crossed my mind.

[SadieKate]

Hey, "Deanna"!

Since you have friction shifting, you don't need to worry about the shifters.

Chain rings - you have to know number of arms on the spider (crank set) and the BCD (the bolt hole diameter). I suspect Sheldon has a huge article on this.

Front Der - the curve of the front derailleur needs to match. Frequently, the front derailleur for a standard double is too shallow an arc. Plus, the back plate of the front der may need to be deeper. Again, because of the friction shifting you may be able to get away with more.

I'll go see what I can find on Sheldon's site.

[SadieKate]

Start here.

http://sheldonbrown.com/gloss_bo-z.html#bcd

[SadieKate]

Sizing and curvature of the front derailleur

http://sheldonbrown.com/gloss_ca-g.html#capacity

You may not be able to get small enough chainrings on that crankset and you may be able to get a new crank for less than the cost of chain rings. I suspect you have a square taper bottom bracket on that bike.

[DebW]

To replace the whole crankset, if that what you decide to do, you either need to match the spindle of the old bottom bracket or replace the bottom bracket. You almost certainly have a square-tapered spindle. You can still get Sugino and possibly other cranksets that take a square-tapered spindle. Even if you can use the same bottom bracket, you may get a better chainline by going with a slightly shorter spindle since the smaller inner ring of a compact can sit closer to the frame, which would give you more usable cogs with the big ring.

[KnottedYet]

My compact double was a used double crankset for a mtb. Suntour. With labor and a new small chainring, switching cost me $50 in all.

[melissam]

Thanks everyone!

This is great info -- I'll have to take some time when I'm not @ work to peruse Sheldon Brown's site. Thanks for the links, SK!

Knot, I'll keep the MTB crankset in mind. Sounds like you got a great deal!

Deb, your expertise is always welcome. I'll keep the bottom bracket in mind.

-- Melissa (AKA "Deanna")