Triple Chain Ring Sizing Question
 

I have the following older "Flag" Specialized crank. I would like to add a triple chin ring to the inside. What size do I look for and what spacers are needed. (Note that the existing BioPace chain ring has been removed and properly filed)

-GG-

http://gallery.me.com/danspeirs/100191/DSC_0718/web.jpg

http://gallery.me.com/danspeirs/100191/DSC_0715/web.jpg

http://gallery.me.com/danspeirs/100191/DSC_0716/web.jpg

As always, the late, great Sheldon Brown has the answer:

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

As for spacers, I think you'll just have to experiment. Some chainrings have offset built in, others don't, so the width of the spacer is dependent on the chainring.

When in doubt Sheldon has the answer. Thanks for the link :beer:

Now, I measured it and it has the following:
BCD: 110mm
Smallest Ring: 33
Center-Center: 64.7mm

So, if I want to buy a third ring what size should I look for?

-GG-

It depends on the other two, if you want to figure ratios.

But - most standard triples have 30 tooth granny gears, and most compact doubles have either 46 or 36 tooth inner rings. So, your limit of a 33 tooth granny might be what you want to look for.

That being said, your current ring combo looks to be a 52/38, which is close to the "standard" double of 52/39. Instead of slapping on a triple ring to get you down to 33, you might just swap the crank out for either a true triple crank at 53/42/30 or a compact double at 50/34 and get a little more bang for the buck.

Your current rings may be 110 BCD, but the inner holes for mounting the 'granny' gear are probably 74 mm BCD. You should be able to go down to a 24 tooth on the 74 BCD.

Currently I have 52-40 on this crank. Not sure if I need the Granny.

that looks like a sugino AT.

It IS a Specialized "Flag" crank

Here is a similar one. The bolts are Sugino and the date code seems to be similar to Sugino: http://tinyurl.com/crankyone

Interest, it looks to be a Mtn. bike crank.

Correct.

I have a 24 tooth 74 bcd chainring on my touring bike. The big ring is 52 tooth, the middle is 40 tooth and they're both 110 bcd.

Its a crazy large drop from the middle to the granny but I haven't dropped the chain yet.