Chainring compatability
 

I am replacing the chainrings on an old 21-speed (7-speed w/ tripple). If I replace the Big and Middle rings with 9-speed rings, will I need to space them out to run a 7/8 speed chain, or is the spacing on the spider.

Chainring spacing is generally universal by manufacturer. I think Shimano likes a 110 BCD (bolt center diameter) and Campy 144. But don't quote me. So, the most important thing is to do some research to find the BCD of your current crank (Sheldon has a database), and to get chainrings with the same BCD. Chainrings are not speed-specific.

I'm not talking about the BCD spacing, I am talking about the spacing between the rings so that the wider 7-speed chain will won't rub on the 9-speed rings. Technically if you are running a n speed, you want the rings a little closer together so that the chain won't drop inbetween rings. So to clarify a little. If I have a tripple set up for 7-speed. I pull the Large and middle rings and replace them with 9-speed rings will the top two rings sit closer together and leave a larger gap between the 2nd and 3rd ring that could potentially cause problems shifting between granny and middle, or is the lateral spacing dictated more by the spider on the crank than the width of the chainring?

Note to Tellyho: Shimano uses a 130 mm BCD for road cranks and chainring spacing isn't universal.

When they went from 8 to 9-speed, Shimano offset the middle ring (or small ring on a double) slightly toward the outside to get it closer to the big ring. AFAIK, the spider arms remained the same thickness and the big rings didn't change either. So if you want to use an 8-speed chain with 9-speed rings, you will probably need to add thin spacer washers between the inner ring and the spider arms.

BTW, you could use a 9-speed chain and not have to respace the crank. The rear shifting shouldn't care.

Thanks this is what I needed to know.

Also about BCD. For shimano 130BCD standard, but they do make a compact with 110BCD

Lots of people have made cranks with 110mm BCD. Shimano's early mtb cranks had this (with 46/36/26 chainrings), and earlier Sugino triple cranks on touring bikes had 110mm also.

Current Shimano compact-double cranks for road bikes are also 110. FSA and others have been making compact double cranks in 110mm for awhile now too.

Also, Campy's modern cranks have 135mm BCD - 114 is older thing that now only shows up on track cranks.

Correct but Shimano's "compact" road cranks are relatively new products and their older and most current road cranks are all 130 mm BCD.

I stand corrected. Not sure how the 110 figure wound up in my head, as I have an old set of 130 BCD 600 cranks. I had no idea that spider thickness might have changed. I ALWAYS learn something on this board.