Bicycle Mechanics - changing my chainrings: do I need new cranks?

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ncscott
04-03-08, 05:37 PM
Hey all,
My situation. Got bad knees, a road bike, a commuter (LHT). Pedal fast, stay in smaller chainring, never the large chainring. Even with a larger cogset, I still run out of gears on long/steep hills on the road bike. Want to make my road bike gearing more like my Surly Trucker to make it more knee freindly. Road bike is a 15 yr old specialized with Shimano 105 8 spd STI with an older xt rear derailer.
My question. Getting a larger cogset is pretty strait forward, but getting smaller chainrings is not. I would like to keep the crank arms as they fit the bike real well (old as sht) and it would seam to be cheaper to keep the crank arms. Is there anyway to replace the 39/53 combo with 36/48 combo without replacing the crank arms???
Thanks,
scott

Jynx
04-03-08, 05:53 PM
i believe the smallest you can go on the front of a standard 130bcd crank is 38 teeth. Best bet is to get a different rear cassette and a MTB rear derailler. Best you can probably do is around a 38 front 32 rear.
JiveTurkey
04-03-08, 06:22 PM
If your old XT rear derailleur is a long cage (SGS) and has the same geometry as the current design, it can handle 34T with a double chainring. (You may be pushing the capacity of a GS/medium cage MTB RD, but would probably work, too.) What is the largest cog you are running now?

BCRider
04-03-08, 08:16 PM
As mentioned by Jynx your choice of chainrings is set by the BCD of the cranks. You can't have teeth that are inside of the chainring bolts regardless of how hard you try...... :D If your old 105 cranks are the same as mine then you'll need to change since I know that the 39 on mine is darn close to the ring bolts already.

Check the center to center distance of two adjacent chain ring bolts in mm's and then go to Sheldon's web site for the chart that equates that chord measure to the BCD diameter and then go shopping for rings in that BCD size. If you can't find them then you're going to have to go with a new smaller BCD crankset.
Monoborracho
04-03-08, 08:52 PM
Though this link is to the story of a new "mountain roadster", many folks seek the type of gearing you are looking for, and I'm one of them. http://www.bikeforums.net/showthread.php?t=400167