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39 X 15?(5 posts)

39 X 15?SD Hart
Jan 24, 2003 10:37 AM
I am just building up my first fixed gear bike and wanted a gearing combination which would yeild between 65 and 70 gear inches. I had a 39 ring, and thus bought a 15 back cog to give me 68 gear inches. All the postings I have read with similar gear inch set-ups have a larger front ring and therefore a larger back cog. Is there anything intrinsically wrong with using a 39 X 15 set-up?
how fast?DougSloan
Jan 24, 2003 10:54 AM
How fast and at what cadence do you want to ride?

Go here ( ), and plug in the numbers and try them out. For example, 39x15 at 90 rpms= 18.5 mph. Change it to 60 rpms, and it's 12.3 mph.

Probably a good start.

That's what I useTig
Jan 24, 2003 11:12 AM
I had a 39 ring and chose a 15 cog for the same ratio and it works really well. Smaller rear cogs can cause quicker chain and tooth wear, but a 15 shouldn't fall within the "too small" range. I've heard that super high cadences on fixed gear using an 11 or 12 cog can cause the chain to jump, but I haven't seen any such problems with the 15 at 160+ RPM.

This gear combo wouldn't be low enough if you ride in a hilly area, but I like it for the flats and occasional hill/bridge. When mixing it up with a fast group ride or a nice tailwind, I can keep up and get an excellent spin workout. It is a high enough ratio to build excellent strength in headwinds or up short climbs.
re: 39 X 15?pauly
Jan 24, 2003 11:59 AM
I've used 39x15 for years and it's been perfect, both from a gearing standpoint and a durability standpoint. I think the idea of larger cogs and chainrings being more durable is overstated. Make sure you have a good chainline and you'll be fine.
ThanksSD Hart
Jan 24, 2003 12:55 PM