Help understanding Shimano gearing
07-30-09, 02:30 PM
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Help understanding Shimano gearing
My new bike with Shimano Ultegra components has 52-39-30 chainrings and a 12-27 10 speed cassette. I see that the older 9 speed Shimano Ultegra triple has 52-42-30 chainrings. It seems to me that the larger middle chainring on the older Shimanos provides a better range when riding with mild hills or winds. Does anyone know why Shimano changed? Has anyone put a 42 tooth chainring from another manufacturer (such as TA) into a 10 speed Shimano Ultegra triple? Is there a reason why doing this would be a bad idea?
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07-30-09, 02:38 PM
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This is just a guess, but I think they changed the middle ring to 39 because people don't like dropping into the "granny" or "sissy" gear. Even though it comes out to about the same ratio, they feel all manly if they can avoid the small ring more often. I agree that a 42 makes more sense, and it's possible to swap the chainrings and adjust the derailleur to shift reasonably well.
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07-30-09, 02:53 PM
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There's plenty of gear overlap with a triple, using either a 42 or 39 middle ring.
The 39 middle ring leaves:
The 39 middle ring leaves:
2 non-overlapping gears on the granny,
5 non-overlapping gears on the big ring.
The 42 middle ring leaves:5 non-overlapping gears on the big ring.
3 non-overlapping gears on the granny,
4 non-overlapping gears on the big ring.
39 is the standard double chainring size. If you're using a triple with a 39, you'll be shifting the front about the same time as your standard double buddies. This could be a positive thing when you're riding in a group.
4 non-overlapping gears on the big ring.
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07-30-09, 04:45 PM
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The 39T middle ring makes more sense, since it produces a more uniform progression between the rings.
The middle ring of a triple is in nearly the same location as the big ring on a double. With that in mind, the middle ring and largest cog is a relatively extreme chain line, that I would rarely use. Since you're shorted by one cog with the middle ring, the 39T makes even more sense.
I used Campy triples for several years, riding in the Colorado mountains, but I used FSA cranks, since Campy doesn't offeer a 39T middle ring. I used a 53/39/28 with a 12-25 to get the same low gear as a Shimano 30/27, but without the large 21-24-27 jumps between the cogs. Worked great.
These days I use a 50/34 with an 11-25 11 speed casssette.
The middle ring of a triple is in nearly the same location as the big ring on a double. With that in mind, the middle ring and largest cog is a relatively extreme chain line, that I would rarely use. Since you're shorted by one cog with the middle ring, the 39T makes even more sense.
I used Campy triples for several years, riding in the Colorado mountains, but I used FSA cranks, since Campy doesn't offeer a 39T middle ring. I used a 53/39/28 with a 12-25 to get the same low gear as a Shimano 30/27, but without the large 21-24-27 jumps between the cogs. Worked great.
These days I use a 50/34 with an 11-25 11 speed casssette.
