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Old 06-22-08, 06:33 AM   #1
SteveA
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Shimano FC-R600 inner chainring sizes?

Greetings,
My wife and son bought me a great bike for my 50th. birthday. The bike has a Shimano FC-R600 compact crankset with 50T/34T. The jump from the 50T to the 34T is too large for my liking. It shifts fine - but I don't need the 34T in my area and the change in cadence (or the required pre-shifting on the rear) are somewhat annoying. I'd like to put on a 40T inner.

A Shimano tech document leads me to believe that the original configuration is the only workable combination. Looking at the crankset it appears that the 50T outer chainring is designed (ramped?) to ensure a smooth shift specifically to a 34T inner. It apprears that any other inner chainring size may compromise front shifting somewhat.

I realize that a different size inner would fit up to the crankset - but I'm not sure how it would shift.

Does anyone here have experience using a 40T (or even a 38T [or anything other than the 34T]) inner chainring with a 50T outer on this compact crankset (or another Shimano compact crankset)?

I searched the forums looking for this answer and got more hits that I would ever have time to look through. Most were discussing the functioning of compacts (without any changes) or the relative merits of compacts versus triples. If there is already a thread discussing a similar question to mine - someone please direct me to that.

Thanks,
Steve

Last edited by SteveA; 06-22-08 at 06:36 AM. Reason: typos
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Old 06-22-08, 09:17 AM   #2
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I've seen 38 teeth sprockets but no 40's.

Shimano compact cranks have a BCD of 110.
Their standard cranks have a 130 BCD.

The BCD is the mounting Bolt Circle Diameter. That's the limiting factor here.
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Old 06-22-08, 10:46 AM   #3
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I agree that more tha a 10t difference is very frustrating. If you tried hard enough, you could end up with a 38t/48t front crank.
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Old 06-23-08, 05:33 AM   #4
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Thanks for your reply ..

Since I've got the 110 I can always go larger than I have now - right? Meaning that it will fit up just fine?

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Originally Posted by PhilThee View Post
I've seen 38 teeth sprockets but no 40's.

Shimano compact cranks have a BCD of 110.
Their standard cranks have a 130 BCD.

The BCD is the mounting Bolt Circle Diameter. That's the limiting factor here.

Last edited by SteveA; 06-23-08 at 05:37 AM. Reason: add thank-you
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Old 06-23-08, 05:37 AM   #5
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Thanks for your reply ..

Even though it is only -2 teeth from 50T to 48T I think that I'd like to keep the current top-end (I'm used to 52T normally). I'm thinking that the 38T/50T would shift fine (because the 34T/50T shifts amazingly well now [for a 16T difference]).

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I agree that more tha a 10t difference is very frustrating. If you tried hard enough, you could end up with a 38t/48t front crank.
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Old 06-23-08, 04:05 PM   #6
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Keep in mind the FDER moves at an angle between the rings. That angle is probably optimized for a 16T difference. IF the FDER height remains the same, it may actually contact a larger small ring. Therefore, it would have to be moved up a bit. (is it movable?) That means your shifting may be a little worse when going to the big rind.
Just saying. I don't have any actual experience with these though, but something to consider. Maybe a different FDER may be needed?
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Old 06-23-08, 04:17 PM   #7
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With a 38t small ring, you can use a "normal" 130bcd... most people using compacts do so because they want a smaller front ring than 38. That said, there are 36 and 38 tooth rings available, for example these FSA Pro Road.

I did notice, however, that Shimano doesn't offer 36t rings for its 110bcd cranksets. You may have the compatibility issue that you are worried about. I'm guessing that you'll be ok though, with a 36t ring as long as the der. doesn't contact it. I think you'll also find reasonable shifting performance. Any special ramping and pinning optimized for 50-34could be counteracted by the reduced jump in ring size.
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Old 06-27-08, 03:37 PM   #8
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Quote:
Originally Posted by justinb View Post
With a 38t small ring, you can use a "normal" 130bcd... most people using compacts do so because they want a smaller front ring than 38. That said, there are 36 and 38 tooth rings available, for example these FSA Pro Road.

I did notice, however, that Shimano doesn't offer 36t rings for its 110bcd cranksets. You may have the compatibility issue that you are worried about. I'm guessing that you'll be ok though, with a 36t ring as long as the der. doesn't contact it. I think you'll also find reasonable shifting performance. Any special ramping and pinning optimized for 50-34could be counteracted by the reduced jump in ring size.
The bike was a gift and came with the compact crankset. I don't know whether given a choice I would have chosen a compact (never having ridden one at that time). I'm considering putting on a standard Ultegra 53/39 or 52/39 set (that I have easy access to anyway) and keeping the compact for when I get older or move somewhere that has more hills.
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Old 06-27-08, 04:24 PM   #9
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Many compacts are offered with 50/36 rings, that would be my choice. The 14 tooth jump would be the same as the standard 53/39. The 50/34 shift often requires an extra shift at the rear to compensate for the big ratio jump. A 50/36 would match up well with a standard cassette for nice close ratio jumps.
I usually ride a 53/39 crankset with a 13-26 cassette. This setup works well where I ride. If I switch to a 50/36 crankset then I could switch to a 12-25 cassette for slightly lower gearing and better hill climbing. I will not be switching to a 50/34 because in my area a 34 would not see enough use and I would miss my 39.
--just some thoughts.

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Old 06-27-08, 05:26 PM   #10
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Quote:
Originally Posted by Bill Kapaun View Post
Keep in mind the FDER moves at an angle between the rings. That angle is probably optimized for a 16T difference. IF the FDER height remains the same, it may actually contact a larger small ring. Therefore, it would have to be moved up a bit. (is it movable?) That means your shifting may be a little worse when going to the big rind.
Just saying. I don't have any actual experience with these though, but something to consider. Maybe a different FDER may be needed?
You are correct in that some front derailleurs (Campy for one) are optomized for the chainrings on compact cranks and may give problems if used on a standard road crank, However, I believe Shimano never offered a specific compact fd and recommended their stand road fd's for all of their cranks.
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Old 06-29-08, 02:30 PM   #11
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Quote:
Originally Posted by HillRider View Post
You are correct in that some front derailleurs (Campy for one) are optomized for the chainrings on compact cranks and may give problems if used on a standard road crank, However, I believe Shimano never offered a specific compact fd and recommended their stand road fd's for all of their cranks.
The Shimano drivetrain interchangeability PDF shows the standard Dura-Ace, Ultegra and 105 FD's matched to Dura-Ace, Ultegra, 105, and the FC-R700 & FC-R600 compact cranksets.

This guide is from Shimano's European site - I would guess that it applies in the U.S. also.

http://cycle.shimano-eu.com/publish/...y_road.pdf.pdf

Last edited by SteveA; 06-29-08 at 02:32 PM. Reason: typo
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