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Chainring sizing

Old 07-24-07, 11:10 AM
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CliftonGK1
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Chainring sizing

Background:
I've got a stock PDG Series 5 which is poorly outfitted for hills, and I'd like to keep the short cage derailleur to preserve the original look of the bike.
I found that the HG70 cassette spans 13-26 which will give me a better low end for hills by 5 teeth over the stock cassette, but that still leaves me with a 39/53 up front, which is a still a bit tall for my taste.
I'd like to go with a 34/48 or 36/50 combo up front, but I'm not finding anything lower than a 39t ring for the 130 BCD stock cranks.

Question:
Do 130 BCD rings come as low as 34t or 36t, or will I need to swap out for a 110 BCD compact double?
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Old 07-24-07, 11:17 AM
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The standard 130mm BCD was optimized for a 39T small ring. You need to change to compact if you want a smaller double. Another option would be a full on mountain bike cassette for lower range although the gearing would be further spread out.
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Old 07-24-07, 12:09 PM
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The smallest possible chainring for your 130 BCD crankset is a 38.
You could possibly put together a custom cassette with a cog as large as a 30 but this may require a longer "B" screw in the rear derailleur. For sure you could go as large as a 28.

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Old 07-24-07, 12:56 PM
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With the current 39 inner up front, I could always go the long cage route with the RD and put a 12-32 on there, but I'm being picky about maintaining the original look with the short cage 105sc it currently has. The stock cassette is a real corncob, so I'm at least opening that up to the top limit of the derailleur (which I've found listed as 27t max) until I can replace the crank with a proper compact double.
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Old 07-24-07, 01:19 PM
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You could most likely go to a 12-30 or 13-30 and stay with the short cage derailleur. With a 30 you might need a longer "B" screw. Sheldon Brown sells custom cassette like this.
Actually to go to a 32 big cog you'd need a mountain style rear derailleur, the long cage road derailleur does not accommodate a larger cog than the short cage.

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Old 07-24-07, 01:45 PM
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Originally Posted by Al1943 View Post
You could most likely go to a 12-30 or 13-30 and stay with the short cage derailleur. With a 30 you might need a longer "B" screw. Sheldon Brown sells custom cassette like this.
Actually to go to a 32 big cog you'd need a mountain style rear derailleur, the long cage road derailleur does not accommodate a larger cog than the short cage.

Al
I might give the longer B-screw a try, if that's all it will take. The listed spec for the 1991 105sc derailleur is 27t max, but a 39-30 would be reasonable for most of the hills that I'll be riding. BikeMan has the HG70 7spd cassettes in 5 or 6 different cog combos for around $25 each, so it's not even an expensive experiment if it won't work.
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