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Old 02-21-10, 08:13 PM   #1
richardtc
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Changing out a double for a compact double or triple

I am interested in buying a bike from a friend. The only thing holding me back is it has a Shimano 105 39/53T Chainring.

I really need to minimally be using a 34/50 compact or a triple with a 30.

My question is, what are the costs associated with changing that out? I assume that it would involve a change of the 105 front derailleur to go triple. Can I simple change out the rings and not the cranks if I go for the double?

I know my friend will give me a deal, but I do need to consider the additional cost.

Thanks!

Richard
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Old 02-21-10, 08:31 PM   #2
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you could just replace your chainrings, that would be your most cost effective option. you could also swap out the bikes' cassette for one that has a bigger cogs.
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Old 02-21-10, 08:55 PM   #3
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you could just replace your chainrings, that would be your most cost effective option. you could also swap out the bikes' cassette for one that has a bigger cogs.
Its hard to find rings smaller than 52 for standard 130 BCD crank which I would assume the OP'S 53/39 set up uses.(I think sugino makes some 50 rings for standard 130 BCD, but not sure on that); Triple 105 cranks (new or lightly used) can be had often for less than what two rings would cost. For example...

http://cgi.ebay.com/Shimano-105-Trip...item35a70b75d2

even comes with the BB.

edited to add: Looks like FSA makes some small 130 BCD rings http://www.universalcycles.com/shopp...ls.php?id=5616

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Old 02-21-10, 08:58 PM   #4
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A standard double crankset uses rings with a 130mm BCD which can accept a minimum 38 tooth chainring. In order to fit a smaller inner ring, compact cranks use a 110mm BCD.

Your cheapest option for lower gearing is either a compact crankset (which will require only a new crankset and possibly bottom bracket) or a MTB cassette and rear derailler. The latter will give you wider spacing between shifts but retain better spacing between chainrings. The former is the reverse.

Is the bike you are considering 9 or 10 speed? If 10 speed, the crank swap should be very easy as you won't need a new bottom bracket (assuming external bearings). If 9 speed, you'll likely want to swap out the Octalink bottom bracket for an external bearing BB to allow you to use the newer Shimano Hollowtech II cranks.

Converting a triple would at a minimum involve a new crankset plus front and rear deraillers. Worst case, you'd need that plus a new left shifter and bottom bracket.
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Old 02-21-10, 10:41 PM   #5
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Originally Posted by richardtc View Post
I am interested in buying a bike from a friend. The only thing holding me back is it has a Shimano 105 39/53T Chainring.

I really need to minimally be using a 34/50 compact or a triple with a 30.

My question is, what are the costs associated with changing that out? I assume that it would involve a change of the 105 front derailleur to go triple. Can I simple change out the rings and not the cranks if I go for the double?

I know my friend will give me a deal, but I do need to consider the additional cost.

Thanks!

Richard
The cost depends on your bike and what you are prepared to settle for. Ideally, you should use a triple with a triple front derailleur and a compact with a compact front derailleur. You can certainly get away without doing that, but shifting between the smaller two rings on a triple can be compromised if you use a double derailleur. Cranksets and chainrings are two of the most overpriced bike components, in my opinion. The more cost-effective solution is to get a new cassette or freewheel. Unless you live near pretty hefty hills, a 39/27 gear ratio is usually enough. A new cassette will run $25-65 depending on how many speeds it is. New cranksets are well over $50 unless you get lucky.

That said, I live in Oregon and advocate for triples constantly. Too many of my friends avoid beautiful rides over hills because they saved a minimal amount of weight by going to a double.
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Old 02-22-10, 12:28 AM   #6
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I swapped a standard double for a compact double. All that's needed is the new crankset, assuming you find one that uses the same interface (square taper, ISIS, Octalink, external, etc.). A standard front derailleur can usually be set up to work fine with a compact. Mine has been working for a few years now, and the worst was a dropped chain to let me know I needed to set the limit screw a little tighter.
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Old 02-27-10, 12:48 PM   #7
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Thanks all! Not sure what I am going to do yet. I really don't want to rig anything that will just be OK. I would rather pass on the deal and by a set up that is specific for me. Thanks again!

Richard
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