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Old 06-11-10, 07:43 AM   #1
GeorgeV77
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Question on crankset

I have a Scott Hybrid for some time now and I'm looking in changing the stock iron crankset (shimano fc-191 28-38-48 square tapper) to some road compact (eg FSA Vero Compact double 34-50 square tapper). The reason is that I have never used the granny gear, plus I want to get some more top end. The crankset spins on a shimano bb-ty 30 68/122 square tapper bottom bracket. Rear hub is 135mm wide, chainstay length is 45cm, the cassette is 8-sp 11-32, the front mech is a shimano FD-C051 48T and the rear mech shimano RD-M 360.
Is it doable using the FD I allrady have? (by raising it on the seat tube and adjusting the limits)
Is the current bb sufficient for cranks-chainstays clearance?
Will the RD be sufficient for the increased capacity?
Is there anything I'm ignoring in the whole concept?

Any help will be welcome..

edit: Crank arm length 175mm

Last edited by GeorgeV77; 06-11-10 at 07:57 AM.
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Old 06-11-10, 11:43 AM   #2
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1) It should be doable on the existing FD. The question is whether or not the Lower Limit screw will push the Derailleur out that far - that would be my only concern.

2) From a 48 to a 50, there shouldn't be much concern about chainstay clearance, unless that outer ring is really close to your chainstay right now. Buy the crank from someone that has a good return policy - or see if a Local Bike Shop has an extra 50T ring lying around that you can just stick up against your 48T to see how much further it sticks out.

3) The RD won't have any issues. The limits on the RD would come into play if you increased the Cassette's gear below 32.

4) Seriously consider borrowing or riding a bike with a 50-11 combination to see how much more top end you really get. That just seems like a heck of a lot of work for 2 more teeth.
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Old 06-11-10, 11:45 AM   #3
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Your current FD may work but will probably be rough.

Is the current bb sufficient for cranks-chainstays clearance? Depends on the crankset. Crankset determines spindle length.

Will the RD be sufficient for the increased capacity? You are decreasing capacity (currently 20 tooth difference between large and small ring down to 16 tooth with the new crankset).

Is there anything I'm ignoring in the whole concept? Three speed shifters will probably be OK as long as you reset your FD limits properly.
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Old 06-11-10, 11:59 AM   #4
GeorgeV77
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The thing is that since this is going to be more like a "Frankenbike", I'm pretty confident that the recommended bb spindle length won't do. Fsa for example recommends 108mm spindle length but this is for road frames with 130mm rear dropout spacing and not my 135.. I guess its going to be trial and error. Maybe I should go for the crank I like and see what bb length will do the job best to keep a correct chainline and still have chainstay clearance.
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Old 06-11-10, 02:08 PM   #5
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George,

Check out this web site with a gear calculator to see how much more gain you will get with adding more teeth. http://www.sheldonbrown.com/gears/
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Old 06-11-10, 02:14 PM   #6
GeorgeV77
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George,

Check out this web site with a gear calculator to see how much more gain you will get with adding more teeth. http://www.sheldonbrown.com/gears/
Thanx man, I ve already read almost all of sheldon browns' articles the last week I'm thinking about doing this.
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Old 06-11-10, 03:09 PM   #7
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It sounds like you're a good candidate for a road bike. Spinning the 48 ring at a higher rpm will probably produce more speed than changing chainrings. But in the long run you'll probably be happier with a road bike. It's really easy to spend too much on upgrades when for a little more you could have a new road bike and pick up some real speed.
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Old 06-11-10, 03:49 PM   #8
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It sounds like you're a good candidate for a road bike. Spinning the 48 ring at a higher rpm will probably produce more speed than changing chainrings. But in the long run you'll probably be happier with a road bike. It's really easy to spend too much on upgrades when for a little more you could have a new road bike and pick up some real speed.
That's where it will go eventually, but until I'll be ready to invest on a worthy road bike I want to try some things out with the one I allready have. I'm not talking about installing a carbon Campy multi-million chainset, all i want is to try out one in the €100 class to see how it will behave.
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Old 06-11-10, 03:59 PM   #9
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I like to experiment too. May I suggest that you install a computer with a cadence meter and try spinning the cranks at 90 to 100 rpm on a regular basis.
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