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PostPosted: Mon Oct 22, 2012 9:30 pm 
Old School Grand Master
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As per the post title - what is the difference between a compact crankset and a normal one?

Thanks all


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PostPosted: Mon Oct 22, 2012 9:39 pm 
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Location: Chez Vegas, Derbyshire
A compact normally has two rings with 50 and 34 teeth. A modern 'standard' has two rings 53 or 52 on the big un and 39 on the little un.

Compacts are easier on the legs on hills and are popular on modern racing bikes because it makes them easier to pedal.


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PostPosted: Mon Oct 22, 2012 9:44 pm 
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Thanks, so a compact is just a name for smaller chainrings? My new bike has 50 big ring and i have emailed FSA regarding getting it changed to a 52.

Seems a bit strange to use a new name for a change in ring size, obviously the smaller rings would be easier to push.

Thanks


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 Post subject: cransets
PostPosted: Mon Oct 22, 2012 9:46 pm 
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Broadly speaking, there were available on mass-produced bikes ( apart from touring or cyclo-cross setups ) 2 choices of crankset; road double with typical chainring sizes of 52/42 or 52/39, and road triple with 52/40/30.

triples were pretty essential for steep climbing, allowing the rider a low 30 ring combined with a 13-26T rear sprocket which would provide a low 31 inch gear.

Triples however are heavier and put the cranks further outboard...

A compact crankset is a relatively modern option for mass produced bikes and provide 2 rings, usually 50/34.

with a wider range rear sprocket, say 12-29T, the rider would have a 31" gear for steep climbs.
the disadvantage is you have a bigger 'drop' from one ring to the other and often drop from the big ring to the small ring then down to a smaller rear sprocket to main your cadence. Also easier to 'off' a chain! :D


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PostPosted: Mon Oct 22, 2012 9:47 pm 
retrobike rider
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Also most compacts are 110 BCD where as non compact are normally 130 BCD.


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PostPosted: Mon Oct 22, 2012 9:58 pm 
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Thanks, as i am a total novice to this - what is BCD?

Does that mean that my old bike which is 52/42 would not be described as compact then?

I am still struggling to understand this - compact = wider and lower gear range.

Sorry if i appear a bit slow, thanks for the replies


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PostPosted: Mon Oct 22, 2012 10:03 pm 
retrobike rider
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This explains it well;

http://sheldonbrown.com/gloss_bo-z.html


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PostPosted: Mon Oct 22, 2012 10:12 pm 
Old School Grand Master
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Inzaman, thanks for that. Does that mean that any chainring with the same BCD as mine will fit my cranks?


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PostPosted: Mon Oct 22, 2012 10:15 pm 
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BCD = bolt circle diameter.

I guess the name "compact" refers to it being a compact version of a triple.

With a "standard" double there is more overlap of gears between the two chainrings and a smaller range between the lowest and highest ranges.

A "compact" double gives you a wider range and there will be less overlap.

Plot your gears into this calculator and then you will be able to see the difference by changing the size of the chainrings.


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PostPosted: Mon Oct 22, 2012 10:29 pm 
Old School Grand Master
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Joined: Sat Oct 06, 2012 5:17 pm
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Location: Norn Iron
Thanks for the calculator; my lack of knowledge means it does not mean that much to me but i will keep a note of it when i need it - thank you.

Again, i am astounded by the amount of knowledge the members on this site have and i have to smile to myself when i find myself out of my depth when i take a look at some, what i think, minor query and i get overwhelmed by the responses.

Thanks again


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