Compact Chainsets - Idiot's guide please...

[punkypossum]

Can some please explain to me if there is any way of telling from the description of the bike, i.e. those numbers given for the cassette or the cranks if it is a compact or not??? (Some websites seem to tell you, but then on others I can see the same bike and it doesn't mention it, others don't seem to mention it at all...) For example, I'm looking at the SCR 1.5 which tells me it has a HG50 12-25 Cassette and 34/50 cranks...is that compact gearing or a normal double (in fact, which numbers would tell me in the first place?)

Very confused...it wouldn't be so bad if the webpages definitely mentioned it, but like I said above, for some bikes some do and others don't...

Hope this question makes any sense - reading it back, I'm struggling to understand it myself!!!

P.S.: I read HairyJock's link on gear ratio's, but it's all still way over my head...

 

[kyuss]

Compact relates to the more compact chainrings/BCD on the front, therefore it's the second set of numbers that tell you whether it's compact or not. The smaller the number of teeth on the chainring the easier the gearing is, though this is reversed when talking of cassettes where the higher the number the easier the gearing.

A standard road double for instance is generally 39/52 or 39/53. Compacts tend to be 34/50 or 36/50. The size of the chainrings that can be fitted to the cranks is determined by the BCD (Bolt Circle Diameter) which is the five little arms at the end of the crank that the chainring attaches to. A standard chainset has a 130mm BCD while compacts are 110mm.

 

[punkypossum]

Thanks, that makes it a lot clearer...take it that SCR is a compact then...why can they not just tell you in the description???

 

[Nick1979]

Nowadays, most 'leisure oriented' (I mean not hardcore race) road bikes come with a compact crankset as the default option. Some (especially the higher end models) offer the choice of either compact or standard.
As the LBS guy told me the other day "compact is becoming the new 'standard'" :-)