Converting to compact crank?

[Palisades]

Hi I am purchasing a new bike with a FSA K-Force MEGAEXO double crank 53/39T. Is it possible to purchase a smaller chain ring to make it a compact crank? I live in San Francisco, and the hills are pretty tough. If so where would I buy the chain ring?

On a related note, is it only the chain ring sizes that make a crank a compact? Or are the crank arms a different size also? Thanks for the information.

 

[carpediemracing]

the bolts that hold the chainring onto the crank are in a circle of a given diameter.

your crank probably has a 130 mm circle which will allow you to install a 38T ring (39T minimum is more common). you may have a 135mm circle (39T minimum, campy standard) but I doubt it.

a standard compact crank, on the other hand, has a 110 mm bolt circle (hence "compact"), allowing you to install a 34T chainring. 110mm is the standard MTB chainring size. there are some manufacturers who had 110mm bolt circle road cranks (Ritchey, Sugino come to mind) but without pros using them, they never got the attention that the cranks get now.

so, although you can probably install a 38t ring, you won't be able to get the gear or two reduction by going to a 34T ring. you'll have to buy a compact crank.

crank arm length is separate and is related to leg length, pedaling style, and what you want.

 

[branded]

the bolts that hold the chainring onto the crank are in a circle of a given diameter.

your crank probably has a 130 mm circle which will allow you to install a 38T ring (39T minimum is more common). you may have a 135mm circle (39T minimum, campy standard) but I doubt it.

a standard compact crank, on the other hand, has a 110 mm bolt circle (hence "compact"), allowing you to install a 34T chainring. 110mm is the standard MTB chainring size. there are some manufacturers who had 110mm bolt circle road cranks (Ritchey, Sugino come to mind) but without pros using them, they never got the attention that the cranks get now.

so, although you can probably install a 38t ring, you won't be able to get the gear or two reduction by going to a 34T ring. you'll have to buy a compact crank.

crank arm length is separate and is related to leg length, pedaling style, and what you want.

Agree with carpediemracing - if you want compact cranks, you have to change the crankset.

If you know you want a compact crankset, is it possible to order these with your new bike? FSA also make a K-Force Compact Megaexo, see
http://www.fullspeedahead.com/fly.aspx?layout=product&taxid=21&pid=435

Note that available crank lengths are the same for both the normal and compact K-Force Megaexo cranksets, 170, 172.5 or 175mm. So the chainring size doesn't affect available crank length. Of course you would have to replace the crankset to change the crank length.

If you aren't sure you want compact cranks, and stay with the standard 53/39 combination, your next cheapest option is to get a 12-27 cassette. That is the lowest standard double gearing arrangement for hills.

It is always possible to change to a compact crankset later.
I've just done this (yesterday) after 3 years with my current bike with a 53/39 plus 12/27. The botton bracket and chainrings needed replacing anyway.
Just come back from a hilly 70km ride and I think I'm going to be happy with the change, but it will take a few more rides to get used to.
You don't realise how much you just do automatically until making a major change like this. All of a sudden I had to start thinking again about when to change gears. Oh well, practice makes perfect.

branded

 

[artemidorus]

Get the bike store to exchange the chainset before you take it away - that will probably be the cheapest way to do it, as they may do it at no additional cost.

 

[nerdag]

110mm is the standard MTB chainring size

MTB bolt circle diameter is 104mm for the outer two rings, and I think its 74mm for the inner ring.

n

 

[artemidorus]

MTB bolt circle diameter is 104mm for the outer two rings, and I think its 74mm for the inner ring.

n

110mm is the old MTB standard from the days of 46/36/24.

 

[carpediemracing]

re: 110mm vs 104 mm.... oops dating myself. thanks for the correction.