What is Q factor
 

I have seen this in a number of threads. I have done a search trying to find what is it exactly with no luck. Also, I have seen BCD used, what is BCD?

BCD = Bolt Circle Diameter. This is the distance from bolt hole to bolt hole on a Chain Ring.


Q Factor as per Sheldon Brown:

Q-factor
In Fit-Kit © terminology, this is a factor based on the relative length of the femur compared to the rest of the leg. A large Q-factor is interpreted as an indication that the rider in question needs a frame with a shallower seat tube angle.

Also, another name for the tread of a crank set.

http://www.sheldonbrown.com/gloss_q.html#qfactor
http://www.sheldonbrown.com/gloss_bo-z.html#bcd

ahhhhh, i see..much appreciated!

In its most common usage, Q-factor simply refers to the side to side distance between the pedals.
How far apart do the pedals place the rider's feet.

As said above... but, the Fit-Kit definition is not commonly used.
Usually Q factor is tread.
http://www.sheldonbrown.com/bridgest...ne-1991-13.htm

So am I right to assume (great article by the way) that you want your crank-arm pedal spacing, bb spindle to be as close to your natural spacing as possible? Generally speaking, a low Q factor is preferred?

I definitely feel better with low-profile cranks than wider tread cranks.

Goodness! I always thought "Q factor" was OCP related! As in: "I wanted that carbon fiber kickstand, but it didn't have a high enough Q factor." :eek:

mostly right. BCD is the diameter of the imaginary circle that runs through the centers of the bolt holes.

yeah i guess i could have been more specific...thanks!

Generally you want a narrower Q.

Though I've got a set of old Sakae cold forged cranks that when mated to my track bike's BB seat so deep that I have to move the ring to the outside of the spider to get a proper chain line. The cranks have very little flare from the spider, so I'm basically looking at like a 45mm-from-center tread and that actually feels too narrow for me to get decent power.

Modern cranks have much higher Q factor, both because they're designed to provide clearance for bikes with wider rear triangle spacing (as much as 140mm on some mtb's) than old road (126mm) or track (120mm) bi kes. On top of that, cold forged allows for a slimmer profile than the less expensive manufacturing techniques that are popular now.