Following up from our post on crank length, today we will cover bolt circle diameter (or BCD) of your power meter, or cranks in general. The question of compact or standard cranks comes up quite often. Like on the topic of crank length there is no definitive rule on which BCD – compact or standard – to choose. But let’s start out with the basics of what BCD is and what the differences are:
Compact and standard crank BCD explained
The BCD, or bolt circle diameter of your power meter (or normal crank) defines how far the chain ring bolts are located from the center of the cranks. The power2max Type S Rotor 3D+ on the right is a compact model, which means the the diameter of the circle the chain ring bolts describe is 110mm, or 11cm in diameter. The other frequently used format is what’s called a “standard” crank with a BCD of 130mm, or 13cm.
Why is this important? Because it determines the gearing you can use on your bike. The BCD determines the smallest possible chain ring you can use at the front and, to some extent, also the biggest chain ring that’s available to you.
Beyond compact and classic, the most frequent BCDs, there are also other ones in the market: Campagnolo uses 110/113 compact (a slight variation on “normal” compact, so chain rings aren’t compatible) and 135 instead of 130mm. Track cranks come in 144mm BCD, and manufacturers like Shimano and FSA now also introduce their own BCD formats, both in 110mm and with 4 bolts, but neither compatible with the other.
Typical chain ring combinations on compact and standard cranks
With compact cranks you can use a 34 tooth chain ring as the smallest option. With standard cranks 38 is the smallest possible chain ring size.
For the large chain ring, the limitations go in the other direction: big chain rings for compact (110 BCD) drive trains tend to go up to 52 teeth. For standard drive trains you can buy much bigger chain rings, up 56 and often even bigger. The reason that compact chain rings are often not available in bigger sizes than 52 is due to chain ring stiffness: the bigger the distance between the point where the chain rings are installed and the chain, the more chain rings will bend (other things being equal). This can influence the quality of shifting. However, with improved quality of chain rings, such as the Praxis Works chain rings we offer in combination with our power meters, stiffness is becoming less of a concern these days.
Typical chain ring combinations for compact cranks:
- 50/34 or 50/36 (compact)
- 52/36 (mid-compact or semi-compact)
- 52/38 or 53/38 (rarely used)
Typical chain ring combinations for standard cranks:
- 50/38 or 52/38
- 53/39 (most frequently chosen)
- 54/42, 55/42, 56/44 (typically used for time trials)
How to figure out if compact or standard cranks are right for you
The choice of compact versus standard comes down to the speeds at which you ride. If you live in a flat area like parts of Florida or have the legs of a Nairo Quintana or an Alejandro Valverde, then you will likely choose standard cranks. If you ride a lot in the mountains or like to take it at little easier a compact crank will suit you better.
To drill down into the details, use a tool like gear-calculator.com. It’s great at helping you figure out at which speed you will bottom out or spin out with your combination of cranks and cassette. An example:
A typical mountain gearing would be a 50/34 compact up front with an or 12/28 cassette at the back. If you want to maintain at least 80RPM cadence whilst climbing and consider 110RPM to be the upper limit, you see that with this gearing combination you get a range from 7.6mph (12.2kph for the metric minded) in the 34-28 gear at 80RPM to 35.9mph (57.7kph) at 110RPM in the 50-12.
A typical strong rider race gearing might be a 53/39 up front with a 11/25 at the back. Using the same cadence limits we get a lower “speed limit” of 9.8mph (15.7kph) at 80RPM and an upper limit of 41.5mph (66.7kph) at 110RPM. By moving from a mountain gearing to race gearing we lost 2.2mph at the lower end and gained 5.6mph at the upper end.
Mid-compact: the swiss army knife of gearing
Unless you are firmly on either the mountain or race gearing end of spectrum, a popular option is the swiss army knife of crank gearing: mid-compact. Mid-compact has a 36 small ring (so 3 teeth smaller than Classic, but only 2 bigger than compact) and a 52 big ring (1 smaller than classic, 2 bigger than compact).
Mid-compact has become a very popular choice of late, because it hardly sacrifices top end compared to a classic crank, yet almost has the climbing characteristics of a compact crank – a nice compromise!
Conclusion: it’s personal
The choice of compact versus classic is very personal and depends on the terrain you ride in and the level you ride at. For many uses compact cranks are a great choice, as they allow you to climb well in the mountains and can easily be changed to mid-compact.
Have fun, keep the rubber side down and the watts up!