Crank arms design

[sprinter]

The track folks use 130 or best 144 BCD so the chainring doesn't twist. On the other hand for example, you have Shimano 5800 105 cranks which places the crank arms closer at 12 o clock that maximises the power delivery although it's on 110 BCD.

So I was wondering which is better for power transfer, stiffness and everything etc?

 

[BikeGremlin]

The track folks use 130 or best 144 BCD so the chainring doesn't twist. On the other hand for example, you have Shimano 5800 105 cranks which places the crank arms closer at 12 o clock that maximises the power delivery although it's on 110 BCD.

So I was wondering which is better for power transfer, stiffness and everything etc?

Yes, Shimano's asymmetric 4-bolt chainring mounts are now used on many of their road bike cranks (I'd have to double-check for the modern MTB cranks). At the cost of sounding a bit sarcastic: the biggest advantage is that new cranks (and chainrings) are not compatible with the older ones and with competition.

However, they do have two theoretical/academic advantages:

• There is more support for the chainrings when pedals are at 3 and 6 o-clock (when you can deliver most torque). So, with the same amount of material used, and similar shape/thickness, you can get a bit more rigidity.
• Also, the spider mount is moved further away from the cranks. With the old system, the spider mount that is behind the crank was a lot less stiff compared to the crank arm. That resulted in material fatigue cracks propagating in that area. This design, at least in theory, might allow for a more gradual difference in stiffness and perhaps solve that problem (though no one seems to care about the more common problem of the pedal mount interface, so I doubt that this was even a consideration, and I'm not sure it makes a difference - time will tell).

Regardless of the chainring mounting interface, with all else being equal (chainring thickness and stiffness), I would say that the larger the BCD for the chainring size, the stiffer it is. Hence, the stiffest option would be the smallest chainring size that the BCD allows (BCD dimensions and min. chainring sizes charts). For 110 BCD that is 34 teeth, for 130 BCD that is 38, and for 144 BCD that is 41 teeth.

I could be wrong about stiffness - haven't measured it to confirm my theory. It's all stiff enough in practice and I'm yet to notice chainring stiffness causing a problem.

Relja