Drag Torque Experiments

I'm in the process of getting my bikes cleaned up and rebuilt for Spring. I thought that this would be a good time to explore something I've been interested in, namely, the extra drag that I notice on my external BB cranksets and Time ATAC pedals.

My idea was pretty simple: I don't have a torque wrench that's sensitive enough to measure the drag directly, but I do have a pretty accurate scale. So, I hung weights (washers) off of the crank or tip of the pedal until it started to turn, then measured my weights. Since I knew the moment arm, I could calculate torque and then power.**


I checked three cranksets and two pedals. Here's what I found:

Crank 1. No-name square taper cranks: torque was 0.005 N-m, so power at 80rpm is 0.042W. This is an old and cheap crankset/BB that I've never messed with. With no chain on it, the thing spins like crazy... you start to wonder just when it will stop!

Crank 2. FSA (Mega-Exo) Gossamer cranks: torque was 0.060 N-m, power at 80rpm is 0.503W! I just took this crankset apart, so maybe the drag is a little higher than it would be after it had been ridden for a while and everything got re-seated.

Crank 3. Shimano (Hollowtech II) XT cranks: torque was 0.033 N-m, power at 80rpm is 0.280W.

Pedal 1. Shimano M520: torque was 0.653e-3 N-m, power at 80rpm is 0.005W (for 1 pedal).

Pedal 2. Time ATAC XS: torque was 12.4e-3 N-m, power at 80rpm is 0.104W.


Then, I added up the total power (2 pedals + cranks) for my three setups:

1. Road/trainer bike (the square taper cranks and Shimano pedals): 0.052W at 80rpm.
2. 'Cross bike (FSA cranks and Time pedals): 0.711W
3. Mountain bike (XT cranks and Time pedals): 0.488W

So, relatively speaking, there's a big difference in power loss between the square taper and external BB setups (about a factor of 10!), and a big difference between the pedals (a factor of 20!). But, the numbers are all so small, maybe 0.2% of the total power I'm putting out, that maybe other factors need to be considered.

For the cranks, the external BB setups are much lighter than the square tapers. They're also much stiffer, although I'm not sure how significant the difference in stiffness is for me.

For the pedals, the whole reason I went away from the Shimano's was that I had trouble clipping in and out while riding in sloppy conditions. This definitely outweighs a small difference in drag.

So, I probably won't change my setups. If I had seen numbers more like 1W in drag, I might have changed my mind...


** There's a key implied assumption here, that drag doesn't change with "static" load. That is, I'm assuming the drag is the same with no load on the pedals/cranks as it is with someone pedaling the bike. My first thought was that this probably wasn't a significant effect, but the more I think about it, I'm not so sure. I suppose this could potentially change my conclusions...