Short cranks: another n = 1 report
I am conducting a self-experiment this year, and I thought I would post a partial update. I should preface with the obvious statement that this is a completely un-controlled experiment - and you may easily conclude that everything I report is within the noise and insignificant.
I am a 5'9" 145-150 rider with a 32" inseam. My TT bike is a 54cm P3 and my current fit has an an approximate 15 cm drop. My saddle is an Adamo that is pushed almost as far forward as it will go. (The position was developed with Todd from ttbikefit.com)
In 2011, I worked my indoor FTP up to 260 W in the OS, riding my TT bike, but mostly sitting up. When I went outside and started riding in the bars, my FTP dropped to 228-230 (88% of previous value). Over the course of the summer, it drifted up a few watts, but nothing particularly dramatic. Obviously, this large of a drop is frustrating. However, I recognize that I am one of Those Guys who has a higher indoor FTP than outdoor...and this is compounded by the usual loss for going to the aero position.
I considered a number of factors in how to alleviate this drop, and decided to focus mainly on the TT position. I did a lot of reading and messing around with numbers and angles (on paper), and came to the conclusion that it was worth trying to shorten my cranks. The reason for this is that at the top of the cycle, having shorter cranks means that you bring your knees slightly less close to your chest, ie., keep the hip angle a little bit more open.
- My road bike and old TT bike cranks are 172.5 mm
- The shortest commonly available cranks are 165 mm; this is what I purchased.
When you replace longer cranks with short cranks, this is what happens:
- The bottom of your cycle is raised by however many mm from the ground. (In my case, 7.5 mm). The more important thing, though, is that your pedal is that much CLOSER to your saddle.
- To make up for this, you raise your saddle by the difference (here, again...7.5 mm). This preserves your angles at the bottom of the extension. I raised mine by 1 cm because...
- You also have to raise your front end (and I had a handy 1 cm spacer) to keep the drop the same.
- As a result of this, at the top of your stroke, the knee is (in my case) 1.5 cm (but actually 17.5 mm) farther away from your chest. This is NOT insignificant.
- However, your shorter lever arm of the new crank means that you have to spin faster at the same linear force to generate the same watts. If you keep the cadence and force the same, it has the effect of making each of your gears a little "harder"...about by one gear. Thus, if you make this change, your rear 26 gear will feel about what a 25 gear felt like with the longer cranks (force and cadence kept constant). Thus, you have to be that much more sure to have conservative gearing!
So what are the results? There are two important factors:
This winter, I tested at the max of 272 W on my road bike indoors. My outdoor TT bike test came in at 247 W, 91% of the previous test. i I lost a few fewer watts in an absolute sense and in a percentage sense. Does this prove that the shorter cranks preserved more of my power? Not conclusively, but it's in the right direction. Anyone who is nearing his/her potential would take 3% for free in a heartbeat...
The other factor is your fit and the CdA (the effective area of your front into the wind, i.e., your "aeroness"). I did not change my total drop. However, I have a well defined 21 mile route that I use as a benchmark from time to time. This spring, I set PRs on this course, riding "only" in the 200-210 W range, i.e., it was faster than I've ever measured this course without being because of crazy high power. I cannot prove, but I believe that this is because I am riding more aero than previously because I'm not doing funny things with my knees as I get to the top of my stroke and because that comfort level lets me keep my butt in the right spot to have my back flatt(er) more consistently.
So is this enough to prove anything? Definitely not. However, i pass it along to anyone else who has considered the change; I think it's something worth experimenting with if you have the time and resources.
Comments
I appreciate you taking the time to write this up. I have to buy a compact crank for IMCdA, and was considering going shorter. I'm on a new bike so fit is slightly different anyway. Did you have any problems adjusting to your new position and the new crank length?
I'm 5'7" but also have a 32" inseam (very short torso), have been riding 172.5s and am wavering between 165s and 170s.
I can definitely feel the difference between the longer and shorter cranks, but it doesn't FEEL like a big deal.
Effectively, I just look at this as one more way to open up your hip angle while still keeping your position as low as possible. I would recommend that first people move their saddle forward to get the hip angle opened up, and then if you still need more try the shorter cranks. Many people have been riding steep for years and it seems pretty proven to work, shorter cranks are a newer fad that isn't quite as "proven" yet.
That said, when SRAM starts selling 165mm S9XX arms or Garmin gets the Vector on the market, I will be racing with 165s. At the moment I'm beating the 165mm arms are available first as Garmin is being very cautious and wants the vector to be fully tested before they start selling them, but that's a different topic.
I haven't spent a lot of time with the quadrant analysis to look at this yet, but I do keep an eye on it while I ride and pretty much know what's happening. (I tend to have a lower cadence indoor rather than outdoor, for example.) My observation FOR ME is that I'm not spinning faster than before, or at least not dramatically. That said, the difference you expect is only 4-5%, (i.e., about 4 rpm) which is an amount I might miss during the transition from inside to outside, since my cadence generally moves up over the course of a few weeks as a rule.
For what it's worth, I am doing this with a Shimano 105 compact crank. A new one of those can be picked up for $150 or less. Used even cheaper. I decided I didn't want to spend $300 or whatever on an experiment. Shifting is - as far as I can tell - identical to my R700. Maybe not that sexy on a P3, but pretty functional. (a touch heavier, but I decided to live with that.)