Switched to 165mm Cranks

Ah, the crank length debate. Long or short? What's best for you? Well, until a week or so ago I was generally neutral, finding enough pros and cons in each article I read to balance things out and keep me in my trusty 175mm's.

But then an article came out in Triathlete Magazine. In it they outlined how recent research showed that most people don't lose power when they reduce their crank size. In fact, most gained a little. The power itself wasn't all that compelling. But they then continued to say that using shorter cranks made it easier to get into an aerodynamic position.

Bingo. That's been my big struggle for years. I'm always messing with my seat and aerobars trying to get more aero. But because my legs are so long I always end up pinching my diaphragm and losing power. Suddenly the light bulb went off that with shorter cranks I wouldn't have such a large range of motion to handle which might allow me to crunch up more.

Stepping back a little. Aero is huge in long course triathlon. Especially on a flat windy course like Florida. What looks like a few seconds to a roadie's 40km time is minutes to an ironman competitor. With such a tall frame (big surface area) and low weight I was always struggling with aerodynamics.

On the crank side there's the argument that longer cranks give you more leverage. Which means you can deliver more power to the spindle. The tradeoff is that your feet need to move in a larger circle which takes energy. The Triathlete Magazine found that cadence isn't tied to heart rate... foot speed is. Their analysis is clearly geared to triathletes interested in constant power output over time. A roadie may be better suited with bigger cranks.

So I got a set of 165mm's on order. Not an easy task at all. Nobody in town has them.

Before I made the switch I benchmarked my current aero position with a set of photos.

The switch was easy. I took the chain rings off and will eBay them. Put my 54 tooth set on and was up and running in a matter of minutes. I've now officially taken apart every piece on my bike. When I painted the bike I didn't know how to get the cranks off so I took it to a shop. This time I did the whole thing myself. Easy. There's still some bearing packing I could learn but overall I've installed every part on the bike. Cool.

As I hopped onto the new cranks I could quickly tell the difference. It is a different stroke. I mean, it's still riding a bike. Just somewhat different.

With no changes to my aerobars I felt a lot more room in the bike. My knees weren't anywhere near crushing my diaphragm any more. I took off some spacers under my elbow pads. And then moved the stem down two spacers.

For only 10mm in crank size I was able to drop my front end by 44mm! That's a pretty good exchange. I didn't mess with my saddle height although the article says I should be able to lower it by a similar amount.

So I rode on the trainer pretty hard last night. I noticed some different muscles getting fatigued but they were supporting muscles. Afterwards I ran and felt great. Check one.

Then this morning I rode outside very hard for an hour. My legs screamed but I was able to output power and didn't feel too cramped in the cockpit. Check two.

The pedaling stroke is more compact. So you can get behind the power curve in any particular gear when the terrain changes. This is because you don't have the torque you're used to with the longer cranks. So you have to be more attentive or you risk a higher overall VI rating for your ride... which isn't good in an ironman. For me this translates to spinning more. If I don't pay attention I'll quickly find myself going much slower... well... or my gps unit's realtime display sucks... which may be the case.

In the article they said that you can turn a particular gear at a lower heart rate/rpe. It was impossible for me to verify this on the ride today. I'm at the end of six days of hard training. Discerning the crank change vs. the fatigue vs. increased motivation from new gear vs. etc was impossible. Just no way to tell.

But I did turn out an average pace of 20mph for one hour. Not even half ironman pace but for a workout it isn't bad.

I may have some more room to drop the front end a bit. I'll experiment with it.

How smart is it to change crank size so dramatically weeks before my A race? I don't know. Every year I mess with something and it doesn't seem to screw me up. And I can always back out of the change in a week if it's not working. I'm hoping the aero payoff is worth it. I'll put in a lot of riding between now and then and hope to strengthen the supporting muscles that are now being hit differently. Dave Scott says it takes between three and six weeks to adjust to a new crank size. I'm at three weeks right now.

Update: It's about a week or two later and I've done a number more rides on the shortys.

They definitely put me into a better aero position. My ride numbers lately seem good and I doubt that's a sudden bump in fitness. Aero is king in long distances and I seem to have inched forward a bit.

But the way these things feel is still different.

On downhill they're like butter. I think this is where the aero advantage kicks in most. Overall they just feel good.

But uphill is much more difficult. I don't have the leverage on the crank to turn over slower cadences. And if I try to spin it out I get frustrated by the sensation that I'm doing 120 rpm with no pressure on the cranks. I have a much more narrow band of cadence to work in and when I fall outside of it things feel bad.

Changes in terrain and pace are much trickier. Without the leverage I still find myself getting "behind the gear" a bit and having to exert more power to catch up... to get my stroke into the ideal zone.

I'm clearly hitting some new muscles which is scary this close to a race. My gleuts hurt near the beginning of rides but seem to loosen up after a while (or go numb.)

For the IMFla course I think I'll be ok. But for the Gaps I think I'll be very, very frustrated the first time I ride on the shorter cranks.

Given the numbers of whole rides I seem to have moved in the right direction and made a good decision. The aero advantage seems worth it. But whether or not there's a hidden muscular cost that affects the run I just don't know.

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