Monday, November 28, 2005

Crankarm Length

As I mentioned a few posts back, I've been experimenting with changing crankarm length on my bikes. I have been riding my road bike, with 170mm cranks, the majority of the time lately, and it feels a little awkward to switch to my mountain bike, with 175mm cranks. I feel like I'm losing some of the pedaling efficiency I've developed on the road bike.

Incidently, I don't buy this business about needing 5mm longer cranks than normal for "extra leverage" while mountain biking. While it's true that this difference does result in more leverage (about a 3% easier gain ratio), I don't really think that this is a factor on multi-geared bikes.

I happened to have an extra set of 170mm cranks, so I put them on my mountain bike a few weeks ago. I think that this switch did solve my pedaling efficiency issue; I notice very little difference in pedaling motion now. There is a problem though. I had to raise my saddle another 5mm to keep the same relative saddle height, and now it feels really high. This really hasn't been an issue with the mostly non-technical riding I've been doing lately ('cross racing included), but I think it will be once I get back to some more serious trails.

In fact, now that I think about it, this could be the cause of another problem I've been having this year! I've been feeling less and less secure about my descending this year, and definitely much worse than I felt last year. It occurs to me that I've also been gradually raising my saddle height this year, in the search for "optimal" pedaling efficiency. Based on the marks on the seatpost, it looks like I've raised it about 3/4 of an inch from the end of last year to this year.


Probably at this point in my racing "career" (ie- very early), I need to worry more about being comfortable on the bike rather than squeezing the last drop of power out of my legs. I could also try to go the other way and put 175mm cranks on my road bike (looks like it's time to go scrounging on ebay). That gets me 5mm back, and then I might consider dropping the saddle a bit more, we'll see. I'll just have to watch for pedal strike on the road bike.

1 comment:

Ashwin Amanna said...

I went through this all last year. My conclusion was going to 170s all around. I had a much better pedal stroke off road and the only liability was when I was bogged down in some rock gardens. the 170s didn't have the same torque at low speed that the 175s had. But now I can't tell at all.

I didn't really notice the saddle height change affecting handling.

I know it costs some $ but I highly recommend a Serotta bike fit from a fitter with a mountain bike background. One of the biggest changes we made was bringing the bar in and up. It took a little while to adjust to for climbing but in the single track it is sweet.

Regardless you'r doing it the right way. try different things out and compare.