Short cranks on TT bike?

William Jenks

My season having wound down with IM WI, this is the time of year where I like to experiment with changes if I'm going to make them (like switching running shoes, etc.)

I have been giving some thought into trying shorter cranks on my TT bike.  I'm 5'9" and ride a 54 cm Cervelo P3 with about 15 cm drop and the seat slammed almost as forward as it will go.  I use 172.5 mm cranks and have no complaints about them.  I am basically very happy with the comfort and shape of my fit.

However, like most folks, my TT FTP is/was lower than a more relaxed position, which I attribute to the more folded up position.  I am considering experimenting with notably shorter cranks, e.g., 165 mm, on the theory that it will mean that the top of my "circle" will have a more open knee-hip-torsoe angle, and thus maybe somewhat more powerful. By raising the seat (and presumably cockpit) another 7.5 mm, I get 15mm lower upswing at the top.

Is there anything to this idea?  I know some people have tried shorter cranks, but I've not heard how happy they are with them.  Since I can't figure out a way to try them without buying a set, I'm very curious to hear what people have to say about their experiences.

Note - this is not about making a small change like 2.5 mm because I'm slightly unhappy with something - this is more like switching to a compact crank, i.e., something kind of knowingly drastic to try to fix a specific problem.

Thanks for any feedback!

William

Tyler Robinson

@William,

I took the plunge this year and went from 172.5 cranks on my 51cm P2 (172.5 cranks stock on a 51cm TT bike - really, Cervelo???) to 165mm's and couldn't be happier. I'm 5'7 with legs longer than torso and I always found that I felt crunched at the top with the 172.5's, to the point of having trouble breathing, hip issues and frequent knee-to-elbow contact. I took a bit of a flyer with the 165's against the advice of my local bike fitter (but in accordance with some advice I received here on EN) and everything is waaaaay better. I feel stretched out, no more breathing issues and no more appendage battles. I was concerned about losing leverage on the hills but I frankly haven't noticed any difference. Of course I also switched to a compact crank, an Adamo saddle and a 12-27 cassette at the same time, so I have no idea what variable is making what difference, but I find I can now comfortably stay in aero for 5-6 hours where previously I had to take breaks every 20 minutes or so. Whether I actually gained any power from this move is questionable, but being able maintain aero position for an extended period is a benefit in and of itself.

Tyler

Tyler

William Jenks

Thanks Tyler. I'm also already using compact...and ran 11-26 for WI....and even an Adamo saddle :-) Do you train with power? Did you notice any change? What I'd really like to do is check some parts out of the bike gruppo library and try them, but somehow we don't have one of those here! :-)

kjanracing

I had 165's on my track bike. Fine for racing track, but when I rode it on the road it felt like I had no power climbing. I could spin them up on the flats or slight rises but for climbing and out of saddle I felt like there was just no torque. does the P3 have just a standard English BB? You could probably get a BB and crankset pretty cheap and if you liked it, spend the bucks on a nicer setup.

Chris G

 I am 6' with a 33 inch inseam and have 170 cranks on my tri bike.  I have 172.5 on my road bike.  I think they are great as it helps with the hip angle as I am running a lot of drop on my bike.  Small adjustments in crank length make a bigger difference than you would think.  I recently had the oppurtunity to ride a bike with a 172.5 drive side crank arm and a 170 mm non drive side bike [don't ask].  You could tell a difference and that there was something very wrong immediately.  It was very odd and far more pronounced than I would have ever imagined.

Remember that changing the crank length will require changes in seat height and likely fore and aft position as well.  Good luck.

Tyler Robinson

I do train with power - sadly, on the trainer for 8 months of the year since we often have snow into May. Sometimes July... Anyhoo, I had the 172mm cranks on the bike over the winter and managed to increase my FTP from 180 to 218 over the course of the OS (I weigh between 135-140). I switched the cranks around May, just as I was starting to get outside and what I noticed was that I didn't get that 10% FTP bump that everyone talks about when moving from indoors to out. I was almost perfectly flat at 218-220 when I finally had the chance to retest outside. So, I guess you could indirectly conclude that my power may have dropped as a result of the switch but I'm way more comfortable at the end of the day.

William Jenks

@Tyler... If I had held my indoor test I'd have been thrilled! I dropped 10%+ going from inside to outside on my bars on the TT bike! Mostly positional I think, since like most people I can push more sitting up. If you look carefully at those anecdotes, you'll also see people saying their TT bike FTP is lower than their road bike or sitting up on the TT bike in most cases. .but it's a lot more fun to talk about the outdoor boost that some people get...I have to imagine mostly on their road bikes...but I won't seeder that's always the case.

kjanracing

I'm probably too dense to notice if I had a different lenght crank on one side. With the Octalink bottom bracket I do seem to notice something's not quite right if I install the cranks a tooth off. If I get 2 teeth off, now I really start wondering what is wrong. Easy fix though.