Do I Need Shorter Cranks?
 

My bike is a Cervelo P2C with 175 mm cranks. I'm 5' 10" with an inseam of 30.5". I'm a very good Senior Games time trial racer having won several medals at the local and state levels for the past several years. When I race, I'm riding on the nose of my saddle. I'm also soon to be 77 years old. All of the above is to set up my problem. As my knees (either one) approach the 12:00 position, they begin to significantly flare out so that my lower body becomes sort of parachute-like. NOT GOOD! This doesn't happen "much" when I'm on my road bike. I absolutely can not raise my seat post and saddle any higher on the time trial bike. So I'm thinking maybe 170 mm cranks might help solve the problem or should I just start racing using my road bike?
Tsapp

I'm about that height, it is essentially a super easy gain in terms of hip flexion or whatever you call it. Either more aero at same hip angle, or more power at a better angle.

I had 175's and went to 165's. It's a legitimate and accepted change to make.

IMO if budget on selling/buying your cranks to swap is good, go for it. Also, if you have more money to spend, a really low stack pedal setup like Speedplay Zeros can get another mm or few over SPD-SL's.

My inseam is 30.75" and cranks are 170mm. I have found that during winter backpacking wearing heavy insulated boots and snowshoes. the muscles that raise the legs can tire. I used to do leg exercises for this with 10 pound leg weights. Those same muscles raise the legs during cycling. On long rides, when I begin to tire, I've noticed that the leg at the bottom of the pedal stroke begins to relax and becomes dependent on the down stroke to raise it. Consequently, I guess that a shorter crank would save the legs just a bit so they would not need to flare out at the top of the pedal stroke.

I'm 5'11 and run 165 on road and tt bikes.

I'm not an expert on bike fit by any means. But, I just want to point out a couple of observations. The 5mm difference between 175mm and 170mm is less than a quarter of an inch (0.197 inch). Do you think that will give you the desired affect you're looking for on the TT bike? Also, I'm wondering if your "knees flaring out" on the TT bike as opposed to on your road bike might be the result of being down on the aero bars and your body thereby being more 'scrunched up.' As such...the problem might not be caused by crank length or seat height, but by the height of your aero bars...and maybe the distance between the seat and the aero bars. You might consider altering the length and/or height of the stem, and/or the fore-aft position of the seat. I was wondering about obtaining a longer seatpost. But it looks like that Cervelo you have has a pretty specific aero post that might not be available in longer lengths.
Dan

The difference is 10mm, almost half an inch.

I went 175 to 165. It's actually the difference, times two. The way I see it.

You gain 10mm in aero by raising the saddle for the 10mm less at BDC. Then you gain 10mm towards hip angle at TDC.

I may be wrong on that, but feels that way to me!

This is also why a low stack pedal like a Speedplay Zero Aero matters. Even 1mm there you can essentially double.

Uh, that's what I just said.

Not sure I understand the "times two" :foo:. If you gain 10mm in aero, it means you did not raise your stem so your hip angle is more pronounced than it was at BDC. But because your crank arm is 10mm shorter, hip angle at TDC is probably similar than what it was with the 175 crank. To me the real advantage is that you can decide what to do with the 10mm (aero, hip or split between the two).