I noticed that Carlos Sastre uses 170mm cranks for road races but 165mm crank for TT races.

 Normally, riders use longer crank in TT races than road races.

For examples, Andreas Klier, Heinrich Haussler and myself.

My body height is same as  Carlos Sastre and my road racing bike setup is exactly same as Sastre.

But my TT bike uses 175mm cranks?

Any secret inside?

 Here is some data copied :

Carlos Sastre

- Race setup Crankset Rotor Agilis 170 mm

-  TT Setup Rotor 3D Aero 165 mm

Heinrich Haussler

 - Race setup Rotor Agilis 172.5 mm

-  TT Setup  Rotor Agilis 177.5 mm

Andreas Klier

 - Race setup Rotor Agilis 172.5 mm

-  TT Setup  Rotor Agilis 175 mm

 I'm taking a stab in the dark not knowing Carlos's riding style too much, but I would guess he spins at higher RPM.  Smaller circles are easier to spin faster.  I went from a 175 to a 170 and I could spin allot more RPM allot easier, but torque for hill climbing dropped off a bit. 

The "secret" is that with smaller cranks you can lower your aerobars while still maintaining the same hip angle (upper thigh to torso) at top of the pedal stroke.  I imagine for a climber the loss of leverage is outweighed by the reduction frontal area (drag).

 Not much of a "secret" lots of tri folks are doing it now.

jmh:

The "secret" is that with smaller cranks you can lower your aerobars while still maintaining the same hip angle (upper thigh to torso) at top of the pedal stroke.  I imagine for a climber the loss of leverage is outweighed by the reduction frontal area (drag).

 Not much of a "secret" lots of tri folks are doing it now.

Short cranks are more and more in vogue in triathlon. Another benefit is that a shorter crank lowers seat height and lowers your  aerodynamic profile. Studies have also shown longer cranks can restrict breathing in the aero position by rotating the knees closer to  the rib cage and diaphragm, reducing oxygen uptake. Another observation comes from studies showing maximum power output and maximum pedaling efficiency do not occur at the same cadence (max power occurs around 120-130 rpm, max efficiency is around 70 rpm). Studies suggest longer cranks do not tend to generate more power, but do increase pedal speed (the velocity of the pedal along its axis of travel) therefore increasing the rider's oxygen requirements. (See Triathlete Magazine Nov 2008, pp 122 "The Crank Length Debate Comes Full Circle"). Anyway, I'm sure some of this depends on the rider's body dimensions and biomechanics and bike geometry, as well as the rider's comfort of fit, so this may not be true for every rider.

hwstony:

For a 50km time trail, which type of crank length is better... when compared with one' road crank?

Can't really say, but 50 km is a fairly long time trail, so aerodynamic profile, efficiency and oxygen uptake will tend to be more important. However, if your bike position is not fit exactly or your position is not comfortable enough that you spend a lot of time out of the best aero position, all those small adjustments are probably a waste of time.  I'm guessing the professionals at that level get thier bikes "dialed in" (fit perfectly) by trial and error at a wind tunnel.

hwstony:

For a 50km time trail, which type of crank length is better... when compared with one' road crank?

An advice, do not think excessively, because the difference is really thin

Lolita90:

hwstony:

For a 50km time trail, which type of crank length is better... when compared with one' road crank?

 

An advice, do not think excessively, because the difference is really thin

This is true ... it generally matters only if you are one of those people who time trials at 45+ km/hour. I was speculating why the likes of Carlos Sastre would TT with a shorter crank, not why any person would or should make the same adjustment.