crank length

Crank lengths come in 3 different sizes , how do you determine which is the most suitable for you, 170 ,172.5 or 175.

 

Actually, they come in more sizes, but that's beside the point.

Trial-and-error is how I've done it. Some cheap 105 9sp cranks (Octalink BB) with crank arms that are easily interchangeable makes changing different lengths a snap.
Having a power meter facilitates dialing-in bike fit (saddle position especially) to a certain length crank.
Otherwise, there are formulas based on measurements of parts of your leg and foot (i.e. femur, total leg length, foot length, inseam, etc...).
5mm difference is not all thought much to be overly concerned about unless your at either end of the extreme on the height spectrum - your musculature will adapt to the length you settle on...

 

Get measured. Cranks come from 155 to 180 apart from the mass produced stuff. What you need depends on you and your bike use. Trial and error can be expensive and painful. No doubt you'll probably drop in those figures, but the best thing I ever did was get measured by knowledgeable professional bike builder.

 

Seeing as Chrislake has started a thread about crank lenghts I too have a few questions I would like to ask.
Tiagra 9 speed 170 mm. 105 10 speed 172.5. I have these two cranks on two different bikes. The 105 is a regular double 39/53. The Tiagra is a compact 34/50. I would like to swap them from one bike to the other. Is this possible without changing out all the other components?

 

Zinn, one manufacturer that readily comes to mind, does 130-220mm for reference - these are custom, apart from the mass produced crankset lengths.

Expensive and painful are relative terms; I can acknowledge that. But definitely, switching cranksets in and out doesn't have to be either or both. Ebay can be your friend (new 105 cranks for $30-40/set), and once you notice a twinge, you stop (obviously) before something becomes painful. 5mm either way should not cause any pain unless your seat height is totally out-of-whack.

If trial-and-error seems beyond your abilities, then by all means, get fitted by a pro who should take aforementioned measurements...

 

TA do a 185mm crank and have done for years. It's a little weighty by today's standard and required a square taper BB but it's pretty nice looking.

I don't buy into the 'increase crank length gradually' thing. I'm guessing that was first muttered by Mr Campagnolo... I used to have no problems going from 185 on the TT bike to 175mm on the road. Similarly, I've no worries other than having to makethe odd roadside stop when changing the adjustable length powercranks from 165mm to 220mm. IMHO you'll only run into injury issues if your body to operate in a range on motion that exceeds your capability. Different lengths may seem odd but as long as you're able to pedal without overly reaching for, of being squished by the pedal at various stages of the pedal stroke you should be fine.

Just pedal in a relaxed manner for a little while and make sure that the side-to-side float on your cleats isn't being maxed out if you go to a longer crank.

 

You should be able swap the cranks without changing anything else. You will have make some adjustments.

 

I remember reading about a study done last year that found that crank length had no discernible effect on performance for cyclists of average height/inseam length. The author implied that crank length was simply a matter of preference (within limits - like some other posters have said, if you are at either end of the leg length spectrum, ie, overly short or tall, then that would be a special case).

Specifically, I recall reading that longer crankarms tended to have the users spinning a bit slower, but they produced the same power as if they had been using shorter crankarms (and thus, spinning quicker). In other words, there's no free lunch