Road Cycling - 165mm to 170mm crank upgrade??

I have fairly short legs that barely stand over a 50 cm frame. I've had several racing folks tell me I should upgrade my current 165mm cranks to 170mm. Can anyone tell me if this is such a big deal, and whether this is a good idea or not. I'll be buy new cranks regardless, just wondering if I should change sizes.

Thanks.

Cat

In my opinion, that's a huge change. I went from a 170 to a 172.5 last winter, and it took me a quite a while to get completely comfortable with it. (55 cm frame, 31" inseam.)

Some things to consider in making the decision are ...

Do you have any knee problems? This is the biggie. Going from a 165 to 170s will force you to lower your saddle about 5 mm to maintain the same reach to the bottom of your stroke, but at the top of the stroke the reach will be 5mm shorter, which means more knee bend. For someone with short, or even an average length inseam, that's a significant change.
Your pedaling style... are you more of a spinner or a masher? Longer cranks favor the masher. It's harder to spin longer cranks, but they give more torque for the amount of effort. Beneficial when climbing or fighting stiff headwinds...
Are you a weight weenie? That's a total of 10 mm of pretty beefy additional aluminum. If you aren't a gram counter, the other factors are much more important. In any case, find out what the difference will be.


Considering how much a crankset costs, it might be worth seeing if someone has a used set of 170s you could borrow or buy cheap for an extended on-bike trial.

If you do make the change, I recommend doing it when your average weekly mileage is at a minimum, rather than making the switch while you're still cranking out a lot of miles. You'll find that several aspects of your bike set-up will have to change.

Hope this helps, and good luck!

From the information you have provided, you seem to be using cranks within the normal range for a person of your size. Your friends' advice is misguided and naive, based on reading too many issues of Bicycling magazine.

170 is the normal size for male riders of average size. An average rider may chose to ride on shorter or longer cranks , based on riding style.

For a shorter rider, the normal size should be proportionately shorter. For a short rider, 170mm would be a long crank.

Get your friends to calculate their crank size as a proportion of leg length. Calc what 170mm would be as a percentage of your leg length, and work out how that translates to your friends leg size. If they follow their own advice, they may well have to "upgrade" to a 190mm crank !!

Two article well worth reading.
The first contains the recomended crank sizes by Specialities-TA, a French company, who are the only crank manufacturer who make a decent range of sizes.

http://simon.trinhall.cam.ac.uk/bike/cranks.html

The second reading contains a critique of much of the bogus research into crank size. There really hasnt been any valid scientific work in the field.
http://www.nettally.com/palmk/crref.html

If you do buy 170 mm cranks and keep your old 165 mm cranks, you'll be tempted to switch out cranks whenever something doesn't go right for you in your cycling career. You need to resist that temptation. In my experience, it's very important to pick a crank length and stick with it. Constantly switching crank lengths is extremely counterproductive.

If you put on the 170s and like them, get rid of the 165s.

Thanks guys! Your advice makes a lot of sense. I'm very comfortable with the set up of my bike. And being a spinner, I think I'll stick with the 165's.

Thanks a bunch.

Cat