Crank arm length?

Question

I need to know what you think i should be using for the length of my crank arm. I ride road bikes. and i do time trials and triathlons. I have a road bike and a TT bike. Both of them i have a 172.5 mm crank arm. I am 5' 8" and 152 lbs. I do not get tired at all from turning this size crank but i have had many people tell me that i need a smaller crank arm on my road bike.

My ?'s are :

would a smaller crank make that much difference on the raod bike and what would it do for me.

Just other info you might need to make a more accurate calculation.

I have a 11-28 cassette on the road bike and a 12-27 on the TT. the bikes are the 2008 Specialized tarmac elite and the 2006 Specialized Transition Comp

Thanks and hope someone has a good answer that makes sense

Answer 1

It only makes a difference if you are really sensitive about adjustments on your bike, like a pro racer. It may alter seat height a little, if any.

If you race in criteriums, it will make a difference, because when you turn, the guys riding the 165 mm cranks will be able to lean deeper into the turn than you can, especially with the inside pedal down, evn though you should not do this.

Keep it in the big ring.

Answer 2

I'm 5-8 too. And, surprise I use a 172.5 too. I rode 170's for years and bought my last bike with 172.5. Very little difference IMO. I've ridden 1000's of training and racing miles and i could use either.

If ya think about it 2.5 mm? 1/10 of an inch is all it is.

As far as cassette ratios, doesn't matter. Again any affect is so very minor.

That's my story and I'm sticking to it 'cause I've road decades with plenty hard and fast miles and used both crank lengths and I'm your size as well.

Answer 3

My girlfriend's Terry has a 165 crank, and she's 5'2".  Unless you have inordinately short legs, keep riding what you have.

Okay, I'm thinking you have an inseam of around 32 inches.  By Lennard Zinn's formula -- 32*.216*25.4 --you would need cranks of 175.5.

So, the Velonews tech editor says your cranks are just fine.  As did the other fine gentlemen who have answered.

I run 205s.  I've been told they're too long, that I don't need more than a 180.

Go with what is working for you.

Answer 4

First I have to agree with the previous answers, that it really isn't going to make a huge difference. But when you get to a certain level you start to look at every little thing, and one of those things is crank arm length. Many years ago when I started racing the standard length was 170. Being 5' 9" I raced for several years with 170's. I did eventually make the switch to 172.5's, but I can't say that it made any noticeable difference in performance. I have ridden 172.5's on all the bikes that I have had since, from TT bikes, to criterium bikes, to hillclimb bikes, and I have always used 172.5's.

Neither did it make any difference in cornering. I can't say that I hit my pedal any more with the 172.5's than I did with the 170's. Hitting your pedal is always good for a quick spike in the old heart rate, but has never once caused me to crash. And believe me, I have crashed in corners my fair share of the time, for many different reasons, but it was usually because I had my weight on my lazy a**, instead of on the outside pedal where it should be.

Like MR said, it doesn't matter what your gear ratios are, and it won't make any difference in your saddle height or bike setup. So stick with the 172.5's. At least that's my opinion.