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crank arm length(9 posts)

crank arm lengthdan ida
Feb 28, 2002 7:24 PM
I have noticed that triathlon racing bikes sometimes have longer cranks than would be expected. Does it have the effect of making the 53 tooth gear bigger? If not- what is the logic? Thanks you
DAN
Interesting observationcyclaholic
Feb 28, 2002 10:32 PM
That's an interesting observation. I'm wondering where you found this info.

I have experience with 3 crank lengths (170, 172.5, and 175). The "book answer" would be that a shorter crank length promotes a better spin. I don't know if I can discern that.

What I can discern is that the 175 helps on hills. It gives you just a bit more leverage as you pedal. So, on the small chainring, I do notice that grinding climbs are just a touch easier. I guess you could say that, in this way, long cranks increase leverage making the low gear seem just a bit lower (even though it doesn't really).

So, I guess you could also figure that when you are pushing a big gear, the added leverage makes it just a little bit easier. Speaking from my own experience, I notice no difference on the flats on my 175 cranks. I do notice, though, that my legs have a larger range of motion.

I prefer the 172.5 length, mostly because I don't notice anything. It seems normal to me.

But when I ride the bike with the longest length, I do have to be careful about bottoming out the pedal as I crank hard through a turn. A couple of months ago, I cut hard through traffic, bottomed out, and very nearly crashed. My left pedal has an ugly scrape on it.
I have both the 170 and 172.5, and have noticedPaul
Mar 1, 2002 7:44 AM
that I'm faster on the flats. I'm not a spinner nor am I a masher, somewhere in the middle. I have been clocking myself, and have the best times with the 172.5. The circumference is only 5mm greater, but it seems to affect my speed, probably due to my style of riding. I have the same size cassettes on both bikes. I plan to replace my 170 this year with a 172.5 on my other bike.
re: crank arm lengthRolf Sindoe
Mar 1, 2002 9:30 AM
I changed from 170 to 175 last summer. It took me 14 days to get used to my new crankset. It felt like my gearing became noticeable higher and my legs ditto weaker. I got uset to it, so it's not a problem anymore, and I like the crankset cosmetically- However, I never noticed any difference in my speed or power - bummer? - Well, I got curious and found these pages:
http://www.bicycling.com/ubb/Forum2/HTML/000403.html
http://www.velonews.com/tech/rev/crank.html
http://www.nettally.com/palmk/crankset.html
http://www.thankstomycranks.com/cranks.htm
http://www.xsystems.co.uk/machinehead/cyclist_crank_length_calculator.html
http://www.murraytourdeforce.co.za/cranks.htm

Maybe you'll find some good info.?
Thanks, but now I'm even more confused...laffeaux
Mar 1, 2002 10:16 AM
These various sites give me anywhere rom 170mm to 188mm as my "best" crank size. I assume that means that no one really knows.
DING!!!! We have a winner!Kerry Irons
Mar 1, 2002 4:41 PM
Well, yes and no. Everyone "knows", it's just that the answers they give all differ from one another. In a legal analogy, crank length is not "settled law." The best studies show no clear benefit from any particular crank length, and the body is highly adaptable. As a result, good studies are extremely hard to conduct. It is generally accepted (though some disagree) that longer cranks result in lower cadence. Your power output doesn't change so if your crank is longer (more leverage) then the force at the pedal tends to decline if the force delivered to the chain (and eventually the road) remains constant.
re: crank arm lengthwsexson
Mar 1, 2002 10:20 AM
Interesting topic. What companies make aftermarket road cranks longer than 175? How can you figure out how long of cranks you can use safely based on your BB height?
CranksBirddog
Mar 1, 2002 7:57 PM
I used to race triathlon and duathlon, and my own experience was that the 172.5's were best for me. I raced for about 4 years with 170's that came stock on my 56cm Giant that I had set up for racing. I bought a Softride with 650 wheels and put 172.5's on it. I instantly noticed a difference. I had always been more of a "spinner" before, but with the 172.5's I was a spinner with power too. I definitely improved my speed, but was it the bike or the cranks or something of both? After some reading and questioning, I decided to give some 175's a try. I got slower!!. I had more power, but I lost my ability to spin fast. You wouldn't think it would make that much difference, but it did. My test was not scientific, it was just my perceived experience. I think that just like an automobile engine's stroke and bore ratio, we all have an ideal size for our leg length and our weight. Nevertheless, I feel that by trial and error, I found that 172.5's were right for me. BTW, Triheads use the larger cranks because they feel it is better to have a little slower cadence with more power, because it saves a little more leg power for the run portions of the race.
re: I must be insensitivedzrider
Mar 4, 2002 9:33 AM
I have cranks of differing lengths, 170 on my commuter, 172.5 on my beater, and 175 on my good bike. I'm aware of this only because I put them on. I never feel any difference. My cadence stays much the same. My legs find the right gear to spin where they like to spin regardless of crank length.