Knee pain from cycling

[jim-ratliff]

Living Proof:

I started out with the intent of just addressing Lynn's comment about longer cranks create torque on the knees.  There really isn't any torque on the knees from a pedaling motion (not like pivot slipping skis, which does torque the knee I think).

Then I got carried away.  The link that BushWacka included makes the point that crank length is more of an arrived at compromise (much of which is ground clearance) rather than an optimal setup, and he recommends much longer cranks in general (and sells same).  Kind of an interesting read.

But in general my assumptions have been the same as yours, that there was a reason that shorter cranks were used on smaller frames and for shorter people and longer cranks on larger frames and for taller people, and that the difference may not be performance driven.

[Svend]

I'm not sure I have anything intelligent to add to this discussion of physics and bio-mechanics in cycling, other than to say that my cranks are longer than I thought.....and my wife says my cranky moods are longer too...   

Seriously, I actually measured the cranks on my 29er, and found them to be about 178, center pedal axle to center spindle.  The shop must have put on longer ones than stock, to make them fit me better.  I am grateful for that, as it seems I've hit the sweet spot in seat position, bar reach and height, and crank length.  Since I made the adjustments last week, the knee pain is all but gone.  Pedaling is MUCH smoother and more powerful, stability is better, and most importantly, the stress on the knees is noticeably diminished. I think I will move my shoe cleats back to bring my feet more inward again, but that is all I will do for now.  May play around with bar position at some point, but just want to enjoy the riding for a while and not make too many changes at once.

I went on my first single track ride tonight, and the bike just felt great.  So stable and smooth, steering was spot on -- agile, yet not squirrely -- and hill climbing was great with the new alignment.  It's all good. 

 LP and JR:  as for your discussion of crank length, it seems you are both coming to the same point from a different direction -- it is true that a longer crank creates more turning force (torque) than a shorter one for a given force on the pedal; and conversely, it is also true that a longer crank requires less force on the pedal to create the same amount of torque as a shorter crank.  For a taller rider, having a good geometric fit that a longer crank provides seems (logically) to have the added benefit of making pedaling easier.  OTOH, does that mean a shorter rider has to work that much harder? Not sure about that one, as, like Jim said, smaller riders weigh less (usually), and the geometry of the bike frame is totally different for the smaller size.  Seat tube angles, wheelbase, etc., are different for smaller frames, but I really have no idea what effect that has on pedaling efficiency.

Interesting discussion, and having made some minor tweaks to the alignment of my bike, I am amazed at how much benefit (or loss) can be had from just a few centimeters adjustment.  Kinda like skis and boots too......same analogy.  A small change can translate into big gains. 

[bushwacka]

i have extremely strong legs, but even last year when I played around with 180cm VS 175 and 172.5 cranks on my road bike with an SRM the 180s had one downside. My 5 second power(IE sprinting)  dropped by 223 watts from 175 to 180, IE the 180 was less power, My 1 minute, 5 minutes and functional threshold all went up with the 180s. the 172.5 were not better in any time range in fact they for me the jump from 180 to 175 was less than 175 to 172.5.

I have no idea how to make a graph for this from my SRM files but here is what I have all number in watts, realize that outside of the 5 second time these numbers are nothing special for a 175lb cyclist.

180
5 second  - 1621
1 minute - 767
5 minute - 409
FT - 313

175
5 seconds  - 1844
1 minutes - 721
5 minutes - 393
FT - 298

172.5
5 seconds - 1656
1 minutes - 656
5 minutes - 310(biggest drop off)
FT - 274

the deal is no matter what I did on any of my bikes my knees just simply would not agree with 180s. Not to mention the clearance problems when MTBing, Cross riding or Crit racing. In crit racing your 5 second power and sprint is the most important thing.  I really need to get a pair of 177.5 to try out and see if I can make them work. BYW Sven your cranks are 177.5 they dont make 178 and I bet it says it on your cranks somewhere what size they are.  I also want to add that my average cadence when road riding is 97 rpms depending on the day. I can grind with the best of them but can spin with the best of the as well. Its what riding a SS bike will do for your pedaling.

I do find it funny that hard core roadies insist on running 172.5 so that can spin faster and smoother, which doesnt mean more power.

[jim-ratliff]

Bushwacka:


Thanks for posting the numbers, that's very interesting even if mostly over my head.  Some curiosity questions.


Did you need to adjust alignment (i.e. seat position) with the various cranks?


Did you feel as if you had worked harder when pedaling the 180 crank?  Or did it feel like a similar level of effort resulting in higher watts? (except for the 5 second number)


Were you able to maintain cadence with the larger crank, or was cadence a little bit slower since your feet had to make a bigger circle?

[bushwacka]

Once I had my position as comfortable as could be on the 180s I just used it for everything.

For me the 180s actually felt easier to produce power on, my cadence dropped slightly in fact so slight that its not worth mentioning. My leg were underworked but my heart was screaming on the 172.5..

[jim-ratliff]

My legs were underworked but my heart was screaming on the 172.5..

I find that a really interesting assessment from a pretty fit biker, especially given the old mantra about "don't push the gears, spin them". 


And I'm very surprised that there's that much difference for such a small difference in length.  1 inch = 2.54 cm = 25.4 mm.  So 2.5mm increase in crank length is only 1/10th of an inch.

[bushwacka]

Jim keep in mind that I am no means a grinder either. On my road bike I average 95 rpms which is pretty normal compared to other Cat 2 roadies, Cat 1 cross riders.

[meput]

Crank arm length has always been a controversial topic. When I rode in grad school in the mid '70's, 165mm - 170mm were the norm. Now the norm is 170-175 for men's road bikes OEM. Shimano Dura-ace cranks (Hollowtech) are currently available 165mm - 180mm in 2.5mm increments. General rule of thumb is the taller the rider, the longer the crank arm length. You are looking at spending big bucks to change crank arm length in the aftermarket. As a result, most riders accept the length that comes with their bike. The question as to length comes up most commonly when a rider is doing a custom bike (either with a custom builder or building a bike up from a frame), or when a rider has a problem (like knee pain or back pain), or when a rider just wants to experiment (usually a racer with access to enough other bikes/crank arms of different lengths with mechanical expertise to do the swapping).

Of interest, a lot of the riders that I know are trending to go shorter (if they have a choice: replacing a crankset or getting a new bike) as they age (50's +). Easier to spin and less stress on the knees.

Lennard Zinn recently wrote on the topic in his column on the VeloNews site: http://velonews.competitor.com/2011/06/bikes-and-tech/technical-faq/technical-faq-with-lennard-zinn-when-it-come-to-crankarm-length-no-easy-answers_178528

FWIW, I am 5'4" and spin a 165mm crank.

[jim-ratliff]

Jim keep in mind that I am no means a grinder either. On my road bike I average 95 rpms which is pretty normal compared to other Cat 2 roadies, Cat 1 cross riders.

I should have explained.  My amazement was that the spinning on the short crank was an aerobic (heart) challenge without really taxing the leg muscles.