Jim’s Tech Talk
By Jim Langley
It was so nice of many of you to weigh in on our discussion regarding whether bicycle gearing is affected by crankarm length, which was the technical question we attempted to provide help with last week. Thank you for the helpful and interesting comments, some of which we’re sharing today. Here’s last week’s column in case you missed it: https://www.roadbikerider.com/does-crankarm-length-affect-gearing/.
Let’s start with comments from Larry English and “Chris”
“Re crank length – surprised you do not mention ‘gain ratio’ by Sheldon Brown. It’s fascinating. It includes crank length AND tire diameter into one common number.”
“How could you omit mentioning Saint Sheldon Brown? One can input all sorts of variables and see the objective result. Here’s Sheldon’s Gain Ratio Calculator.
Sorry Larry and Chris, I didn’t mention it because I forgot all about it, doh! Thanks for the link Chris. So that anyone interested can learn more, here’s a link to the article Sheldon penned on his Gain Ratio idea for Adventure Cycling magazine.
It’s an interesting and perhaps superior way of expressing gearing and it does put numbers on how crankarm length affects gearing.
Russ Marx wrote
“Higher RPM = more power. Shorter crankarms allow more RPM. Change the gearing to place your cadence in the speed range or power range you are looking for. It’s more than just crank length.”
Yes, it’s interesting reading about the setups the pros use. They do choose their gearing, crank length and cadence carefully.
Here’s a report on Fillipo Ganna’s World Hour Record breaking Pinarello. Can you imagine being able to hold 98RPM pushing a 64 tooth chainring and a 14 tooth cog with 170 crankarms? That’s a 123 inch gear. Or in Sheldon’s Gain Ratio terms, a 9.0. Had he used 165 crankarms it would be a 9.3 and for 175s 8.8.
Big Ring Bob looks at it comparing the work done per pedal revolution
“Many production road bikes today come with 172.5 mm cranks. If you reduce this to 165 mm, you are shortening the vertical stroke by 15 mm, a little over 1/2 inch. This means your knee is going to move a little less than 1/4 inch lower at the top and higher at the bottom of the stroke. This is the change in vertical motion of the knee. Work done to move the pedal through one revolution is changed by (172.5- 165)/165 which is about 4%. Shifting between a 16 tooth and a 17 tooth gear gives you a change of approximately 6%. Hopefully this demonstrates the relationship between crankarm length and gearing.”
Bike Fitness Coaching (who asked the original question last week) weighs in
Regarding crankarm length, I won’t get into this too deep since I actually teach this in my bike fitting course. I’m surprised Mr. Burke discusses biomechanics and doesn’t mention the real issues, Range of Motion of the knee. The human body was built to walk and run, not ride a bicycle. Walk around the block and/or run around the block and measure knee flexion. Now get on your 175-220 mm cranks and measure knee flexion. There’s much more on the bicycle.
Now, getting into anatomy, everyone knows about the knee joint (i.e., patella, patellofemoral head, acl, mcl, pcl, lcl, *patellar ligament/quadricep tendon), but not too many know how the knee actually works, i.e., what makes the lower leg extend on the downstroke? If you studied anatomy, which most fitters know nothing about, then you would tremble in sheer horror knowing how long crankarms are destroying one’s knees. You mention the older riders talking about the benefits of longer cranks, then why did most world tour pros retire in their mid 20’s with blown out knees?
Power is basically Force x Velocity. How hard you push on the cranks x how fast you spin the cranks. I’m sure everyone remembers the Commutative Law in math? I think it was taught in 3rd grade.
For example, If you want to produce 10 units of power, it can be 5×2 (high force x low cadence) or 2×5 (low force x high cadence). Believe me and every orthopedic surgeon and physical therapist, it is much safer and healthier on the knee joints to use shorter cranks and spin them faster.
In closing, I am 6′ tall and use 170mm cranks on all my bicycles except my track bike with 165mm. No more knee pain!
* Everyone knows the difference between a tendon and a ligament. What I see often is that the medical industry combines the patellar ligament/quadricep tendon calling this the patellar tendon.”
I was surprised by the comment “why did most older world tour pros retire in their mid 20’s with blown-out knees.” I remember some pros with knee issues but not most. Googling I found this on bike tech guru Lennard Zinn’s site explaining why some older pros were able to ride longer crankarms without issues.
Lennard’s a good source for different length crankarms and bike fitting solutions: https://zinncycles.com/custom-cranks/.
“The great Tour de France riders of the 1980s, Bernard Hinault, Laurent Fignon, and Greg LeMond all were able to ride with very low handlebars and a nearly horizontal back with 175mm cranks that were about 20.5% of their leg lengths simply because they all had extremely long thighs relative to their lower legs.”
I do know that Hinault suffered from tendonitis in his knee but Fignon and LeMond didn’t have issues as far as I know. Speaking of Greg and Laurent, if you have access to it, The Last Rider on Netflix, which tells the tale of their Tour battle, is worth watching. I won’t spoil the punchline except to say there was more to Greg’s Tour-clinching time trial victory than Fignon refusing to use aero equipment.
The last word
Let’s let reader Gary Keene have the final say
“First, let’s stipulate that most of us aren’t worrying about bouncing our knees off our chest in a deep TT tuck, so, good on you Jim! Second, I am surprised the issue of fit has not yet been addressed, specifically, that–within the dynamic relationships of the body’s contact points, the length of the crank is relative to the length of the femur = there’s a range, and there’s adjustments at the saddle, and there are preferences, but it is a vital factor.
Finally, all of us fall on a range between say Contador and Ullrich: spinners and stompers. The former puts more of the load on the lungs, the latter on the legs. While gears can be changed in the moment, crankarms cannot, so wise personal choices are the order of the day. Bottom line: we have options, not rules.”
Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.