Is there a standard crank arm length?

TomH · Jan 9, 2011

I didnt mean the impact of the change in angle from a larger crank, I was referring to the impact that traveling a larger circumference circle, albeit easier, has on endurance/power expended. If you're tall enough to push a 175 comfortably, arent you just looking at the difference in leverage vs circumference of your foot circles?

It kinda ties in with the high cadence style of riding.. Shorter cranks mean your legs are moving less per rotation. For whatever reason, I suck at spinning a high cadence.. in that case, wouldnt it make sense to run the longest comfortable crank?

rx-79g · Jan 9, 2011

TomH said:
I didnt mean the impact of the change in angle from a larger crank, I was referring to the impact that traveling a larger circumference circle, albeit easier, has on endurance/power expended. If you're tall enough to push a 175 comfortably, arent you just looking at the difference in leverage vs circumference of your foot circles?

It kinda ties in with the high cadence style of riding.. Shorter cranks mean your legs are moving less per rotation. For whatever reason, I suck at spinning a high cadence.. in that case, wouldnt it make sense to run the longest comfortable crank?

Not necessarily. You can ride at 75 rpm with any crank length by shifting to a higher gear. Does making a bigger circle with your foot make you more efficient or powerful? If your legs don't like moving quickly, why would making the distance they have to move be advantageous?

danl1 · Jan 9, 2011

rx-79g said:
Just a thought - one can achieve "proportionally less force" by shifting. Crank arm length should always be about pedal stroke, not gear ratio.

At least until one runs out of shifts. That's actually why I switched to longer cranks back in the day: I lived at the top of an awful hill, componentry wasn't as diverse as it is today, and that small bit of extra leverage was just enough to get me up and over without stalling out (and my knees rather liked me for it.)

Obviously, a minor consideration, and I'm tall enough that going to 175's was moving from too-small to correct, not from OK to too-large for may size. But yeah, gear development is the last reason to make that switch.

cda 455 · Jan 10, 2011

TomH said:
With a 175mm vs 170mm crank, your feet move ~31mm further, linearly, per rotation (the difference in circumference). At 80rpm, 2325mm more per minute (7.6 feet).. or 139,500mm more per hour. 457 feet further per hour.

Since its a longer arm, its easier to push the same gear, but you are spinning around a larger circle. 5mm doesnt sound like much, but 7.6 additional feet per minute does.

I havent thought it out enough to come to a conclusion about how this added distance really impacts your body, or the amount of energy expended.. but thats the numbers.

Very interesting perspective.

I just started riding my wife's Cannondale hardtail with 170mm cranks ( My bike has 175mm cranks) and I've noticed I haven't had any deadspots spinning 170mm vs. spinning my 175mm.

And just to throw it out for discussion; Since I've a Big Mig fan (Miguel Indurain), if I ever buy a dedicated road bike for doing TT or short races I would definitely use 180mm cranks (As well as a 55+T chainring) like he did to take advantage of the leverage as well as the advantage of the leg power I have.

cxwrench · Jan 10, 2011

Lookbiker said:
I was reading that Cavendish uses 170mm cranks. He is 5' 10''

No power issues there.

Just use what works for you and don't treat these charts and formulas as the final word on the subject. Adding a few mm might not generate any more power for you but going longer might send a troublesome knee over the edge.

Cav is 175cm, 5'9", and probably uses 170 because he's raced on the track so much. his sprint doesn't produce huge watts, most likely because he sprints very low, and his leadout is usually very fast. his acceleration is brutal, so you'd think he'd show more max wattage but he's around 1400. low, aero, and he doesn't weigh much.

muskat · Jan 10, 2011

Your legs will feel the difference.

divest · Jan 10, 2011

I just switched to 172.5 from 170 and the things that I have noticed are: I get noticeably more force when climbing, esp when climbing out of the saddle (feels much more natural and efficient)

In terms of spinning I felt more comfortable spinning at 110-115 rpm with the 170 cranks, I feel like more of a masher with the longer cranks but I've only had them for a few weeks and I think I'll adapt quickly in my base period coming up next month.

The reason I switched is because I have big feet for my height.

black_box · Jan 11, 2011

danl1 said:
As Sheldon Brown notes, we're all different sizes, but all use the same staircases. And the loads we put on our knees on the stairs are much higher than average loads when riding. Shorter folks adapt to the same stairs tall folk do, and curiously enough, it's taller folk that tend to have more knee problems. Oh, and the typical riser height on a staircase is ~ 7-3/8", or 187mm.

With your feet on opposite ends of this 187 total crank diameter, that would be a 93.5mm crankarm. If you take steps two at a time, it would be 187mm. 93.5mm is well under any crank we'd see on a bike and I would guess that taller folks have more knee issues due to their extra weight (same with larger dogs and their hips).

Stairs could be a nice comparison though. Ascend a few flights of stairs one day using one step at a time and with a comfortable pace and see how you feel. Then do them two at a time with half the cadence so you finish at the same time? Technically, you've done the same amount of work in the same time, so equal power output. You could then try it with a comfortable cadence for 2-stepping and then double it for the singles. Then compare your perceived effort after the 4 runs.

rx-79g · Jan 11, 2011

black_box said:
With your feet on opposite ends of this 187 total crank diameter, that would be a 93.5mm crankarm. If you take steps two at a time, it would be 187mm. 93.5mm is well under any crank we'd see on a bike and I would guess that taller folks have more knee issues due to their extra weight (same with larger dogs and their hips).

Stairs could be a nice comparison though. Ascend a few flights of stairs one day using one step at a time and with a comfortable pace and see how you feel. Then do them two at a time with half the cadence so you finish at the same time? Technically, you've done the same amount of work in the same time, so equal power output. You could then try it with a comfortable cadence for 2-stepping and then double it for the singles. Then compare your perceived effort after the 4 runs.

It wasn't intended as a direct mechanical comparison between steps and cranks, but a general comparison that we all use standardized things like stairs, hammers, door knobs despite different body sizes.

black_box · Jan 11, 2011

rx-79g said:
It wasn't intended as a direct mechanical comparison between steps and cranks, but a general comparison that we all use standardized things like stairs, hammers, door knobs despite different body sizes.

I figured, I just thought it was interesting and my mind wandered. Why not 220mm steps? It seems when one-size-fits all we want 187mm steps. When 3 sizes fit all, we want 170, 172.5, and 175mm cranks. If everyone can be put onto one set of steps, why bother with different cranks at all? If taller people have more problems with their knees, shouldnt they have shorter cranks to reduce the strain?

rx-79g · Jan 11, 2011

black_box said:
I figured, I just thought it was interesting and my mind wandered. Why not 220mm steps? It seems when one-size-fits all we want 187mm steps. When 3 sizes fit all, we want 170, 172.5, and 175mm cranks. If everyone can be put onto one set of steps, why bother with different cranks at all? If taller people have more problems with their knees, shouldnt they have shorter cranks to reduce the strain?

Really, most everyone DID use 170s until fairly recently, and longer cranks do create more strain. So the answer to your question is not that it isn't a bad idea, but that cyclists are always grabbing for the newest widget, and crank length is another thing to play with.

I'm not saying it is pointless, or that someone who's 6'2" with 172.5s is straining their knees more than a someone 5'6" on 170s. But right now it's free-for-all with little data on the benefits or negative effects.

orlin03 · Jan 11, 2011

I like choices

As a 6'1" guy with long legs for his height, when it was time to upgrade the crank on my roadbike, I went with a 175. The difference was almost nonexistent, until I hit the hills; climbing seemed stronger, and out of the saddle efforts seemed especially improved & balanced. Fast forwarding to my first TT build, I almost went with another 175, but decided to dig deeper and research this decision. I ended up going with the opposite approach and went with 170 cranks, with the idea that holding a higher cadence would be easier, and that finding a lower position on the bike would be more comfortable (since the crankarm comes up lower at the top of it's travel). It was also a good choice for me.

Since I liked two very different cranks, I swapped the cranks on both bikes to test the differences. Truthfully, I prefer the 170 on both bikes for spinning at high speeds/high outputs; my typical 40k race-pace cadence is above 95, with 97-105 feeling optimal. The 175s, after the saddle height had been lowered, made me feel more crowded by my knees (edit: it just felt harder to "get on top" of the gear with the 175s). At normal-fairly fast riding speeds and cadences between 90-95, I couldn't tell a difference between the cranks. Everytime I got out of the saddle, though, I felt most comfortable on the 175s regardless of the bike I was on; I simply felt like I had a more complete stroke. Seated climbing was not as different as I previously thought, but when the incline got really steep and my cadence dropped, I did feel a bit better with the longer arms; I can't say which one would've been better after a day of repeated long climbs though. Basically, I'm happy with the 175 arms on my roadbike, though if roadracing was my main focus, maybe I'd think differently. I definitely wouldn't want them on my TT bike for my riding style; I really wish I had the opportunity to test a smaller crank to see what would happen if I went even further.

I believe that having choices is a good thing. Try matching your decision to the school of thought that most fits your own perceptions, riding style, and intended use.

Cycling for Cancer · Jan 24, 2011

Stronglight adjustable crank arm length

came with my Aerozine X-13 crankset with ceramic BB bearings. I rode with the adjustment set for 170mm for a couple of weeks, then changed it to 175mm. Finished the season at 175mm (at least 1000 miles). I did not mind the extra motion of the knees, in fact my quads kind of appreciated the extra "excursion" and I could feel different parts of my quad developing in response to the longer crank length. I also appreciated the longer length on climbs. Perhaps a minor difference, but noticeable.

I am 5' 7" with a normal sized torso and relatively shorter legs (29" inseam). Typical ride 20+ (median ride length 37.8 miles, longest ride 50 miles). Overall I liked the longer crank length, it allowed me to use my leg strength more effectively and did not seem to hurt my knees.

chas0039 · Jan 24, 2011

One of the most important things for me is to set up the correct seat setback from the pedal with the thigh horizontal. This in turn is effected by the length of the pedal and then, in turn, I need to adjust the stem length to get the overall spacing correctly set up, from seat back to bar. On some frames, the top bar length really limits what I can do, given that the Brooks saddles I use only go so far back. A change of a few mm in pedal length can make a difference and I have found some common frames where I really couldn't use 170mm and still get the whole thing to fit right.

I may be wrong, but I think a lot of people don't really look closely enough at the seat to pedal length when they set things up. At least for me, this is an important set up measurement.

chas0039 · Jan 24, 2011

delete, I keep getting double postings for some reason. Sorry

Camilo · Jan 25, 2011

Mapei said:
Every time I start my bike out of my driveway, or I do the 270 degree turn from the Griffith Park Bike path onto the sidewalk of Western Avenue, one of my shoes scrapes against the front tire, causing me to momentarily lose my balance and curse "G***d**m toe overlap!"

Slow learner!

(ducking and running...)

Is there a standard crank arm length?

TomH

rx-79g

danl1

cda 455

cxwrench

muskat

divest

black_box

rx-79g

black_box

rx-79g

orlin03

Cycling for Cancer

chas0039

chas0039

Camilo

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