The Effect Of Crank Length On Performance
 

I've seen a lot of holy wars started over crank length on different forums over the years, but the tl;dr is - if you're fairly normal proportioned (not extremely tall or short) you can ride pretty much anything (commercially available anyways) without it having much if any effect. The current thought is to use crank length as a fitting tool (and also taking special consideration for something like track where too long and you risking clipping a pedal) and letting the rest sort itself out naturally.

Watch this:

I sometimes feel like I'd rather have shorter cranks on my road bike, as my track bike just feels better to pedal with 165s versus 172.5s on the roadie but haven't had the means to try it. There could also be a multitude of other things at work there. Shorter cranks don't require as much movement at the hip to spin, so it does make sense that they'd be easier to pedal at higher cadences. Interesting little convo on crank length from a bike fitter's perspective here, around the 4:00 mark:

These are both true. In one of the other threads around here, someone ( I think Carleton) calculated the gain ratio difference between 165 and 170 cranks, as well as the difference in foot speed given the same rpm for each crank. If I remember correctly, the gain ratio wasn't a huge difference, but the foot speed differential was greater than one would think.

As far as the the gain ratio and "feeling easier", the cranks do give you a tiny increase in leverage, but that 5mm might be able to be offset by the fact that your hip angle opens because your TDC drops 10mm, which moves your weakest part of the pedal stroke into a stronger range of motion, and shortens the the window of time you spend here because you reach fuller feg extension earlier in the pedal stroke.

In other words, although longer cranks give you more leverage at the pedal interface, shorter cranks allow more of the pedal stroke to be spent in a more powerful muscular range.

Yeah. That sums it up.

In my experience, it comes down to riding style and event focus and less about height.

For example, bike check interviews have had Sarah Hammer on 172.5mm cranks and Jamie Staff on 165mm cranks.

Crank length is important for unlocking that last 10% or so of power and speed. The thing is, you may have already lucked-up and started with your optimal crank length...you just have to experiment.

If I remember correctly, for every 2.5mm step down in crank length, you have to go down 2 gear-inches to have the same torque (feeling on the pedal).

So, this is why racing 170mm cranks on a 98" gear feels like racing 167.5mm cranks on a 96" gear.

You lose leverage...but you gain more effective cadence ranges and knees don't come up as high, so you can bend over more.

Then there is "tangential velocity" of the foot spinning around the shorter or longer cranks.

It's a mess of calculations :foo: I've done them. There is also mention of it in the Bicycle Science book. I'll see if I can dig out the formula they came up with that involved crank length, chainring, cog, and torque.

I am 5"5" tall and currently using 165 mm cranks. I am thinking of changing to 150 mm for next year. I haven't been very successful in finding a 155 mm set in 144 BCD. Does anyone have any suggestions?

ROTOR "3D" or "ALDHU" or "whatevername" cranks used to come in 150mm, 155mm, 160mm, 165mm, 170mm, 172.5mm, 175mm sizes.

Their website is jacked up now. So, I'm not sure what their offerings are.

Here is a pic I snapped of Ken Lo's cranks. They are 150 or 155mm (I can't find my notes). I don't think they are custom. You can (or at least could) get them without a power meter.

https://i.imgur.com/l4M2NDa.jpg

Maybe ROTOR stopped making them?

FYI, Ken Lo is one of the fastest masters Man 1 riders in the US right now.

You can pretty much use any correct length 3d+, 3d30 rotor crank and buy the 144mm spider separately.

I personally opted to take a Dura Ace 172.5 (solid) and cut it down to 150mm. At my height 5'4", it allowed me to run a bigger effective gear (gain ratio) than when I was using a 165mm. 165 or 170 is okay if you can spin and finesse a lower effective gear, but it didn't really suit my high torque mashing style.

Depending on the shape of the pedal end of a crank arm, it may be possible to shorten a crank set. The minimum amount you can cut off is around 22 mm. So if you can find a reasonably beefy 175 mm crank, you can make a perfectly good 153 out of it.

Cutting and tapping the arms is not rocket science, but it helps to have a spare set of steel crank arms to use as a guide so the new pedal threads are at the right angle.

The Rotor ALDHU comes in 150 and 155mm. To put together a crankset for a BSA 68 track bike, you need :

Crank arms https://rotoramerica.com/collections...ts/aldhu-track
Track Spider https://rotoramerica.com/collections...-track-30-axle
Track 30 Axle https://rotoramerica.com/collections...u-track-spider
BSA30 Track BB https://rotoramerica.com/collections...ts/bsa30-track

Beware and get the correct bottom bracket. The axle is 30mm, so get the new track BSA30 BB with the red seals, not the older 24mm track BB with the blue seals (online vendors may not yet know the difference). The Rotor rep said the road BSA 30 does not work.

Also, the spider is thin and not countersunk for captive nuts. Dura Ace chainrings don't cinch down with normal track bolts. FSA are reported to work as well as the Rotor track chanirings. https://rotoramerica.com/collections...ack-144bcd-1-8

The older Rotor 3D24 crankset came in 155mm. The spider is removable, and one version of the Power2Max track powermeter used the Rotor 3D24 road crankset. 24 refers to the 24mm spindle. Power2Max also offered a track powermeter using the Rotor 3D+ crank set which used a 30mm spindle. That's what is shown in the Ken Lo photo above.

The 3D+ road cranks are currently on sale on the Rotor website, so this may be a good way to go.

https://rotoramerica.com/collections...ts/3d-crankset

Check to see if you need a special tool to replace the spider. I know the 3D24 did.

At the same cadence a shorter crank requires lower foot speed and higher torque. Longer crank is the opposite, higher foot speed and lower peak torque

In reality someone running longer cranks is likely to use a larger gear at a lower cadence, effectively equalizing the peak torque and foot speed. Someone running shorter cranks is likely to use a lower gear and higher cadence, also equalizing peak torque and foot speed. In the end the difference is the cadence and actual gear used.

I think the most important thing is that your cranks on all your bikes should be no more than 2.5mm different from any other bike. I can't remember what study this was but that was the main take away I had from reading it. The participants could the produce the same power at any length once acclimatized but changing by more than 2.5mm resulted in a loss of performance until they re-adjusted to the new length.

Thanks for this.

I've always been perplexed by ROTOR's offerings and their mix and match style. I like the idea of mix and match. Maybe they can do a better job of presenting how it all works and how one might craft a solution.

Ken is also like 5 feet tall.

Leg length is a factor. I'm currently running 170s, and my fitter said normally she would scream about someone running cranks that long, but they work okay for me because I have such long legs. Still thinking about going down to 165s though because the latest science says they are faster. (Per the video I posted earlier.)

Thanks everyone for all the information. Fortunately, I also know Ken Lo and am going to talk to him about his setup. I never thought to talk to him, which I guess just makes me unaware!

Good to know that Shimano chainrings need a different bolt because most of my rings are Dura Ace. I have some old superbe pros and am hoping they work. I am trying to keep the cost of the conversion down to something manageable. As you get to be an older Master, and living on a fixed income, expense begins to be important!

[QUOTE=gl98115;20633322]The Rotor ALDHU comes in 150 and 155mm. To put together a crankset for a BSA 68 track bike, you need :

Crank arms
Track Spider
Track 30 Axle
BSA30 Track BB

Beware and get the correct bottom bracket. The axle is 30mm, so get the new track BSA30 BB with the red seals, not the older 24mm track BB with the blue seals (online vendors may not yet know the difference). The Rotor rep said the road BSA 30 does not work.

Also, the spider is thin and not countersunk for captive nuts. Dura Ace chainrings don't cinch down with normal track bolts. FSA are reported to work as well as the Rotor track chanirings.

Is the above crankset a power meter (links removed since I am not allowed to post links)? I am currently riding a 3D Inpower road adapted with a spider adapter to get to 144BCD. I would love to have a dedicated track crankset that is 155 and a power meter.

Just to clarify, Dura Ace rings work fine with the Power2Max powermeter spider that you can get with a Rotor 3D+ or 3D24 crankset. It looks like Joe Lo is running a Campy ring on his Power2Max. It's the Rotor ALDHU track spider that needs fiddling. I've never used a Rotor 3D+ or 3D24 track spider, so have no comment on their compatibility.

Power2Max now offers two track power meter spiders for the Rotor ALDHU crankset.

https://cimg8.ibsrv.net/gimg/bikefor...796aa0a0c3.jpg

https://power2max.cc/collections/nge...-3d-plus-track
https://power2max.cc/collections/ng-track-power-meter

Interesting, it looks like you use it with the ROAD ALDHU axle and appropriate bottom bracket instead of the track axle and BB.

Looks like the ALDHU cranks are only available in 170-175.

You can buy the ALDHU crankarms in different lengths (150, 155, 160, 165, 170, 172, 175) from other vendors.

https://rotoramerica.com/collections...ducts/aldhu-3d

I see now. I did not scroll down far enough: http://rotorbike.com/catalog/default...3d-cranks.html. It lists only 170-175 in the written description, but the chart farther down shows the other lengths.