Benefits to a larger crank arm?

[Mr lee]

I see that the XT M8000 crank is also offered in 180mm. I currently have a 175mm and am a touch over 2m tall.

 

As the 180mm is more expensive, what benefit would I see with the extra 5mm or is it negligible in the bigger picture of things?

Edited April 10, 2017 by Mr lee

[Captain Fastbastard Mayhem]

3% difference in leverage over a 175 which makes it "easier" to turn a gear.

 

Down side... Very bad pedal clearance

[Mr lee]

3% difference in leverage over a 175 which makes it "easier" to turn a gear.

 

Down side... Very bad pedal clearance

 

 

Would it not be a 3% difference in pedal cadence as well and is that significant enough to "very bad"?

[ChrisF]

uhm ja ... could be interesting to truly understand all the aspects of this ...

 

 

Longer crank :

 

= lower gear ratio (more torque for the same input from the rider)

 

= higher cadence for top speed (would mostly negated by just running a different gear, so only an issue at the ends of the gear ratios)

 

= your toes are closer to the front tire when turning ... may or may not be an issue dependant on rider and bike geometry

 

= your pedal/shoes are jut that tad lower in turns .... 

 

 

 

as the tire size dont change with the rider/frame size the pedal height in turns may be a real issue.

 

Distance to the front tire ... larger rider = larger frame, thus maybe not an issue in all cases ....

 

 

the impact of cadence vs torque ... now here is a much more interesting topic for research .....  and as we are now moving away from hard engineering to bio-engineering, the answer wont be that easy to peg ....

[100Tours]

Sheldon Brown's conclusion is that crank length doesn't make a lot of difference, but the clearance issue could be a real issue. Note that increasing your crank length may require you to change your gearing and to drop your seat height (and potentially handlebar height too).

 

As I remember, with a shorter crank you can achieve a higher cadence (so a higher top end speed if you can put the power down). Shorter cranks require less leg 'lift' over the top of the pedal stroke, so you will notice that more work will be required from your hip flexors with the longer crank, and potentially more stress on your behind with long cranks due to the increased range of leg motion. 

 

A longer crank may help you put out more power (but slower cadence), but if you're not already a strong climber or you don't spend a lot of time in the big blade then it's not going to do much for you.

 

At 2m tall you may find some advantage from getting an extra cm of range between top and bottom of the pedal stroke. But if you're spinning out on shorter cranks, longer cranks may be a step backwards (slower cadence and more saddle bounce). Thinking aloud it would be interesting to find out the longest crank length for which you could maintain a cadence of 100 for example - if you couldn't get that cadence then the crank would perhaps be too long.

 

I do use different length cranks on my tandem, and making this change was a good move. 172.5 in the front and 165 on the back. This is to get our achievable cadence ranges aligned (my wife can match my cadence when she is using shorter cranks).

Edited April 10, 2017 by 100Tours

[BaGearA]

pedal strikes 

[intern]

Don't think crank length makes much difference. After being on 172.5 on my RB for absolute ages - like two decades - a bought a bike with 167.5 cranks on it. Felt a bit funny the first few rides, but made no difference to ability to go fast and be comfortable etc.

 

Coming atcha from a sample of one and purely subjective feedback, natch.

[openmind]

I'm 1.88m tall with a 94cm inseam and have used 180mm cranks on all my bikes for more than 15 years - MTB and road. Some time before that I moved up from 172.5 to 175 and liked how much better it felt on climbs. On the simple principle of "if a little is good then a lot is better" I moved up to 180 and the benefit (for me) was amplified even further. I easily spin at 120rpm and have learned to avoid pedal strikes on my MTB, even the full suss (at 3 o'clock my pedal is the same height as any other crank length). 

 

Here's the last word on crank length: - in a nutshell - use what works for you, but do experiment.

https://www.stevehoggbikefitting.com/bikefit/2011/06/crank-length-which-one/

 

If you buy them at the right place they are the same price as 175mm cranks. More tall people should try longer cranks in my opinion, they do make a difference. 

[Captain Fastbastard Mayhem]

Would it not be a 3% difference in pedal cadence as well and is that significant enough to "very bad"?

No. Chainring size affects cadence at a given speed. Not crank length.

 

You may be more abke to maintain a given cadence with a certain crank length due to certain physiological differences, but it will not change how many rpm you need to maintain to go Xkph. You're still turning the same amount of teeth.

Edited April 11, 2017 by Myles Mayhew

[scotty]

Explained quite well here, taller does not always mean longer and vice versa. 

 

[myveonix]

['Dale]

Is 177.5 not an option for you?

[Captain Fastbastard Mayhem]

https://youtu.be/Yl90KFqDW-Y

So much fail in this video. Some stuff he gets right, but other stuff is just way off and he conflates several terms and concepts.

 

Let's start.

 

Crank length has fokkol to do with gearing. The variables determining gearing are: wheel circumference; chainring size; cog size. The end.

 

A smaller crank arm may allow you to spin at a higher max rpm due to the smaller distance your foot is travelling but it will not change your required cadence to hit a certain speed. Nor will crank length change your cadence. If you're pedalling at 100rpm you're doing that regardless of length. And if you're doing that with a 53t ring your chain is moving Xcm per revolution regardless of crank length.

 

A smaller crank will also require you to put out a higher net power figure to overcome the loss of torque, due to the shorter lever. If you increase rpm, you increase speed.

 

 

The crank is just the lever that you use to turn that blade.

Edited April 11, 2017 by Myles Mayhew

[Jurgens Smit]

So much fail in this video. Some stuff he gets right, but other stuff is just way off and he conflates several terms and concepts.

 

Let's start.

 

Crank length has fokkol to do with gearing. The variables determining gearing are: wheel circumference; chainring size; cog size. The end.

 

A smaller crank arm may allow you to spin at a higher max rpm due to the smaller distance your foot is travelling but it will not change your required cadence to hit a certain speed. Nor will crank length change your cadence. If you're pedalling at 100rpm you're doing that regardless of length. And if you're doing that with a 53t ring your chain is moving Xcm per revolution regardless of crank length.

 

A smaller crank will also require you to put out a higher net power figure to overcome the loss of torque, due to the shorter lever. If you increase rpm, you increase speed.

 

 

The crank is just the lever that you use to turn that blade.

 

Won't a longer crank arm be slightly more difficult to turn over at 12?

 

Obviously the lever action will be better on the rest of the pedal stroke, but that's about the only downside I can think of.

[Erroli8a8]

the longer the crank length the lower the seat hight.

the lower the seat hight the smaller the hip angle to your leg to at the top of the peddeling cycle.

 

these points are more important to remember. as a new bike fit might be in order.

[Captain Fastbastard Mayhem]

Won't a longer crank arm be slightly more difficult to turn over at 12?

 

Obviously the lever action will be better on the rest of the pedal stroke, but that's about the only downside I can think of.

potentially, but only due to the fact that you will be slightly more cramped due to the more acute angle of your hip flexors and / or knees at the top of the rotation.

 

But again - that would be down to a physiological difference between 2 people. Some are more flexible, others not. Likewise with muscle development. 

 

Point is, crank length has nothing at all to do with cadence. 

 

It DOES, however, affect your foot's angular velocity - ie the speed at which your foot is rotating around the circle you're pedalling. But that's not cadence. 

Edited April 11, 2017 by Myles Mayhew