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Crank length change .


FirstV8
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Longer lever makes more torque, shorter lever requires more torque to do the same work.

The lever is a torque multiplier - the longer the lever the higher the multiple. 

The interesting thing is with a longer crank your rotational speed decreases proportionally. The net effect is that within quite a large range of crank lengths, there is zero/negligible change in power output. ie: torque goes up, rotational velocity goes down in  "zero sum" way. This holds true over a MUCH larger range than the typical spectrum of 170-175mm. Some tests indicate that there is only a noticeable loss in power output once you get down to about 145mm crank length. Apparently anyway, I read up on it quite a bit when building a up a bike during lockdown and the only cranks I could find were 170mm (I'm 1.84cm so "shouldn't" be using cranks that short). 

 

Interesting read:

https://www.bikeradar.com/advice/sizing-and-fit/what-is-the-best-crank-length-for-cycling/

 

Bottom line: crank length within the commercially available range has almost zero impact on power output. However what is noticeable is comfort. Shorter cranks result in angles that are a bit easier on the joints. 

Edited by NickGM
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Thanks Hubbers , i really put the cat among the pigeons with this question but i think its given a lot of us a good idea as to what will work best for your  personal ability .I decided to take advice from many suggestions and that is to leave my saddle height as is and enjoy the comfort as that is what seems to be the general consensus .   

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Thanks Hubbers , i really put the cat among the pigeons with this question but i think its given a lot of us a good idea as to what will work best for your  personal ability .I decided to take advice from many suggestions and that is to leave my saddle height as is and enjoy the comfort as that is what seems to be the general consensus .   

You also need to take into account you specific needs. You've had a Hip replacement, so the hip muscles etc. have had added stress.

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As someone whose idea of bike fitting is "does this bike fit in my car, in my garage, and in with my bicycle gang", personally I don't see benefit in changing crank length (not at 2.5mm anyway) for the purpose of pedal efficiency or fit. Especially on an MTB where there are so many other factors at play.

 

However, as 95% of my riding is enduro/trail rides in forresty singletrack, laden with roots, rocks, berms, jumps, and all manner of fun, I do see a benefit in shortening my crank length (currently at the OEM 175mm) to 165mm to reduce pedal strike on trails.

 

That 10mm extra clearance could be the difference between shredding and tasting dirt.

Edited by patches
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I have ridden 175mm cranks my whole life , on road , track and now Mtb . I recently bought a Dura Ace group set with 172.5 length cranks . I have been riding them for about 2 months on my road bike and feel comfortable and have no niggles in my legs . My Mtb still has 175 cranks . Do i leave the  saddle on my road bike as is or should i raise it the 2,5mm difference . Will it help anyway or i just leave well alone . . 

 

yes raise the saddle (you're not getting the same full extension of your leg on the downstroke any more).

 

But the biggest difference is on the top of the pedal stroke, where your foot now crests the pedal arc 5mm lower than before. that means your leg doesn't need to get so close up to your torso during the pedal rotation. 

 

And I'd check your track bike cranks again. those are almost always less than 175.

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My vrou geniet ook my langer voe.... Ag nevermind.

Kan jy daadwerklik n verskil tussen die verskillende lengtes voel?

Eerlike vraag.

Jy het my aan die dink.

 

jy weet mos lengte maak saak

 

With the longer cranks I feel like the power I put down is used/transferred more efficiently.

 

BUT, I'm not sure if it's because of the smaller bike and the slightly different body position. (ek's nie 'n ekspert nie, ek like net bike ry)

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imho, it is a very person specific thing, but surely length (leg length) plays the biggest role in this equation.

 

Someone with short legs will battle to turn over longer cranks and someone with long legs will spin like hell on short cranks.

 

For the pros, obviously there is going to be a sweet spot, but for me as a "fun rider" and slightly taller specimen (1.87m), it's 175mm all the way.

Edited by TheoG
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My vrou geniet ook my langer voe.... Ag nevermind.

Kan jy daadwerklik n verskil tussen die verskillende lengtes voel?

Eerlike vraag.

Jy het my aan die dink.

 

:lol:  Ek dink jy verdien alweer 'n warning point of twee vir daai een ...   :lol:  :whistling: .

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Interesting comment. I would have thought that 172.5 would require more power/torque than 175 as the lever length is now shorter. I would believe that it would be easier to spin and maybe there is where the difference is.

You are correct and that's what I think he tried to say.

The longer cranks certainly have more leverage and will therefore give you more torque through the pedal stroke and also give you more power in the downstroke. The shorter crank length will have less torque.

However, due to the longer crank circumference, it might also slow your cadence down a bit.

also, any crank length change should always result in a saddle height adjustment.

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If it was all in my head then 175mm would be the same length as 172.5mm

So you are out of your mind, Cupcakes

Of course I'm out of my mind! I'm contributing the this ridiculous thread. 

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Of course I'm out of my mind! I'm contributing the this ridiculous thread. 

 

I can dance all night, Baby.

 

Someone asks an honest question about something technical and subtle and you think it's a ridiculous thread. Swak.

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Longer lever = more torque. 

Agreed, but that means you are able to exert more torque to the wheel with less effort from the legs. That means better for the knees etc, but worse for spinning.

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I upgraded from 172.5 to 175 for a while on my previous bike. what I did find though that my cadence speed dropped noticeably.  I also found a noticeable dead spot when pedalling (specially uphill).
Since back to 172.5's my cadence is again 80-85 range.  Cant say for dead spot now, I installed ovals.  So not apple vs apple comparison.

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I have 165mm cranks on my enduro bike and 175mm on all my other bikes. Riding the enduro bike is definitely harder work than any of the others but I couldn't tell you whether that is due to the weight and slow-rolling tyres or due to the crank length. 

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This thread reminds me of the Princess and The Pea. You can feel the difference in 2.5mm?!?!?!

I did.  for sure.  My garmin proved it.

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