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Crank lengths - 170mm vers 175mm
NB: Originally posted elsewhere on the Global Riders Network and appears via syndication.
I'm curious to know what the Pros and Cons are and when I would choose to run 170's over 175's etc?
Cheers
Phil
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I changed my trance from 175mm to 170mm and now run this size on all my bikes. I think my road bike might be 172.5mm because that's what came with it. I'm not sure I wouldn't really notice the difference but I initially changed it as it was having knee issues and I know I have short legs for my height so thought shorter ones wouldn't really hurt.
Shorter gives you a touch more ground clearance. Downhill bikes often run shorter cranks.
Ive always been curious about this as well. Do you get any power difference between 170 - 175 or is it just a matter of length of legs to suit length of cranks?
Being towards the other end of the spectrum from Brian at 185cm (6'1"), I prefer the longer cranks. My young bloke had 170mm XTR cranks on his old bike, a medium Jekyll, and it felt harder to pedal against at low cadence, but easier to spin. This was consistent with what I had been led to expect from the roadies at the predominantly road and commuting forum I hang out at, at bicycles.net.au.
Now that Josh is on my old Jekyll, being almost my height (at 15! - need to put a brick on his head), he much prefers the longer 175s.
Fixie and track riders prefer shorter cranks to avoid pedal strike and (I believe) make high cadences in the sprints easier. MTBers tend to longer cranks to make it easier to torque over trail obstacles, also having the widest variation in cadence of all the disciplines, with roadies closer to MTB but sometimes in between.
Both my bikes are 175's. A mate had (till his frame snapped) 170's and whilst he was arguably fitter than I he would struggle to keep up on longer distance rides 100+. No doubt there must be a benefit to running longer cranks over distance. I failed A level mechanics but it must something to do with levers and efficiency. Any maths wizz'z on the forums care to share some insight?
In all honesty, what ever feels more right will be better. If you do change the size then you need to remember to adjust your saddle fore/aft and height.
Normally bigger cranks for bigger guys, but torque as mentioned earlier is very important, Especially on SS. I am 182cm and ride a SS 29er and I found that the extra torque, just gives me that little bit more energy to spin up the wheels.
As for clearance 5mm difference to the 170's. That is right 5mm! I reckon BB height has more weight to clearance theory.
I am sure some mathematician will come along and do the maths for torque values.
The frame designer for my latest bike recommended 160 mm cranks. His argument was on the basis of ground clearance. As a rigid bike the BB is low(er than a hardtail or FS bike) always, not just when the suspension would be compressed. Thus stuff you'd normally step over with the suspension up in the travel can hang up on a pedal.
Like always, I ignored the advice and am running 175 mm cranks. I notice a few more pedal strikes than on the hardtail 29er, but not significantly different to on the 575. All my bikes run 175s.
There's a wealth of text on the why's and wherefors of crank length.... lots to do with roadies and maintaining cadence etc..... not that i've read it.
Crank length has a lot to do with leg length. Shorter legs get a shorter crank, to keep the leg within it's efficient angle range to maximize the power produced by the various muscles involved. If a short leg uses a longer crank, the knee has to come up too far reducing the efficiency of the quads. For the average person, this won't matter a great deal, but for endoros or racers this will make a big difference. 5mm in crank length is 10mm + in overall vertical movement.
For me, however I am 6'4 and could prolly go a little longer on my road bike. My new road bike will be a 61cm frame I think!
You wanted some maths to this?
170mm v 175mm cranks
The longer moment arm will give more torque. How much?
Difference = 5mm (per crank arm) =0.005m
Let's assume you weigh 80kg, so your maximum force through the pedal would be when you are standing on it. ~=800N
Torque=Force x Distance
=800 x 0.005
=4 N.m per crank arm.
So for the 2 legs, you'll get 8 N.m of extra torque.
What does this mean? You'll get about a 3% increase in torque. How the human body uses energy is anyone's guess, but as some have pointed out, you may lose that gain in the longer stroke by having to lift your legs higher.
Longer cranks give better torque.
A lot has to do with cadence or the type or rider you are.
Some people like to crank along in a bigger gear, which a longer crank will be better for you, as mtbers generally have a lower cadence.(nothing wrong with this)
Shorter cranks will help with people who know how to spin a higher cadence. Also for short people.
Dowhillers like shorter cranks for better ground clearance.
People read way too much into it,175 to 170mm crank swap gets you 3% less leverage which wouldn't even force you to pedal a lower gear.
I run 155mm cranks and I'm six foot tall, yeah it's on a DH bike but I would run them on an XC bike as well being able to pedal in rough sections is more of a benfit to me than the 13% leverage I lose.
If you have enough gears crank leverage doesn't mean as much as people on the internets will tell you it does.
I don't see why. there is no XC trail at Glenrock, that is that rough, that you need shorter cranks.
I use 175 and have rarely had a strike.
Yeah that is fair enough but as people are starting to shy away from 44t big rings and instead opting for things like a 38x10 setup bike manufacturers may start dropping BB heights as you don't need the clearance of the current bikes.
I use my DH bike on the XC loop from time to time and with sag it sits with about a 10.5" BB height so short cranks are a big help, I still find myself doing a quick backpedal to miss rocks here and there and they often scrape the ground on Twisties.
With short cranks you can pedal through corners without worrying about clipping, I have run every size between 175-155mm(except 160) and I like the 155mm the most, I have 165mm cranks on my hardtail and my DJ bike only because getting CR-MO 155mm cranks is difficult and expensive.
Shorter cranks allow you to spin faster meaning a more consistant power output. Shorter cranks are also better for your knees if you pedal properly.
Anyone comming from a roadie background will know the benifits of spinning with effiency of pedaling and not crunching big gears.
Mountain bike riders are the least efficient peddlers around. The beinifits of longer cranks are flawed as far as efficiency is concerned.
I disagree, especially spinning up 29er wheels. I do have a roadie background in my shady past and agree spinning is good. But it just doesn't seem to gel with me, with 29er wheels.
I agree to disagree.
Im not a fan of 29ers.
Not sure, what riding you do, but have you tried one?
It may or may not suit your riding.
I hate to admit that I have tried one.
The type of riding I do? Id call it back country riding, Long rides with some light freeride thrown in for some fun.
29ers are just not suitable for freeriding.
I do have a background in road with some expert coaching thrown my way.
I ride a 29er and started out with 175mm but decided to try 170's cos I had a set not doing anything else. Have decided to stick with the 170's as they are easier/better for me to spin up the bigger wheels. They're FSA K Force Light so dropped a few extra grams too! (I prefer to ride higher cadence and am 6'3").
Give 'em a try if you can do it without buying a set and make up your mind I say
I can't bring myself to ride a 29er, seems like cheating to me, suspension well that was a tech evolution and I get that. 29ers are nothing but a bigger wheel, I understand it rolls over things easier and hold speed better and for that reason I think they should have their own race category like 24" BMX bikes have.
They seem to sell really well to fat blokes that ride just so they can shave their legs and wear lycra.
I'm starting to remember why I'm not allowed on Rotorburn anymore.
LOLZOR!!!
Way back when I were a lad I was told to go for 180 cranks on my first MTB seenas how I was 6' 2". After a while I rode a borrowed bike with 175s and everything pedalwise felt better. I also tried 170s once but they didn't feel as efficient.
I put it down to physiology. My legs work best within a narrow band of movement and cranks of 175 allow me to pedal within that band. The greater leverage effect of the 180s is wasted by the additional movement required to turn them.
If this theory adds up someone shorter should generally be riding shorter cranks than someone tall. No surprise there then. A high cadence rider may also prefer slightly shorter cranks.
29ers.
Lets face it, unluss you're a bearded rigid framed simglespeeder, ride a Cuban pimp cruiser or have a fixie with dildo bars, going faster with the same amount of effort is the purpose of almost every upgrade I know as far as I can see.
It's not a cheat or crime (Yet). On some races there's a definite advantage to big wheels but a big wallet also helps even on 26" wheels so maybe a seperate class may be demanded for both. At the end of the day the fittest bloke with the fastest bike wins.
I tried a 29er at the 12hr last year and loved it. I was expecting a barge but found it handled well, I even took in some air so at the moment I'd add a carbon niner jet duallie to my collection today if I could afford it.
Btw. You don't need a 29er to be fat, shave your legs and wear lycra. Just take a trip to Subiaco any Sunday.
Given that you move around a hellofalot more on a mtb than a road bike, it makes less difference what crank length you use.
I would only bother looking at changing crank length if you were experiencing issues of hitting rocks (and would 5mm make a difference?) or if you didn't feel comfortable due to leg length. Unless you are racing big time, I doubt it would matter.