crank length, gearing, etc-
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  1. #1
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    crank length, gearing, etc

    I've got a SS Inbred out with a journalist. He asks:-

    1. Why 180mm cranks? I fully understand that longer cranks effectively reduce the gearing. But I'd rather ride 170mm cranks with 30x16 gearing instead of 180 cranks and 32x16: same pedalling effort, only very marginally spinnier, and easier on me knees. I'd even prefer 170mm cranks and 32x16 - a bit harder in terms of effort, but closer to my normal pedalling style.

    I reply:-

    quick answers cos it's friday.

    you can't easily run a 30t front cog. or if you can I don't know how. it's a low speed leverage thing, and historic reasons though I guess... kind of like how we ended up with 26in wheels :-)

    He asks again:-

    I know the low speed leverage thing. But what we're worried about isn't leverage but the REAL gear ratio, or 'gain'.

    gain = wheel radius x no. of chainring teeth
    crank length no. of rear sprocket teeth

    It's no easier to push a 32x16 with 180mm than it is 30x16 with 170mm. It is, in fact, the same gear.

    You could say you're less likely to spin out at speed with slower turning 180mm cranks, but then you can spin shorter cranks faster more easily so I don't buy that.

    Speccing is a problem, I admit. 30T inners are common on road triples. Or you could get Chris Bell of Highpath Engineering to make one. But probably it's a case of EITHER elegantly OR cheaply and not both. 32x17 would do it too, as well. But you don't see many 17T freewheels either.

    Personally I'd settle for 170mm and keep 32x16.

    Don't forget that you're about 6'2" or something. For you 180mm cranks is roughly the same as 170mm cranks for me.

    30T inners are common on road triples.

    I say:-

    But to line a triple inner up with the rear hub on a SS hub would require a narrower hub flange spacing causing a weaker rear wheel. Or a long axle and a higher "Q" factor, which I know you're going to turn around and tell me doesn't actually biomechanically affect anything, but it does make building with standard parts.

    I've kind of run out of steam... but anyone got anything to add? Comment? "Just because" doesn't seem to be the right answer, but not the wrong one either :-)

  2. #2
    Am I getting too bulky?
    Reputation: Bikehigh's Avatar
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    The only advantage I can really see for longer cranks is low speed gruntability. Short cranks are easier to spin, because the pedal/foot move less for the same about of ground covered, and so have an bit of an advantage in high speed situations. In low speed situations with longer cranks, the pedal/foot moves more, making it easier to keep the cranks moving while mashing and trying to keep the cranks turning.
    I gotta roll, can't stand still, got a flame in my heart, can't get my fill.

  3. #3
    Nat is online now
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  4. #4
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    I tend towards the "just because" argument myself, and other very scientific personal feelings.

    Engineering and physics and Q-bert factor answers just invite the worms Nate has linked us to.

  5. #5
    No Justice = No Peace
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    Feb 2005

    Who wants to know?

    Tradition, ease and availability of parts, and comfort - I am six feet plus.

    I worked for a motorcycle magazine and I saw journalists do this over and over. They would get on a perfectly adequate bike, spend $2,000.00 making it more like what THEY would ride, then pan it in the press because it wasn't up to snuff.

    Why would a journalist who is unfamiliar with SS mountainbiking come out with this argument? He is apparently familiar with road bikes, where 170 is a way more common crank, and where 30 tooth rings are not odd. Why can't he just ride, evaluate, learn about the sport as it exists, and make his uninformed opinions known?

    We ride what we ride, and an honest journalist would be investigating that, not trying to recreate his road riding experience on your bike....

    Just my opinion, anyway...
    "Welcome to my underground lair...."

  6. #6
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    Quote Originally Posted by Lutarious
    We ride what we ride, and an honest journalist would be investigating that, not trying to recreate his road riding experience on your bike....

    Just my opinion, anyway...
    I know what you mean. But then we sell handlebars that are a bit odd that came about because I wanted to meddle with "the norm".

    For me, the crank thing is because, in my mind, when I'm grinding my tits off going up something far too steep, even on 32:18, I think "well, at least I've got my 180's on"... rather than having it niggle me that I've "only got 175s :-(" or worse!

  7. #7
    No Justice = No Peace
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    Speaking of weird Bars.....

    [[email protected]]I know what you mean. But then we sell handlebars that are a bit odd that came about because I wanted to meddle with "the norm".

    Do you know where I can get my hands on a set of mary Bars for my girl? It looks like just what she needs for her injured Wrists. What exactly is the rise on them anyway? Looks to be like 2.5 in. which would be nice, and the extreme sweep will be awesome. PM or Email me if you know where I can get some right away.

    Lutarious ( )
    "Welcome to my underground lair...."

  8. #8
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
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    Dec 1998

    On the gearing: tell him to swap to a 34x18. Much easier to do. Then throw in that larger ring/cog combos are more efficient and smoother feeling.

    Crank length: Most SSers setup for the climbs, not the flats. Low cadence, out of the saddle climbing is easier with longer cranks. High cadences are only slightly affected once the rider adjusts to the length.

    At 6'1" I am currently running a 38x21 with 185 mm cranks. My effective cadence range is 40-170 rpm. My knees get sore when I ride my geared road bike with 175 mm cranks. They fine are fine on the SS (and I had major knee surgery before I rode MTBs).

    It did take me 3-5 weeks to adjust the 185s from 180s.

    This guy must be mostly a roadie.
    The trouble with common sense is it is no longer common

  9. #9
    Recovering couch patato
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    Jan 2004
    Over the past years, I;ve managed to bring my 175mm iding down to road-duy bikes only. Now I got a fully built bike to test, first 175mm equipped dirt bike for me in years.
    Swapping bikes between the new 175mm bike and my Karate Monkey (both geared now BTW, sorry), me and my buddy are for the first time ever really getting the "feel" for the difference. Before it was hardly noticable, but now it's totally in our face. The 180mm's just "roll", where the 175mm's seem like a lever that won't turn, high or low gearing. My buddy's not happy, his bike is on 175mm's as well, and he's been riding one of my bikes for the past weeks, so always swapping between cranklengths. Not only does he now dislike his 26" wheels, but also the to break it to his lady?

    A factor I hardly ever read about regarding crank length, is leg muscle's "preference" for gruting or spinning.
    Even riding geared, I like to throw my upper body into the climbs. The short cranks are immune to that, seem to resist my input, while longer cranks just give way and drive the bike forward.

    Another factor often overlooked is out-of-saddle freedom. With hypothetical 50mm cranks, standing out of the saddle cranks level will hardly give any room for the " boys", getting ready for a bunny hop or taking blows from the terrain. Long cranks offer more stability and hoppabilty IMO.

    Could for each rider, any crank length be given a range or combined rpm's and watts that work?
    Like for me, 175mm cranks are cool, but only if I don't drop cadence under like 80rpm, while power output exceeds 250W. For me, that means hard accelerating out ot corners, or any climbing, are pretty much out. Longer cranks allow me to drop rpm's lower at higher power outputs.

    Perhaps for road TT's, long cranks are not ideal for me, I wouldn't know yet. At 100-105rpm (SS) and 450W (lasts maybe 5-10mins, don't worry), I don't feel bohered with 175's at all. Same bike, in crits, getting out of corners is a big pain.

    My recent experience with 175's has given me new inspiration to somehow start using 185's (that I have laying around) again. Just need to find me a wide square BB, good thing the new Fisher 29"ers have such tall BB's, it should be fine.

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