crank length: does 2.5mm make a difference?

[3cb]

I had my road bike (vamoots sl) fitted by Toga Bikes in Manhattan. They dialed in everything just so. However I cannot justify another $250 fitting charge for my $300 commuter bike (a Soma ES courtesy of Craigslist). I thought I would just transpose the measurements/positions from my road bike to the commuter. The commuter came with 170mm cranks while the road bike is set up with 172.5. Given that I'll be clocking 100 miles a week on the commuter, does it make sense to get a larger crank, or is 2.5mm difference not material?

[deechee]

depends on the person I think. I notice small changes; saddle being a bit off center, thicker bike shorts etc. so I notice the 2.5mm That said, commuting, if you're going easy and you feel no pains, why bother changing?

[Ken Robb]

I have ridden cranks from 170-172.5-175-180mm. I can detect 5mm difference but not 2.5mm. I got noticeably more leverage with the 180mm. and found it easier to spin higher cadence with the 170mm. but I was ok riding on all of them and I suppose that 175mm is my best compromise.
I did get some knee pain after muscling up hills in higher gears than normal with the 180 set. I don't know if it was due to me delighting in the increased leverage while showing off for my ride partners or because of the increased bend it caused in my knees.

I have a pal who got on one of my bikes and immediately recognized that it had 172.5 cranks rather than my usual 175. Princess and the Pea to me.

I would suggest you ride the bike with the "wrong" cranks and see if the difference matters to YOU.

[Pete Serotta]

I would agree...have the seat adjusted a little higher since the cranks are a little shorter. As a commuter I do not believe it will make a difference if I was riding it. Many triples now only come in 170 or 175 so 'lots" of folks are going longer or shorter quite often. In theory the longer crank gives a little more leverage and the shorter one will spin easier..... :confused: In real life my old legs can not tell a difference.

PETE

[RPS]

I have both and don’t mind the difference. Also have 175. They all work for me.

Considering the difference is only 1.5%, how certain are you in their ability to be that accurate. Also, do you know if the perfect theoretical size would have been 171.5 and they rounded up to 172.5 because that’s what is available? Is it possible that rounding down to 170 would have been almost as good?

If you are concerned about one being better than the other, I’d ride what you have and never look back. If you are concerned more about uniformity so that you always ride the same, then I’d change them.

Personally, I think too much uniformity is not a good thing when training.

[regularguy412]

I could tell the difference. I switched from 170's, that I had been riding for about 8 years, to 172.5's when I got fitted for my CSI. For me, the change was a God-send. I'm not a great 'spinner', that is, I really don't have a 'great' VO2max. Consequently, I don't feel really efficient when I get too far above a cadence of about 105 rpm. The slightly longer crank helped slow down my stroke a bit and seemed to help me develop better power through a wider range of cadences. I find that I don't have to shift quite as often when riding through rolling terrain.

Just remember that the longer crank causes your foot to traverse a larger circle, on the order of about 16 mm larger. Some riders might not notice much difference, but it did make a difference for me. The 172.5's are probably right on the upper limit of the crank length that I can comfortably use.

Mike in AR :beer:

[SpeedyChix]

If you have a high cadence and aren't accustomed to changing back and forth from road and mtb it MIGHT bother you. it is one of those 'it depends' things. I run 175 off road and was at 172.5 on road for a time. Cadence wasn't there so I went to 170 again on the road. Some day I may try a shorter crank off road.

The no cost trial: Set up the saddle to pedal axle measure of your fitted bit, give it a try. Then if it bothers get a different crank and change your seat height.

[Ti Designs]

It you've wasted some time working on a good spin it makes a difference. If you pedal ugly at 90 RPMs it probably doesn't matter much. The same could be said for carbon wheels...

[johnnymossville]

I switched to 172.5 from 170 and I swear I could feel a difference the first couple of rides. Real or Not I don't know, but now it feels completely natural to me, just as it did before.

[bagochips3]

I switched from 170 to 172.5 per my fitter and could defiantly tell the difference. I felt like I was getting a bit more power and pedaling at a slightly lower cadence.

But if I found a bike with 170 crank arms that I intended for commuting, I doubt I would change them. Heck, I have 165's on my fixie.

[Matt Barkley]

'going back to 170s from a long(ish) commitment to 172.5s and attempts at 175s... Saving the knees and getting my spin back on with inconsistent riding.. Also better sprinting and corner clearance. :beer:

[bironi]

depends on the person I think. I notice small changes; saddle being a bit off center, thicker bike shorts etc. so I notice the 2.5mm That said, commuting, if you're going easy and you feel no pains, why bother changing?

+1 on what deechee said. I do notice the difference between my 165 and 170 cranks on my fixed gear, and I have made the change to make both my fixed gear bikes 165's.

[Lincoln]

<SNIP>
I would suggest you ride the bike with the "wrong" cranks and see if the difference matters to YOU.

+1
Road, fixed & mountain bikes, NONE have the same length cranks. I can tell a 5mm difference (never tried 2.5) but it does not bother me one bit.

[wasfast]

It you've wasted some time working on a good spin it makes a difference. If you pedal ugly at 90 RPMs it probably doesn't matter much. The same could be said for carbon wheels...

Well, they're just cranks afterall. :) But carbon wheels, well, they're just so bling......

[giordana93]

2.5 mm is one tenth of an inch, so the short answer is it doesn't matter.
I'm a spinner and have used 165s on my road bike the last 15 years, and have a mtb with slicks that has 175s used for commuting. and a second road bike I've been playing with different fits on, with good old 170s (and at times 172.5s); and for sure there is a difference, huge even, between the 165s and 175s--that I think about for a good 15 seconds before completely forgetting what length the crank is. so unless you are dying to get a new crank, let it be; read sheldon brown's discussion of this for some perspective

[Marcusaurelius]

I can tell the difference. I tried 172.5 but switched to 175mm. It just feels natural to me. I just feel like I'm missing something with the shorter crank not unlike riding a bike that doesn't quite fit.

[Ti Designs]

If you use a normal sized keyboard, open up a laptop with one of those compact keyboards, look at the screen and type like you always do. That gibberish you see on the screen is what happens when your brain learns a certain timing and firing sequence for muscles (some call it muscle memory - I tired to plug my quads into a USB port the other day...), and then things are moved. If you pedal by almost randomly pushing in some direction and let the cranks direct the movement of the pedals, any change in crank length probably makes no difference at all. If you hit random keys in the hope that something intelegent comes out (like I do), switching to a different keyboard isn't going to make any difference either.

I have a hard time playing a piano that's 3/16" wider across 88 keys and I can't spin 165mm cranks as fast as 170's - I spend a lot of time practicing both. One of three conclusions can be reached: 1) humans can be trained to move with accuracy (thus the need for consistant crank lengths), 2) I'm an idiot, or 3) I'm a trained idiot.

[Grant McLean]

2.5 mm is one tenth of an inch, so the short answer is it doesn't matter.


2.5 is the radius. the difference is 5mm in the diameter of the circle your leg makes.

-g

[Ti Designs]

2.5 is the radius. the difference is 5mm in the diameter of the circle your leg makes.

So it doesn't matter twice???

[Viper]

For all we know, he might belong on a 170mm. We don't know his leg length/inseam, length of femur etc. We know dookey and other than Thundercats, Superfriends, James Bond, Miami Vice or U2, I don't offer opinions unless I know the facts. :)

FWIW:

http://64.233.169.104/search?q=cache:Yze9_CQt12QJ:www.billbostoncycles.c om/crank_length.htm+170+crank+length+lower+seat&hl=en&ct=clnk&cd=8&gl=us


:beer:

[crossjunkee]

Raise the seat, and start pedalling, you won't notice the difference on a commuter.

If you were locked in the saddle for endless miles, you "might" notice the difference. You'll be unclipping and moving around on the saddle a lot. It won't be a big deal.

[RPS]

I have a hard time playing a piano that's 3/16" wider across 88 keys and I can't spin 165mm cranks as fast as 170's - I spend a lot of time practicing both. One of three conclusions can be reached: 1) humans can be trained to move with accuracy (thus the need for consistant crank lengths), 2) I'm an idiot, or 3) I'm a trained idiot.Can you walk "and" run? Seriously.

I’d agree with your logic except it isn’t supported by nature. As an example, when humans and most animals walk or run, we tend to take longer steps as we simultaneously increase cadence to go faster, so the premise that somehow we should train our muscles to fire precisely in order to support only one predetermined sequence of events seems illogical. :confused:

Besides, just like cross training is good for us, and runners jog on easy days taking shorter steps than when they do fast intervals (doing both successfully without falling over their own feet), I think having bikes with different length cranks and often spinning at different cadences may actually be beneficial.

The idea that someone can predict an exact perfect crank length for a given rider in a matter of hours seems a little questionable from the onset.

[johnnymossville]

...
The idea that someone can predict an exact perfect crank length for a given rider in a matter of hours seems a little questionable from the onset.

I agree. In fact, I seriously doubt a 10mm crank length difference will ever turn a loser into a winner or a winner into a loser of a bike race. hey, could be wrong.

[flickwet]

My name is Erik and I am a long cranker,

I have 180mm cranks on all my bikes, I've got a 33" inseam and have always thought that the slight moment arm advantage of a 180mm crank length outweighed the downside of the greater circle and presumed reduced cadence, Is there a torque vs Horsepower correlation? I just feel faster on 180's but don't really know if I am. Someone with more time than I may be able to show data based on newton/meters over time, needless to say for me it feels like I am faster with the longer cranks, at my personal cadence. Which begs the question, Is there an "optimum cadence" for each individual and thus one could say that the longest crank that one can spin at optimum (within reason of course) would prove fastest over time? Does 5 or 10 mm buy me another gear? Mentally yes, reality? and finally, maybe 180 is not a "long" crank for someone of my inseam, I've done the calculations and anything from 179-181 seems to be the calculated length, yet everyone seems to be anti longer crank because of potentially lower cadence speed, this is a long rambling post for me but those few mm have always been a hot topic of curiosity for me

[dekindy]

If my cleats (Shimano SPD-SL) get too worn my knees start hurting (only let this happen one time) so yes, I can tell the difference. It would be my guess that the difference was less than or equal to 2.5mm.

It was interesting. I had my Serotta fitter install the cleats and he noted the wear and said, "I am surprised that your knees had not started hurting". Do Serotta fitters know their business or what? Totally rhetorical question at least for my fitter. I have absolute confidence in his ability since he has done wonders improving my riding position and resulting efficiency and more importantly comfort!

I guess that I am saying you should probably account for the difference in crank length by adjusting your seat height so that the distances are equal for both bikes or it might cause a long term injury. As far as power, I doubt if it makes any difference.

[RPS]

If my cleats (Shimano SPD-SL) get too worn my knees start hurting (only let this happen one time) so yes, I can tell the difference. It would be my guess that the difference was less than or equal to 2.5mm.IMO that’s an unfair comparison. I’d agree it’s doubtful your cleat ever wore down 2.5 MM but that doesn’t mean the cause of knee pain can not be due to something other than the effective length of the crank. Something simpler like the angle of the shoe or extra play is more likely to cause problems.

Besides, do worn cleats change the functional length of the cranks, or do they remain the same?

[RPS]

Is there a torque vs Horsepower correlation?Erick, I think you asked rhetorically, but for those who may not know:

Power is a function of torque multiplied times RPM. If you increase one by 5 % and reduce the other by 5% you end up almost in the same place.

As it relates to "gearing" and therefore torque at the rear wheel -- which is where it counts the most -- it’s safe to say that using 180 vs 170 MM cranks is like the difference between a 17T and an 18T rear sprocket.

Sheldon Brown’s gain ratio takes crank length into account in that it compares how far the bike travels relative to how far a pedal travels. If a rider rode the same 20 MPH with 180 MM cranks and 17T versus 170 MM cranks with an 18T, our feet would be traveling the same speed (i.e. – lower cadence/RPM but a longer circumference). Unless the rider was limited by flexibility, I doubt it would make a huge difference in the amount of power generated, and hence torque at the rear wheel. Torque at the BBKT would obviously be higher with the longer cranks.

[dekindy]

IMO that’s an unfair comparison. I’d agree it’s doubtful your cleat ever wore down 2.5 MM but that doesn’t mean the cause of knee pain can not be due to something other than the effective length of the crank. Something simpler like the angle of the shoe or extra play is more likely to cause problems.

Besides, do worn cleats change the functional length of the cranks, or do they remain the same?

The cranks would obviously stay the same length. But with worn cleats your leg is straighter at the bottom of your crank stroke, everything else staying equal, which is the same as lengthening the crankarm. Since I ride thousands of miles and the only time my knee has hurt has been when my cleats were badly worn, then that is the culprit since they quit hurting after getting new cleats. You are, his chart lists 6 other possible causes of frontal knee pain.

According to Chris Carmichael one of the possible causes of anterior knee pain (front) can be excessive crank length.
Ulitmate Ride, p. 165.

Crank length has obvious implications for saddle height. Any change in crank length requires a corresponding change in saddle height. Ultimate Ride, p. 172.

Crank length can be generally recommended as a function of frame size. Time trial cranks are typically 2.5 to 5.0mm longer than road cranks. p.172

chart on p.173 indicates < 54cm = 167.5-170mm; 55-57cm = 170-172.5mm; 57-61 = 172.75-175mm; > 62cm =175mm. p.173.

[dekindy]

Erick, I think you asked rhetorically, but for those who may not know:

Power is a function of torque multiplied times RPM. If you increase one by 5 % and reduce the other by 5% you end up almost in the same place.

As it relates to "gearing" and therefore torque at the rear wheel -- which is where it counts the most -- it’s safe to say that using 180 vs 170 MM cranks is like the difference between a 17T and an 18T rear sprocket.

Sheldon Brown’s gain ratio takes crank length into account in that it compares how far the bike travels relative to how far a pedal travels. If a rider rode the same 20 MPH with 180 MM cranks and 17T versus 170 MM cranks with an 18T, our feet would be traveling the same speed (i.e. – lower cadence/RPM but a longer circumference). Unless the rider was limited by flexibility, I doubt it would make a huge difference in the amount of power generated, and hence torque at the rear wheel. Torque at the BBKT would obviously be higher with the longer cranks.

That is about as clear as I have ever seen it explained. Could we simply say that changing the crankarm length effectively changes the gear ratio? I would have to read more to refresh my memory as to why time trial cranks are recommended to be longer than regular road cranks. It probably has to do with the body position being different and the resulting changes that causes.

[Ti Designs]

Can you walk "and" run? Seriously.

I’d agree with your logic except it isn’t supported by nature. As an example, when humans and most animals walk or run, we tend to take longer steps as we simultaneously increase cadence to go faster, so the premise that somehow we should train our muscles to fire precisely in order to support only one predetermined sequence of events seems illogical.

Poor example. With running or walking the foot is free to move in any direction while it's in the air. If you take someone with a significant leg lenght difference and plot their stride on a treadmill you'll find it's staggered. The same condition causes pain over one hip on the bike because the travel of the foot is controlled. You're comparing an activity with free motion with one where a person must interface with a machine which controls movement.

My claim is that the most efficient pedal stroke is an accurate one, even at high intensity. I think humans have the ability to be that accurate - at least the idiots who practice things like piano and pedal stroke are.



The idea that someone can predict an exact perfect crank length for a given rider in a matter of hours seems a little questionable from the onset.

It's not so much predicting the perfect crank length as it is picking one that fits into the rider's parameters (range of motion and femur length) and then having them learn the pedal stroke. People can at least understand that there's something to the first part, very few people spend any time on the second part.

[regularguy412]

I'd like to add to my original post that at the same time I moved to 172.5's, I also changed my position on the bike substantially from where it was before. I now have a much more rearward position in that, my knee is (for me) pretty far behind the pedal spindle. I have a fairly horizontal body position; not a perfectly flat back in the drops, but almost. I'm also somewhat barrel-chested, so with the longer crank, my femur comes up and touches my chest at the top of the pedal stroke. I can't use a longer crank with my position as it is, because my body would have to rock to clear my femurs on every stroke. If I had retained my old, more forward position, I could use a longer crank because my chest would not get in the way of the pedaling motion and my saddle could be marginally higher.

As has been stated in other posts on this thread, crank length choice is a combination of body geometry, position on the bike, pedaling style and cadence efficiencies.

Mike in AR:beer:

[dekindy]

Oops, wrong thread.