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Discussion Starter · #1 ·
Im looking at upgrading my current crankset. One of the many choices i face is the arm length. Ive searched and searched for a reliable/credible source for deciding on which one or how to decide which length to go with. Many sites or forums Ive checked mention 160mm or 190mm arm length, both of which i have never seen.

It seems as though the only arm lengths available are 170, 172.5, and 175mm. So what is this 160 or 190mm business, a bunch of nonsense?

How do you choose the right arm length?
 

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Look harder--there's a bunch of stuff out there on this.

As I recall, there is a lot of disagreement (like there is about KOPS).

Conventional wisdom is that if you're shorter or building a fixie, go for 170mm. If you're extremely short (think 650mm wheels), get a 165mm crankset. If you're medium build, consider 172.5 mm. If you're 6' or over, go for 175mm.

Most bike buiders will chose the conventional-sized crankset for the size frame they are putting together. So your old crank should be your guide.

I'm 6'4" and use 175mm cranks on my mountain, road, and hybrid bikes; 170mm on my fixie. You don't want to use longer on a fixie for fear that you'll hit a pedal in a turn, and it will lift the rear wheel and cause a crash. (Not an issue if you have a freewheel, 'cause you can level or raise the inside pedal in the turn and hold it there.).

How tall are you?
 

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Discussion Starter · #4 ·
drewmcg said:
Look harder--there's a bunch of stuff out there on this.

As I recall, there is a lot of disagreement (like there is about KOPS).

Conventional wisdom is that if you're shorter or building a fixie, go for 170mm. If you're extremely short (think 650mm wheels), get a 165mm crankset. If you're medium build, consider 172.5 mm. If you're 6' or over, go for 175mm.

Most bike buiders will chose the conventional-sized crankset for the size frame they are putting together. So your old crank should be your guide.

I'm 6'4" and use 175mm cranks on my mountain, road, and hybrid bikes; 170mm on my fixie. You don't want to use longer on a fixie for fear that you'll hit a pedal in a turn, and it will lift the rear wheel and cause a crash. (Not an issue if you have a freewheel, 'cause you can level or raise the inside pedal in the turn and hold it there.).

How tall are you?
I am 5' 10'' , so I should go with 172.5?
 
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At 5' 10" I prefer 172.5 but rode 170's for quite some time, you probably won't notice that much difference. But my guess is unless your legs are unusually short for that height that the 172.5's will be fine.
 

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Definitive length advice

adamgvc said:
I am 5' 10'' , so I should go with 172.5?
You should go with what you like. It's personal preference, and nothing more.

Some reading:

The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.

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Fred Matheny Summary: There have been studies of crankarm length, but the results aren't consistent. Some show that longer cranks provide greater leverage for turning big gears. Some show that shorter cranks foster greater speed via a faster cadence. And some show that crank length is completely individual.

So, longer crankarms aren't a panacea for time trialing. In fact, there are dangers associated with them. The added length makes your knees bend more at the top of pedal strokes and extend more at the bottom -- both of which can lead to biomechanical injuries if you jump from 170 mm to, say, 180 mm.

Also, longer cranks reduce cadence -- and a brisk cadence is the key to good time trialing.

All this said, many time trialists use crankarms 2.5 mm longer than those on their normal road bike. Because 2.5 mm (one-tenth of an inch) isn't much, it rarely causes an injury. But the jury is still out on whether that bit of extra length actually improves performance.

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Jim Martin tests as reported in VeloNews: 16 bike racers of various heights doing maximal sprint power tests of under four seconds duration on cranks of 120, 145, 170, 195, and 220mm showed no statistical difference between crank lengths. Seat height to the pedal was maintained throughout, but fore-aft saddle position and handlebar height were not readjusted with crank length changes, despite variations with crank length of pedal-to-knee relationship and saddle-to-bar drop. This also led to Martin’s assertion that he could see no point to positioning the knee over the pedal spindle.

Further Martin tests showed no statistical relationship between metabolic cost and either pedaling rate (RPM) or crank length, using nine trained cyclists riding 145, 170 and 195mm cranks who pedaled at 30-, 60-, and 90 percent of their lactate threshold at 40, 60, 80 and 100 RPM. On the contrary, power output and pedal speed (pedaling rate times crank length), accounted for over 98 percent of the variation in metabolic cost.

In another test, Martin had 10 racers perform a 30-second maximal sprint on 120mm and 220mm cranks at 135RPM for the 120mm and 109RPM for the 220mm. he found that, while the rate of fatigue was less for longer cranks, the fatigue per revolution was identical. This led him to suggest that track sprinters, rather than spinning at high RPM, should select the gear at or just below the one at which they produce maximum power output. The higher gear, as fatigue per revolution would be constant, would get the rider to the finish sooner, as fatigue would take more time to set in.

Finally, Martin’s studies of pedaling technique indicated that regional cyclists had “better” pedaling mechanics than elite cyclists. It indicated that elite riders pull up less on the pedals on the backstroke and push down harder on the downstroke.

By studying 13 trained cyclists and 35 fit athletes who did not own bicycles, he also showed that non-cyclists, who started out lower on the first day, produced higher power outputs by the 4th day than trained cyclists. They also hit their maximum power at a higher RPM than the cyclists. The total time to learn to produce more power by the non-cyclists was three days and a total of 36 seconds of hard pedaling! This seemed to dispel the ideas that cycling adaptation takes time, that pedaling technique refined over time is important, particularly to learn to pedal efficiently at high RPM, and that avoiding “working against yourself” on the backstroke (revealed in graphs showing a net negative torque past bottom dead center) is useful.
 

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Larry Lackapants
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I'm 184 cm tall and 70 kg heavy and use 170 mm cranks. At some point on a hill near by (15-20% short climb) I just feel that I can't propell the bike forward even though standing on the pedals (39x25). It's the only place I feel I'd rather have longer pedals. Although I'm quite sure that my technique for pedalling out of the saddle sux.
Besides, I'm quite sure that spinning on the flats would be somewhat negatively affected by longer cranks.
 

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I'm 5'10" with somewhat short legs for my height (32" inseam).

I've tried 175mm cranks... they felt too long and unwieldly. 172.5mm was perfect though.

It's amazing how much of a diff 2.5mm can make with crank length... though I guess to be fair the 'crank circle' was altered by 16mm by the change, which sounds less trivial.

Whatev. I could definitely feel the difference, am happy with the result.


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I've got bikes with 175, 172.5, and 170. I used to think I could tell the difference - I don't know if that's the case any more.

175 - regular road crankset
172.5 - cyclocross crankset
170 - compact on CX bike

I used to be really picky about that stuff. I'm a spinner though, so I'm probably better off just a little shorter than I need anyway.
 

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the difference between a 172.5mm and a 170 and 175 is 2.5 mm that is about 1/10 of an inch. The thickness of 2 nickles. 2 brands of pedals could vary that much in height, orthotics could give that much variation, thick socks could get there, different brands of shoes can get there.

I use 180's on my SS and barely tell the difference. You gain some ground clearance going shorter, especially in extreme cornering but otherwise if is 2.5mm

Supposedly shorter cranks spin up quicker and longer cranks give better leverage, you just have to decide if you are that sensitive or in need of that little of an advantage before buying cranks, otherwise just replace with what your bike came with (unless you are a giant then get as long of cranks as you can).
 

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rockcrusher said:
the difference between a 172.5mm and a 170 and 175 is 2.5 mm that is about 1/10 of an inch. The thickness of 2 nickles. 2 brands of pedals could vary that much in height, orthotics could give that much variation, thick socks could get there, different brands of shoes can get there.
And yet, small amounts can make a diff, surprising as it sounds.

Bernard Hinault suffered a major injury during his racing career by having his seat accidentally set just a few millimeters too high. He was out for months.

The body's very adaptive usually, but it's touchy about some things.

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