Crank Arm Length?

[Max]

Is there a rule of thumb,when selecting the crank arm length?

[Max]

I found what I was looking for

http://www.airborne.net/eready/janette/store/sizing_step3.asp

[Kevin]

Max,

Below is a website with crank information. The general rules for crank size are as follows:

Frame Size---------Crankarm---Inseam----------Crankarm
54 cm or less------170 mm-----80 cm or less---170 mm
55 - 58 cm--------172.5 mm---81 - 86 cm------172.5 mm
59 cm or greater--175 mm------87 cm or above-175 mm


http://www.cptips.com/crnklth.htm

Kevin

[Too Tall]

Don't want to sound cynical but that one size fits all *this size is misleading. Femur length varies widely even though inseam may be the same....see the problem? Bill Boston is a pretty savy fit man and he has a reasonable method of fitting cranks based on femur length....in brief you measure femur lenght in inches and choose cranks in centimeters. Hey, it's just a way to view the problem! Try it. If you have doubts, your best bet is a fit expert.

[Smiley]

Shimano just added a new 177.5 crank length to go along with their 180 mm length. Too Tall is correct there are more choices today , gear mashers will love the choices.

[Bruce K]

Kevin;

The chart/info you posted is interesting.

I have had 3 different fits done by 3 different fitters over 4 years. One for original Concours purchase, one 2 years later to verify sizing, and one "pro fit" prior to the Ottrott.

All 3 came out the same with one guy differing on stem length (10 mm longer). I tried his idea and dumped it for the original and pro fit versions.

The point is, based on an inseam of 78-80cm and a frame size of 53, your info suggests a 170 crank. All 3 fitters suggested 172.5 and that is what I have been using.

I guess there must be something in my body dimensions that puts me on the longer cranks but I can'r figure out what that might be.

BK

[Kevin]

Bruce K,

I can't take credit for the chart, I just lifted it from another source. I also agree that the chart simply provides a starting point for looking at cranks. The chart is certainly not the bible for cranks. According to the chart I should just sqeak into a 175. However, I use a 172.5 as fitted by my LBS. I would certainly take the advice of my fitter over the chart. I find the shorter crank better because I am more of a spinner and have knee issues.

Kevin

[Matt Barkley]

I believe shorter cranks are better in more situations than longer cranks. If you are new to cycling - go with a shorter crank so you will more easily learn to spin - and develop a pedal stroke with the smaller circles - all tends to keep riders in the smaller gears and therefor higher cadence results. Also shorter cranks for some of the same reasons are less stress on the knees. Shorter cranks allow longer sprints and more efficient endurance riding.

Having said, what size crank? We have 165 through 180 available. 170 through 180 (or 175) are the most common. 172.5 being the average. I usually say unless you are TooTall start out with 170s. If you have a nice pedal stroke and want to use your slower cadence power go for 172.5 or even 175. TooTall and long femure gals/guys, depending on your height start with 172.5 or 175 and go up.

Lance is going shorter and shorter on his cranks (started out 175 always - now is 172.5 ot 170), but remember Ulrich is longer (up to 177.5 for climbing stages, and maybe tts) and Ulrich got 2nd. In other words he would have won if Lance wasn't there - so his style is diferent and damn good.

[Too Tall]

Right on! I'll take a set of 190's. Matt, just for giggles I sprint the top of Great Falls climb for ~ the last 50 yards and regularly hit 145-170rpms. Would I spin faster with more watts with shorter cranks?

My old track bike has 170's and a 68" gearset. It feels like I can not make good watts.....muscle memory issues?

If this adversely affects my freakfactor, I'll retract the last statement.

PS - Maybe you should come by my house...we have a shelf full of cold Belgians.

[zap]

Jan is bigger than Lance. So in addition to Jan pedaling at a slower cadence than Lance, it would make sense for Jan to use longer cranks.

I started out with 170 and quickly moved up to 175. My lieght weight project (climbing) bike has 177.5. I'm 6'1" and try to keep my cadence around 90.

I started spinning more last year but have given it up. As Too Tall mentioned, muscle memory probably has something to do with it. I also don't have Lance's patience to train my body to spin faster.

For those that have the means, there is a carbon crank out of S. Africa that allows you to change the crank length by moving the pedal up or down the crank arm. I believe Pezcycling.com has a review.

Too Tall, keep your cranks. You don't need to spin any faster! I don't know what turns faster, a circular saw blade or Too Tall's chainrings when he's cranking it out on rollers!!!!!

[MadRocketSci]

I think that shorter cranks should allow you to have more bend in the waist, since your knee won't be coming up as high as with longer cranks. Better aero, more power?

[Ken Robb]

see my sizing info on post about saddle height but at 6'1 I started on 175 cranks, then got one bike with 172.5 and 2 bikes w/170. I expected that I would want to swap out the 170s. After quite a few miles I think they may be my favorites. I don'tnotice a lack of leverage as I had feared. This may be due to them keeping me from bending my knees farther than my most powerful range of motion. While I sometimes notice a slight tenderness in my knees after a hard ride on the 175s I've never have a twinge after riding the 170s. The 172.5s are on my wife's bike so I don't have enough miles on them to say if they might be the magical compromise.

[Russ]

This has always been a much discussed topic. This is what I have noticed:

On the average European peloton, I have always seem riders with either 170's of 175's (more so in the 80's and 90's, than today). Also, I know that riders went with larger cranksets for time trials. When I rode amateur, I started with 172.5, but soon moved to 170's and that is what I use today...

I have a 81.5 inseam and a 52.5 frame... So the formulas given above don't quite match my preference. IMO, however, I have a short femur for a cyclist.

I find 170 cransets more "comfortable" for spinning and overall better leg movement. Recently, I have occasioanlly ridden on 172.5's and I don't think I was bale to feel as good.

But once again, this is all personal.

[sharky]

the rule of thumb is: 20.8% of inseam, +5mm for long femurs, -5mm for short femurs. also considered is ankling and toes pointing down while pedaling

[Keith A]

As Russ says "this is all personal" with everyone having their likes and dislikes.

My CSi is a 54cm and I'm using 172.5 and would consider myself a spinner with my average cadence between 95 - 105. I started off with 170's and rode these for years. Then I bought a new bike that came with 172.5's and thought I'd give them a try and see if I liked them or not.

At first, the 172.5's felt uncomfortable and my spin didn't feel smooth. However, there was noticeably more leverage and I liked the cranks when sprinting and climbing hills. So I persisted and adjusted to the new length and am very happy with the way they feel.

I still have the bike that has the 170's on it and take it out for an occasional ride. Every time I do this, I feel like I'm not getting anywhere because of the reduced resistance. I'm sure that I could adjust back to the shorter length, but I'm not interested in trying.

[MadRocketSci]

here's another take...

http://www.cranklength.info/

[Keith A]

Originally posted by sharky
the rule of thumb is: 20.8% of inseam, +5mm for long femurs, -5mm for short femurs. also considered is ankling and toes pointing down while pedaling

So what is consider a long or short femur?

[Too Tall]

! Good question.

[Russ]

Originally posted by sharky
the rule of thumb is: 20.8% of inseam, +5mm for long femurs, -5mm for short femurs. also considered is ankling and toes pointing down while pedaling

Where did this formula come from?

I seem to be fine at 20.9 (almost like Hinault, oh boy) until I add the - 5mm. Then it gets kind of funny.... I just don't see myself on 165's!

[Russ]

Originally posted by Keith A
So what is consider a long or short femur?

Yeah, what about "short" femur and ankling (which is what I do)
or "long" femur and "pointy toes" technique...

In the web site http://www.cranklength.info/crankCalc.htm They do not have this equation.

[sharky]

I think (can't remember where I read this) that a long femur is considered one that the distance from crotch to knee is greater than knee to ankle ?

[Richard]

This is a very old memory, but I believe the average ratio of femur to tibia is around 1.05 to 1.10 or so and greater than that is a long femur