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Crank Arm Length
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This has been probably asked to death and then some but please bear with...

So I've recently changed from 172.5 crank arm lengths to 170mm on my Time Trial bike and seem some positive improvements. Cadence is up, power arc is more consistent and overall times are down.

I was wondering on moving further down and using 165mm on the TT bike and 170mm on the roadie. The roadie I'm not concerned about, its the TT bike I want to get the cadence up from 80rpm so the two choices are to reduce the chainring size from 55t to 53t, not a fan of as I find the 53 a bit gutless so my next stop is the crank length.

I've read and read and read until the cows come home and the consensus is that it will achieve what I want, with dropping the saddle height slightly.

Whats the forums thoughts on this?
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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Your first assertion was correct. This has been asked to death. And recently as well. A simple search will show that.

But that aside, chainring sizes aren't about "guts". They're about whatever puts you in the right range for your target cadence and power. As long as you stay on a line of thinking that associates them with your self-esteem, you are going to hold yourself back.
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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Also, if you shorten the crank arms, you would raise the saddle, not lower it.

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Re: Crank Arm Length [kaihirst] [ In reply to ]
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If you reduce crank length the then you should be reducing chainring size correspondingly and increasing cadence. It's all interrelated.

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Re: Crank Arm Length [JoeO] [ In reply to ]
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Mate, I have read every article possible on this and come up with nothing other than its down to inside leg length.

What I'm looking for is examples of peeps that may have switched to shorter lengths. I've already switched down and seen some positive results on going smaller by 2.5mm so I'm wondering if there are any more benefits on moving lower by 5mm.

Self esteem doesn't come into it at all. At the end of the day I can run a 56t and do so in winter in training with no issues. What I'm looking for is a more efficient delivery and am focussing on crank length...
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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I switched from 172.5 mm to 165 mm in 2014. Haven't looked back since.

I didn't lose any power on my tri bike but it was more about being able to get lower on the front end (or more drop from saddle to pads) without closing off the hip angle too much. Although I don't have any wind tunnel testing to prove it, but I feel my CdA improved after switching to shorter cranks which allowed a faster position.

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Re: Crank Arm Length [brider] [ In reply to ]
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Not necessarily, mine stayed the same. Lost a little extension at the bottom of the stroke which had little to no impact on sustained power, but lower front end and more comfortable back position.
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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I'll give you my example, I went from 175's to 155's, and couldn't be happier. My aero position got way more comfortable and more aero. Dont have a power meter, but safe to say no loss of power. And yes, you raise you saddle when you go to shorter cranks, usually the amount you shorten, you lengthen on the seat post. If it is a lot like mine, I also went a bit forward on the saddle too..
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Re: Crank Arm Length [JasoninHalifax] [ In reply to ]
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JasoninHalifax wrote:
If you reduce crank length the then you should be reducing chainring size correspondingly and increasing cadence. It's all interrelated.
Yes, its all interrelated but not quite as simple as one might expect. If you go to shorter cranks then obviously you have a shorter lever arm and therefore will produce less torque for any given force. But, if you raise your saddle so that you keep the max extension the same from crank to crank, then something else happens. Your leg will be more extended throughout the cycle (except right at the point of max extension). With a more extended leg, you can produce more force. Think of a partial squat vs. a full squat. So, these two factors, shorter lever but stronger leg, combine in such a way that you only need to reduce gearing by about half the difference in crank lengths to maintain things. For example, if you went down by 10% in crank length, you would only need to go down in chainring size by 5%.
Cheers,
Jim
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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I've been playing with crank length for four years now and from starting at 170 I tried as low as 150 but found too short and settled on 160 for two seasons but my last few rides they felt too long so did my first ride 4hrs on 155 today and loved it. Over that period with shorter cranks my cadence has dropped on average of about 16rpm there about and I have gone to a bigger chain ring. Read this article.

http://www.cyclingutah.com/tech/crank-length-coming-full-circle/
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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You've gone to 170 and like it. You're considering going to 165 - here's why you might do that.
If you have a low handle bar elevation, like 10-14+cm and even now, with 170s you still feel a slight impingement in your hip as you come over the top of the pedal stroke? Then go shorter.

Many of us have (see Monty's post...and others) and have loved it.

What I find interesting is when an athlete is hesitant to go to 165 but in fact if they went to 160 or 155 they'd love it even more.

Ian

PS. I'm not just making this a study of one. I've been a professional bike fitter for over a decade and teaching fit for 6 years. Shorter cranks isn't a fad or a mystery - it's logical.
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Re: Crank Arm Length [ianpeace] [ In reply to ]
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I agree with you 100% but I found that a to bigger jump too soon may not be the wisest. Each time I dropped 5mm felt amazing until past 160 and 150 felt too short and I struggled at times to stay in sync with my pedalling. As I posted in a previous post recently 160 didn't feel as efficient any more after over two years and that shorter would be better. I put 155 on last night, put the seat up and a little juggling with the setback but my pedal efficiency felt amazing. Hence it may be an expensive process but I would work my way down and a lesser adjustment in bike position each time. I think the muscle memory you have for pedalling comes into play especially when it comes to fatigue and running off the bike.
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Re: Crank Arm Length [kaihirst] [ In reply to ]
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1. The only way to intuitively understand the benefits of shorter cranks is to have a competent fitter utilizing proper equipment take you to the limit of your current crank length before making a change. In other words, if your fit is not at least in the ballpark of optimized, you probably won't notice much, or even worse, you might have a different experience than if you were close to optimal.

2. When done as described above, about 80-90% of riders are going to prefer a crank length from .5 to 1.5 cm shorter than what would typically come on a stock bike in their size.

3. Many of the things previously mentioned are important, especially riding a lower / more aero position, but the first thing the fitter and rider need to determine is "Do you prefer this?". It is the same question asked when seat height, setback or drop is changed.... How does this effect your intuitive sense of pedaling/comfort? After which, we can look at the power, how much drop you can ride, etc. First is always rider feedback.

4. There is no perfect crank length, or if there is I don't know how to find it. I do know how to find an acceptable range for a given rider. Acceptable here is synonymous with appropriate, and NOT the same thing as aggressive. The #1 thing I look at is how close to 100 degrees of hip angle can we achieve with this rider? Is their lack of mobility or excess belly fat getting in the way of this angle? If we have a lean and reasonable limber rider who just can't seem to pedal effectively when we dip below 105 degrees of hip angle, crank length absolutely becomes the prime suspect.

5. Crank length is often the most important metric I change for riders under 5'6" or so. The shorter the rider, the more thigh-torso clearance and knee flexion are potentially compromised at the top of the pedal stroke. The majority of such riders are what I term "massively over-cranked". These riders typically reduce crank length from 1-2cm, with the shortest commonly available length of 145mm (Cobb) being frequently used for riders under 5'3".

6. Generally, not one watt is gained or lost from a crank length change. Now if the crank was so long that it was basically contributing to a lousy fit, sure, we will probably find some power, but that is more a function of fixing the overall fit. This leads me into the final points...

7. Crank length IS fundamental to your bike fit. Seat height, setback, reach to bars, drop to bars are your primary fit coordinates. Nail those four and you've got a fit. Saddle selection, aerobar shape/tilt/width, cleat position etc are all secondary factors. Crank length is the rug that ties the room together. What this means is that if we determine 160mm is in your ideal range, your bike fit still exists on 175mm cranks. It is not optimized though. Saddle height will change, drop will change, set back will change and even reach may change with the less optimal length, but the fit isn't completely ruined by the wrong cranks. If cost is an issue, we adjust the saddle, ride a bit less drop and advise to change the cranks when you can.

8. Adaptation time for changes in crank length is about 3-5 minutes. Seriously, it just disappears. Which somewhat addresses the question, “Do I need the same length crank on my road bike.” Generally, no. For ‘cross training’ triathletes, road bikes tend to be more of a laid back affair. Specifically, we are not trying to ride in our most bent over, reduced thigh to torso positions, as we do on the tri bikes. So it is not usually as big of an issue. Which leads to the final point...

9. Crank lengths on road bikes have been around for over a century, and generally work well. The problem with crank length arose when we transferred those historical lengths to commensurately sized triathlon/time trial bikes, without realizing the inherent difference in those styles of bikes. For those riding a road bike set up low, in a racing position, an exploration of crank length could be useful. Keep in mind the one tangible drawback that I have found with shorter cranks is a reduced ability to perform out of the saddle climbing. This seems to be a situation where one lever (crank length) comes to the forefront, and diminishes our sense of the system of at least six levers* used in propelling the bike.

*Foot, crank, lower leg, upper leg, gears, spokes.
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Re: Crank Arm Length [Bio_McGeek] [ In reply to ]
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Bio_McGeek wrote:
JasoninHalifax wrote:
If you reduce crank length the then you should be reducing chainring size correspondingly and increasing cadence. It's all interrelated.

Your leg will be more extended throughout the cycle (except right at the point of max extension). With a more extended leg, you can produce more force. Think of a partial squat vs. a full squat. So, these two factors, shorter lever but stronger leg, combine in such a way that you only need to reduce gearing by about half the difference in crank lengths to maintain things.

That's the exact analogy I use with my clients! I know I owe you a discussion on this and my belief that knee flexion is the primary driver of crank length. I apologize I haven't been able to begin one; it's been a crazy busy Spring.

Jim Manton / ERO Sports
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Re: Crank Arm Length [Jim@EROsports] [ In reply to ]
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Jim@EROsports wrote:
That's the exact analogy I use with my clients!
Excellent! Seems to be easily digestible when explained this way. If I start talking about biomechanical gear ratios most people's eyes glaze over. The flip side of being stronger with a more extended leg is that your leg, as a unit, is also slower. But don't go there; graduate level muscle phys and biomechanics required!
Jim@EROsports wrote:
I know I owe you a discussion on this and my belief that knee flexion is the primary driver of crank length. I apologize I haven't been able to begin one; it's been a crazy busy Spring.
No worries, glad to hear business is good! I do want to hear your thoughts on this because I am giving a talk on crank length at ISCO in Muenster in Nov. I'll be covering crank length research and how it applies to bike fitting and aerodynamics, as well as the power-aero trade-off.
Cheers,
Jim
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Re: Crank Arm Length [monty] [ In reply to ]
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Monty,
Do you still run 52/34 chainrings with those?

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Re: Crank Arm Length [kaihirst] [ In reply to ]
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kaihirst wrote:
Mate, I have read every article possible on this and come up with nothing other than its down to inside leg length.

What I'm looking for is examples of peeps that may have switched to shorter lengths. I've already switched down and seen some positive results on going smaller by 2.5mm so I'm wondering if there are any more benefits on moving lower by 5mm.

Self esteem doesn't come into it at all. At the end of the day I can run a 56t and do so in winter in training with no issues. What I'm looking for is a more efficient delivery and am focussing on crank length...

2.5mm ???
you know how much is 2.5mm? what kind of advantage would that give you?

i moved from 175mm to 160mm and i can see clearly that now my knees are far away from my elbows on the up phase. that is the most significant advantage i noticed (less compressed on the upper part)

i think you need to make a bigger step than 2.5mm

my cadence, except the first 2-3 months where it raised by 10rpm as average, after a year, it went back to previous averages.
they claim, you can run better...hmmm I don't know, i did not notice that.
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Re: Crank Arm Length [docfuel] [ In reply to ]
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Monty,
Do you still run 52/34 chainrings with those? //

Honestly I see this all the time, and those numbers are just gears, matched with the rear ones gives you gear inches and ratios. Shortening your cranks does not make you magically stronger or weaker, so whatever gears you were using and were comfortable with, you just keep them. If you think perhaps you are going to spin a couple more rpm's, you might lose a tooth on the front chainrings, but really probably makes no difference what so ever.


I have been on compact cranks for a very long time now(since they came out) which just means I can use smaller front chainrings. People like to confuse compact with something else too, but all it does is give you the ability to use smaller gears, and thus closer ratios(smaller jumps from gear to gear)


I figured out very quickly when I retired from being a pro, that I do not need a 53/54/55+ front chainring. I run a 50/52-- 34/32 front set up. I calculated out that would be plenty for me to ride a 55 40l TT, which of course I dont come close to anyone, so it is for tailwinds and downhills now. Rear cluster is an 11/24 and I ride 26" wheels, so all my gears are a bit smaller than the rest of you on 700c bikes. I prefer to spin to get more power, not to push harder..



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Re: Crank Arm Length [kaihirst] [ In reply to ]
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In January, I went from 175 to 165 on my TT bike. The first 4-5 rides weren't that awesome as I was dialing in the seat height. It was not an exact 10 mm raise the seat height thing for comfort. I jacked around with it by 1 mm at a time after the initial 10 mm raise. Then I dropped like 3 mm, then up to 9, down to 8--and I may have changed it to 7.5 or 8.5 after that. Either way, it just took around a week to really feel good on it & adapt. I really didn't notice much of a difference rpm wise from the crank length change aside from the relief in the hip flexor and top of the downstroke. It really was a move I should have done a long time ago as I'm riding way more comfortable now.
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Re: Crank Arm Length [monty] [ In reply to ]
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I know they’re just gears, but people used to say you couldn’t have 18teeth between the big and small chainrings. I thought I remembered you having that big jump.

I completely forgot that you have 650s. I didn’t know anyone was making a 650 in your size anymore. I recall Dan saying that he thought 650s were more aero.

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