Question about crank length

[cloudguy]

I need a new groupo after a crash, and I've come across a pretty good deal on craig's list for a brand new DA set with 172.5 cranks. Problem is I have a 30" inseam, which from what I've gathered on the net means I should ride 170.

How big of a deal is this 2.5 mm, in terms of fit, comfort, etc? Anyone notice a significant difference in knee pain?

p.s. I just checked my old cranks, and it turns out they are 172.5 too, so I guess what I'm asking is if I should keep this length or go down a size.

Thanks in advance.

-CG

[soulspinner]

Buy and adjust to original seat height.

[Pete Serotta]

Buy them and ride them....they will be fine. In theory the shorter crank will assist in spinning if you are a spinner and a long one is better for leverage.


The reason I say theory is that I am far from an expert but have used 172.5 and 175 and 170 over the years. My favorite is 172.5 with a 31 inch inseam.

Campy for their triples only offers 170 and 175. so I went with the 175.


You did not mention you were training for a special timed event or that you had any problems with your old crank so you should be fine.

[rustychisel]

totally agree. If you're especially sensitive you may notice the difference for, oh, less than a week.

[dancinkozmo]

...i dunno; ive got a 32" inseam and after trying 175 cranks , experienced sore knees after every ride. switched to shorter cranks and the soreness disapeared. i read in the Zinn book that the bigger circle you pedal through with longer cranks is more stressful on the knees.
that being said, i do have brittle knees... you're probably going to be okay with the 172's if youre younger and have good knees.

[Ken Robb]

[QUOTE=cloudguy]

How big of a deal is this 2.5 mm, in terms of fit, comfort, etc? Anyone notice a significant difference in knee pain?


Do you mean you have had knee pain for a while and kept riding?

[salem]

In addition to how sensitive your knees, I'd say another factor is how much you ride. I have a 34" inseam and I ride 170mm cranks, but I also surmise that femur length (mine are short, or rather my lower leg is long) is far more important than total inseam. The lower leg doesn't contribute to the up down range of motion, so it really the upper leg is the critical number.

Back on subject, I find the 170 cranks are much easier on my knees and my legs feel fresher at the end of a ride because I am not starting my power stroke with my leg in as deep a bend (think of how squats get harder the lower you go). This was particularly noticeable when touring across PA on a 45+ pound loaded bike with 175 cranks, doing three consecutive 120 mile days. My knees fell apart, while I've done similar efforts on 170s without a hitch. Granted, that example is a little extreme, but the point is your body will handle some amount of mis-fit to a point. For what I do, I won't touch over 170--knee surgery if far more expensive than new cranks.

[slowandsteady]

1) Read the ZINN article on crank length
2) Then go look at a ruler and tell me how big that 2.5mm really is (pretty darn small eh?
3) Does 2.5mm makes any difference...only if you are so dialed on your ride in it is scary otherwise you have more play in your shoes, cleats, socks etc. than the 2.5 mm you will never notice in your cranks - 5mm might be another story.

[Satellite]

Cloudguy,

Someone somewhere on the forum once posted he inadvertently had mismatched crank arm lengths on his road bike and never even knew it. First, I say wow and second I say if he didn't notice the difference while riding with a mismatched set why would you notice the overall change with a matched set? Try'em and see if you like them if you don't it is an easy problem to correct; buy'em off eBay and re-eBay the old ones, more than likely it would be a one-for-one trade (if you don't sit on them like I did read below).

BTW this is a very legit question Serotta’s big push is fit, what could possibly be more important than the crank (ok maybe the frame sheez). I have spent hundreds of dollars on cranks for one bike. Mine came with 10sp D/A 175’s, I bought D/A 172.5 (I did notice a difference) and then I decided to go compact right after the purchase and bought FSA K-Force Light Titanium Crankset in 172.5. I eventually eBay’d the D/A cranks but I only got a fraction of the initial investment back; but I do love the ride.

Satellite

[znfdl]

I have a 30 inch inseam and have been riding 172.5mm cranks since 1990 with no issues.

[Ti Designs]

If only fitting were that simple...

Thus far not a single word about lower back flexability or range of motion at the hip. I guess the only injuries or problems on the bike are knee problems...


Since most people here think that pedal stroke work is somewhere between voodoo and a total waste of time, I'll use the engine analogy, 'cause cranks and rods and pistons are easier to understand than the human body. So let's say I take my engine and replace the crank shaft with one that has more offset without compensating with shorter connecting rods. Somewhere near TDC Mr piston meets Mr head along with his kids the valves, and it gets ugly. It's easy to understand (for motor heads at least...) 'cause there are hard limits, and the damage isn't masked, you can't blame it on an oil leak or bad plugs or the dreaded misfire on cylinder #4.

On the bike, if the cranks are too long or the bars are too low, or a dozen other factors create a condition that you are outside of your range of motion at the hip as you pedal, nothing suddenly fails (we hope). The hip simply lifts a little and allows the pedal to come over the top of the pedal stroke - how bad could that be? Well first, instead of powering the bike you are now pushing one pedal over with the other - not good. Second, think about having your SI joint manipulated 80 times a minute for two hours. The muscles that fan across the back have to fire to stabalize the spine and your back probably lost it's sense of humor about things like this back when you were in your 20's.

Crank length on a bike is one of those subjects that most fitters avoid. When I went to Serotta Fit School they said "what's on their bike is probably fine". I spent most of my racing career trying to convince myself that longer cranks are better. It didn't work, my back is sensitive to constant movement, I'm on 170's on all my bikes. But here's a point I can't stress enough on this forum: What I'm on has nothing to do with what you need, My crank length, my saddle, the color of my bars - that's about what I need. You don't have my back (be happy about that), you don't have my legs (no, not looking to trade), why should you use the same bike?

So, here's my suggestion: Set up your bike on a trainer, get the thing level and have a huge mirror next to you so you can see both your body position and your whole pedal stroke. First warm up for a few minutes, then put it in a very small gear so it almost feels like you are pedalling air. So, it's body position first, we're looking for a flat lower back - no slouching!!! The way you know you have it right is when you can move down or up and the only thing that changes is the angle at the hip. Next, look at your arm position when you're on the tops - square shoulders, relaxed arms, right? The way you get out to the hoods isn't by rolling the shoulders forward and straightening out the arms - that's the most common mistake. First thing that happens is the angle at the hips changes, drop the back position, then drape your hands over the hoods. Your arm position should look the same as it was on the tops. Once you have this we can go onto testing range of motion across the whole range.

This is where things go all wrong, 'cause what people do and what they think they are doing is often two very different things. I only coach in person for this reason, but I've gotten this far...

So, back up to the tops, pedal the bike smoothly for a second (this gets the hips settled on the saddle and as level as they normally are). Clip out with one foot and place that foot on the trainer support at the rear dropout (I glue pieces of an old mountain bike tire to the trainer I use for this - it helps). Here's the trick - you're not going to be pedalling the bike with the one leg clipped in, you're tracing the circle the pedal has to go in. When you first start this you're probably going to notice a few things. First, without momentum your pedal never makes it over the top of the pedal stroke. Second, the pedal drops like a rock from 2:00 to 6:00 and third, there's a hesitation at the bottom. The hip flexors not brining the pedal over the top is because you've always let the other foot push it over. Now we're asking a very small muscle group to run it's full range of motion - don't over do it the first time, you won't be able to walk up stairs the next day if you do - don't say I didn't warn you! Next, that rush of speed at the pedal goes to the bottom is partly gravity, partly human nature. Everything you do is based on the ground - you're always looking for something under your feet. Ever been walking and not paying attention (texting...) and stepped down a step you didn't see? There's a real sense of panic when there's no ground under your foot. Lastly, there's the hesitation at the bottom, which is almost the same thing. In anything else you push off of the groud, cycling is the only case where you're asked to pedal in a circle. Pushing off at the bottom means the crank is held at it's lowest point for a second. OK, back to the test. Still on the tops, good posture, I want you to trace the pedal stroke with your foot at about 30 - 40 RPMs. It should be as smooth and consistant as a second hand going around, no speeding up, no slowing down, no using momentum to get the pedal over the top (This is where I stand there and say "slow down" about 20 times - watch the mirror, you'll see). The next thing you're going to notice is that it gets harder in an easier gear - how can that be? Your body knows where you limbs are based on muscles firing, if there's effort and tension it knows where your feet are. This is simply a learned sequence of muscle contractions, it has nothing to do with that - keep it in an easy gear. If you ever want to see how bad humans are at turning true circles, take the resistance roller off the back tire and try it - humbling, aint it???

Now that you have that (that alone could take a few days) take a look at your hip as you pedal with your hands on the tops. Does your hip lift every time the pedal goes over the top or is it solid on the saddle? If you can pedal with the hips staying level, bring yourself out to the hoods - remember, flat back, square sholders, hands draped over the hoods, and try the same thing. You've increased that static angle at the hips by bringing your position down, now you're checking if you are still within your range of motion. If you can still pedal without the hips moving up at the top of the stroke, we're good. If not, you need to take some angle out of the hips.

There are a few ways of taking angle out of the hips. Think of the static angle of the hips as the angle formed by the bottom bracket, hip joint and shoulders. We can open that angle up by moving the bars up and back, or we could move the saddle forward (which messes with lots of other things) or we can reduce the dynamic angle by shortening the cranks.

So right now someone is thinking "this can also be used to show me if I need longer cranks 'cause I'm not using enough of my range of motion", and this is trut, but fitting isn't that simple. You can't look at one measurment in isolation, it's all about the big picture. For example, longer cranks do have more mechanical advantage - it is a longer lever, but they also mean a greater range of motion. Longer cranks with the hip angle opened up means your center of gravity may not be over the pedals. Which one is better, more leverage or center of gravity falling on the pedals? That comes down to a question of riding style. Most riders will still find their center of gravity over the pedals when they try to go fast, which puts them outside their range of motion and causes back problems. Mechanical advantage is the crank arm length times the gear ratio, so you could use a smaller crank with smaller gearing and get the same thing while staying within your range of motion... And lastly, if you've read this far (probably talking to myself here), why not blow away the idea of the perfect fit? So from off season to mid season most people gain lower back flexability - something about warm weather and more miles. So, if you're set up for mid season position so you're just within your range of motionand your center of gravity is over the pedals, what happens in the winter when you ride the same bike in the same position, but you don't have the same lower back flexability?


Or maybe whatever crank length you have is just fine... Does it seem odd that on a forum where everybody is so up on fitting, yet so many people here have knee problems? (knee problems are a whole different rant)

[rugbysecondrow]

Or maybe whatever crank length you have is just fine... Does it seem odd that on a forum where everybody is so up on fitting, yet so many people here have knee problems? (knee problems are a whole different rant)

I've never had knee problems from biking, but I often tend to blame lack of cross training and strength training for knee and other types of pain. Maybe not 100%, but a contributer.

Best of luck.

[rustychisel]

If only fitting were that simple...

.... ...... ....... ............


Man you're good! Talk a lot, but good. :beer:

[gregclimbs]

<a whole lot of typing>
...but fitting isn't that simple.
<a whole lot more typing>

yes, it is.

It was Kyle who said (and I paraphrase here) the bike is a highly adjustable tool and the body is highly adaptable.

I guarantee you in a blind test on adjustable cranks you could NOT tell 2.5mm at the crank and would put money on it.

Even for someone 5'2" with a 28inch inseam the difference between 170s and 172.5s (5mm over the diameter) is less than 1% of the inseam length.

From a performance standpoint, Martin (et.al) has already shown that cranklength doesn't matter.

I find that anyone that is trying to sell such precision in fit (lasers, motion capture, sEMG etc) is merely trying to make money fitting people.

It is just NOT that precise a "science".

Sorry.

g

[jbrainin]

I have a 30 inch inseam and have been riding 172.5mm cranks since 1990 with no issues.

Me, too--and I'm definitely a spinner. I think that saddle height is much more critical than the 0.2" difference in crank length (diameter), anyway.

[Ti Designs]

I guarantee you in a blind test on adjustable cranks you could NOT tell 2.5mm at the crank and would put money on it.


You might want to check how many other people have lost that bet with me. I work on pedal stroke all winter long, I hold the record in all of the local roller races, I can tell the difference (but I'll take your money if you insist). The body is very adaptable, but limits of range of motion are still limits, and the body can learn very exact motions. I always return to the piano example. I play a Yamaha CP33, I can run a chord progression across 5 octaves in about half a second. A Roland RD300s is about 1/16" shorter across 88 keys. I can tell the difference any day of the week, weekends and holidays, with my eyes closed.

[JD Smith]

You might want to check how many other people have lost that bet with me. I work on pedal stroke all winter long, I hold the record in all of the local roller races, I can tell the difference (but I'll take your money if you insist). The body is very adaptable, but limits of range of motion are still limits, and the body can learn very exact motions. I always return to the piano example. I play a Yamaha CP33, I can run a chord progression across 5 octaves in about half a second. A Roland RD300s is about 1/16" shorter across 88 keys. I can tell the difference any day of the week, weekends and holidays, with my eyes closed.

This goes along with your other statement-"What I'm on has nothing to do with what you need, My crank length, my saddle, the color of my bars - that's about what I need. You don't have my back (be happy about that), you don't have my legs (no, not looking to trade), why should you use the same bike?"
Some people can tell, some can't. Blanket statements concerning sensitivity are meaningless. If you're fortunate enough to have found a comfortable efficient position, it maybe more likely you'll be sensitive to an uncomfortable, inefficient position. Some people find out the hard way, all by themselves, though trial, error, expense, and discomfort. A good fitter who doesn't tell someone what's comfortable, but is able to help them use their own sensations to find what's comfortable, is most valuable.
I'd think that a person who puts in 10k miles/year will have a greater opportunity to become sensitive than someone who puts in 3k/year.
I have one bike set up with 170mm cranks. I'm building up a similar bike. Due to not paying close attention, I jumped on an ebay deal for a 7800 crankset, later noticing they were 172.5. I'll see if I notice a difference.

[gregclimbs]

You might want to check how many other people have lost that bet with me.

Ok, I'll bite - how many?

I work on pedal stroke all winter long

Seems a waste since all literature suggests it DOES NOT MATTER. But hey, enjoy your winter.

I hold the record in all of the local roller races

What does that have to do with the cost of tea in china?

I can tell the difference (but I'll take your money if you insist).

Do I get to design the test?

The body is very adaptable, but limits of range of motion are still limits

And adjusting crankarms/saddle height so that max extension are equal between 170s and 172.5s does not come ANYWHERE near "limits of range of motions" unless we are talking a little person.

and the body can learn very exact motions. I always return to the piano example. I play a Yamaha CP33, I can run a chord progression across 5 octaves in about half a second. A Roland RD300s is about 1/16" shorter across 88 keys. I can tell the difference any day of the week, weekends and holidays, with my eyes closed.

Poor example. Impossible to do a blind test. So many other factors confound that example.

I stand behind my statement - anyone who over-emphasises the exacting nature of a fit is either blowing smoke or selling snake oil. Or both.

g

[Ti Designs]

I stand behind my statement - anyone who over-emphasises the exacting nature of a fit is either blowing smoke or selling snake oil. Or both.

I don't think snake oil sales will ever take off, the sheer number of snakes it takes to make a single barrel should see to that.

As for an exacting nature, I don't think you read what I wrote. I laugh at the people who have to be within 2mm of their "perfect position" - I have bar tape that's thicker than that. I fit based on dynamics of cycling and limit how far I can go in any direction with the limits of the body. Exact or not, being beyond your limits is somewhere between uncomfortable and injury causing.

I noticed your signature - you're one of those power meter guys who says pedal stroke work is a waste of time. I think that the power meter is one of many tools I use in coaching and training, but one of the things it tells me is that the human body puts out so little power that an efficient pedal stroke is always my first step. We can agree to disagree, you can point at test results, I can point at my rider's results, who's right?

The answer to your first question is 4, but only the last one put money on it.

Now if you'll excuse me, I have to go squeeze some snake oil into the smoker.