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which crank length????

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which crank length????

Old 11-14-11, 03:35 PM
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rotti
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which crank length????

I'm going to be buying a new crankset. I'm going from compact to a standard. My compact have 165mm cranks. My options are 170, 172.5, or 175. What is the standard that comes on most road bikes? I'm guessing 172.5 because it's middle of the road, but I'm looking for some opinions before I order. Thanks for all the help.
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Old 11-14-11, 03:43 PM
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165 mm? That seems very small. What size of bike do you have?
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Old 11-14-11, 04:24 PM
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I loved my 165's and I'm 6'2" (188cm)
I am now running normal 175's for my height, but only because I'm too cheap to go buy another set of 165's until these wear out first.
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Old 11-14-11, 05:05 PM
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I'm 5'9 and I ride a 54 Trek Madone 4.5. I contacted an LBS and they recommended 170 for a 54. He said I could get away with 172.5.
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Old 11-14-11, 05:53 PM
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170s
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Old 11-14-11, 06:20 PM
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Inseam is a good starting point. I use 172.5's on most of my stuff and I'm the same height and bike size.
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Old 11-14-11, 06:24 PM
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keep your 165mm.

This is what John Cobb recommends to many triathletes. I don't do try's, but i do spend a lot of time in a relatively aero position (CdA of ~0.32). i'm 5'8.5" and used to run 172.5mm. The 165mm allows me to get a lower position and have more knee clearance. I have not noticed any drops in power while climbing either.
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Old 11-14-11, 08:11 PM
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Originally Posted by echappist View Post
keep your 165mm.

This is what John Cobb recommends to many triathletes. I don't do try's, but i do spend a lot of time in a relatively aero position (CdA of ~0.32). i'm 5'8.5" and used to run 172.5mm. The 165mm allows me to get a lower position and have more knee clearance. I have not noticed any drops in power while climbing either.
Agreed. There isn't a magical formula out there that says if you're X then you need y. However, there have been studies that show crank length doesn't affect power production. So, you should ride whatever length works for you. Like echappist said though, if you like an aggressive position on the bike, 165's would certainly help. FWIW, I ride 165 on all my bikes (road and TT), I'm 5'9"ish, and Cobb has fit me.
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Old 11-14-11, 09:29 PM
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I would ride 165's if I wasn't too cheap to buy them. I have 172.5's on my road and 175 on my TT and I can't tell the difference. 165's would allow more drop on the TT bike, which would be nice. 165's also give a little more pedal clearance in turns. Most people wouldn't even notice a 5mm difference though.
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Old 11-14-11, 09:52 PM
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For me it doesn't make much difference. I have 165mm cranks on our tandem, 170mm on my touring and folding bikes, and 175 on my road bike. They all feel fine and I doubt there's much difference in my power output. I do tend to spin a little faster in a bit lower gear using the 165s on the tandem and think I have a slight preference for longer cranks. But it hasn't caused me to consider switching out the cranks since we got them 37 years ago.
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Old 11-14-11, 10:02 PM
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Originally Posted by echappist View Post
keep your 165mm.

This is what John Cobb recommends to many triathletes. I don't do try's, but i do spend a lot of time in a relatively aero position (CdA of ~0.32). i'm 5'8.5" and used to run 172.5mm. The 165mm allows me to get a lower position and have more knee clearance. I have not noticed any drops in power while climbing either.
Originally Posted by Nate552 View Post
Agreed. There isn't a magical formula out there that says if you're X then you need y. However, there have been studies that show crank length doesn't affect power production. So, you should ride whatever length works for you. Like echappist said though, if you like an aggressive position on the bike, 165's would certainly help. FWIW, I ride 165 on all my bikes (road and TT), I'm 5'9"ish, and Cobb has fit me.
I'm 5'8" and also have a fairly aero position on the road bike. I'm currently on 172.5 cranks and recently purchased some 165s to try out. Last season I was constantly bothered by my legs hitting my chest when I'm in the drops or IAB. I've been hesitating about putting them on but your experience has convinced me I need to give it a try.

p.s. When I first started riding 6 years ago I started on 175s and never had a problem with them until I started to get more aero.
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Old 11-15-11, 07:43 AM
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I'm thinking of going back to 170s, but only after Bethel. The 175s are so nice there I can't help but keep them for that hill. But after... 170s. Maybe. I told myself this last year but never switched, always thought, "Okay, next week, after this race."

Crank length is individual, but I'd also say it's related to age. 20 years ago I was a gazillion times faster on 167.5s. Then I went to 170s as I slowed, then 175s when I was soooo slow it was sad.
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Old 11-15-11, 09:30 AM
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always raced and rode 172.5s

not by conscious choice or anthing but just thats what my bikes (54s and 56s) always came with.

well this season i bought a 52, (supersix) just to get a little lower headtube, and lighter stiffer and tighter, etc...

in a fairly glaring oversight i never checked into the crank length, and only discovered towards the end of the season that i was on 170s all year.

i've talked about it here a fair bit but there was a huge discrepancy this year between both my TT perfomance (really good) and my road/HC performances (middling to embarassing, depending on the event), and also a discrepancy between this year and last year: basically my TT times and results improved from last year, and my hill climb times tanked (same weight)- mount evans in particular i lost 8 minutes off of last years time. got really frustrated and desperate trying to figure out what was going on, including bike fit and hand wringing.

finally bought some new cranks last week, 172.5s, and went back to a 52/36 compact like i raced last year, and the initial results are really promising. obviously its off season but i felt smoother and stonger on the bike than i did most of the season, and it seems i eliminated some nagging back pain as well.

lesson learned. 2.5mm can make quite a difference

Last edited by badhat; 11-15-11 at 09:33 AM.
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Old 11-15-11, 09:34 AM
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I've ridden crank lengths from 165-180mm.
I've had seat heights between 72-84cm.
I've had seat setbacks between 3-12cm.
I've had a reach between 54-62cm.
Nose up, nose down, nose level.

I can't remember riding any better because of position or crank lengths.
It all came down to whether I was training, or not.

Don't fret the small stuff.
Set up a position that feels right, grab some cranks that make you feel fast/strong/whatever, ride bike, adapt.

Bike fitters be damned.
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Old 11-15-11, 09:50 AM
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Originally Posted by Nate552 View Post
Agreed. There isn't a magical formula out there that says if you're X then you need y. However, there have been studies that show crank length doesn't affect power production. So, you should ride whatever length works for you.
Tell that to Leonard Zinn.
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Old 11-15-11, 11:17 AM
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It's all about leg speed for me. My road bikes I use for long training rides and road races have 175mm cranks (5'11", 32" inseam). You trade speed for torque with longer cranks, and when I am on the open road, I like to grind. I have big, heavy, sprinter's legs; I have found I TT better if I keep my legspeed down. On my track bikes I have 165mm cranks. I race match sprints and it's really tough to get up into the 140+rpm range on longer cranks; I tried it, the longer cranks make it feel like my feet are made of lead. I also have a crit bike which I use 165mm cranks. I figure I use 165mm on the track so there is no harm in going small on a crit bike, and I get another ~10deg of pedal-through corning angle out of it.

What a smaller crank does do, however, is make any particular gear combination feel larger. You don't have as much leverage on the drivetrain and it's noticeable (especially changing a full cm like I do). You will need to shift down one cog (to a cog with more teeth) to feel the same amount of torque, however, you'll be able to spin that smaller gear faster, so there is no power tradeoff.
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Old 11-15-11, 12:00 PM
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Originally Posted by STP View Post
Tell that to Leonard Zinn.
I very much respect Leonard Zinn when it comes to wrenching, but his opinion can go to hell when you have an aero and bike-fit guru like Cobb suggesting otherwise.
Originally Posted by Brian Ratliff View Post
It's all about leg speed for me. My road bikes I use for long training rides and road races have 175mm cranks (5'11", 32" inseam). You trade speed for torque with longer cranks, and when I am on the open road, I like to grind. I have big, heavy, sprinter's legs; I have found I TT better if I keep my legspeed down. On my track bikes I have 165mm cranks. I race match sprints and it's really tough to get up into the 140+rpm range on longer cranks; I tried it, the longer cranks make it feel like my feet are made of lead. I also have a crit bike which I use 165mm cranks. I figure I use 165mm on the track so there is no harm in going small on a crit bike, and I get another ~10deg of pedal-through corning angle out of it.

What a smaller crank does do, however, is make any particular gear combination feel larger. You don't have as much leverage on the drivetrain and it's noticeable (especially changing a full cm like I do). You will need to shift down one cog (to a cog with more teeth) to feel the same amount of torque, however, you'll be able to spin that smaller gear faster, so there is no power tradeoff.
Regard the last point, it does take quite a bit of adaptation (i say 2-3 weeks) to get used to the 165mm

As for the issue of speed/leg speed, i think we should clarify. There's cadence, which is rate of rotation, and then there's tangential velocity/circumferential pedal velocity.

CPV = C*CL*2*Pi/60
Where CPV = circumferential pedal velocity (in m/s), C = cadence (in rev/min), CL = crank length (in m), and again the constants 2, Pi, and 60 serve to convert the data to the proper units. While technically muscle shortening velocity or at least joint angular velocity should be used instead of CPV, CPV has been shown to be an excellent predictor of joint angular velocity and, by extension, muscle shortening velocity. Indeed, since crank length is generally constant, especially for a given individual, one could just as well use cadence instead of CPV. However, the latter has been used here to be consistent with scientific convention and to emphasize the relationship of such cycling-specific plots to the more general force-velocity curve of muscle.
the above is from Coggan's discussion on quadrant analysis. People who are interested should also take a look at this
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Old 11-15-11, 12:16 PM
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Yes, I mean cadence, rotational (angular) leg speed. Tangential velocity doesn't really change when you go to different sized cranks. Longer crankarms, same tangential velocity, equals lower cadence.
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Old 11-15-11, 01:09 PM
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This question is on the aero implications of a shorter crank length, primarily for Nate and R.Ex.

If you go with the shorter crank, then that implies you will raise your saddle height by the same amount to compensate. Assuming you leave the bars in the same position, you'll have more saddle-to-bar drop with shorter cranks. While I know there are damn near no absolutes in the wind tunnel, does this trend you towards a lower drag setup? Layman fits equate a lot of drop with better aerodynamics, but is this something that actually tends to hold true?

I have no idea on the matter, just I am interested in your guy's experience.
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Old 11-15-11, 01:55 PM
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seems backwards to me to predicate saddle to bar drop on crank length though

in the first place we're talking about 1cm difference in bottom of pedal stroke between 165s and 175s, when represent the extremes for practical discussion. unless your stem/bars are already completely slammed, you can compensate with a spacer or two to find the efficient profile regardless of crank effect on saddle position.

the more relevant concern for aero position wrt crank length would be the top of the pedal stroke where a longer pedal is gonna force your upper leg into your abdomen more. especially if you have any body fat.
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Old 11-15-11, 02:17 PM
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Originally Posted by badhat View Post
seems backwards to me to predicate saddle to bar drop on crank length though

in the first place we're talking about 1cm difference in bottom of pedal stroke between 165s and 175s, when represent the extremes for practical discussion. unless your stem/bars are already completely slammed, you can compensate with a spacer or two to find the efficient profile regardless of crank effect on saddle position.

the more relevant concern for aero position wrt crank length would be the top of the pedal stroke where a longer pedal is gonna force your upper leg into your abdomen more. especially if you have any body fat.
I found that shorter cranks do allow me to raise my saddle relative to the bars. Even though I was heavy when I first tried it (190 lbs, give or take), I could rotate my pelvis forward, allowing me to flatten my back regardless of my then current "gut intrusion" factor.

The top of the pedal stroke doesn't make as much difference, at least to me. Yes, there's more room with the shorter cranks, but the bigger benefit was the flatter back, more comfortable drops.

In fact this is the reason I'm considering the 170s. I've gained back 1/2 of that "gut intrusion" and am starting to approach the limits of my current saddle/bar drop height. 170s would allow me more room to maintain a flat back.
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Old 11-17-11, 10:00 PM
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Crank length his highly personal and as others have already stated, personal preference based on feel trumps any leg length formula.

Those who mash and grind lower cadences will likely prefer longer. Those who "spin to win" with higher cadences will likely prefer shorter. Ironically, for a given cadence, longer crank arms require higher footspeed than short crank arms because the foot has to cover the longer radius in the same time period.

It's easier to cover moves by quickly accelerating with shorter crank arms. It's obviously easier to climb with the mechanical advantage of the longer lever of longer arms.

Consider the terrain when choosing a crank length. If you ride/race a hilly course, maybe longer will be better. If you ride/race a flat course, maybe shorter will be better. Maybe have 2 sets of cranks for different styles of riding/racing, just like people have compact cranks and big cassettes for when they ride in the mountains and a different setup for everything else.

For example, maybe have a set of 165mm cranks for that flat crit and 172.5 or 175 for the hilly road race. Neuromuscular adaptation won't be an issue. People often ride road, MTB, TT, or track bikes interchangeably through the month all with different crank lengths. You will have to adjust saddle height, but that's not a major issue.

I think one setup doesn't have to work for all terrain and types of riding. I think that's where all of the different opinions come in. A guy racing in Florida is gonna LOVE 167.5s whereas a guy in North Georgia cringes at the thought.
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Old 11-17-11, 11:31 PM
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Right on. Doing more hilly stuff I've been loving 175s. But, now when I go down to 170s I actually get a little cramp mid-quad..my stroke seems stunted. At 6', 172.5 seems my minimum. I will say my pace on the flats has dropped, but I gladly trade that for the uphill leverage.

Last edited by FrenchFit; 11-17-11 at 11:35 PM.
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Old 11-18-11, 12:22 AM
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Originally Posted by FrenchFit View Post
Right on. Doing more hilly stuff I've been loving 175s. But, now when I go down to 170s I actually get a little cramp mid-quad..my stroke seems stunted. At 6', 172.5 seems my minimum. I will say my pace on the flats has dropped, but I gladly trade that for the uphill leverage.
Did you raise your saddle height when you changed to a shorter crank? By shortening your cranks 5mm, you've effectively lowered your saddle by the same amount. This might be the cause of your pedal stroke feeling shortened.
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Old 11-18-11, 06:50 AM
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Thanks for all the great replies. Very interesting hearing all the different opinions. I decided to go with the 170's. I have bad arthritis in my hip and I have limited range of motion. I was concerned about the extra flexion over the top with the 175's. Thanks for all the help.
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