[cwg_at_opc]

i went to Saint Sheldon's and built a table for this discussion:

 

[EverydayRide]

Help me out, I don't understand a single thing on your posted chart.

 

[Stogaguy]

Sheldon Brown

The chart is from the late great Sheldon Brown's website. Here is a couple of links.
http://www.sheldonbrown.com/gears/
http://www.sheldonbrown.com/gain.html
Sheldon was a long time bike industry professional. He passed away early last year long before his time. His website is a wealth of information on a wide variety of cycling topics.

The gear chart shows the speed achieved in each gear on the bike at a given rpm at the crank (cadence). Personally, I find this form of reference very helpful when planning/comparing bicycle gearing.

 

[cwg_at_opc]

Help me out, I don't understand a single thing on your posted chart.

i highlighted(bold and italicized) the key speeds which i was referring to
earlier. the first column is cassette, a 12-25, the next four columns
are two sets: gear inches and mph@90rpm for a 34/50. the last four
columns are gear inches and mph@80rpm for a 39/53.

Basically, at 10pedal rpms faster, i can pedal one(or two) gear higher
at the same or less perceived effort, which results in a slightly higher
speed. on Sheldon Brown's website, there is on online gear calculator
which generates these tables. for everybody, i also tried these tables
with the crank length changed to confirm that that has no effect(which
obviously it doesn't, since we're talking about gear inches and mph@rpm)

(p.s. - stupid excel truncated 172.5 to 173 when i upped the font size)

 

[STARNUT]

Can't lowering the saddle compensate for having "too long" a crankarm?

This is how we are taught to set a saddle height, and it's incorrect and assumes a lot about foot size and cleat placement. And your assumption is incorrect. It may be fine and BCD but you knee angle at TDC will be "wonky" and ay more than 110º.

moving on.


as Kerry points out; crank length has jack to do with power. Watts = Force (N m) x Velocity (m/s). Pretty simple if you think about it. It's damn near impossible to buy a crank to short but is possible to buy on to long. The highest ever in-competition recorded wattage was set on a set of 160s and was 2700+ watts. Force AND Velocity. People should be buying cranks based on their knee angle at top-dead-center not knee angle at bottom-dead-center. Buy your cranks based on injury prevention not power.

Anyone in doubt, check out Jim Martin's paper.

Starnut (who *gasp* uses a shorter crank on his TT bike than on his road bike)

 

[oily666]

All the grand numerical and scientific wazoos aside, I rode 12 years on 175's. This year I'm using 172.5's and everything is better. Better spin, power through a longer part of the pedal stroke, less hip and knee discomfort, better climbing out of the saddle. Average speeds are about what they'd be in early June. Just bought 170's for my touring rig.

Talk amongst yourselves

 

[Fredrico]

Eddy Merckx once said, "If you want to go fast, pedal fast." If you can pedal faster on a 170mm crank, go with it. You're obviously generating more power at the higher cadence on the 170mm crank, because you're going faster.

I'm 5'8" and have a 31" inseam. 175 and 172.5 cranks provide great leverage for my legs, but there's no question for me, 170s are easier to spin. Eddy used 172.5s, Fignon, the guy who lost the TDF by 8 seconds to Greg LeMond in the final TT, rode on 175s. Alot depends on leg length and pedaling style. Trackies use 165s, not only because they're easier to spin on a fixed gear, but also to avoid hitting the track embankment. Forget about leverage. Those guys have the most awesome quads and explosive power, and they're doing above 100 rpm most of the time.

Makes perfect sense spinning a tighter circle at high rpms is more efficient than a with a larger circle. At high rpms, your legs are following the crank around, not taking advantage of the slight leverage advantage of the longer crank arms that would give a slight advantage pushing up a hill at lower cadence.

 

[heathb]

Just get the 170's and go out and ride. You'll have no knee pain, you can hammer just as hard and these types of threads will be a thing of the past.

You get used to what you ride all the time. I just started back on 170's and I'm loving it for racing. Of course today I had a bad day as I popped three of my chainring bolts off my new DA crank climbing and bent the inner chainring almost in half. So the force is there with these 170 crank arms as I've never done that with my 175 or 177mm DA cranks.

 

[dd74]

Yeah. I'm going to give 170mm a shot.

If anyone has a 170mm 53/39 Ultegra or DA crankset for sale, please PM me.

 

[EverydayRide]

i highlighted(bold and italicized) the key speeds which i was referring to


(p.s. - stupid excel truncated 172.5 to 173 when i upped the font size)

Thanks everyone, that includes Starnut and cwg_at_opc too for the added explanations to the charts.

As what oily666, Fredrico and others have pointed out, crank arm length was a serious topic but seemed to slip through the cracks of bike science this past decade. You buy a frame at X height, you get a crank arm at X length. I've seen the sizing of cranks increase over these past years on road bikes when before it was rare to see anything bigger then 172.5's. You sometimes see 56cm [centre to top seat tube] frames come equipped with 175mm cranks ......I even have a Trek 2100 ZR9000 56cm with 175's as standard. So it's expensive to move around standard features, especially the cost of crank arms now-a-days just to make the fit "FIT."

 

[Kerry Irons]

Doing the math

nobody said anything about generating more power. however i'm able to spin faster, easier, in a higher gear, thus can go faster, how hard is that to get?

not a math major, nor physicist

Your second point emphasizes what you don't understand about your first point. If you increase the speed you go on a bicycle, assuming aerodynamic and friction losses don't change, then the ONLY way that can happen is if you are generating more power. So when you say "nobody said anything about generating more power" you are completely incorrect. YOU said you're generating more power when you say you are going faster (given the numbers you state, about 30% more power). There is no reasonable way you can claim that this came from a 2.5 mm change in crank length.

You need to find a time trial course and do a series of TTs in similar weather conditions with the two different cranks. You will find, I am sure, that your time differences are all within normal statistical variation, regardless of which crank length you use.

 

[Kerry Irons]

Reading backwards

Yeah, Kerry, what's up? I pretty much said that as the crank arm length increases the maximum RPMs drop.

Sorry, I read the first part of your post thinking you were commenting on an increase in crank length. That said, I can't believe for a minute your statement that "maximum smooth cadence . . . went down about 10-20 RPM per 2.5mm increase in crank arm length."

 

[cwg_at_opc]

Your second point emphasizes what you don't understand about your first point. If you increase the speed you go on a bicycle, assuming aerodynamic and friction losses don't change, then the ONLY way that can happen is if you are generating more power. So when you say "nobody said anything about generating more power" you are completely incorrect. YOU said you're generating more power when you say you are going faster (given the numbers you state, about 30% more power). There is no reasonable way you can claim that this came from a 2.5 mm change in crank length.

You need to find a time trial course and do a series of TTs in similar weather conditions with the two different cranks. You will find, I am sure, that your time differences are all within normal statistical variation, regardless of which crank length you use.

ok. i can agree with that. it's a semantic disconnect here and a lack of clarity
in my verbiage. so i prefer not to call this an argument, because it's not really.

when i think of 'generating' more power i always think in terms of 'quick, easy
weight loss' kind of people who are looking for free no-effort improvements. this is
certainly not the case here, some additional watts are going to the pedals, but
they're spread out over more of the pedal circle, so it seems like less work.

so yeah, i'm pushing a bit harder(not a huge amount), i'm spinning a lot faster
'cause it's easier, and now i'm going faster. this all makes perfect sense to me
even/especially with your correct explanation.

spinning faster is easier with a shorter crank,
it's easier to spin a higher gear at a higher cadence,
higher cadence, higher gears, slightly higher power output, ergo, more speed.

the next argument i expect to hear is: why don't you just spin the 172.5s
at 90rpm and go faster? it's easier for me to spin the 170s in the 90s.

are you happy now?

 

[Squidward]

Sorry, I read the first part of your post thinking you were commenting on an increase in crank length. That said, I can't believe for a minute your statement that "maximum smooth cadence . . . went down about 10-20 RPM per 2.5mm increase in crank arm length."

Yeah, I was just joshin' ya.

About the cadence dropping: it's all from memory. Now that I've given it some thought (and the cup of coffee I drank half an hour ago is kicking in) it was a 10 RPM drop per 2.5mm increase. I think my confusion was that between 170mm and 175mm there was a 20 RPM drop so that's where that number came from.

 

[Wayne Jacobsen]

Sorry for piling on, but the percentage increase in power needed to increase speed from 18 to 22 mph is way more than the percentage difference in speed. If I filled in the boxes correctly on analyticcycling.com, it takes about 103 watts to ride at 18 mph, and about 175 watts to ride at 22mph. With respect, I think this could not have been accomplished by a change in crank length.

 

[cwg_at_opc]

Sorry for piling on, but the percentage increase in power needed to increase speed from 18 to 22 mph is way more than the percentage difference in speed. If I filled in the boxes correctly on analyticcycling.com, it takes about 103 watts to ride at 18 mph, and about 175 watts to ride at 22mph. With respect, I think this could not have been accomplished by a change in crank length.

there's no question it will take more watts to go faster, but what would it take
to spin 10rpm faster? it might not feel like a lot more effort since the work is
spread out over more of the rotation.

more question(s):
- is it easier to spin 90rpm(or 100 or faster) on a 170 than a 172.5?
- which is easier to sustain 100rpm for extended periods on a 170 or a 172.5?

 

[SystemShock]

Eddy Merckx once said, "If you want to go fast, pedal fast." If you can pedal faster on a 170mm crank, go with it. You're obviously generating more power at the higher cadence on the 170mm crank, because you're going faster.

I'm 5'8" and have a 31" inseam. 175 and 172.5 cranks provide great leverage for my legs, but there's no question for me, 170s are easier to spin. Eddy used 172.5s...

Not sure about that. If Bernard Hinault's classic book Road Racing is to be believed, Merckx used 175s. Of course, not everyone uses the same crank length throughout their entire career, it's possible he was on 172.5s for awhile.

But with Eddy's long legs (91cm inseam), 175s aren't really a stretch– literally.
.

 

[DM.Aelis]

there's no question it will take more watts to go faster, but what would it take
to spin 10rpm faster? it might not feel like a lot more effort since the work is
spread out over more of the rotation.

more question(s):
- is it easier to spin 90rpm(or 100 or faster) on a 170 than a 172.5?
- which is easier to sustain 100rpm for extended periods on a 170 or a 172.5?

:idea:

Part of me doesn't even want to join in on this...
but your question(s) make no sense.

The question you should be asking is

- Is it easier to generate x number of watts at 90rpm or at 50 rpm?

In the example you listed, you preferred shorter cranks because they enabled you to utilize the higher cadence that you found more comfortable. Frankly, watts are watts; but if you like generating the same amount of power at a higher cadence, go for shorter cranks that lend themselves to that faster spin.

It really doesn't matter at all.

 

[Fredrico]

Should've looked it up.

Not sure about that. If Bernard Hinault's classic book Road Racing is to be believed, Merckx used 175s. Of course, not everyone uses the same crank length throughout their entire career, it's possible he was on 172.5s for awhile.

But with Eddy's long legs (91cm inseam), 175s aren't really a stretch– literally.
.

I have that book too. The general idea is long leg levers work fine with long cranks, and short legs with short cranks. At a certain point, as some posters have suggested, the circle the legs have to scribe gets too big for an efficient spin, and power starts to drop off.

 

[SystemShock]

The $64 Dollar Question...

I have that book too. The general idea is long leg levers work fine with long cranks, and short legs with short cranks. At a certain point, as some posters have suggested, the circle the legs have to scribe gets too big for an efficient spin, and power starts to drop off.

Which brings to mind the 'beeg' fit question: Should crank lengths vary more than they traditionally have?

The 'traditional' thing, after all, was to have almost everybody on 170, 172.5, or 175s... a 3% variation in crankarm lengths. But, ppl's leg lengths obviously vary by much, much more than that.

So, perhaps it's wiser to have a range more like 165mm to 180mm? Even that doesn't match the variation in riders' 'lever' lengths. :idea:

But then I guess we have to

- ask the really short riders (inseam below 75cm) if 165 feels better than 170 to them
- ask the really big riders (inseam 95cm+) if 180 is better than 175 for them
.