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Banned Sock Puppet
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16,379 Posts

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Banned Sock Puppet
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The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks tend to mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard.

There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything.

You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

A 2008 study by Jim Martin, Ph.D., from the University of Utah shows zero correlation between crank length and any performance factors.
Performance is one thing. I believe @wettek is speaking from an entirely anatomical perspective. IOW, if something hurts, then you change something and it no longer hurts, as long as it doesn't complicate something else, the change is worthwhile IMO.
 
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Banned Sock Puppet
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My inseam is 30.5" and the calculation I have done put me at 167.5 crank which with the supply issues would be almost impossible to find . Research also shows I could go with a 165 .
My oldest bike which I still enjoy riding and put around 3k miles a year on could honestly use some new chainrings , so now may be the time to try this
If I were you, I would try a 170mm crank for awhile and see how that works for you. Small changes are best.
 

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Banned Sock Puppet
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OP, did you adjust your saddle height? You should have raised your saddle 2.5mm to adjust for the shorter crank length.
Is there really a need to? If yoy have shorter crank arms, you will also be extending less, so it's really a wash.

I would say leave well enough alone and if you still have pain in front of the knees, then raise the saddle a bit.
 

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Banned Sock Puppet
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As I said, I have not adjusted my saddle height yet. The only 2.5mm change so far is my crank length.
Were you able to find the individual crank arms or did you have to replace the whole crank set?
 

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Banned Sock Puppet
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Searched the internet to see if I could get the drive side spider and non drive side arm separately, but it was by far easier (and cheaper) to get a complete set (chainrings and cranks)
Yeah, I see lots of 5800 non-drive side crank arms on eBay for $50-60. The correct drive side spiders are much harder to find.
 

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Banned Sock Puppet
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I figure that in the big scheme of things, a $240 US ($320 AUS) crankset that you can resell for $150 if it doesn't suit, is bugger all when you consider how much we "happily" drop on a new bike or wheelset.
Were you able to find the exact same crankset with shorter arms? If not, did you notice any difference in front shifting?
 

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Banned Sock Puppet
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Yep, I went from 52-36 @ 175 to 52-36 @ 172.5, so I didn't have to adjust anything. Stock was limited here in Aus for a while, but came good.
Same generation?
 

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Banned Sock Puppet
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I wouldn't recommend raising your seat to simulate shorter cranks, your hamstrings will suffer.
Yes, this would be trading one problem for another. But I don't think that was being discussed. Some were suggesting raising the saddle AFTER getting shorter crank arms which I think is changing too many variables at a time anyway.
 

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Banned Sock Puppet
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Yes R8000.
I have 5800 and 6800, but those are impossible to find anymore. I would need to go to 7000 or 8000. As long as they are Hollowtech II, I presume they should work.
 

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Banned Sock Puppet
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I really like the 8000. Probably the best (and last) mechanical Ultegra. Although a crankset will always be mechanical!
How are the 8000 and 6800 cranksets different other than aesthetics? They're both 11 speed, correct?
 

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Banned Sock Puppet
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You're gonna buy those cranks after all the debonding and corrosion failures? 105 is cheaper and better.
Debonding and corrosion failures? Are you talking specifically about 8000 or are you talking about Ultegra before that? I have over 10K miles on a 6800 Ultegra crankset and never had any debonding or corrisions issues.

That being said, I can't see any reason to spend over $100 more to buy the 8000 over the 7000 when all you save is 39g. I don't know anything about either of those first hand. I do have bikes with both 6800 and 5800 and they both shift flawlessly.
 

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Banned Sock Puppet
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Good find!. Be interested to see how your change goes. Yeah, Shimano had the debonding issue, but I am pretty sure they sorted it.
Doing a search, the debonding issue was on 6800 and 9000 cranksets. Apparently 105 is one machined piece whereas Ultegra and Dura-Ace bond two pieces together and that design somehow saves weight. Not sure if the newer generations are the same way.

Keep in mind if you search the internet hard enough, you will find examples of failures for SRAM and Campagnolo as well as Shimano.
 

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Banned Sock Puppet
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2.5 mm is a tiny change in crank length. Its like a 1.5% change. The sole thickness on different shoes probably differs by a lot more than that. I think its all in your head.
2.5mm in the world of bike fit is not insignificant. Try raising or lowering your saddle 2.5mm and tell me that won't make a difference in how you feel.
 

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Banned Sock Puppet
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1.5% is insignificant in my book.
Your book is wrong. So tell me, is moving your saddle up or down 2.5mm insignificant? I know I can feel the difference here.
 

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Banned Sock Puppet
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Statisticians draw it at 5%

An inch is 2.54 cm. 10% of 2.54 cm is .254 cm, or 2.54 mm, Right? So we're talking 1/10 of an inch. I think that's right. Get a tape measure and measure out 1/10 of an inch. Mark that distance on a piece of paper. It's tiny. It would difficult to even raise your seat post that little with any accuracy. Why don't stems come in finer increments than one cm if 2.5 mm is such a significant distance?

I know you consider yourself the god of bike fitting, but look at the paper again and tell me you'd really be able to tell a 1/10" difference in your crank length, or seat post length. I sure couldn't.
First off, in the world of bike fit, 1 inch is HUGE. 1/10 of an inch is significant.

I just measured between the marks that I put on my seatpost. I needed to raise the saddle 3mm when I use my winter bike shoes. I noticed the difference immediaty when I got on the bike with my winter shoes and felt "low" and needed to raise the saddle to compensate. No psychology was involved here as I didn't make the adjustment until after I felt the difference. Comprende?
 

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Banned Sock Puppet
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Thickness of chamois, shoe soles, gloves, etc. 1/10" is pretty small.
Try riding without all of those and you will need to adjust your saddle.
 

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Banned Sock Puppet
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Oh thank you. More golden nuggets from the God of Bike Fit. I'll adjust my saddle height 0.1" when I wear different shorts.
Where do you come up with this? Do you really think there is a 0.1" difference between the chamois thickness any of your bike shorts? That is compressed under your butt.

"God of Bike Fit"? That one is rich.
 

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Banned Sock Puppet
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Well go and count your road bikes again and inflate your ego a bit more.
The MO of @pmf is predictable. When others here tell him he's wrong, he gets butt hurt and calls others "know it all". He's done this in other threads. In one word - LAME.
 
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