Reborn

[daveryanwyoming]

Nice post, but Sheldon's term is 'gain ratios' not 'gear ratios'.

I think folks forget that the crank length is just one lever in a series of 'levers' between the pedals and where the rear tire meets the road. Choosing crank length impacts cadence (for the same CPV as you point out) and it effectively changes your gearing as Sheldon points out and it can sometimes help with fit, especially TT bike fit as you also point out. But overall there's no magic in longer or shorter cranks in terms of free power or huge power losses within reasonable ranges provided by common commercially available crank lengths.

-Dave

Originally Posted by joroshiba .

Ok lets think about the whole picture. First of all it is well established (thanks to Jim Martin's research) that as you increase crank length or decrease it you CPV (circumfrential pedal velocity) stays the same. This means that in order for the pedal to move at the same speed, when you move to a shorter crank you are going to increase cadence and the opposite if you decrease. In the same gear you have essentially changed the gearing of the bike now, that is to say for a distance of circumfrential movement you are moving the wheel farther. So it is natural to spin in lower gears, at a higher cadence at the same steady speed with shorter cranks. Now, upon switching cranks, it is possible that muscle firing patterns could take some time to adjust if you have lots of time on different length cranks as you are now spinning different size circles.

Now lets get to the "torque" issue everyone likes to discuss. A longer lever does create more torque, but we must remember that you are not interested in creating torque around the BB, but around the wheel axle (the stuff that makes your bike actually move). Given what we know about how the gearing of the bike physically changes when you switch crank lengths, we will find that torque around the hub does NOT change. Yes it is easier to push a given gear with a longer crank, because that given gear is essentially smaller and you are going slower in it. (Sheldon Brown had a great way for accounting or this in his gearing calculations known as "gear ratios". The best way to compare gearing!!)

In the end, what it comes down to is ride what you feel comfortable riding, and fits your body. An overly long crank for your body size will result in a hip angle that is very acute on the top end of your stroke for an optimal extension. Too short and you have the opposite problem. For this reason, many people attempting to get a very aggressive TT fit will go with a shorter crank: it allows them to get aggressive without running into hip angle issues. The academic research I have seen points to limited to know power generation difference in the range of crank lengths people actually use. http://www.plan2peak.com/files/32_article_JMartinCrankLengthPedalingTechnique.pdf

 

[limerickman]

Originally Posted by daveryanwyoming .

Nice post, but Sheldon's term is 'gain ratios' not 'gear ratios'.

I think folks forget that the crank length is just one lever in a series of 'levers' between the pedals and where the rear tire meets the road. Choosing crank length impacts cadence (for the same CPV as you point out) and it effectively changes your gearing as Sheldon points out and it can sometimes help with fit, especially TT bike fit as you also point out. But overall there's no magic in longer or shorter cranks in terms of free power or huge power losses within reasonable ranges provided by common commercially available crank lengths.

-Dave

In respect of the people who took over Sheldon's site, it was really generous of them to keep his very informative website going after his death.

 

[daveryanwyoming]

Originally Posted by limerickman .


In respect of the people who took over Sheldon's site, it was really generous of them to keep his very informative website going after his death.

Agreed, he posted a wealth of great information on many cycling subjects and the folks at Harris Cyclery have done a great job of preserving that information and keeping it available to folks.

-Dave

 

[joroshiba]

Originally Posted by daveryanwyoming .

Nice post, but Sheldon's term is 'gain ratios' not 'gear ratios'.

I think folks forget that the crank length is just one lever in a series of 'levers' between the pedals and where the rear tire meets the road. Choosing crank length impacts cadence (for the same CPV as you point out) and it effectively changes your gearing as Sheldon points out and it can sometimes help with fit, especially TT bike fit as you also point out. But overall there's no magic in longer or shorter cranks in terms of free power or huge power losses within reasonable ranges provided by common commercially available crank lengths.

-Dave

Thanks for the correction Dave. (I edited the original post to reflect the correct information) I think it is far too easy to forget that there are a serious of levers in the drivetrain, and that some of them are adjustable on the fly!

FWIW: I personally find that I like "long" cranks for my height. (I'm 5'6", ride a 50cm frame and use 172.5mm cranks) It works for my fit and body, I prefer the feeling of the tight hip angle and I'm more effective at producing power at tight hip angles than the average person for some reason.

 

[daveryanwyoming]

Originally Posted by joroshiba .....FWIW:... I prefer the feeling of the tight hip angle and I'm more effective at producing power at tight hip angles than the average person for some reason.

I find that interesting in terms of the bike fitting I do. Some folks can clearly ride effectively, stay comfortable and sustain good power with tighter hip angles than others particularly minimum hip angles measured between their torso and femur at the top of the pedal stroke. When it comes to TT fitting a lot of folks dismiss the value of being flexible as you should just be rotating a road position forward around the BB into a TT position and be able to maintain the same working angles. Trouble is, some folks can work well with tight hip angles even in the road position and others need to ride more upright and open road positions to begin with so even when you rotate the position forward around the BB into a TT position there will still be folks that have to ride pretty high and others that can ride much lower up front.

I figure it has to stem from some form of functional flexibility or functional operating range for different folks but don't have a good theory for what is different. I've seen riders that can stand upright with locked legs and put both palms flat on the floor that have to ride relatively open hip angles or their comfort and sustainable power is compromised and others that can barely touch their fingers to the ground from an upright stance that can ride lower. So it makes me wonder if it's not posterior muscle groups like calves and hamstrings that limit functional ranges but maybe things like hip flexors and psoas or something anterior that sets limits on different riders.

Regardless it seems like a good thing that you can ride tight if you choose from both a road position and TT position standpoint. Personally when I get much below 45 degrees minimum hip angle I suffer and run 165mm cranks on my TT bike to open that up but still keep a relatively low front end and relatively flat torso angles in the TT position. Now that I've observed that and have found I don't lose power and can ride a lower TT position with the shorter cranks I'm considering going back to at least 170mm cranks on my road bike to see if opening up the angles there allows for a bit lower drops position with reasonable comfort and good power.

Interesting stuff for sure but this is pretty far off topic from the original crank length question. FWIW I run 165mm cranks on my TT and track bkes, run 172.5mm on my road and cross bikes and run 175 mm cranks on mountain bikes and having run power meters on all of them (not on the MTB any longer) my sustainable power is the same regardless of crank length. But from a positioning standpoint shorter cranks can open up some possibilities regarding hip angle where it's useful (not useful with the upright cross and MTB positions which already have very open hip angles).

-Dave

 

[An old Guy]

Originally Posted by dot .

difference between 175 and 170mm on my hardtail was about 1.5-2 min per 47 min lap, that's why I'm worried.

The gross math is simple and does not explain your time differences. That suggests that the gross math is not sufficinet to explain what is happening.

At the minimum one needs to examine the kinematics a bit more closely to determine the cause of your difference.

One posibility is that the time difference is simply normal variation. Very likely.

On the otherhand it is possible that the geomety has changed enough to cause some component of your body to reach a limit. Unlikely. But easily testable. Simply make some minor adjustments in position.