[PolarBearWY]

I notice cranksets as coming in 170 and 175mm sizes. What is the deal with that? What is standard? Physics would state that you would be able to crank harder easier from a dead stop or in a tough gear with a larger size, but when peddling at higher RPMs that would be more movement for your feet and lower legs.

Where do you guys find the balance?

I suppose I'm really only looking for answers from people who have bought a crankset and changed or didn't change the size of the new one compared to the size of the old one. If you didn't buy a crank with size in mind, then I shouldn't need your opinion.

Merci en avance.

 

[TM1]

I'm going to have to assume this is a newby question.

You pegged the physics, but missed the sizing issues and the 180mm size available in most cranks. It is a matter of compromise.

175mm has been standard for most mtb applications since the early 90's. Before that 170mm and even 165mm were common. Some taller riders like the 180mm as long as BB height isn't an issue.

 

[Softy]

If you are a strong, tall ,stomper or SS then the extra leverage of 180mm is good.

175 is the norm . Industry standard.

170 ? if you are a roadie background,you are short or like to sit and spin out the climbs. Also better for tech, rocky rooty. DH racers often go short.

Its really hard to notice the difference between 5mm.

 

[PolarBearWY]

Originally posted by Softy
Its really hard to notice the difference between 5mm.

Yup. That's barely 3/16" of an inch.

Thanks guys. I thought I had seen some 180s but didn't see any 180s at JensenUSA in the cranks I was looking at. No idea what's on my new Trek 8000. I'm sort of a newby I guess. Put about 16000+ miles on my '94 Trek 930, but have never paid attention to components like I do now.

 

[Ridemonkey]

Crank length has been debated for years. I was an advocate of long cranks for many years. I'm 6'2" with long thighs so I thought long cranks was the way to go. A couple of years ago I started developing bad ankle pain. I switched to 175's and not only did the ankle pain subside but my riding improved a fair amount.

Velo news did a study a while back that concluded that long cranks were over rated. You might want to search their site for more info.

 

[Wumpus]

Dear Lennard;
Because of your experience building Big bikes I was curious of your opinions on crank length. Should 175 cranks be used by riders of a certain height, or leg length? Is there anything to be aware of (like knee pain) when switching from 172.5 to 175 cranks? --Mark

Dear Mark;
The short answer is that crank length should increase as the rider's leg length increases, but there is probably no predicting the one that will work best for you. To get the ideal length for yourself, you would study it over time with a power meter and heart monitor (and maybe while surrounded by mirrors with plumb bobs over your bike - see below), using various lengths.

In general, for people over 6 feet 6 inches, I often use cranks of 195mm-200mm, sometimes even 210mm or longer (custom-made). A 175mm normally fits a rider between perhaps 5 feet 8 inches and 6 feet.

I think a possible rule of thumb for crank length is 20 percent of inseam length. That is a good place to start anyway.

Usually, you will not see a knee pain problem with a crank length increase, especially one of 2.5mm, if there is not already a knee problem with the rider. A longer crank will make the knee bend at the top of the stroke sharper. If a rider has a problem with chondromalacia, which is the deterioration or inflammation of the articular cartilage on the back of the kneecap, sharper knee angles will increase the pressure on the back of the kneecap and hence the pain and inflammation. A higher seat and shorter crank are usually what the doctor orders for this condition.

Now for a longer answer
There is also reason to believe that changing the crank length can even out asymmetries in a rider's bike position. In other words, I have seen examples where you can move a rider who is sitting over to one side of the bike to a centered position and even move them over to the other side of the bike by adjusting the crank length. This sort of asymmetry was heretofore always corrected with cleat shims and cleat fore-aft position changes to one foot, asymmetric chainrings, asymmetric crank lengths, asymmetric pedals, and other methods designed to alleviate leg-length discrepancies. I am interested enough in this that I plan to do more testing with it for a future VeloNews article.

Another advantage of a long crank for a tall rider is that you can improve the handling performance of the bike. Assuming the rider has a custom frame designed to accommodate the longer crank (built with a correspondingly higher bottom bracket), the frame size can be smaller. In other words, if the rider uses a 200mm crank rather than a 170mm crank (or a 210mm rather than a 180mm) and has a 3cm (30mm) higher bottom bracket, the seat tube can be 3cm shorter. The rider will still have the same seat height with the same amount of seat post extension, and the center of mass of the bike and rider will be the same height above the ground.

However, the smaller frame will tend to twist back and forth less. This is a huge advantage for a tall rider, as tall frames are notorious for their tendency to shimmy - i.e., build up a back-and-forth shaking oscillation that increases in amplitude rapidly, especially when riding at high speed with the hands off of the bars.

If you keep back issues of VeloNews (and everyone should be doing that, right?), here are some articles I did on the subject of crank length:
#12 July 23, 1990
#6/April 10, 1995
#7/April 29,1996
#11/July 1, 1996
#3, 3/1/99
Thanks and good luck. --Lennard

 

[Wumpus]

http://www.velonews.com/tech/report/articles/5257.0.html

 

[PolarBearWY]

Just an FYI...

I know "rules of thumb" should often be taken with a grain of salt, and this one is no different. You said that a 175mm crank would generally fit 5'8" to 6'0".

I'm 6'0" with fairly long legs and arms. My inseam is 34.

34 in x 25.4 (mm/in) x 20% = ~172.5 mm.

For a 175mm crank to generally fit people as short as 5'8" they'd have to have some serious disproportionate leg lengths, and would look funny (a 5'8" with 36" inseam would be worth a picture)

So, either the rule of thumb isn't a good rule of thumb or your previous sentence is incorrect. I didn't really want to post this to correct you, but to correct the information for others seeing it.

All-in-all, I'd love to check out a 180mm crank just to see if I could tell a difference between my 175mm, which is equivalent to 3/16" of an inch.

 

[Wumpus]

Originally posted by PolarBearWY
Just an FYI...

I didn't really want to post this to correct you, but to correct the information for others seeing it.

Correct all you want. That was a cut and paste from Velo News.

I couldn't find the article that RideMonkey was talking about.



BTW - Inseam is measured from the ground with feet at shoulder width straight up to the goods. It is not equal to your pants inseam.

 

[Wumpus]

Inseam is C in the drawing.

 

[PolarBearWY]

Thanks again.

 

[SuspectDevice]

to furter add to the melee.
Crank length is incoseqential and has no significant effect on cycling power output, as the main limiter for endurance cycling is largely metabolic parameters (i.e. lactate threshold) and the main limiter for power production in slalom type setting is leg speed (the v in the power=forceXvelocity realtionship).

 

[PolarBearWY]

Agreed, which is why I posed the question in the first place. I realize that physics would say there is some difference, but ~3/16" would make very, very little difference.

 

[Wumpus]

Going from a 170 to 175 would probably be useless, but going from a 175 to 190 is an 8% increase in length.

Torque = lever length x force (when force is perpendicular)

So you get an 8% increase in torque allowing you to run a larger gear.*

Miguel Indurian, five time tour de france winner, and some other road racers use longer cranks.





*I am not a physicist.

 

[SuspectDevice]

Originally posted by Wumpus
Going from a 170 to 175 would probably be useless, but going from a 175 to 190 is an 8% increase in length.

Torque = lever length x force (when force is perpendicular)

So you get an 8% increase in torque allowing you to run a larger gear.*

Miguel Indurian, five time tour de france winner, and some other road racers use longer cranks.





*I am not a physicist.

But as muscular strnght alone does not propel a bicycle you would see no differnce, except for knee pain and a sever hindrance in your ability to ride in the drops, as your knees would hit your chest, thus any potential gain in power is going to be outweighed by an increase in frontal area...

 

[Wumpus]

Originally posted by SuspectDevice
But as muscular strnght alone does not propel a bicycle you would see no differnce, except for knee pain and a sever hindrance in your ability to ride in the drops, as your knees would hit your chest, thus any potential gain in power is going to be outweighed by an increase in frontal area...

Miguel Indurian, one of the greatest time trialist ever, rode longer cranks. Didn't seem to hinder him.


This is about taller people riding longer cranks in proportion to their leg lengths.

 

[PolarBearWY]

Originally posted by SuspectDevice
...as your knees would hit your chest

If increasing a crank by 1.5 cm would cause you to hit your knees on your chest then your seat may be a tad low. That might be the cause of the potention knee pain you were speaking of. I could probably put 230 mm cranks on my bike before hitting my chest with my knees.

Originally posted by PolarBearWY
Agreed, which is why I posed the question in the first place. I realize that physics would say there is some difference, but ~3/16" would make very, very little difference.

And that is a completely different type of biking.

 

[SuspectDevice]

Originally posted by PolarBearWY
If increasing a crank by 1.5 cm would cause you to hit your knees on your chest then your seat may be a tad low. That might be the cause of the potention knee pain you were speaking of. I could probably put 230 mm cranks on my bike before hitting my chest with my knees.

Then your bars are too high!

Listen Kiddo, Numbers don't lie and I've fit, or been around when other people have fit like 1/2 the cat 1/2's and Pros in New England. Crank Length makes a very big difference in how low you can get on the bike before you compromise power output and comfort significantly. And as Wind restiance is cubic function of velocity, having a lower frontal area is going to be far more benefical than some non-existent increase in power...