large crank length is bad for the knees, true/false?

[jimboalee]

I've only ever used 170mm cranks. But if CRC deliver tomorrow one bike will have 152mm cranks (due to pedal strike issues after changing to narrow slicks on an mtb).

I suppose it will feel strange at first. But, after reading this, may improve my spinning?

6" cranks They're for seven year olds...

 

[ASC1951]

please explain?

Kidney damage caused by repeated and sustained exertion while deliberately or accidentally dehydrated, although this was certainly much commoner when performance-enhancing drugs were widely used. Professional cyclists often suffer from over-developed hearts, too, which can be a problem if exercise isn't maintained after retirement.

 

[Mycroft]

Kidney damage caused by repeated and sustained exertion while deliberately or accidentally dehydrated, although this was certainly much commoner when performance-enhancing drugs were widely used. Professional cyclists often suffer from over-developed hearts, too, which can be a problem if exercise isn't maintained after retirement.

cheers

thought it might be something to do with dehydration, explanations are always good though

 

[Fab Foodie]

I've used long cranks of 180 for a while and it just meant i found it harder to spin freely.
I think the theory goes that if you are riding with long cranks its harder to spin so riders tend to mash the gears which is rumoured to be bad for the knees.

I don't think there is any science either way, just a lot of speculation.

Mike Burrows goes with shorter cranks and spinning a bit more as being more efficient. He's written a few articles on the subject... but I can't find one right now

 

[MartinC]

Using longer cranks will increase the range of movement your knee goes through repeatedly and under load. As others have pointed out this has the potential to damage the soft tissue involved. Whether or not the damage will occur in practice or not depends on so many things that that one can only speculate on whether it will or not.

 

[GrasB]

which is not dissimilar to Sheldon's observation, who I believe said that we all climb stairs of different pitch, ride using different gearing with hardly a second thought.

Apparently this is a bit of a red herring this one. I've been told if you look at someone who is climbing stairs where the step is really to big for them you'll see they rotate forward until the knee is in front of the ankle then push up, if you're on a bike you can't do this adjustment unless you're honking all day long.

 

[Fiona N]

Using longer cranks will increase the range of movement your knee goes through repeatedly and under load. As others have pointed out this has the potential to damage the soft tissue involved. Whether or not the damage will occur in practice or not depends on so many things that that one can only speculate on whether it will or not.

This was my point about the gym bike - the cranks were so long that, for folk with short legs, on the upstroke your knees had to come up above the hips to avoid excessive compression of the joint. There are arguments for moving joints through their full range of motio, but, in some cases, this principle doesn't apply. E.g. deep squats are a specific exercise to change the emphasised muscles, you don't do 1000's of reps of this exercise if you want to maintain knee health and function into old age.

 

[jimboalee]

Tractors and tug boats use very long cranks, produces huge amounts of torque, good for pulling heavy loads and steep hills but engine speeds need to be very low and give poor acceleration. Racing engines go for short cranks, perfect for turning power into acceleration but the downside is they are uneconomical and engine life is short. Don't think that answers your question and not sure how it relates to cycling.

An F1 engine is as you say, short throw and produces about 700 horsepower at 18,000 rpm.

On the other hand, a Keith Black 500 cuin ( 8000cc ) drag race engine has a fairly long throw crank to get the displacement.
Two drag race engines going away at 8000 rpm each produce about the same power as the entire field of last weekend's Monaco GP off the start line. ( 15.5 thousand horsepower )

Power is the torque on the crank x engine rpm, all divided by a constant.

The torque and power involved in accelerating a Top Fuel dragster up the 1320 ft racetrack is such that if the team chief gets the clutches wrong, the car completes the first 100ft or so without the front wheels turning because they left the ground on the start line.

The Pro-stockers have 'wheelie' bars which ALWAYS hit the track to stop the car standing up and blocking the driver's view.

This has bugger all to do with cycling, but is a nice 'appetiser' to get me in the right mood for Santa Pod this weekend

 

[snailracer]

Well I have ridden on 140mm and 175mm cranks, on the same bike (not at the same time, I should add). I have fairly short legs, and bad knees from football.

It is easier to spin faster, and harder to "grind", on the shorter cranks.

With regards to knee pain, I found that seat height adjustment is much more critical, because the onset of pedal pressure is much more abrupt than with the longer cranks. I don't really agree with those who claim that longer cranks result in over extension and a higher risk of injury - the overextension is offset by a gentler transition in pedalling forces as the pedals rotate. Overall, I did not notice much difference.

It takes a bit of getting used to, as well.