Wierd discovery on crank lengths

mangizmo

OK...bear with me here, I am quite a strong rider, just ridden the Dartmoor classic 105 mile sportive in 7hrs 07 mins having punctured...so I am not amazing....but a reasonablly strong rider
Now, since riding with my club, several riders had commented that I rode with a skewed over to the right style, I have seen myself on video, and my style was really wierd
I was born with a club foot, corrected at age 2 years...no real impact, apart from the fact that I have a slightly shorter right leg....which has a less developed calf muscle....hence I assume the right leg is also weaker
Somebody suggested that I put a 175mm crank on the left (long strong leg) and a 170mm crank on the shorter (weaker) leg....this I tried, and it just felt wrong, actually my style was worse
SO...I tried a shorter crank on the left (longer stronger leg) and a 175mm on the shorter weaker) side....and BINGO...but why does this help ?, it seems like the opposite to what you would expect
I THINK I have worked it out, my wonky style was not a result of the difference in leg length (some 2-3mm, ie quite small), but rather a difference in power.
According to Sheldon Brown, he says that a shorter crank gives a bigger gear ratio...counter intuitive I know,so by putting a shorter crank on the stronger side, I have evened up the power....it definately works, everyone has commented that my style is smoother and it just feels "better"....I rode the Dartmoor Classic with this configuration.....still cant see why a shorter crank gives a bigger gear...but hey it works....this is what Sheldon Brown says in his section on gear ratios
"All of these systems share a common inadequacy: none of them takes crank length into account! The fact is that a mountain bike with a 46/16 has the same gear as a road bike with a 53/19 only if they have the same length cranks. If the mountain bike has 175's and the road bike 170's, the gear on the mountain bike is really about 3% lower! "
Just thought I would share that with you all

mangizmo

Sorry, my leg length difference is 2-3cm, not 2-3mm

Alex_Simmons/RST

Glad you found something that appears to work for you.

I would however suggest considering seeing a specialist bike fitter but I don't know any in the UK although I don't know where you are. Certainly someone with the expertise of Steve Hogg here in Sydney would be what you'd need.

Using different length cranks is a fairly radical solution (I have used it for myself until I had the knee flexion to get back to full length cranks) and I would suggest there might be other avenues to pursue that could provide you with a sustainable outcome than using different length cranks, which I suspect is going to possible create other problems down the track. Properly designed/fitted shoes, cleat shims, cleat position, saddle position and so on would be a better place to start IMO.

As for the gears, maybe you are talking about gain ratio and not gear ratio? Gain ratio is relatively unimportant. Gear ratio has nothing to do with crank length - it's just a measure of how far the bike travels with one full revolution of a crank, which depends on the chainring & cog used and rear tyre circumference but not crank length.

softlad

mangizmo wrote:

The fact is that a mountain bike with a 46/16 has the same gear as a road bike with a 53/19 only if they have the same length cranks. If the mountain bike has 175's and the road bike 170's, the gear on the mountain bike is really about 3% lower! "

I think you are confusing lever ratios with gearing. Personally, I would be a bit careful reading too much into what Sheldon Brown says, as I think a lot of his comments can be misleading...

mangizmo

Thanks...its a wierd one, I get along really well, the sportive that I recently rode was a very hilly course on Dartnoor, my time even with a puncture (lost about 12 mins) put me in position 332 out of 950 keen riders...so I dont do too bad, if I could nail this problem, then I reckon I could do better as my cardio fitness is very good and the top of my right leg is just as strong as the left (ie very well developed and powerful) , and with my "disability", its a really good idea to use the leg as much as possible, to keep it strong, I am convinced that cycling has prevented me from becoming quite disabled, I look and feel completely on a level playing field with everyone else...I would never wear shorts though, as the right calf muscle is considerably smaller than the left, and years and years of excercise has never changed this...but thats just vanity I suppose
I will look into consulting a specialist as you suggest...I just lived with it before, but now I am riding with more competetive riders, I am looking to maximise my riding....and its fun too

scherrit

Hi,
Wot Alex said.....

I would prefer to see a solution that would preserve similar joint angles for left and right hips and knees (erm, respectively).

We are more than happy to work with athletes to sort out even fairly extreme biomechanical problems like this. We're based in West London. Steve Hogg is world class and has much experience with this sort of problem, he does video fittings still I think, it's just a faff to burn him a CD etc.
Cheers,
Scherrit.
we are on 020 8998 7271,
web site(www.thebikewhisperer.co.uk) is down at the moment

58585

Shorter cranks (relative to longer cranks) make it harder to push a given gear simply because the lever you are using to turn the rear wheel is shorter. To keep the same speed in a particular gear you need to keep the rearwheel rotating at the same rate. If you use "long" cranks you need to spin them faster to give the desired speed (at the rear wheel), but it takes less force. "Short" cranks can be spun slower but need a higher force to give the same (rear wheel) speed.
As mentioned gear ratio is calculated on wheel size and chainring/sprocket size; it doesn't tell you anything about how hard it is to push a gear, which is directly related to your crank length.
If you plug the numbers in, you will see the difference between 170mm and 175mm cranks equates to just a few rpm difference in cadence to give the same speed.

mangizmo

58585 wrote:

Shorter cranks (relative to longer cranks) make it harder to push a given gear simply because the lever you are using to turn the rear wheel is shorter.


Right, I have been thinking about this and I now reckon that my strange riding style was due not to a difference in leg length ( not really that great, maybe 2cm or so) but rather in the fact that the right (shorter) leg is weaker, this caused me to have to push harder with the right leg, causing me to skew to the right and sit on the right of the saddle with a slightly twisted back....not a great style...I still rode pretty quick, but it was just ungainly, and I did get some back ache
When I put a shorter crank on the weaker (shorter) side, and a longer crank on the longer (stronger) side, this made the situation worse because the right (weaker) leg had to work even harder to push the crank round, thus increasing the disadvantage of the weaker leg
When I fitted a shorter crank to the stronger leg side and a longer crank to the weaker side, the situation was evened out because the weaker leg has to push less hard (on a longer crank) and the stronger leg has to push harder to move the crank round....I think that is why my style is more even and I climb hills especially better when I sit down and just power away, previously, I would get out of the saddle to even out the power.
There is no doubt whatever in my mind that this has helped, I am now going to experiment with 165mm & 175mm cranks to see what effect that has
....Do you follow my reasoning ?

BikeDynamics

Mangizmo

You have not mentioned if you changed your saddle height whilst swapping cranks.

As an alternative hypothesis I suggest you may be balancing yourself up by reducing the power from your longer leg. To stop your short leg knee and ankle angles from over extending, your saddle needs to be low enough so you can reach down to the 175mm crank. The longer leg meanwhile only needs to reach down to the 170mm crank so is probably under extended. As a rough guess I would suggest your short leg knee is opening to around 150 degrees whilst your long leg is probably no more than 125 degrees. Similarly, asuming your saddle is set based on your short leg, you may find that the long leg hip and knee angles get a bit tight at the top of the stroke. This could result in you 'bobbing as the long leg knee comes up to the chest. A shorter crank would help if this was the case and make you more stable.

If you would like to be filmed and measured come along to http://bikedynamics.com/

Regards

Mike at BikeDynamics.co.uk

mangizmo

BikeDynamics wrote:

Mangizmo

asuming your saddle is set based on your short leg, you may find that the long leg hip and knee angles get a bit tight at the top of the stroke. This could result in you 'bobbing as the long leg knee comes up to the chest. A shorter crank would help if this was the case and make you more stable.

Thats right, a shorter crank on the stronger/longer side seems to have two effects....firstly it makes the stronger side harder to turn, which balances out the weaker leg....secondly, at the top of the stroke, the knee does not come up as far, hence this also reduces the tendancy for me to skew over to the right to compensate, I had a tendancy to let the longer leg knee stick out a bit due to the higher rise of the leg at the top of the stroke....it has certainly helped, but strangely it is exactly the opposite to what I had in mind, I expected that I would use a longer crank on the longer leg, and a shorter crank on the shorter leg.......even with all this, I manage to ride as one of the faster riders in quite a fast club.....I think I would have been pretty good if I had not had this disadvantage....but I love my cycling and thats the main thing, its a great sport, I tried running marathons....but I was really bad at it, I can ride a bike on an equal footing with the better riders, and thats a great satisfaction for me, It is only recently that I have gotten around to this fine tuning of my bike....next event is the Exmoor Beast in November...thanks for your interest