Wierd discovery on crank lengths
mangizmo
Posts: 39
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
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
0
Posts
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.
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...
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
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
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.
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