Crank lengths !

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Re: Crank lengths !

Postby hondated » 6 Aug 2020, 6:19pm
Once again I appreciate all advice & opinions offered
I never mind being wrong either. That's how you learn. Funny I seem to be learning a lot lately.
This is where I got my information from.
Whether the information is correct or not even taking the point on board whether you feel comfortable or not surely there must be a basic formula to begin with.

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Re: Crank lengths !

Postby hondated » 8 Aug 2020, 2:12pm

bgnukem wrote:The formula in the original post is gobbledigook.
E.g. My inside leg is 36" or 91.44cm.

Multiple by 1.25 gives 114.57cm. Add '65' (I assume mm) gives 116.85cm.

So my crank length should be 1.16m then, or longer than my actual legs.....

Didnt even get an level in maths then :lol:
91.44 x 1.25
+ 65
= 179.3
Really did not appreciate your gobbledigook comment :evil:

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Re: Crank lengths !

Postby bgnukem » 10 Aug 2020, 2:05pm

Yes, an 'A' level at 'A' grade! Forgotten most of it now though, 30y later.....

Clearly the formula assumes converting leg length from cm to mm, then adding 65mm, as Brucey suggests.

So the original post didn't make sense, hence my comment.

179mm sounds about right. Got 180 cranks on one bike but hard to find anything over 175 normally (or less than 165). I think St John Street/Thorn can oblige. Someone's probably already suggested them but I can't see the whole thread while replying.

I think the Spa Cycles triples are available in shorter lengths too, maybe 165 min.?

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Re: Crank lengths !

Postby ANTONISH » 10 Aug 2020, 5:56pm

There may be a ideal crank length but it's much down to personal preference.
As I'm short I was persuaded that a shorter crank (165mm) would be a good idea. I persevered for a year and never really liked the feel of it.
I went back to 170mm and was happier.
As an aside to this Jean Robic ( won Tour de France in 1947 ) who was very small used 180mm cranks throughout a long professional career.
Nearer home the short Dave Bedwell (British pro) used very short cranks (6 1/8 inches around 155mm ) - he was a very good sprinter.
Later in his career he reverted to normal length cranks.
I think it depends on yourself although there may be a marginal gain for a racing cyclist in choosing the "optimum" crank length .

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Re: Crank lengths !

Postby fausto99 » 12 Aug 2020, 9:14am

Brucey wrote:trying it out is the only way to be sure. ..... I've used 165mm to 175mm cranks on different bikes and it does make a difference (for me) to the way it feels. However most of the time I ride 170mm these days and I don't think it is a limiting factor in any aspect of my bike riding

+1 on all that. Over the years I settled on 165, having had bikes with cranks from 165 to 175.

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Re: Crank lengths !

Postby cycleruk » 12 Aug 2020, 9:58am

Having longer cranks will cause more of a knee bend at the top of the stroke. A 5mm longer crank will have a 10mm overall difference over a complete revolution. So assuming you lower the saddle height to compensate for the 5mm at the bottom that means your knee will be 10mm higher at the top. How much of an effect the extra 10mm of knee bend has I have no idea on health or function. Obviously a shorter crank will have the opposite effect and less knee bend.
I have 175's on my mountain bike and 170's on my road bikes. As I ride the MTB very infrequently I don't feel any ill effects from it. I did try 165's on a bike thinking that it would help to "spin" easier. However it felt somewhat harder to pedal so stayed with 170's.
I would add to check your saddle height as if that's too low it can also cause knee problems.
You'll never know if you don't try it.

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Re: Crank lengths !

Postby recumbentpanda » 13 Aug 2020, 8:05am

This discussion comes up a lot in recumbent circles where many of us use short cranks to avoid excessive knee bending. This is because on a ‘bent one can put alarming forces through the knee by bracing against the seat back. At an acute angle the leg muscles do not have good mechanical advantage and therefore strain can occur.

This leads to a point I have been known to make before in said discussions: while short cranks may reduce leverage, the system includes (at least) two other levers -the thigh bone above the knee, and the tibia/fibia below the knee. The mechanical advantage of the muscle groups moving these varies according to the angle of bend. At a high angle, the muscles do not have good leverage. A shorter crank can improve this.

Results vary because humans vary. Some may have very strong muscles that cancel out the low leverage issue. Some may be tall but have relatively shorter legs or vice versa. What works for you will depend on the proportions of all three levers in the system.

The potential of short cranks to improve leverage across the system as a whole explains why ‘you’ll need lower gears with shorter cranks’ isn’t usually true. Both my ‘bents came with 175 cranks that I converted to shorter. In neither case was it necessary to change the gearing.

Incidentally, some people obtain short cranks by having long ones expensively cut down and re-drilled. This isn’t really necessary. As noted up thread the Spa Cycles cranksets come in various lengths. These I believe are made by Sugino, who offer them also under their own name. Another option is that some BMX brands make cranks of varying lengths with 110mm bcd spiders. I have used those by Sinz very successfully in the past for single front chainring set ups, and it should be possible to build up a double or a triple on them as they are just a standard spider.