Crank Length

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Stradageek
Posts: 913
Joined: 17 Jan 2011, 1:07pm

Crank Length

Post by Stradageek »

My wife at 4'10" has always looked 'awkward' on any bike she rides. Eventually her day to day commuter became an ancient three-speed folder with cottered cranks. This both looked, and to Jean felt, the most comfortable ride.

I noticed that the cranks were only 150mm long so fitted 160mm cranks to her Dawes tourer - 160mm being the shortest I could find on a standard triple from a child's bike. The Dawes still wasn't comfortable.

I then discovered this https://highpath.co.uk/crank-length-calculation/ which estimated she needed 135mm cranks!

We purchased some crank shorteners and fitted them to her Kettwiesel trike using the nearest 130mm option. This shortened the cranks by 25%.

Her cycling experience is reborn! No more aching hips, no more struggling to push off and much faster overall as a much more comfortable higher cadence, is not only possible but preferred.

Interestingly, I imagined I'd have to reduce the bikes gearing to compensate but as Jean can now put optimal power in over the whole pedal stroke no changes have proved necessary.

So it strikes me that whereas much progress has been made with women's frame sizes, reach etc.a lot more should be done to make genuinely 'short' cranks available
Halla
Posts: 194
Joined: 27 Apr 2008, 9:28pm

Re: Crank Length

Post by Halla »

Have you looked at the SPA Cycles website?

They have crank lengths from 130mm right through to normal adult sizes in 5mm increments..

The shorter cranks are shown as kid cranks, seem to be for double chain rings.
Stradageek
Posts: 913
Joined: 17 Jan 2011, 1:07pm

Re: Crank Length

Post by Stradageek »

I think my beef is with the bike designers. By my approximate calculations, any bike designed for a small person (say 5'2" - 5'4") should be fitted with 150mm cranks as standard with options to go shorter.

This blind spot in bike design ranks alongside the obsession with over-gearing. I've lost count of the number of bikes I've repaired that have a completely unused 'big' ring - why does a hybrid need a 120" gear!

My first change to my lovely Pashley Postie cargo bike was to fit a small chain ring. With the original ring I can't imagine any postman tackling even a modest slope - no wonder they were always pushing them :D
Bmblbzzz
Posts: 4089
Joined: 18 May 2012, 7:56pm
Location: From here to there.

Re: Crank Length

Post by Bmblbzzz »

It's good to hear this story. :D I'd agree that by and large there's a tendency to both over-crank and over-gear most bikes.
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531colin
Posts: 13792
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Crank Length

Post by 531colin »

Stradageek wrote: 5 Jun 2021, 8:15am I think my beef is with the bike designers. By my approximate calculations, any bike designed for a small person (say 5'2" - 5'4") should be fitted with 150mm cranks as standard with options to go shorter.

This blind spot in bike design ranks alongside the obsession with over-gearing. ............
Tell you what.....you source the cranksets with crank length 150mm and shorter and a BCD less than 74mm.....and I'll design the bikes!

The Stronglight Kid cranks are doubles 110/74 BCD.......minimum ring size is 34, which is absurd for a touring bike.
mumbojumbo
Posts: 781
Joined: 1 Aug 2018, 8:18pm

Re: Crank Length

Post by mumbojumbo »

I believe that bikes should be individualized for each rider,but a bespoke solution will seem dearer. Many seem to want to save and shops sell a standard bike .So much better to buy used bike and modify using the sums saved.
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pjclinch
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Re: Crank Length

Post by pjclinch »

Stuff like the Islabikes main range is notionally for children but in reality it's for "people of smaller stature", and unlike many kids' bikes they don't take shortcuts because you'll grow out of it in two years anyway. And cranks down to very short are readily available as a result. So they are there, but IIRC Isla Rowntree had quite a job getting smaller than "standard" but high quality parts when she started the business.

Pete.
Often seen riding a bike around Dundee...
Tricyclebell
Posts: 9
Joined: 1 Feb 2019, 8:50pm

Re: Crank Length

Post by Tricyclebell »

Hi, I am 4ft 10inch tall and have cycled for over 40 years. I started on 170 cranks because I knew no other and that's what the small bike came with. I often wondered why the small people in the group were out of the saddle first on the climbs. Many years later thanks to TA I ended up with 160 cranks which provided me with a far more comfortable riding position and my pedalling became more productive with less fatigue. My last four frames have been custom build because I have found the efforts of 'the big boy frame builders' fall short of anything comfortable ie full size wheels, at best 165 cranks, the idea was good but the compromise on frame geometry and crank length was very evident. There is now greater understanding (scientifically based )relating to crank length, there is a formula available to work this out. I now ride 145 cranks which have transformed my cycling (I use BOX chainset and cranks which actually go much lower than 145 ). Shorter cranks provide me with greater power particularly on the climbs and I arrive home from rides with less fatigue. The shorter cranks are also much kinder to my hips and knees which must have suffered from huge compression forces over many years. I know have osteoarthritis and believe if it were not for the shorter cranks I would not be riding the bike now. I looked at Isla Rowntree bikes but still the appropriate bike for my size had cranks which would have been too long. Frog do children's bikes and I actually bought one as an interim measure and their bikes were perfectly sized with cranks. I hope your wife has many more comfortable years cycling and perhaps one day the cycle industry will accept the fact that even short people deserve to be comfortable on a bike.
Stradageek
Posts: 913
Joined: 17 Jan 2011, 1:07pm

Re: Crank Length

Post by Stradageek »

Good to hear your story Tricyclebell, the more we ride (with Jean on her shortened cranks) the more she feels and the more I can see the dramatic difference.

Her cadence is faster and her whole body posture is more relaxed but best of all is returning from the latest 35 miler with no aching limbs :D
Bmblbzzz
Posts: 4089
Joined: 18 May 2012, 7:56pm
Location: From here to there.

Re: Crank Length

Post by Bmblbzzz »

Tricyclebell wrote: 16 Jun 2021, 8:27pm Hi, I am 4ft 10inch tall and have cycled for over 40 years. I started on 170 cranks because I knew no other and that's what the small bike came with. I often wondered why the small people in the group were out of the saddle first on the climbs. Many years later thanks to TA I ended up with 160 cranks which provided me with a far more comfortable riding position and my pedalling became more productive with less fatigue. My last four frames have been custom build because I have found the efforts of 'the big boy frame builders' fall short of anything comfortable ie full size wheels, at best 165 cranks, the idea was good but the compromise on frame geometry and crank length was very evident. There is now greater understanding (scientifically based )relating to crank length, there is a formula available to work this out. I now ride 145 cranks which have transformed my cycling (I use BOX chainset and cranks which actually go much lower than 145 ). Shorter cranks provide me with greater power particularly on the climbs and I arrive home from rides with less fatigue. The shorter cranks are also much kinder to my hips and knees which must have suffered from huge compression forces over many years. I know have osteoarthritis and believe if it were not for the shorter cranks I would not be riding the bike now. I looked at Isla Rowntree bikes but still the appropriate bike for my size had cranks which would have been too long. Frog do children's bikes and I actually bought one as an interim measure and their bikes were perfectly sized with cranks. I hope your wife has many more comfortable years cycling and perhaps one day the cycle industry will accept the fact that even short people deserve to be comfortable on a bike.
Do you know the formula, or know where it might be found?
TIA.
Jdsk
Posts: 8292
Joined: 5 Mar 2019, 5:42pm

Re: Crank Length

Post by Jdsk »

A collection of formulae and tables:
https://bikedynamics.co.uk/FitGuidecranks.htm

Jonathan
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