I have been assembling a bike for my daughter for which I’ve decided to shorten a pair of cranks.

She is 116 centimetres tall and this piece suggests ~10% of height is a good starting point. and slightly shorter is better than too long. So my plan is to do 115mm…

I can probably source another pair to modify in time for when she’s a bit taller, but nobody does anything quite as short as this and certainly not in 104 BCD, which is my preference…

Anyone else gone through this exercise? How short did you go? Any other pointers?

Check out the crank lengths (and suggested height range) for Islabikes, they seem to be pretty much the standard for kids sizing and work well enough. Off the top of my head I think she’s about right for a Beinn 20 small, which comes with something around 115 (100 on our Cnoc 16, 125 on our B20 large), so you’re in the right ballpark. Remember kids grow though, and they will generally cope fine with something a bit too big.

Yeah the isla cranks were one of the things that underwhelmed me a little about them, yes they get the lengths about right, but they’re still using fixed chainrings, I much prefer the idea of being able to change chainrings like I can on all of my own bikes…

I got her an old hotrock 20, it’s not too bad, I like the frame, some stuff is being changed right away though, and it came with 127mm cranks, she’s definitely a shortarse they’ll be too long for at least another year I reckon…

Why?

Spawn cycles do kids cranks.

@aracer – Why not?

@peabrain – I am aware of spa cycles and SJS options for kids cranks but neither was quite what I wanted…

Doesn’t really answer the question – surely there has to be some perceived advantage for it to be a reason to dislike Islabikes.

kids cranks

Don’t Profile make short cranks?

As you say, I think 115mm would be ideal for your daughter. If you have access to a pedestal drill I’d recommend modifying a pair of Sram S600 cranks (£30ish). To go down that short you’ll have to spotface the hole, but that’s not a big issue.
You’ll need a 13mm drill and some pedal taps (superstar sell the taps).

These sites might be of interest:

http://faqload.com/faqs/bicycle-components/drivetrain/crank-arm-shortening

http://bikesmithdesign.com/Short_Cranks/s600.html

Not a child, but my 5foot wife has 140mm cranks.

(Apologies for mixing unit, I think she’s about 1m55)

She’s using sram s600 cranks, they’re actually really nice cranks! Shortened by highpath – who did a bloody great job)

Right now, I’ m trying to shorten some new shimano cranks, for her new Giant Lust, which she’s refusing to ride till I sort out the cranks…

I already managed to get some cheap old sugino cranks, shouldn’t be a problem laying my hands on a pillar Drill and the local bicycle kitchen reckon I can use their pedal taps…

aracer, no issue with Isla bikes…

Edinburgh bike do a Junior Chainset which is good value, but 150mm ish, as do SR Suntour, they turn up on ebay. 24″ Trek MT220 kids bikes used to come with cranks with two drillings, at about 120mm and 150mm, but I’ve never seen them sold individually. I know they had replaceable rings though.

Has anyone for a single ring crankset for Kids? Any help would be appreciated… Would love it if it was GPX or Hollow Tech compatiable…

I’m thinking of building a 1 x 10 speed from my lad on 24inch wheeler – found here – super light frame set – http://www.poison-bikes.de/shopart/2015000021/Rahmen-Rahmen-Kits/MTB-Rahmen/HARDTAIL-RAHMEN-_Teilgefedert_-24-Zoll/Zyankali-24-Rahmenkit-Kid.html?&d=1

use a thorn crank for a 1×10 on my son’s ripcord with a 13-40 range rear, 30t front. would get him an 11sp XT if I could afford it. There is a suntour triple that is a bit heavier than the thorn but only £20 from germany through amazon.co.uk

http://www.amazon.co.uk/Kettenradgarnitur-SR-Suntour-T20242-Z%C3%A4hne-schwarz/dp/B0029MP3AO

The chainstays on that frame are huge for a 24inch kids bike as is the reach

Not a child, but my 5foot wife has 140mm cranks.

That’s about right, approx 20% of inside leg seems to work out as a guide, children up to adults. I’ve had a play with bike fitting calcs that put me on 185mm or something crazy, I use 170mm on my road bikes.
We use 92mm on 14″ bikes, 102mm on 16″ and 114mm on 20″. They look quite short but they mean kids don’t struggle to get the pedal over the top particularly when the saddle is low for more confidence.

James, thanks for your reply.

As this threads representative of ‘the industry’, is there anything you can do about the total lack of size appropriate cranks?

i can’t think of any other bike-fit-critical component that’s available in such a narrow range of sizes.

grips, frames, stems, saddle-width, bar-width, wheel-size, etc. all available in a wide range of sizes.

cranks? a whopping 6% difference from 175-165. I’d need to try 230’s to experience just how awful 165’s must feel for a 5footer.

if we (for now) go with the 20% rule of thumb*, and consider that the average woman is 5’4″, then most (if not nearly all) women will be riding bikes that simply cannot be made to fit them comfortably – in a way that 6footers take for granted.

(*and as you say, 20% puts us taller chaps on 185’s, **** that. If anything, 20% is on the long side)

Pulse Crank Shorteners

https://crosscountrycycle.wordpress.com/2014/08/25/crank-length/

https://books.google.co.uk/books?id=VB_kAwAAQBAJ&pg=PA176&lpg=PA176&dq=phil+burt+crank+length&source=bl&ots=Q2TxDBCwn7&sig=O18GAPsu3w05-nwyjgwFPpjZAaA&hl=en&sa=X&ved=0ahUKEwjPxpixsabJAhUHNxQKHc92AMMQ6AEISjAG#v=onepage&q=phil%20burt%20crank%20length&f=false

https://www.powercranks.com/cld.html

Has anyone for a single ring crankset for Kids? Any help would be appreciated… Would love it if it was GPX or Hollow Tech compatiable…

I just used a 170mm SLX crank, with a Uberbike NW chain ring. Has been perfect, not dropped the chain once. He’s no giant, I’m not convinced the crank length is worth worrying about beyond the shortest one you have in the spares box (unless you’re a manufacturer trying to be absolutely spot on in all regards, which is fair enough).

I think too long isn’t too bad either – my boys tend to climb out of the saddle, maybe a throwback to running single speed till they went to 20″ wheels. Extra leverage seems handy.

My tallish 8 year old is on a 150 thorn crank with a 30t NW ring. It’s a good fit for him, but a reduced q-factor would be preferable. I imagine this is where the fixed ring cranks like those on islabikes have an advantage as they can custom spec a small q-factor without worrying about a the clearance of a traditional triple spider set up.

The thorn is square taper though.

is there anything you can do about the total lack of size appropriate cranks?

Me, no not with our volumes. It’s about what’s available and there is a lack of short-enough options. An example – Shimano STEPs for E bikes. Nothing under 170mm available. I don’t even need any real leverage on them.. and the spacing is wide so to get the BB lower (ie all the weight) I need short cranks. 165mm seems as short as most suppliers offer in e-bike spec.

20% puts me on 175mm at a bit over 6′ and works out at around 152mm for a 5’4″ rider of average inside leg. It’s just an easy calc to divide IL by 5. Could be refined but that puts us into ‘fit calculators’ and I’d then have to ignore it on principle : )
Agreed though, decent cranks should come in a wider range of sizes in 5mm increments. Could be 7.5mm gap at the extreme ends. 2.5mm increments on top end road stuff seems silly and about limiting tooling, it’s nice to have that choice if you’re an average male buyer but perhaps big brands could cater for more riders in total by offering 160-165-170-175 or 155>170mm.

a reduced q-factor would be preferable

True, both need considering. Frog have been promoting this fact for kid’s bike fit recently and it’s an area I’ve found tricky to get the spec right in the past. Choice is improving with the growth of quality kids bikes.

if cranks are too long the rider will bounce up and down. lewis did his with 165mm but not with 150. I use 165 cranks and ride better than I did on 175.

I’d need to try 230’s to experience just how awful 165’s must feel for a 5footer.

I’m 5’6 with a 31″ inside leg (cycling, not clothing measure). I like 165mm cranks but haven’t tried shorter.

Pedalling with 175s feels to me like running up a steep flight of stairs two steps at a time.

I need my seat low enough not to over-extend my leg at the bottom of the stroke (or I get knee pain) which makes the top of the stroke feel very cramped – with 175mm cranks, effectively my seat is 20mm too low (try it sometime). 165mm cranks are like a breath of fresh air, and I can ride faster and for longer than with 175mm ones (same bike / tyres / route).

There are many more length options in square taper if you can find a decent BB. I just wish this extended to ‘affordable’ 2 piece cranks :/

No not spa cycles – SPAWN CYCLES 😀

Cool Canadian bike company. Iirc they don’t do 115mm though – only 89, 102 and 127.

My two rode on the kiddyback tandem with SJS crank shorteners for years (came with adult stoker cranks). The increased Q factor of about 1.5 cm never bothered them. They rode it from age 4 to about 12 without any issues. Q factor is over-rated outside of performance cycling. EXtra holes in good quality cranks is a nice touch, and our old 20″ wheeled Trek MT60 was one of the very first.

@peabrain – Doh! The spawn 102 might have been a better option, but I am half way down the shortening route now…

I might look at getting the 127 for when she’s a bit taller… 75 canuki-sheckles is ~£38 plus some P+P, that isn’t bad…

I started doing this a while ago building a bike for my son. Shortened the crank re drilled and tapped to change to 150 mm
I don’t have a left hand thread tap to do the other crank + I wanted to run a pretty small chain ring that wouldn’t fit on a 104 bcd arm.
The tap was going to be about £20
The simple answe was to buy 150 mm cranks from Isla bike with a 28 tooth ring. They have been on with a superstar top guide same as their creig 26
Gives good ratios he can get up most hills and it never drops the chain.

absolute black do a 64BCD 28t NW that you can run on the granny position on a lot of cranks inculding the thorn one from SJS