little person bike...

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Oct 22, 2019
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My wife's sister is a 'little person', aka a dwarf. she has extremely short upper legs and arms, she's not quite 48" (122 cm) tall overall.

When she was in her 20s, I found a kid's bike, and fixed it up for her, put better wheels on the frame, put a bigger chainring on the small kid cranks. I forget if it had gears, but I remember she really enjoyed being able to ride it....

now she's in her 40s, and undergoing rehab from hip surgery (and has a 2nd round to go on the other side). but once she's recovered from that, I was thinking adapting a bike for her would be a cool thing.

a few years ago, my wife bought a Giant folding mini-bike. She never rode it much. I think it has 20" wheels, 7 or 8 speed derailleur. The crank is normal-adult sized, so SIL's short legs can't handle it. Tonight, I dropped the seat and bars as low as they went, and it was pretty low. if I can find some short cranks for it, like from a youth BMX, and put a little hub motor on it, maybe 300-500W, with a twist-throttle, heh, she could get around and get some exercise too.

(she has a car, a Corolla with adaptive controls, so its not like she's a shut-in).
 
I've done bikes for folks who were not munchkin-short, but had extremely short legs. Short cranks are one part of the equation, but low bottom bracket height and low ratio gearing are also important. The only ways you can lower the BB of a given frame are to use smaller diameter wheels or swap in a shorter fork. There are technical complications for both.

One of the points against using a folding bike with small wheels for your project is that the wheels are already small, and when you use even smaller wheels, things get very silly very quickly.
 
Would she fit on a BMX bike or Chinese minivelo ( 20 inch wheels, unusually small frame ) ? because that would be a good place to start.
 
it was like 25 years ago, but I think the bike I built her was based on a kid sized BMX, that I retrofitted a classic 'banana' seat on and some really small cranks, which IIRC were ashtabula since the bmx frame was built for those. I think it might even have been a 16" wheel bike, or maybe I put a 16' wheel on the front and it had a 20" on the rear. I dunno, that was a long time ago.


anyways, upon daylight, and looking at the Giant Expressway folder again, the stepthrough is too tall even if I solve the crank problem.
 
Normal crank arm length is 170mm (6.7 inches)but I seem to remember seeing them for sale down to 140mm (5.5 inches).

edit:
https://www.ebay.com/itm/124669946841
 
LewTwo said:
Normal crank arm length is 170mm (6.7 inches)but I seem to remember seeing them for sale down to 140mm (5.5 inches).

127mm is a common size for kids' bikes and unicycles. I have that size on my adult sized Big Wheel trike.
 
Did you consider Cleary or another manufacturer of pretty good kid's bikes?
 
There are lots of options.


150mm
https://www.ebay.com/b/Industrial-150-mm-Crankarm-Bicycle-Cranksets/109118/bn_7116570326?mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5337590774&customid=&toolid=10001

140mm
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2499334.m570.l1313&_nkw=140mm+bicycle+crankset&_sacat=109118

I'd lean towards something like these 140mm Promax cranks:
https://www.ebay.com/itm/194231984517?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D233120%26meid%3D332fe422a4214a2ea4bb39d7a1a48344%26pid%3D101195%26rk%3D3%26rkt%3D12%26sd%3D283552361094%26itm%3D194231984517%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv9PairwiseUnbiasedWeb%26brand%3DPROMAX&_trksid=p2047675.c101195.m1851&amdata=cksum%3A194231984517332fe422a4214a2ea4bb39d7a1a48344%7Cenc%3AAQAGAAACAN%252Bgz7uJIME1eQqLCCN9U8qiclUryu0Z7oOIjcouyT%252BFKPFmXn9I2XTHAH4YerhWQiqwv7c%252B9UftLGOf7XVxewKtaTsMUci%252BvtFLrsvwulsYsYCdSmiILxsdzcDEnMiskCx7Ct5w0DsfKdoJnoyhqTMG9hHogLfDM4O46UZcz3jFGBxdobv9A8XhXG1epWsoFZNPbQcbqutt6PZ6HzuYdHz%252BbFFTwRTwsoxl3%252BE6qMzZfX1f9pE%252FffDMLoUgw6mdk85AHslYJ%252BC3gbmjIeOsASGG5GCadIf6juqmz7tiEbZwLfNkk5LlnWd%252BaSPE%252BU0ejimMjGc7Ny1%252FdmYeEyjDtes%252FxP1sioOompTYWA8G4Y0Jm5b%252FKi5eq9SxN0Nenv8%252BaFVF9JvTijIz8HKBMjfGBY6AHL8pL3WtwIDKdlBTmSpiEK3zbqsdOEbuGRSoFZZioUrkjpq1KvL%252FN1wcQO6Sj0lHA1DsA1s07qkXX0dn9WZPy36rpbc%252Fi6ZZTOTKRfC1503f7PNmODR%252F6BPAgJcAMgM0qASJ3Wov2ACYVUEPiGqyRN3T4owwxIrBwLaI6lmz9QMep8MIH44lkkNIDcOj13q55mSuhzANsNMuLJ50w9STjGH8JR741Jn1aftuIXoTwi9TI3w%252FoF%252Bdooaja5joE0KaNYs%252BNJoGucG%252BQVJUdlc2%7Campid%3APL_CLK%7Cclp%3A2047675&epid=1512250662

They take a 5 bolt 110mm BCD chain ring on a 113mm JIS bottom bracket. Q-Factor and chainline will be about right.

...

With very short legs she probably won't want excessive Q-Factor (width between pedals) which can be uncomfortable so be mindful. If the crank arm has a lot of offset it may be too wide. Straight-ish arms.

Mostly looking at square taper cranks, be aware there are JIS and ISO standards that are not interchangeable. Match the crank and bottom bracket.

Also want to ensure the chain ring is 7 or 8 speed (or however many gears it has) compatible. Seven speed chains are used for narrow single speed (eg BMX). Eight speed and up are narrower.

Probably want about 45T chain ring, but it wont set any speed records. Up to about 25-30kph pedalling, so won't need a ton of power.

Sounds like a candidate for something like a simple Bafang hub with a baserunner controller, either front or rear and thumb throttle. I would fit a low power rear at about 250W (electrical) to reduce the chances of surprises for a inexperienced rider with a double hip replacement. Power (and speed) is easily limited in the baserunner.

BTW, depending how much you love or hate her :wink: you might consider buying her hip armour. A crash can be an absolute disaster with hip replacements.
 
It is definitely a custom job, or a kid’s racing specialty bike. I mean, most of kid's bikes are weak, poorly built for a small adult.

The example of a BMX is good, because racing grade BMX bikes are tough and very well built. Any racing quality bikes of other specialties for kids would be good too. If you need full suspension, there are kids DH racing bikes on 20’’ wheels. Commencal Clash, Prodigy Lil Shredder, Norco fluid, Meek Boys... More choice in 24’’ kids DH, and they could be built on 20’’ wheels for lower height.
 
LeftCoastNurd said:
anyways, upon daylight, and looking at the Giant Expressway folder again, the stepthrough is too tall even if I solve the crank problem.

These things could be solved by cutting the seatpost down, and also the handlebars.. or replacing/modifying the handlebar tube.
You could also drop the height further by going with 18 inch wheels, if you have the capability to run disk brakes on a bike like that.

I have weird fitment issues myself. Long legs and short torso means i have to modify anything i ride right off the bat. All kinds of tricks are possible.

A kid's bike is a good place to start!
 
LeftCoastNurd said:
When she was in her 20s, I found a kid's bike, and fixed it up for her, put better wheels on the frame, put a bigger chainring on the small kid cranks. I forget if it had gears, but I remember she really enjoyed being able to ride it....
Go the same route again. I've seen some very good kid's bikes (16 or 20" wheels, 7-8 speeds, disk brakes) available from REI and Specialized.

For power try a roller drive. The QiRoll is one I've had experience with, and it works OK for low powers (a hundred watts or so.) I assume speed and/or hill climbing is not her primary goal. You can definitely get a front hub motor for a 20" wheel, and perhaps even a 16" wheel (I have a 750 watt DD hub on the smaller of the two 16" wheels) so that's doable as well.
 
re: Q factor she has nearly an adult size torso, including pelvis. its her UPPER leg bones (thigh) that are extremely short. So, width isn't a problem, its leg stroke. I suspect even 140mm cranks are way too long, thats an 11" diameter circle.

re riding after hip surgery, I figure at least a year of recuperating before we even think of this.
 
I think you’d better ask her first if she wants to pedal. I mean, maybe she likes riding more than cycling. It would give a different approach to the project.
 
while she long ago 'lost' the bike I'd put together for her, she has a bike my wife bought for her when she was in massachussets, and A-M used to ride it. she hadn't ridden it in about 10 years due to gaining too much weight, but she's lost all that. obviously she can't ride now... the ebike I built my wife has a twist throttle and a 1000W rear wheel motor on a step through 26" 'comfort bike'. I turned the power down to 500W, and its still a rocket, will do almost 25MPH on flat land with zero pedaling.

so, yeah, 250W, and I think for her, a thumb throttle (her fingers are somewhat deformed) rather than a twist. sigh, I bet nobody makes sub-20" wheels with hub motors for cheap.
 
Lots of cheap small motor wheels with integrated rim, 10 inch being the most popular and makes about 15 inch OD with the tire on.
 
The other problem not mentioned here is how will the builder test and adjust it being that the bike is probably too small for him to ride? I will be facing a similar problem when I build one for my 5'7" girlfriend. It will be too small for me to ride it. Usually for a pedal bike one can do pretty good on a stand but I wonder for an e-bike. I guess a simple go/no-go test of the PAS and/or throttle will work on a stand. But you won't be able to do much else.
 
I'd ask if she wouldn't mind a bicycle-based scooter? Get a 20-inch ladies frame, and put 16 inch wheels on it, that should make the step-over height very low.

I dont recall where, but there are kits that put two fixed pedals into the bottom bracket, with no crank-arms. Looks like a straight bar in the BB shell.
 
spinningmagnets said:
I'd ask if she wouldn't mind a bicycle-based scooter? Get a 20-inch ladies frame, and put 16 inch wheels on it, that should make the step-over height very low.

I dont recall where, but there are kits that put two fixed pedals into the bottom bracket, with no crank-arms. Looks like a straight bar in the BB shell.
Would be a simple matter to design a 3D printed plug to go in each side of the BB.
Could even size the ID so that the existing pedals could be used for the foot rests.
 
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