Hisamatsu

I have a 2017 Brompton and looking for shorter crank arms. The stock ones are 170mm, and I am looking for 130-140mm length.

Seems to be square tapered BB with 130BCD. Has anyone replaced the crank arm or know where to buy one?

badmother

There used to be somebody who modifyed crank arms. Cut them shorter, drilled new holes and new threading.

Hisamatsu

Yea that is also an option, tapping/threading existing cranks then trimming the edge shorter.


Not sure who/where to get it done. I don't have the tools to do it myself.

fietsbob

there are shorter crank arms I think J&B a US company has 150s , Origin 8 is one of their brands.. Imported..

yes custom machine shops can make a second set of threaded pedal holes.. In the Shop we have some children's bikes
with cranksa with 2 sets of threaded pedal holes..

badmother

https://no.search.yahoo.com/search?f...rtening+cranks

If this is something you want done on several bikes maybe getting the tools is worth it (if you have a place to work) but if it is possible to buy qualituy cranks in your length at a fair price I guess it is tempting.

Have you searched E-bay, Alibaba and similar sites?

blakcloud

2017 model uses JIS.

blakcloud

Just out of curiosity why so short?

Thorn make cranks that look half decent in 140.

Hisamatsu

I only need it done on two bikes. If I can find someone that know how to do it that would be awesome.

I see many cranks that are 135mm like this one.

Promax SQ-1 Square Taper JIS Cold Forged Crank Arms 135mm Black | 365 Cycles

But unsure if it will work with the brompton. Are all square tapper JIS the same?


My SO has short legs. 700mm long.
https://www.stevehoggbikefitting.com...gth-which-one/

According to this the formula is

crank length = (inseam in mm) * 0.216

According to https://www.junik-hpv.de/ specializing in short people bike

crank length = (inseam in mm) / (between 5 or 5.5)

But definitely a shorter than 170mm default brompton crank is more suited.

https://www.sjscycles.co.uk/search/?...%20140&geoc=US

I only see 110 BCD on their site. Will this even work with a brompton that use 130?

Guess I'll just have to find new chainring and shorten the chain?

badmother

Should be the same if tey are not cheap and made in somebodys backyard

blakcloud

For those cranks that you posted, yes you will also need a new chainring. You won't have to shorten the chain if you keep the same number of teeth or close to it when you purchase a chainring. In the link you posted the Origin8's look like they would be the best choice.

Yes all JIS are the same. No need to worry about that.

Hisamatsu

Read that chainline alignment might be an issue with replacing cranks, how would I know?

The origin8 alloy crank spec says 107 SQR JIS, what does the 107 mean? I think Brompton BB is 119?

My brompton has 44t, if i go 34t do i need to shorten chain?

blakcloud

It becomes trial and error at this point. You install your new crank and hope that it works. If it is off you replace the bottom bracket for the corrected length. Phil Wood makes a bottom bracket with 5mm of leeway but that is a very expensive way to go.

No matter which way you slice it, this is an expensive upgrade and I am not convinced it is needed. Not trying to change your mind (or am I?) but Steve Hogg is a professional bike fitter for racers who want to maximize their potential. The Brompton is not for racing (except Brompton World Championships). So using his formula is being done in the wrong context. The other website is in a language I don't understand so maybe there is something there that would make me change my mind. Again this is a Brompton. If you were looking for the most efficient bike, this isn't one I would chose and I own a Brompton.

I get your SO is on the smaller side but is she really complaining about the cranks or is it you who thinks it would be better to make them shorter? Why not just let her enjoy the ride.

Can this upgrade be done? Absolutely. Throw enough money at it and it can be done. Could I be wrong about crank length? Absolutely. I am not an expert but there is a reason that even bikes made for smaller women don't have 140mm length cranks.

If you do the upgrade let us know how it works. It would be great to hear what she thinks after the upgrade.

blakcloud

107 and 119 is the length of the axle end to end. Most single speed bikes use a 107 but as you can see Brompton is different. As I said in my last post it is trial and error. Chain line depends on the crank and bottom bracket in unison.

You could buy a few lengths of bottom brackets and return the ones that don't work. This eliminates going back to the bike store multiple times.
@1nterceptor has a video on changing his crank, I think. He posts on here hopefully he will chime in.

If you go 34, you have to shorten your chain. By how much, I don't know. Again trial and error.

Hisamatsu

170mm is definitely too long. While still ride-able, kids even ride adult size bike, but comfort isn't there. My SO has basically the leg length of a 10-12yr old. We got her a islabike designed for kids with 155mm crank and it felt much better.

I was hoping to not do Trial/Error and also replace the BB. Hoping someone has done the crank replacement before and tell me which to get

fietsbob

Bear in mind, a different crank may not fit , not because the square taper is not matching, ISO/JIS,

but because it will move the chainring out of the straight chain-line. having a different profile...

So how about adding Pedal Blocks? your pedals will be closer, to the seat, the power stroke will still be 170, as the arm is unchanged.
so like an oval.




....

blakcloud

Well that makes perfect sense now. Your SO has tried the shorter crank and liked the smaller crank. The Islabike was a good choice. Hopefully someone has the numbers you are looking for. I would still buy the crank and chain ring and try it on the 119, you might just get lucky. Good luck.

fietsbob

there are crank shorteners for a Kid back tandem , its not a permanent hole

it just lets you screw the pedal into it, which in turn is screwed into the regular crank arm pedal hole..

pedals are unavoidably further apart..

tcs

And of course:

shorter cranks, bottom bracket same place > foot further from ground @ maximum extension (bottom of stroke) > saddle higher > rider position higher > bars higher, bigger step up&down when starting/stopping, less leverage (torque) on cranks

None of this is a showstopper, just stuff to consider.

badmother

https://www.bikefriday.com/bicycles/models?select=17550

Hisamatsu

I think the Brompton has a 119mm spindle. The promax sq-1 works with 113. Could I just use chainring bolt spacers to get the chainline correct?



https://store.bikefriday.com/product...fql57trnv97p04 $150, I could get a crankset like the origin8 or promax for less, just unsure how well they fit.


So the pedals are ~20mm wider on each side, will that still be comfortable?

12boy

A couple of point to consider...different cranks require different bottom brackets. Generally speaking it is good to have a straight chain line so that the sprocket(s) on the back are in the same plane as the chain ring. It is entirely possible that any of these will work with the Brompton bottom bracket, and once you have the crank on the bottom bracket spindle you can see if a shorter one is needed. There is a concept called "q" factor which is how wide the pedals are apart. With a fat bike, for example the q is large to clear the tires. Some feel the lower the q the better, although I personally can't detect a small difference. Wide q means you are riding more bowlegged than narrow q. Vuelta was selling bottom brackets for $13 or so, and I have been running a 118 one for several years in my B with no problems. At that price a replacement one is not a big expense.

12boy

The other thing I forgot to mention is that longer cranks have more torque but spin less easily than shorter ones. If you drastically reduce the length of the crank it could be harder to take off in a given gear, but once up to speed, easier to pedal at a high RPM.

Hisamatsu

Ordered the Promax SQ-1 to try out, hoping it will fit as is or with some spacers.


I would assume Q-factor comfort will be more noticeable the shorter you are

edit: someone on the brompton yahoo group confirmed they are using the Origin8 crank fine. Maybe I should have gone with that one instead of Promax

Rick Imby

It is a problem since you are focusing on it. Human bodies are very good at adapting. If she did not have the option of a 130 and you had not pointed it out that she should have a smaller crank arm she would never have thought of it.

Do you redesign buildings with shorter or taller stairs? Does she have a problem walking up stairs?

The standard stair in the US is 7" which equals 177.8 mm.

I believe it can make a difference it performance and it is great that you are spending the energy and money to build it for her.

Have fun and report back---

Hisamatsu

There is a reason why kids bikes don't come with adult 170mm cranks, because they are smaller and shorter. My SO is basically kids dimensions, she had issues while riding regular adult size bikes, knee pain, discomfort etc, we didn't know why, just assumed it was how it is. Took a couple years for us to find islabike which has shorter cranks, they are basically bikes designed for kids. She was happier with it, less discomfort and complaining means more willingness to ride. Got a brompton for travel and knee issue again. All the effort is worth it as I hear less complaining from her while riding =]