Crank shortening?

Anyone had this done? There’s a company called Highpath Engineering which offer the service but unfortunately their pricing (£62 for a pair of cranks) and lead time (4-6 weeks!) makes it more attractive just to buy a new set of cranks…

Any alternatives?

152mm – https://www.amazon.co.uk/d/Sports-Outdoors/Kettenradgarnitur-SR-Suntour-T20242-Z%C3%A4hne-schwarz/B0029MP3AO

https://www.decathlon.co.uk/36t-single-chainset-140mm-id_8012956.html?iv_=__iv_p_1_g_25792541576_c_93735761456_w_pla-163027379096_n_g_d_c_v__l__t__r_1o6x_pla_y_15177021_f_online_o_586867_z_GB_i_en_j_163027379096_s__e__h_9046062_ii_9046042_vi__&gclid=CjwKEAjw1qHABRDU9qaXs4rtiS0SJADNzJisciBhvkeQrdnItOV73DrLWZRfW7IE-v6KX_4RK0pdnBoCHsbw_wcB

I shortened some myself; a pillar drill is very useful, otherwise just hacksaw and file. I could only find square taper short cranks for sale and needed to fit an external BB. I used Shimano Deore, 175mm cut down to 150mm. The alloy can be tapped with (for example) these. I put a pedal in the original thread, lined the pedal axle up with the axis of the pillar drill, clamped the crank in the vice, removed the pedal then slid the vice 25mm and drilled it, tapped it by hand then cut off the end and reshaped the crank.

The cranks I used have a recess on the back so there’s only 80% of the original thickness in the new location. I filled the recess with epoxy metal. For the rider and use it will get, I’m not worried by the reduction in strength, but if you’re landing big air you might need to find a more solid crank to modify.

http://singletrackworld.com/forum/topic/shortening-cranks

http://singletrackworld.com/forum/topic/shortening-cranks-for-kids-how-short-to-go

http://singletrackworld.com/forum/topic/shortening-cranks-1

http://singletrackworld.com/forum/topic/shortening-cranks-2

http://singletrackworld.com/forum/topic/cranks-that-can-be-shortened

http://singletrackworld.com/forum/topic/which-cranks-for-shortening

http://singletrackworld.com/forum/topic/have-you-ever-drilled-and-tapped-cranks-to-make-shorter

http://singletrackworld.com/forum/topic/machinist-to-alter-kids-bike-cranks

http://singletrackworld.com/forum/topic/are-slx-crank-arms-hollow

http://singletrackworld.com/forum/topic/options-for-kids-cranks-should-adults-be-using-short-cranks-too

http://singletrackworld.com/forum/topic/kids-bike-crank-sets

Right, that’s me finished reading all Paton’s links 😉

Doesn’t sound like I should really be thinking about it, I only wanted to shorten by 5mm! All a lot of hassle/expense for something that maybe won’t make much of a difference…

5mm? Youd need to be shortening by a greater amount than the pedal insert anyway..otherwise you’ll have an hole with only one half.

Yeah, I’d thought maybe it would be possible to drill the hole into an oval and fill the gap with a CNCd insert or something, doesn’t seem that difficult for someone handier than me (and with the right tools…).

I guess there isn’t much of a market for shortening such a small amount.

Depending on the cranks, it might be possible. Plug the original hole and then drill and tap. I made some 165mm fat bike cranks that are still working well. If I can get the pictures off Photobucket, I’ll post them.

Seems easier to get an off the shelf solution. Nx cranks are 155mm and up.
What cranks are they? as with many having pedal inserts or pedal washer recess, I can’t see the cost making sense unless it’s something boutique.
Screw in pedal adusters to shorten crank lengths are available, but increases the q factor

Well, that was a PITA! I’ve moved the pictures of my crank shortening experience from photobucket(spit) to Flickr – I got fed up trying to find affordable shorter cranks for my Farley 5 fat bike, so decided to take matters into my own hands and try and plug the existing 175mm cranks and re-drill them at 165mm (I’ve found I get a sore knee on 175mm cranks, but 165mm suits me fine).

The implements of destruction:

DSC_4392 by tillydog[/url], on Flickr

(Mostly from a buying frenzy on ebay).

From left to right:

15mm 6061 T6 aluminium round bar (note that it needs to be something pretty hard, like T6 temper to avoid deforming under the load from the pedals – plain aluminium would be no good).

9/16″ x 20 tpi dies – left and right hand

Loctite 638 retaining compound

13mm bullet point drill

14mm drill

Pedal thread repair reamer/tap (left and right handed)

Pedal thread repair inserts (left and right handed)

The 15mm bar was too big to tap directly, so I had to ask a favour of someone to cut a piece off and turn the ends down to 14.3mm so that I could tap LH and RH threads on it and split it into two:

DSC_4337 by tillydog[/url], on Flickr

DSC_4338 by tillydog[/url], on Flickr

After thoroughly scrubbing and degreasing the aluminium plugs and the crank threads, the plugs were fitted to the cranks with the Loctite 638 (really strong threadlock) and set so that the tidy face was level with the outside of the crank:

DSC_4339 by tillydog[/url], on Flickr

DSC_4340 by tillydog[/url], on Flickr

After letting it set overnight, I had a damn good try at unscrewing the plugs to confirm that the Loctite had worked  – it had! Then I trimmed the excess off as best I could:

DSC_4344 by tillydog[/url], on Flickr

One day I will learn to cut straight… (my metalwork teacher would be spinning in his grave.)

DSC_4345 by tillydog[/url], on Flickr

To try and make sure that the new holes were parallel to the crank (nobody likes a wonky pedal) I made a wooden jig with a hole through it which was a close fit on the crank spindle (24mm). This took a bit of care to make, as I wanted to get the hole as square as possible, but it had to be drilled in stages as the drill bit wouldn’t reach through the full thickness. The crank itself can be used to check that the finished item is square.

DSC_4394 by tillydog[/url], on Flickr

Note that the weight of the cranks is taken on a separate block of wood under the drill spindle – the hole is only for alignment.

DSC_4397[1] by tillydog[/url], on Flickr

A bit of careful measuring and the location of the new hole was marked and centre-punched. It is quite close to the shoulder of the pedal washer recess.

DSC_4347 by tillydog[/url], on Flickr

I used a centre drill to start the hole to try and ensure that the drill bit didn’t get kicked over by the recess, then drilled through with the 13mm bit (it is a “bullet point” bit, and doesn’t need a pilot hole to drill accurately).

DSC_4365 by tillydog[/url], on Flickr

No going back:

DSC_4366 by tillydog[/url], on Flickr

I could have tapped these holes for pedal threads directly, but was bothered that the wedging effect as the pedals were tightened would fall across the glue line for the threaded plugs, so wanted to bush the holes with crank repair inserts to reinforce them.

This meant opening the holes out to 14mm

DSC_4351 by tillydog[/url], on Flickr

and using the combined reamer / tap to cut threads for the repair insert:

DSC_4373 by tillydog[/url], on Flickr

(It isn’t in the drill here! The drill chuck is a loose one that I was using as an improvised tap wrench as my tap wrench was too small to take these taps).

All through this, I was chanting to myself “left crank – left hand thread – left crank – left hand thread – … …” (I hid the wrong tap from myself and then had a cup of tea before re-checking that I had the correct tap… twice).

DSC_4358 by tillydog[/url], on Flickr

DSC_4364 by tillydog[/url], on Flickr

DSC_4361 by tillydog[/url], on Flickr

Looks about right…

DSC_4360 by tillydog[/url], on Flickr

Same again for the other side, then some more Loctite (after cleaning and de-greasing everything again) to secure the inserts:

DSC_4379 by tillydog[/url], on Flickr

DSC_4380[1] by tillydog[/url], on Flickr

The pedal threads were heavily oiled and fitted to the inserts and tightened up to make sure that the inserts were fully home and tight.

DSC_4381 by tillydog[/url], on Flickr

DSC_4382 by tillydog[/url], on Flickr
I checked that I could still loosen the pedals after a few hours and then left it all to set overnight.

Pedal side:

DSC_4385 by tillydog[/url], on Flickr

I got lazy and decided to use the belt sander to clean up the back of the cranks. This actually worked OK once I got the hang of it:

DSC_4402 by tillydog[/url], on Flickr
Unfortunately the first one didn’t go so well:

DSC_4403 by tillydog[/url], on Flickr

Still, it’s only cosmetic.

Pedals fitted and back on the bike:

DSC_4404 by tillydog[/url], on Flickr

DSC_4406 by tillydog[/url], on Flickr

A brief test-ride around the block in the dark confirmed that nothing wobbled or fell off, and they’ve been been fine ever since (18 months and about 1300 miles).

I considered taking some material off the end of the arms, but decided that this risked weakening them too much. In practice, the slightly long arms haven’t been an issue at all.

Don’t try this at home, kids.  😉

I’m vicariously enjoying the satisfaction of a job well done there Tilly, but there is 0% chance of me trying that at home, my garage currently consists of one wobbly set of shelves and a bit of old carpet covered in my begged borrowed or stolen tool kit.

Interested in your knee experiences, that’s why I was thinking of shortening, although I suspected, and have had many people since confirm, that 5mm wouldn’t make a difference. Still might do it if I can find some cheap 170mm cranks.

Thanks – 5mm makes a huge difference to me (at 5’6″). I’m just about OK on 170mm now, but 165mm are definitely more comfortable. 175mm are a no-no, but it took me a *long* time to figure this out.

I’d encourage you to find a way of trying it out to see if it works for you. The people who say it doesn’t make any difference are the ones who don’t have a problem – If you have a “normal” width BB, XT cranks are available in 165mm, and there are a fair few available second hand if you just wanted to try something.

I’d encourage you to find a way of trying it out to see if it works for you. The people who say it doesn’t make any difference are the ones who don’t have a problem

agreed I went from 175mm to 165mm and found I was less stretched out which resulted in less fatigue on long decents and feeling more stable when cornering. I aslo now find sitting climbing easier as i can spin the cranks a bit faster I’m 5’11”