The work on little smurfs bike continues (20″ full suspension MTB).

I’ve now got the old crankset and BB off the frame (thanks to help from on here) and attention is now turning to replacements.

As standard it has a 68mm x 116 square taper BB (might be 113 – I need to double check tonight) and a double chainring crankset with 145mm crankarms.

It all weighs a lot (crankset = 742g & BB = 309g), it’s worn out, and I’d like to replace it with:

– A new, lighter BB
– A new crankset, ideally with 140mm arms and which can take 2 chainrings – I think I need 22 and 32t. I’ll be using a 9 speed XT 11 – 34 cassette from the spares box and the bike has 20″ wheels.

I worked out I needed 140mm cranks arms from here:
http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html

145mm arms won’t be the end of the world as little smurf is growing quickly. I’ve also seen others use a triple crankset but replace the outer ring with a modified bashguard, so that’s an option.

I’m struggling with finding short crank arms that will work with small rings and I’d rather not buy a BB until I’ve sorted out the cranks. Light and cheap would be nice as well.

I’d also like to avoid a 1 x 10 set up as I’m using spare 9 speed shifters and a cassette. I’m also guessing a granny ring and 34t rear is going to be needed on the steeper hills.

Does anyone have any suggestions on what to get?

Take a look in the “families and riding with kids” forum over in mtbr, there’s been plenty of discussion on short cranks. I think people end up buying some short suntour 150mm cranks or something similar and then cut them down further, drill a new hole and to a thread.

Highpath Engineering may be able to help, they shortened so cranks for Mrs Bruce

http://www.highpath.net/highpath/cycles/shorten.html

Thanks for the leads.

I’ve been reading on MTBR so I think I’ll post there as well and send some PMs. Folks often skip over the exact spec so I’ll follow up to see if I can get some more details.

I’ve just called Middleburn…they can only get down to 160mm, or 155mm in special cases.

A call to SJS cycles has turned up some good options and helpful advice…..looks like I’ll be able to run 140mm arms and a 24 and 34t setup, which is quite close to what folks on MTBR are recommending.

I couldn’t find anything shorter than 150mm that would take compact rings (Thorn from SJS – very nice too). That’s on a 24″ setup though – but 9spd double + bash working well. Had a set of cranks ready to shorten but decided 150s would do (started with 165mm!).

Best pic i could find 😉

Push by Jonathan Bateman, on Flickr

@jonathan – what cranks did you go with?

The Thorn cranks are a fair bit more than I was hoping to pay….by the time I’ve added rings etc it’s another big bill.

I think 150mm are going to be too long though. Various online calculators suggest I need 135 – 140mm. The standard ones are 145mm.

With the low BB height, a few test rides in the driveway suggests pedal strikes could be an issue as well.

Dear bike industry; please make short* cranks, loads of people have money they want to throw at you.

(*135mm as a suggestion)

i’m looking at you, Shimano.

@ahwiles

I talked to a helpful lady at Middleburn yesterday and said the same thing. I think they would be an ideal company, given their parts are available via lots or retailers and direct, they make the parts themselves, and they could offer custom chainsets using all of their chainrings etc

Perhaps we should all ring them up 😉

I started with some RF downhill cranks (165mm) which were a bargain for cranks, bb, rings and bash, but I eventually had to admit they were too long. I was really looking around for some other HTII style cranks I could shorten (mates with skills/tools), although I had some old ST ones knocking about. Eventually (due to various circumstances) I just bought some Thorn ones (150 is the shortest I think they do). It helped that I’d already bought rings etc with the RF set, so just had a bb to buy.

They are very nicely finished and I reckon you get a good price if you sold them on when done. The demand is out there!

Might also be worth a chat with Spa Cycles. Their own brand Sugino copy cranks go down to at least 160mm – £22 plus a chainring. They might also have other stuff hidden away. It is old school mtb bolt pattern, so goes down to 24t granny / 36t middle (I’m running 28t ring with 11-34 cassette on 26″ wheels).

22:34 is probably lower than needed with 20″ wheels. We previously modified a 20″ Islabike so it could actually climb off road hills – think that was 24 or 26 ring on the front and 28 on the back as the lowest gear.

This is them after I’d made an outer chainguard:-
http://www.whiteflyerphotography.com/events/Horwich%20Humdinger%20Cyclocross/Under%2012/slides/HHDXC-116.html

I’ve bought a set of Sram S600 square taper cranks that I’m going to shorten (hope fully this week if I find time). Even with buying the pedal taps (cheap enough on CRC), the cost will be pretty reasonable.
Should end up with something like this:

Thanks for the leads. I did a bit of research last night, and here’s what I’ve found so far:

Cranks:

Forza cranks (145 or 155). Cheaper than many. Takes 130/74 mm. Q-factor is 159 on 110 mm SQ taper so among the narrowest triple chainring cranks.

Sugino Xd600. I believe 152mm is the minimum. Takes 110/74 chainrings. Q-factor is 164 mm on 110 sq taper but it might be possible to install them with 107mm or 103mm BB to get fairly low Q factor.

Shimano road triples. Minimum length is 165 mm. Use an external BB which. These have Q-factor of 157-158mm.

Thorn 140mm: They take 110/74. Available from SJS cycles. Nice but fairly expensive.

Thorn Mk2 crank arms. 150mm minimum. Take four arm 104/64mm BCD rings, allowing the use “regular” mountain bike rings, including a 22t smallest chain ring. Black only. Cheaper than the Mk1 version

FSA Gossamer megaexo: Minimum length is 165 and Q-factor of 160mm wide. External BB cranks

Shimano mountain bike cranks. I think 165mm is the minimum. Use four arm spiders and 104mm – 64mm (I think). Allow the lowest gearing (22 on granny) but have a wider Q factor.

DaVinci crankset – Various options available including lengths of 150mm -160mm – 165mm – 167.5mm – 170mm – 172.5mm – 175mm – 180mm – 200mm and a 130/150/170mm triple hole or 120/140/160mm triple Hole Child/Full-size-person Crank. They can also do different spiders and 104/64mm and 110/74mm. They are $220 though!

ACE RPM Cranks crank arm set, 5 arm spider. Various lengths from 140mm. BCD not known. Found here – http://americancycle.com/product/ace-bmx-ace-rpm-cranks-crank-arm-set-3134.htm

Various other shorter cranks available from the US – http://americancycle.com/sitesearch.cfm?search=cranks&goSiteSearch.x=-241&goSiteSearch.y=-167

LT1 crank arms. 145mm length. From http://www.greenmachinebmx.com/shop/product/kuwahara-lt1-crank-arms-1556/

SINZ SQUARE CRANK ARM BLACK. Various lengths from 125mm. From here http://www.greenmachinebmx.com/shop/product/sinz–square-crank-arm-black-1560/

Spa cycles. 160mm seems to be the minimum. Do cater for double and triple chainsets though. 110/74mm. http://www.spacycles.co.uk/products.php?mode=search

Vega Cranks. Minimum is 155mm (and they are special order from here http://www.chickencycles.co.uk/index.php?cat=8&sub=97&ord=0)

Carmina cranks – various lengths and spiders. Expensive though

Notes:

– 130/74 mm chainrings could make the jump from granny to middle ring rather large and problematic for indexed shifting.
– Middleburn (and possibly others) make 110/74 chain rings with 24t and 34t for a fairly low geared 2 ring set up, if your cranks can take more than one ring
– Interesting information on selecting a crank length – http://bikedynamics.co.uk/FitGuidecranks.htm

I’ll remeasure my daughter later tonight and use the site above to see what crank length I should be looking for. The Thorn Mk2 looks like a good bet if she can use 150mm, but I suspect she’ll need something shorter.

smurf – when your kid outgrows this bike, I’m buying it, ok?

http://www.edinburghbicycle.com/products/budget-r-junior-chainset

£21.99 152mm, sorted

Raleigh 140mm single 40T, £19.99
http://www.cyclelife.com/Product.aspx?pc=2&pt=68&pg=7457

That’s a really useful list Smurf – cheers!

A lot depends on what your overall outlay will be, which depends on what bits you’re already got already knocking about. I was determined to go 104BCD as I had loads of rings and guards for that. I had a few 110BCD rings knocking about, but realistically I was going to have to buy at least 1 new one. But if you’ve got some 110 stuff then all those lovely BMX cranks start to make a lot of sense.

I was suprised that the Q factor on the Mk2 Thorns wasn’t much different from the Q factor of the Raceface Respond DH cranks I tried first which ran with a pile of spacers between bearing and crank to take up the long spindle (for 83mm shells) slack. I can measure if you want numbers.

@midlifecrashes – Some good finds there

@jonathan – I’m finding that my overall outlay is getting higher all the time! I’ve no spare rings, but it does seem sensible to go for standard MTB BCD if I can as that will give me access to a much wider range of new and little used rings.

However, the triple chainset that midlifecrashes linked to is a tempting price if I can get away with the length (both in terms of leg length and pedal strikes on a 20″ bike)

Great research!

That Edinburgh one is rivetted on steel chainrings and heavier than your original crankset……

I couldn’t work out the BikeDynamics formula on the web site, but looking at the table on the website I’ve assumed little smurf will need something around 125 – 130mm.

Comparing that to a few other sources of info:

125mm – 130mm – BikeDynamics
136mm – from http://www.machinehead-software.co.uk/bike/cranks/cyclist_crank_length_calculator.html
127mm – Crank length on Isla bikes Benin 20 large
140mm – Crank length on Islabikes Benin 24
145mm – Original Scott crank length

Little smurf has a Benin 20 large now and it won’t be long before she will be too large for that (plus this bike I’m building if I don’t get a move on!).

Given that, and I think I should provide a “low range” gear set in the 2 x 9 set up, and all of these parts will be on 24″ wheeled bikes in the next few years (and used by 2 kids), I think the 140mm Thorn cranks are looking like a good option – as short as I can find that will take 2 rings with low gearing. Shame they aren’t one of the cheaper options!

Hi Smurf,

Shame I missed this from the off as I converted old XT cranks for lads 20″ + had bespoke bash// granny ring chainset unit mad up (to eliminate the ugly & redundant middle & outer spider arm sections).

A very neat and effective off-road upgrade – I’ve been going on about how carp (yes, even Isla bike) standard kids ratio’s fail to ‘match’ real trails with ‘proper’ climbs.
“but kids don’t have the power” was the collective marketing reply. No, but boy do they have exceptional balance skills! My two were & continue to climb very steep section at incredibly slow rates on account of their amazing ‘righting’ skills.

..sorry, went off on one. I have all the correct taps, no need to shell out for fancy cranks.

Final job looks like….

Smurf,

Please ignore all of my waffle….I didn’t read your thread….can see your goals differ from mine.

Offer to loan out my taps (bit expensive to buy for effectively a one off) if anyone’s inspired.

I have some nice phots, but can’ work out how to get them over from google images.

Apologies for the distraction…..please continue without me 🙂

Try this one

http://bikesmithdesign.com

Thorn 140mm cranks, rings, bolts and a new BB on their way…..Not the cheapest possible option, but it should all work together and I need to spend time on other parts of the build.

Tanks all for your suggestions.