Currently embarking on a 24″ build for the 7 year old…I’ve kindly been pointed in the direction of 150mm Thorn cranks but they require square taper BB and having only ever ridden Hollow Tech 2 I have no idea what’s what!

Apparently it needs to be 110mm.

I’m looking for balance between cost effective and light weight…which ones should I be looking at??

Bog standard shimano one tbh.

They all weigh a bit due to the design, but the shimano ones tend to last a bit better than the cheaper ones.

Edit : like this

If you’ve only used Hollow Tech 2, then you will need the tool(s) for square taper as well:

To fit and remove the bottom bracket

To remove the cranks (you can get cheaper versions than Park, and you only need this if/when you need to remove the cranks)

You may also need to buy crank bolts if they are not supplied with the cranks, since I don’t think they are supplied with the UN55 bottom bracket.

Thanks…so any Shimano one will do? I’ll go for what ever is cheapest then.

Thanks for the heads up on tool and caps. I knew it would be a different tool but weren’t sure which.

so any Shimano one will do

Shimano square taper BBs are ‘JIS’ (Japanese Industry Standard*) taper, which is what the Thorn cranks taper is according to their website, but you can get other brand JIS taper BBs. The other square taper standard is ISO (which was what Campagnolo used on their square taper cranks and BBs).

* This is a real standard, which has stayed the same for decades. It’s not like modern standards which change more frequently than some people change their underwear.

Spa cycles JP400 Plastic should be plenty good enough FOR a tenner.

Man, why mess? Just buy a real Shimano one. They’re cheap enough and last forever.

That’s 68mm BB shell, OP’s frame might be 73mm. You need one or the other, not just a case of shifting a spacer across.

Spa cycles JP400 Plastic should be plenty good enough FOR a tenner.

For a kid’s bike where it’s likely that it will never need to be removed, it should be OK. Edit – but only if, as Montgomery points out, it is a 68mm shell.

However, I don’t like the plastic cups. A friend had a Giant with a Shimano BB with plastic cups, and they had been damaged when the bike was built in the factory. He only found this when realised the BB was loose and he went to tighten it up with a tool like the Park one, which could not get enough purchase because too many splines in the cup were missing or damaged.

He was only able to tighten it by using my tool, a Var tool with a spring loaded bolt which threads into the BB axle and holds the tool in place against the cup.

For the extra £5 odd (less when you factor in CRC’s free postage), I would get the UN55 with metal cups.

Shimano UN55. About £15, fit and forget

Yeh I bought a cheaper one from Halfords for about a tenner, as they didn’t happen to have the shimano one in stock, so I thought ‘whatever’.

I forget the make, VP or something but it was getting a bit rumbly after seemingly no time, just get a shimano one for a couple of quid more.

Buy cheap buy twice as they say! And the Un55 is only a fiver more than the cheap crap anyway.

UN55. Quality bb for not much £. As above, you will need the tool too.

Cheers all looks like the Shimano UN55 is the way to go.

It is a kids bike…but an XS BFE…hasn’t loaded yet, anybody know if 68mm or 73mm of the top of their head?

Again thanks for pointing this out with HT2 you just add spacers!

UN55 is a troublemaking component, they last so long that when you eventually need to change them, it can be a total bastard to get them out! Do grease it well

Yeh good advice, make sure the frame threads and bb is completely clean before fitting and grease the threads and grease inside of the frame.. A bb that’s welded itself to the frame is no fun!

You don’t need loads, but make sure all surfaces have a film of grease on them.

https://www.cotic.co.uk/product/BFe275#compatibility

Good, that, wish all manufacturers were that specific.

113mm wide for the thorn. Lewis has a Tange Seiki LN 7922 its about 230g

Super cheers…Ill grease it within an inch of its life…in 3 or so years I expect I’ll be changing the cranks to something longer and so probably HT2.

113? On the site it says 110mm!?

You don’t need to go crazy with grease, just make sure there is a good film on the threads, and that the treads are clean before.

113? On the site it says 110mm!?

If this is the right webpage, it says “110mm bracket required to get a 54mm chainline on middle ring, 107mm can be used if only using a single ring in the outer position.”

So you need to check what chainline the front mech you are planning to use is designed for. It’s possible that you might need to use a narrower BB if the chainline of the mech is substantially less, but there’s probably a degree of tolerance for the mech to be used with a slightly wider (or narrower) chainline than it’s supposed to have (front mechs are pretty crude mechanisms). Sheldon Brown is a bit out of date on the chainlines of the latest kit, e.g. Shimano MTB doubles are 48.8mm (or 51.8mm for Boost), but it’s probably worth reading – here.

The other thing to watch out for if you go for a narrower BB, is that it positions the crank arms closer to the chainstays, and they might even foul the chainstays. That said, if you can go narrower, then that will give a lower Q factor (distance between pedals), and a 7 year old might prefer that (although they might well not notice it, whereas I imagine they certainly would if taken to the extreme of a very wide Q factor as used on fat bikes).

Lastly, the Shimano BBs are mostly symmetrical, so if a 110mm BB gives a 54mm chainline, then a 107mm will give a 52.5mm chainline (i.e. half the 3mm difference). They do make some asymmetrical BBs, which I think have a special suffix, e.g. UN55 vs UN55K or UN55E, where I think those with a suffix are asymmetrical. I would be 99.999% confident that any Shimano BB sold by CRC etc. would be symmetrical, since the asymmetric ones are intended for oddities like fitting a chaincase which is clamped to the BB shell by the BB cup (like a spacer).

Wow…all very tech.

Front mech…what’s that’s? LOL

Not sure if it makes a difference but she’ll be running a 1x N/W. I can’t be doing with all the faff and extra weight of a front mech. Was even wondering whether to go single speed.

Not sure if it makes a difference but she’ll be running a 1x N/W. I can’t be doing with all the faff and extra weight of a front mech. Was even wondering whether to go single speed.

I did wonder if you might be going 1x. Others with experience of running 1x with chainlines not aligned with the middle of the cassette can probably advise better, but I would suggest that you probably still want a narrower chainline than 54mm even if you don’t need to worry about a front mech. You could always try it and see, but I suspect it might cause problems with shifts (and with back pedalling) in the lowest gears.

50mm chain line with a NW on the thorn with Lewis’s ripcord and his previous Carrera blast.

113mm wide for the thorn.

[quote]50mm chain line with a NW on the thorn [/quote]

SJS are quoting a 54mm chainline with a 110mm BB. How are you getting a narrower chainline with a wider BB?

A Shimano square taper BB will probably outlive you. Weight is just something you need to live with.

SJS are quoting a 54mm chainline with a 110mm BB. How are you getting a narrower chainline with a wider BB

SJS are wrong or I don’t know how to use a ruler ?

SJS are wrong or I don’t know how to use a ruler ?

What’s the Queen got to do with it? 😉

Depending how much you want to lose some weight, if you can find one, the XT models (UN70, 71, 2, 3 etc) have hollow axles and are quite a bit lighter and have been known to last longer than some planets. They’ve been out of production for a while but you might find one in a bin somewhere!

I’ve just measured the chainline on the same crank with a 73×110 UN55 as recommended by SJS. It’s running a 30t, so that has c3mm built in spacers. The chainline with that chainring is about 47mm. So with a non-spaced chainring, that would be about 50mm.

if you can find one, the XT models (UN70, 71, 2, 3 etc)

hahahhahaha! I’ll cut you if you come anywhere near my stash of UN72s!

UN55’s are really cheap … and will doubtless outlast the bike
The best way is trying… IMHO … at least actually have in BB fitted and measure with everything in place and also check how it shifts

I reduced the size going from the stock suntour cranks to some SRAM cranks with NW fitted on the outer on POAH’s advice. At the time I went at the upper limit of what he’d advised as it seemed a big drop but retrospectively I could have gone 2mm less but have no shifting issues even with a 40T expander. (So 1-2mm isn’t absolutely critical)

You will also get 1-2mm difference based on hubs/mech hanger etc…..
I could go and check because the BB isn’t in (replaced with HT) but different cranks and frame … so it won’t add any more value than Yak and POAH

hahahhahaha! I’ll cut you if you come anywhere near my stash of UN72s!

You fool, you foolish fool, you NEVER tell them you have a stash, how on earth do you think I’ve kept mine so safe for so…D’Oh!

takes UN91 out of bottom drawer, feels the axle spin effortlessly then replace it. #myprecious

*Hires private detective to track down Stoner and Amedias*

Not seen a UN72 for ages, apart from the one that’s been in the Rockhopper for years…..

What’s the concensus on the Stronglight ones?

What’s the concensus on the Stronglight ones?

mediocre at best…

UN5X is still the best option unless you want to pay Phil/TA/SKF/White Ind. money, cheaper stuff is cheaper but not as good as the UN55 IME so better to just go with what works.

Not seen a UN72 for ages, apart from the one that’s been in the Rockhopper for years…..

The longest continuously serving one I have is a UN72 that was bought when UN71 became UN72 as I remember being confused that the model number had changed, so that must be around 1997/1998? Must have a out a billion miles on it now…

The stupidest part about using UN7Xs and UN9Xs and having a stash is that they last so long you start to wonder if you should sell some as you’ll not need them, but sanity normally returns quickly 😉

Re tools, as Slowster says there are cheaper versions than Park, so you may wish to save yourself a few quid particularly as you’ll be using the tools very infrequently. The following two tools should be of sufficient quality for the job and come in at a fraction under £10, including free postage if you buy them together

CRC sell a Weldtite version of the Shimano bottom bracket fit/removal tool for £5.99:
http://www.chainreactioncycles.com/weldtite-shimano-bottom-bracket-cartridge-tool/rp-prod86711

…and CRC also sell an X-Tools version of the crank puller for £3.99:
http://www.chainreactioncycles.com/x-tools-crank-extractor/rp-prod10181

Supperleggero’s post has reminded me of something else: my friend’s bike with the plastic BB cups would not accept the tool he had, because the height of the splines was different. It’s just another reason to go with the UN55, since all the tool manufacturers will make sure that their tools do fit the UN55, since it is the most common BB.

Before buying the Weldtite BB tool, make sure you have a suitably sized wrench to fit it, i.e. 32mm. (The Park one will accept a 3/8″ driver, a 1″ wrench or a 32mm wrench.)

As per Slowster’s post stick with the Shimano UN55. If you don’t have anything suitable to drive the Weldtite tool’s 32mm then the cheapest option is probably a 32mm socket such as this one which is £1.72 from B&Q:
http://www.diy.com/departments/mac-allister-1/2-drive-socket-32mm/674069_BQ.prd

This does of course assume that you have a 1/2″ drive ratchet or similar already. If not then a spanner is probably a cheaper option.

On a related note, at some point I need to remove what I think is an ISIS bottom bracket from my road bike (yes, I’ve already stripped the FSA crank threads using a standard crank puller…). Will a standard BB tool as above, which I have, fit – or am I going to need a special tool for this one-off job? Quick search suggests that is the case.

Montgomery, it depends upon the cups: some ISIS BBs used cups with internal splines which will accept a standard tool like the Park BBT-22 (some older tools might not fit if they were sized only to go over a square taper axle, rather than the wider Octalink and ISIS axles), others used cups with external notches requiring a tool like the Park BBT-18.