Shimano UN54 BB Which Size?

[MacB]

Started to build up the 29er and the plan is to put the Stronglight triple chainset on but the UN54 BB I have, at 113mm, isn't long enough. From what I can see I have the choice of 115, 118, 122 and 127...I'm pretty sure that I can discount the 115 as an option. I installed the 113 BB and when I went to put on the drive side crank the middle chainring touched the chainstay just when I pushed it on by hand.

So I'm sort of thinking the choice is 118, 122 or 127 but I'd like to try and order the right one first time. Is there any easy way of working it out and are the increases in size symetrical or not?

 

[MacB]

Ok, a quick measure in the garage tells me I need a 50mm chainline to ensure clearance, it's a MTB so makes sense. Still don't know which length of UN54 though

 

[Stu1961]

Looks like it could be 118 according to this, although I don't know which chainset you have......

Stronglight

 

[RecordAceFromNew]

Hi mate thanks for keeping the chainset model secret!

So, you can take the following suggestion at your peril!

Ok, assuming it is THE* Stronglight Impact road triple the late great Sheldon wrote about here a 115mm JIS gave him a 45mm chainline. Incidentally he also helpfully provided the q factors.

However you say you need a 50mm chainline, given bb spindles are usually asymmetric, one can't just add 5mm or 10mm, but need to decipher the devious codes on page 16 and 34 of this. Assuming you have Sheldon's chainset, then essentially they say you need the 122.5mm UN54 (often marketed as 122mm) to give you the extra 5mm on the right hand side.

Won't blame you if you think the above is as clear as mud...

Hope you have a front mech that works ok with a 50mm chainline.

* Personally I wouldn't automatically assume a current chainset of the same model name must be identical to that in the table.

 

[MacB]

It's the Stronglight Impact, it's all getting very confusing, I agree that it's between the 118 and the 122. The clearance is very close as it's a 113 BB, not the recommended 110, I was trying, so probably gives a chainline of about 47mm rather than the official 46mm. I was planning on using the Tiagra triple FD as well but that's for a 31.8 ST, and has shim for 28.something, but this ST is 34.9. I reckon I can find some old BBs to try for size at LBS, but a couple more questions:-

if I get the required chainline, or clearance from chainstay, with the Stronglight Impact, is there any reason it won't shift with either a MTB or Road FD? This will be via a friction shifter so no concerns around indexing. I'm assuming the chainring gaps are the same MTB or Road?

I was wanting to use the Tiagra, or similar, as it's all silver and the colour scheme I'm going for is silver and red. I was assuming that the additional thickness of the ST would put the Tiagra FD out far enough to work with the 50mm chainline?

I think I've got a MTB triple front mech that will do 34.9, should I forget the road FD idea and just go with that?

If I can get the front shifting and chainset sorted, is there any reason not to use the Stronglight Impact on a MTB, as in too weak or anything?

 

[MacB]

oh yes, it's a standard 68mm BB shell with English thread

 

[MacB]

Right just been to garage and reinstalled BB but left the drive side cup 5mm short of being fully inserted. Chainset, Impact 46/36/24, is now well clear of chainstay, looks like close to 4mm, but very hard to measure, though I did install drive side crank fully. This looks to me like it's about a 52mm chainline done this way, which would make my guess of 47mm previously not far off.

Looking at that link Record Ace, I'm seeing on page 34:-

MM is a 107 or 110 and is the baseline
LL is a 113 and is what I currently have and is plus 3
XL is the 118 and is +5
NL is the 122 and is +8

I'm guessing we half those variances which would mean:-

Stronglight chart shows a 46mm chainline with a 110 BB
I used a 113, apparently symetrical, so my chainline is 47.5(if I'm measuring 47 with a metal rule I can live with that)
If I use an XL 118 that would be +5 from the baseline of 46 so giving a 48.5
If I use an NL 122.2 then that's +8 from the baseline of 46 and bingo we have 50mm

Ok, I'm convinced, I'll order a 122 unless you tell me the above is garbage

 

[RecordAceFromNew]

It's the Stronglight Impact, it's all getting very confusing, I agree that it's between the 118 and the 122. The clearance is very close as it's a 113 BB, not the recommended 110, I was trying, so probably gives a chainline of about 47mm rather than the official 46mm. I was planning on using the Tiagra triple FD as well but that's for a 31.8 ST, and has shim for 28.something, but this ST is 34.9. I reckon I can find some old BBs to try for size at LBS, but a couple more questions:-

if I get the required chainline, or clearance from chainstay, with the Stronglight Impact, is there any reason it won't shift with either a MTB or Road FD? This will be via a friction shifter so no concerns around indexing. I'm assuming the chainring gaps are the same MTB or Road?

I was wanting to use the Tiagra, or similar, as it's all silver and the colour scheme I'm going for is silver and red. I was assuming that the additional thickness of the ST would put the Tiagra FD out far enough to work with the 50mm chainline?

I think I've got a MTB triple front mech that will do 34.9, should I forget the road FD idea and just go with that?

If I can get the front shifting and chainset sorted, is there any reason not to use the Stronglight Impact on a MTB, as in too weak or anything?

STOP don't order the 122mm bb yet!

Afaik there are at least two versions of SL Impact triples. I believe some have 5 arms, and need 115mm bbs to deliver 45mm. 110mm is apparently only for the 4 arm ones for the same chainline, but who knows things might have moved on. The safest bet is probably removing the clashing ring(s), torque everything up and check.

Assuming you have whichever version that does deliver 45mm chainline with a 110mm bb, then the 113mm should deliver 48mm, which is not far from your 47mm measured. So you actually need a further correction of ONLY 2mm to get to 50mm (assuming that is what you want per below), if so you need a 118mm bb.

In other words you shouldn't half those variances.

If the Tiagra FD is for 31.8 I am not sure if it is a good idea to use it on a 34.9 ST, I would have thought the clamp would apply concentrated loads on the seat tube. I wouldn't do it on my machine.

I don't think chainset strength is likely to be an issue, but road chainsets usually have smaller q factors due to chainstay size/profile and rear hub spacing, so you might also want to check for crankarm clashing at the chainstay if you are cutting chainline fine. Also although you have a road chainset you have more or less mtb sized largest ring, which means mtb/hybrid fds would have closer profile and therefore perhaps somewhat slicker front shifts. It is not difficult to move chainline in or out by getting the right bb subject to clash, and since you have the chainset already you might want to see which fd can give you the best compromise.

 

[MacB]

Ah, no, I wasn't going to try and fit the 31.8 Tiagra FD I would be buying a 34.9 instead, but can't find one as yet so we'll see.

Too late, I've ordered the 122, worst that can happen is that I need to order another one or the chainline is a little further out, if I use the MTB FD I have then it should cope with that no problem. I certainly don't want the middle chainring shaving too close to the chainstay for offroad use. I'm likely to be out of the saddle a lot and flex is very possible.

 

[MacB]

Now that's weird, following the info on here I rang CRC the next morning to alter the order to a 118, needless to say a 122 still arrived today. I thought sod it, I'll fit it up and see what the chainline is like. Well I make it spot on 50mm measured to the centre of the middle ring, it's hard to be totally accurate but my clearest measurement is from the inside of the centre ring to the centre of the rear mudguard bridge behind the BB shell. This comes out at 47/48mm using a small steel rule butted up against middle ring.

Anyway, took it out again, greased properly and fully fitted cranks and still get 50mm. I also fitted a Deore FD and, manipulating it by hand it definitely covers the full range of the chainset. As it'll be a friction shifter for the front then that all seems fine. I suppose the only remaining concern could be around chainline to rear cassette. Won't really know that until fully built up, may get LBS to assist to make sure all works as should.

Thanks for the help folks....Al

 

[RecordAceFromNew]

Now that's weird, following the info on here I rang CRC the next morning to alter the order to a 118, needless to say a 122 still arrived today. I thought sod it, I'll fit it up and see what the chainline is like. Well I make it spot on 50mm measured to the centre of the middle ring, it's hard to be totally accurate but my clearest measurement is from the inside of the centre ring to the centre of the rear mudguard bridge behind the BB shell. This comes out at 47/48mm using a small steel rule butted up against middle ring.

Anyway, took it out again, greased properly and fully fitted cranks and still get 50mm. I also fitted a Deore FD and, manipulating it by hand it definitely covers the full range of the chainset. As it'll be a friction shifter for the front then that all seems fine. I suppose the only remaining concern could be around chainline to rear cassette. Won't really know that until fully built up, may get LBS to assist to make sure all works as should.

Thanks for the help folks....Al

Good to know it works!

It would not be strange however if yours is actually the Impact that requires a 115mm bb to give a 45mm chainline, per calculations in post #4 above.

 

[MacB]

Good to know it works!

It would not be strange however if yours is actually the Impact that requires a 115mm bb to give a 45mm chainline, per calculations in post #4 above.

could be, could be, yet I was cautious, when I unboxed the Stronglight chainset I cut out the chart on the back of the box. It lists all the chainsets they do, required BB per double/triple and resulting chainlines. It definitely shows:-

Impact - JIS - Triple - 110mm - Chainline - 46mm

The only chainset mentioned on there that needs a 115mm Axle is the Mygal triple and still giving 46mm. The item on the Spa website actually states required BB 110mm.

However, wouldn't you know it, the plot thickens, I've just gone to the Stronglight website and, for the Impact triple they list a 115mm BB required....so looks like you were right yet again

 

[RecordAceFromNew]

It seems bb length confusion re different versions of the Impact is not uncommon, e.g. here.

What is curious is that the 118mm requirement was calculated based on your derived result of ~47mm chainline measured with a 113mm bb (albeit derived from leaving the driveside cup 5mm out).

Given they have different number of arms, as a matter of interest which one does yours look like?

 

[MacB]

Mine looks like the second one

 

[RecordAceFromNew]

Having looked around a bit, it appears for Stronglight Impact Triples:

1. The 4 arm version is based on/copy of the Sugino XD2, which requires a 110mm JIS bb to give a 45mm chainline.

2. The 5 arm version is based on/copy of the Sugino Alpina2, which requires a 113mm/115mm JIS bb to give a 45mm chainline. In case there is any confusion the 113mm and 115mm JIS bbs have identical spindle length on the drive side, the 115mm is simply longer by 2mm on the non-drive side.

3. Don't trust whatever it might say on the box supplied, or indeed on websites (because picture of the crankset don't always seem to be consistent with the recommended bb length as stated). Also there are many reports of version supplied not always the same as pics in online adverts. Horrendous confusion abounds, seemingly because two very different Impact chainsets are available on the market at the same time, and neither Stronglight nor vendors appear to be clued up (see e.g. pic and bb recommendation by Stronglight here)...

4. The above appears to be correct, from a fair amount of anecdotal evidence (that experienced by the OP is just one of many), but there is no guarantee it is.

Generally, to increase the chainline by 5mm, a 110mm JIS bb should be replaced by a 118mm JIS bb, while a 113mm/115mm JIS bb should be replaced by a 122mm JIS bb.