Shimano BB-UN55; actual widths

For discussions about bikes and equipment.
Brucey
Posts: 44456
Joined: 4 Jan 2012, 6:25pm

Shimano BB-UN55; actual widths

Post by Brucey »

the actual axle lengths and symmetry of UN55 BB units has been discussed quite a lot recently but without the benefit of any accurate measurements. I just spent a while in front of a pile of BB units with a set of verniers in my hand and I have the following information (all measurements in mm);

Nominal size---actual length------RH stickout-----LH stickout----Equivalent (symmetric) length

107---------------108.0---------------21.0-------------19.0--------------110.0

110---------------111.5---------------21.0-------------22.5--------------110.0

115---------------115.6---------------24.0-------------23.6--------------116.0

118---------------118.5--------------25.6--------------24.9--------------119.2

122---------------123.2--------------28.6--------------26.6--------------125.2

Note that

- the stickout measurements are w.r.t. the BB shell.
- if you need to know the measurement vs the RH end of the BB cartridge, subtract 3.6mm (the thickness of the flange on the RH cup) from the stickout value.
- the LH stickout value is calculated assuming a 68.0mm BB shell; real BB shells vary somewhat.
- all measurements are likely accurate to about 0.2mm, but the LH stickout measurement will vary by more than this because of cumulative measurement errors, plus any variations in the BB shell width.

- the equivalent (symmetric) length can be used to compare between other BB units (with a similar taper, and symmetrically constructed) on a 68mm shell, for chainline purposes. Variations in equivalent length produce variations in chainline of about half the equivalent length variation. Thus running through these BB units, the chainline varies by 0.0mm, 3.0mm, 1.6mm, 3.0mm respectively. Finer increments than this are possible if spacers are used between the RH cup and the BB shell. The scope for using such spacers is limited however, because the present design of LH cup also has a flange on it.

NB; as mentioned by others in previous discussions, the aluminium LH cup in several of the units I looked at was a very loose fit, so loose that the BB unit is almost certain to work loose in service (and probably wreck the frame).

What are shimano thinking of...???? :shock: :shock: :shock:

If you must use such a BB with a loose LH cup, I'd advise only to fit it with Loctite and/or shims between the LH cup and the cartridge unit.

cheers
edit for typo
[edit: two points to note

a) that a 113mm unit gives almost identical chainline (within 0.5mm) as a 115mm unit and
b) that (as of early 2019) shimano appear to have (finally) sorted themselves out; recently supplied UN55 units have LH cups that fit the centre units better than they did before. I think there has also been a change in the centre body dimension but I've not measured up exactly yet.]
Last edited by Brucey on 1 Jun 2019, 9:16pm, edited 2 times in total.
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
ANTONISH
Posts: 2958
Joined: 26 Mar 2009, 9:49am

Re: Shimano BB-UN55; actual widths

Post by ANTONISH »

Thanks for that useful information Brucey. I've saved it for future use.
User avatar
NATURAL ANKLING
Posts: 13780
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: Shimano BB-UN55; actual widths

Post by NATURAL ANKLING »

Hi,
Yeh, the dims you give follow a pattern..........not.

I just got a box of units through all the sizes and offer them up after a quick compare to suit the cranks I put on.

I always end up spacing the left hand pedal about 2 - 3mm.

Thought I might make up a table to aid me when I build up a bike, its alright talking about chain lines but try and get your pedals equi distance :?
NA Thinks Just End 2 End Return + Bivvy - Some day Soon I hope
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.
Brucey
Posts: 44456
Joined: 4 Jan 2012, 6:25pm

Re: Shimano BB-UN55; actual widths

Post by Brucey »

BB-UN55 has changed; I've just added a further note to the post above to reflect that. I'll add more details as I have them but recently supplied BB-UN55 units have had LH cups that fit better and (I think) a different centre dimension. Spindle lengths appear to have remained the same.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
finch
Posts: 5
Joined: 25 Jul 2019, 9:34am

Re: Shimano BB-UN55; actual widths

Post by finch »

Brucey wrote:If you must use such a BB with a loose LH cup, I'd advise only to fit it with Loctite and/or shims between the LH cup and the cartridge unit.


Thank you for the very informative post. Just to make sure there's no ambiguity, does this quoted line mean using Loctite between the cup and the cartridge? What strength of threadlocker would be suitable there?
User avatar
squeaker
Posts: 4112
Joined: 12 Jan 2007, 11:43pm
Location: Sussex

Re: Shimano BB-UN55; actual widths

Post by squeaker »

Brucey wrote:....
- the equivalent (symmetric) length can be used to compare between other BB units (with a similar taper, and symmetrically constructed) on a 68mm shell, for chainline purposes. Variations in equivalent length produce variations in chainline of about half the equivalent length variation. Thus running through these BB units, the chainline varies by 0.0mm, 3.0mm, 1.6mm, 3.0mm respectively....

Interesting, but in real life doesn't the installed chainline depend on the taper dimensions? Maybe Shimano gauge / classify their BB widths using a standard taper gauge? More work: take a crankset and fit to all the BB units then remeasure? (I'm sure you've got nothing better to do :lol: )
"42"
robc02
Posts: 1824
Joined: 23 Apr 2009, 7:12pm
Location: Stafford

Re: Shimano BB-UN55; actual widths

Post by robc02 »

Very useful information here, thanks Brucey.

Does this belong in "Too Good to Lose"?
Brucey
Posts: 44456
Joined: 4 Jan 2012, 6:25pm

Re: Shimano BB-UN55; actual widths

Post by Brucey »

finch wrote:
Brucey wrote:If you must use such a BB with a loose LH cup, I'd advise only to fit it with Loctite and/or shims between the LH cup and the cartridge unit.


Thank you for the very informative post. Just to make sure there's no ambiguity, does this quoted line mean using Loctite between the cup and the cartridge? What strength of threadlocker would be suitable there?


it depends on the clearance between the parts.

As noted above the most recent parts I have seen no longer seem to have the same problem and will probably work OK without such remedial action.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
finch
Posts: 5
Joined: 25 Jul 2019, 9:34am

Re: Shimano BB-UN55; actual widths

Post by finch »

Brucey wrote:As noted above the most recent parts I have seen no longer seem to have the same problem and will probably work OK without such remedial action.


Didn't want to order a whole new UN55, but in the end went with a plastic cup from an old UN26 that seemed to fit fine. One creak eliminated, it seems.
User avatar
531colin
Posts: 16036
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

ANYBODY KNOW ABOUT 73MM SHELLS?

Post by 531colin »

This may be equivalent to wanting the "moon on a stick" ....but anybody know anything about units for 73mm bracket shells?

and can we have this in "too good to lose", please?
slowster
Moderator
Posts: 4612
Joined: 7 Jul 2017, 10:37am

Re: ANYBODY KNOW ABOUT 73MM SHELLS?

Post by slowster »

531colin wrote:This may be equivalent to wanting the "moon on a stick" ....but anybody know anything about units for 73mm bracket shells

I think that the chainlines for 68mm UN55 and 73mm UN55 bottom brackets with the same axle lengths are the same.

Is this for your Longitude? If so, a 122.5mm UN55 (both 68mm and 73mm) combined with a Spa RD/TD chainset and the chainring in the outermost position will provide the requisite (55mm IIRC) chainline for a Rohloff with the supplied standard splined sprocket carrier (as opposed to the aftermarket slimmer carrier which reduces chainline by 2mm). As best I could tell the resulting chainline was pretty much perfect, and I did not need to bother with spacers on the chainring bolts.
User avatar
531colin
Posts: 16036
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: ANYBODY KNOW ABOUT 73MM SHELLS?

Post by 531colin »

slowster wrote:...Is this for your Longitude? If so, a 122.5mm UN55 (both 68mm and 73mm) combined with a Spa RD/TD chainset and the chainring in the outermost position will provide the requisite (55mm IIRC) chainline for a Rohloff ....

Thanks, thats great information; I may well just copy you!

slowster wrote:...I think that the chainlines for 68mm UN55 and 73mm UN55 bottom brackets with the same axle lengths are the same...

Ah! I might have had a lightbulb moment :idea: does that mean they have kept the centre of the axle on the bike midline and added 2.5mm to each half of the bearing unit/sleeve thing?
All I have done so far is bung on an old LX triple with its BB unit (which might have been for 68mm BB shell?...Raleigh Mtrax more than 20 years ago!). I was somewhat concerned because the left crank was set to foul the chainstay but the big ring was about 3mm too far out for the Rohloff. So maybe (if I'm right) all I need to do is get a BB unit for 73mm shell with the same axle length?
.....making my head spin!....L crank will still be awful close to the chainstay? but I would quite like to use a crankset which is currently languishing in a bits box rather than buy new! AAAARGH!!
Brucey
Posts: 44456
Joined: 4 Jan 2012, 6:25pm

Re: ANYBODY KNOW ABOUT 73MM SHELLS?

Post by Brucey »

531colin wrote:
Ah! I might have had a lightbulb moment :idea: does that mean they have kept the centre of the axle on the bike midline and added 2.5mm to each half of the bearing unit/sleeve thing?...


re the RH end of the cartridge, that is how I have always assumed they are made, but I have not measured up to be sure.

[although thinking about it, the spindle may be identical and the LH end of the centre unit may not be displaced leftwards; the LH cup is different between 68 and 73mm variants

https://si.shimano.com/api/publish/storage/pdf/en/ev/BB-UN55/EV-BB-UN55-3281.pdf ]


BB-UN55 is now discontinued and the (probably inferior) replacement is not yet available in all lengths, so near me LBSs are busy tearing their hair out; they would normally fit about a dozen a week.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
nsew
Posts: 1006
Joined: 14 Dec 2017, 12:38pm

Re: Shimano BB-UN55; actual widths

Post by nsew »

Nominal 127.5 ...... actual 128.0 ....... RH 31.0 .......LH 29.0 ...... (68mm)

Purchased a couple of UN55’s recently with a date code of early 2020. Took quite a search.
colin54
Posts: 2517
Joined: 24 Sep 2013, 4:34pm

Re: Shimano BB-UN55; actual widths

Post by colin54 »

531colin wrote:This may be equivalent to wanting the "moon on a stick" ....but anybody know anything about units for 73mm bracket shells?


Does this link from the Thorn Forum by PH (of this forum as well I think) add anything to your quest ?
Lots of useful Rohloff info on there btw.
I'm not sure whether PH is talking about a 73mm BB or just about BB chainlines in general, though that is how the question is worded that he has replied to, he may comment if he sees this post.
Wheel Easy !

http://thorncyclesforum.co.uk/index.php ... #msg102527
Nu-Fogey
Post Reply