Shimano BB-UN55; actual widths

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slowster
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Re: ANYBODY KNOW ABOUT 73MM SHELLS?

Post by slowster »

531colin wrote: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?

I don't know that it is the case for all axle lengths, but certainly from personal experience and measurement of the resulting chainlines it's true for the 122.5mm length, i.e. both ~56.9mm with outer chainring on Spa TD/RD chainsets according to the notes I've just checked.

I think it was the case that the middle ring of Shimano MTB triples would give the required chainline for a Rohloff. That was the route that most Rohloff users took, and Rohloff even supply/supplied 5 x 5mm chainring spacers with the hubs to take the place of the outer ring. I think that it's not quite so straightforward now because the Rohloff chainline increased slightly when they introduced the splined sprocket carrier to replace the threaded sprocket carrier. I think the threaded sprocket chainline was 54mm, and having just checked the manual it's now 57mm, or 55mm with the aftermarket slimmer carrier. I presume the slimmer carrier was developed to offer backwards compatibility for Rohloff owners whose threaded sprocket has worn out and who need to switch to a splined sprocket, i.e. avoiding a need to change the existing chainset or BB presumably because the difference between the 54mm and 55mm chainlines is within tolerance for the hub.

I had a similar concern about fouling the chainstays, but I found that in my case I had enough clearance that if I used chainring spacers I could have possibly even fitted a 113mm axle based on Brucey's measurements and nsew's measurement of the 113mm here. I would have done so to reduce the Q factor, but in the end I never got round to trying it because it looked like my shoes would probably rub against the chainstays.

BTW Brucey, thank you for going to the trouble of taking and posting those original measurements, which saved me wasting money guessing which size UN55 I would need. I was able to measure my chainline with an existing UN55 which I already happened to have spare but which was far too short, and then use that measurement to calculate that I needed a 122.5mm.
Brucey
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Re: ANYBODY KNOW ABOUT 73MM SHELLS?

Post by Brucey »

slowster wrote: …. saved me wasting money guessing which size UN55 I would need. I was able to measure my chainline with an existing UN55 which I already happened to have spare but which was far too short, and then use that measurement to calculate that I needed a 122.5mm.


that was exactly the point in taking these measurements; the chainline doesn't vary with changes in length as you might expect, and all you need is one example of this BB (and it doesn't have to be much better than scrap condition) and armed with the measurements, you should be able to shoot for the correct length without too much of a palaver.

There is an implicit assumption that shimano make their ST spindles consistently; if other manufacturers make their ST (JIS) tapers the same way then similar measurements can be made. However a tiny variation in taper dimension will make significant differences to the chainline, not least because the crank will 'resize' to a different taper the first time it is fully tightened.

FWIW the new model shimano BB spindles may (like many of their similarly constructed rivals) be readily reversible, which means that asymmetric spindles may offer more choices of chainline, just as was possible with BB-UN72 etc BITD.

cheers
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531colin
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Re: BB UN300 73 x 118 first look

Post by 531colin »

I'm adding this to the existing thread about UN55 measurements; I'm happy for it to be moved if that seems sensible.
After a bit of searching I got a UN300 118 spindle for 73mm bracket shell. (Bikester)
I'm following Brucey's measurement scheme as far as I can.....
Nominal axle width 118 ....measured 118.6
RH stickout 22.7 from BB shell (+ 2.5mm for the wider shell makes 25.2mm, compares to 25.6 for the 68mm UN55)
LH stickout 23.1 from BB shell (again + 2.5mm = 25.6 cf. 24.9 for the 68 UN55)
These measurements after fitting into a BB shell which measured 73mm pretty accurately.
So, chainline will be comparable if not precise to the decimal point...L crank a shade further out on 73mm.


The actual unit is built from pretty standard-looking manufactured caged ballraces, labelled 6903RS....

ImageIMG_5301 by 531colin, on Flickr

.....a steel sleeve and alloy cups; the alloy cups have a rubber (?) seal, below is the left cup showing the seal, some threadlock on the threads and something looking similar inside the cup; maybe this is to stop the bearing rocking in the cup?

ImageIMG_5299 by 531colin, on Flickr

ImageIMG_5300 by 531colin, on Flickr

It seems to me that the seal in the alloy cups will run dry pretty quickly and become useless. For the left cup I can simply put some grease behind it. The right side of the axle has one of those strange collars that Shimano sometimes fit...

ImageIMG_5305 by 531colin, on Flickr

I have never known what they are for, but this one levered off easily with 2 screwdrivers so I was able to lift the seal lip with a tiny ball-end Allen key and inject some grease behind it. A minimal intervention which might do a bit of good, and probably won't do any harm. (you can see the shoulder which I think the collar butts up to rather than fits on)

ImageIMG_5308 by 531colin, on Flickr
Brucey
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Re: Shimano BB-UN55; actual widths

Post by Brucey »

that is interesting, 6903-2RS (and 61903-2RS) bearing size is 17x30x7mm.

Lots of other units built in this style use a 173110-2RS bearings which are 17x31x10mm. In theory the latter should be stronger, but in practice other things may trump the small difference in bearing dimensions.

cheers
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531colin
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Re: Shimano BB-UN55; actual widths

Post by 531colin »

It doesn't look like you could get any 1/4" balls in there!
Brucey
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Re: Shimano BB-UN55; actual widths

Post by Brucey »

interestingly 173110-2RS bearings are usually built using 3/16" balls. One of my mad ideas is to build a loose-ball BB built around modified 173110 bearing parts.

cheers
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LancsGirl
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Re: Shimano BB-UN55; actual widths

Post by LancsGirl »

And to add more info. I've just snagged a NOS UN-72, nominally 115mm.

Actual axle length 115.5mm

Right hand stick-out, from inside face of RH cup lip to axle end, 23.5
gregoryoftours
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Re: Shimano BB-UN55; actual widths

Post by gregoryoftours »

That was a good find!
TheBomber
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Re: Shimano BB-UN55; actual widths

Post by TheBomber »

I also recently bought and fitted a UN-72 (not NOS) and it is also slightly wider than the 52 it replaced despite being nominally the same size. The front mech had to be wound out a bit and I feel the extra Q factor as I seem to be sensitive to that. Not attempted any measurements but probably 1.5 - 2.0mm wider.
LancsGirl
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Re: Shimano BB-UN55; actual widths

Post by LancsGirl »

gregoryoftours wrote: 5 Feb 2023, 3:14pm That was a good find!
Yes, wasn't it just? £55, which seemed an OK price, if they are as good as people seem to say.
De Sisti
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Re: Shimano BB-UN55; actual widths

Post by De Sisti »

LancsGirl wrote: 4 Feb 2023, 5:51pm And to add more info. I've just snagged a NOS UN-72
From where?
LancsGirl
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Re: Shimano BB-UN55; actual widths

Post by LancsGirl »

De Sisti wrote: 5 Feb 2023, 9:31pm
LancsGirl wrote: 4 Feb 2023, 5:51pm And to add more info. I've just snagged a NOS UN-72
From where?
eBay. A shop in Bradford I think. Paul Milne Cycles. I think it was the only one, sorry. At least they aren't showing any more now. Though if there had been more than one I'd have bought them all.

There's actually a 110mm NOS one in the US for sale, but it's the E version, which I think means you have to put some spacers behind the RH cup, or something (?) And it's a lot more expensive, especially with shipping and import duties.
Brucey
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Re: BB UN300 73 x 118 first look

Post by Brucey »

531colin wrote: 18 Dec 2020, 5:08pm..... some threadlock on the threads and something looking similar inside the cup; maybe this is to stop the bearing rocking in the cup?.........
shimanio do something similar with their HT2 bottom brackets. I've always assumed that this was to give an allowance for BB shells that are not machined properly, thus perhaps allowing the bearing to sit at a very slight angle to the cup.
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