Measuring Square Taper Bottom Brackets

[CREPELLO]

How are square taper bottom bracket axle lengths worked out? I understood it that the axle length increased symmetrically, to the left and right side.

I've just replaced a UN54 107mm BB with a UN55 110mm. I did this because I wanted to gain that marginal bit extra heel clearance at the RH heal/dropout on my Hewitt Cheviot. Assumed it would be 1.5mm, which I felt was enough.

What I've found is that the crank front face at pedal - chainstay distance differs each side, but all in the wrong direction - 27mm on the right, 30mm on the left!

I did make measurements ages ago with the old BB in place - of course, I never wrote them down
But looking at the new 110 BB, all the extra length appears to be on the LH side. Visually, there appears marginal clearance between the RH crank and BB cup, just as the old 107 BB. That much I can remember.

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OK, I've now measured both 107 and 110 BB's. The critical drive side axle measures 17mm on the 107 BB and 17.5mm on the 110 BB (measured to the thick washer that butts up to the crank). The non drive side axle measures (107) 25mm to the BB body and (110) 28mm respectively. So all the extra length is basically on the non drive side

[Brucey]

if the 110mm UN55 model is (unlike the 107mm UN54) the type that is meant for a chaincase bracket or an 'E' fit front mech then this would explain the discrepancy you have seen.

If so, to get the extra heel clearance you desire, you can fit a spacer on the drive-side.

There may be a cryptic marking on the BB indicating this. IIRC CRC have occasionally sold these BB units in the past.

cheers

[531colin]

Do the axles actually measure 107 and 110mm?

[CREPELLO]

Do the axles actually measure 107 and 110mm?

Oh yes, that measurement. The 107 BB axle measures overall 108mm. And the 110 axle measures...111.5mm

I've got 4 UN** BB's to compare, one each of 52/53/54/55 series (2 each of 107 and 110mm axle). Just to add to the confusion, the earlier Japan 52/53 look like they have a different axle profile, compared to the Singapore 54/55 models. Visually, it's almost like comparing JIS to ISO axles - the flats on the 55 are clearly wider than on the 54.

Confusion piles on some more when I look at the box of the UN54 - it says on the front "For bracket type front mech [E]" and "107/68BSA". The BB markings make no mention of the E type spec. It's identical to the other 107 BB in all critical measurements.

I wrote a thread about E type BB's back in January after buying a UN E type BB. IIRC, the DS axle was the same as a standard 110 BB axle. The extra length was on the the nonDS - which makes sense.

What doesn't make sense to me is a standard BB axle putting on most of the extra length on the non drive side. As I said above, there appears to be a difference between the 107 and 110 axle of .5mm.

[CREPELLO]

If so, to get the extra heel clearance you desire, you can fit a spacer on the drive-side.

Yes, I was thinking of that - I've done it before. For some reason, I feel nervous about doing this now. The threaded section of the drive side cup measures about 11mm. I have a spacer of 2.46mm. I'm concerned that the load concentrated on the DS of the BB shell is rather higher with the spacer in place. It's a touring bike after all, where heavy loads are common place.

[Brucey]

IIRC there is an undercut on the threads, which is about the same size as a 1.5mm spacer (which would bring the axle roughly central).

If you are worried about the fit (which you shouldn't be unless the frame threads are bad, I'd say) , use some loctite on it.

BTW I've seen these BBs listed as suitable for a 70mm shell and/or for use with a chaincase bracket.

cheers

[CREPELLO]

IIRC there is an undercut on the threads, which is about the same size as a 1.5mm spacer (which would bring the axle roughly central).

Yes, of course, I overlooked that undercut. I've now fitted the BB with the 2.5mm spacer. The chainline is 48mm and I don't get the heel of my shoe clipping the rear dropout anymore (wide heels don't help)

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I'm still interested in how Shimano decide how the axle length is pitched. Why is there more axle exposed on the non drive side than the drive side?

What would a 113mm or longer axle look like in this respect? (I've got one 113, but it's in a bike)

Below is a row of UN** BB's - from left to right: UN52 110 / UN53 107 / UN54 107 / UN55 110

Shown is the non drive side, clearly showing the longer axle of the 110. But the drive side is only 0.5mm longer. So why do SHimano do it?

The taper flats are also quite different in appearance.
Also shown are the three different sleeves/cups - plastic, steel and the latest aluminium.

[james01]

[
Below is a row of UN** BB's - from left to right: UN52 110 / UN53 107 / UN54 107 / UN55 110

BB2.jpg

Shown is the non drive side, clearly showing the longer axle of the 110. But the drive side is only 0.5mm longer. So why do SHimano do it?

.

Baffling. Could it be a simple manufacturing error where the axles were reversed in the shells?

[iow]

some good info HERE

[CREPELLO]

A good link there, thanks; not seen it before.

Unfortunately, it didn't mention the anomaly I've found, but it did have a list of Shimano UN lengths and codes. Interestingly, 107 and 110 are coded the same (MM), whilst other lengths all carry their own code. This suggests some association between these two lengths, that the other lengths don't have. Or why would they have the same code?

Also mentioned (confirming Bruccy's advice) is that the use of a 2.5mm spacer is accepted, but only 1mm if a plastic retainer cup is used. Now that UN55's come with an alu cup, there is that flexibility again that was lost for a few years with the placcy cup.

[Brucey]

Re the plastic/alu cup thing; there are three solutions if you are using a driveside spacer;

1) buy a 110-70 unit instead of a 110-68 unit. The alu cup will work fine, or

2) fit a suitable spacer of similar thickness inside the alu cup, too, or

3) remove the flange on the alu cup, to make it more like the plastic one in shape. This will weaken it, but it will only cause a problem if you didn't need to remove the flange in the first place, in which case the weakened region is less well supported when you come to use the extractor tool.

Re the anomaly; dunno whether to vote for cockup or conspiracy. It occurs to me that the extra length on the left might give useful chainstay clearance on the left side with some frames/cranks...?

cheers

[CREPELLO]

Brucey, are you saying that the alu cup isn't sound with the 2.5mm spacer? Lack of bearing support? I'd hoped that it would be alright. But I do have a spare steel cup I can use.

[Brucey]

if the shoulder on the outside of the LH cup bears against the BB shell before the shoulder on the inside bears against the end of the bearing unit, it is possible for the bearing unit to be less secure than it might be otherwise. In any event a spot of loctite on this part of the assy isn't a bad idea.

cheers

[CREPELLO]

Makes sense. So does Loctite mix alright with grease, or should that be thoroughly cleaned off? TBH, it's mainly waxoyl.

[531colin]

...........

Below is a row of UN** BB's - from left to right: UN52 110 / UN53 107 / UN54 107 / UN55 110

BB2.jpg

Shown is the non drive side, clearly showing the longer axle of the 110. But the drive side is only 0.5mm longer. So why do SHimano do it?

......................

I'm not about to suggest that Shimano are infallible, but it seems odd if all their 110 mm BBs are wrong and nobody noticed.

Its possible that both the 110mm BBs are for E type F. mechs....maybe unlikely?

It seems to me that for the comparison to be valid, all the BB units would need to be the same length, and screw in the same distance (from the right side)