External bottom brackets for dummies?

[iamkeith]

Needing some confirmation that I'm doing things correctly with a new IRD  crankset and x-type bb, from SOMA,  that I'm putting on an old bike.  I mostly have square-taper setups, so I'm fairly new to these external setups.  The others that I have are different (and more logical)  but I suspect this new one is actually more standard.

Question has to do with spacers and their configuration.  And lack of symmetry!  The crank and bb fit either 68mm or 73mm shells.  In this case, the spacers go between the cup and the threaded shell.  If there were two 2.5 mm spacers, things would be obvious: one per side, 5mm total + 68mm = 73mm, symmetrical.  But there are three 2.5 mm spacers and, through trial and error, I discovered that I need all three to take up side-to-side play in the crank.

The problem is that no arrangement of the spacers will give me symmetrical, lateral positioning of the crank arms.  If I use two spacers on the drive side and one on the n-d side, the arms are offset 2.5mm to the left (more space between the left chainstay and the crank arm),  if I put all three spacers on the right, it is 1.75mm  too far that way but, more important, seems fragile in a sort of cantilevered way, with not enough threads engaged between the cup and the shell.

Do people usually just live with the crank being offset to the left?  The perfectionist in me has a hard tine accepting that something isn't wrong.  I've offset  phil-type bbs, with separate retaining rings, to adjust chainline or clear a chainstay, but never this severely.

Do shops keep thinner spacers around and just use those instead, for better adjustment?

(My other x-type bbs are on mountain bikes with 73mm shells, and the spacers went between the cups and the arms, and were thinner, to allow much more  fine-tuned adjustment).

[iamkeith]

[aeroperf]

Yes, they do make 1.25mm spacers.  My Shimano MT-800 came with one (and three 2.5mm).  Whether it is symmetrical or not, it is all about the chain line.  With my Homer, I used 2 of the 2.5mm on the right to get the center chainring of a triple at 47.5mm.  My Soma Saga required 2 of the 2.5mm on the left and one 1.25mm on the right.  Go figure. If your front drive is a double, check your LBS for the thinner spacers and put them on as required by the chainline.  Hope this helps.
3.  Exploded view of Shimano MC-M590 showing (2 #8) on right, one on left   2.  Soma, 2 on left and 1.25 on right    1. Homer bottom bracket 1 on left and 2 on right.

[aeroperf]

And of course, the system reversed my photos versus their names.  1 - Soma.  2 - Homer.  3 - EV.

[iamkeith]

Thanks.  I didn't even think to see if the chainline was matching some standard or other.  On multi-gear drivetrains, I've always been more concerned with, and prioritized, arm- or chainring-to-chainstay clearance.  I might have compounded my problem by using a modern, wide (mtb or modern gravel bike?) crankset with a 130mm hub.  Crank was such a good deal on sale that I couldn't pass it up.

Funny, I meant to post this question on the ibob list.  But Brenton's thread - asking for advice on chain sizing - threw me off and made me think that's where I was.  At least the bike is a Rivendell.  :-)

[Nick Payne]

The spacers should go between the crank and cup, not between the cup and BB shell. Also, external BB cups are much more reliant on the BB being accurately chased and faced than cartridge bearing square taper BBs. If the cups aren't co-axial and square, the bearings will have a much shorter life.

When I fitted a Deore XT crankset to my Riv road frame about eight years ago, I didn't use the spacers intended to be used with a 68mm BB shell. I used a Dremel with small grinding wheel on the LH crankarm to remove about half of the length of the splines (see first photo), so that the crank could slide further onto the axle. Omitting the spacers on both sides gave a better chainline for a road bike, and also narrowed the Q of the crank. My modification has now lasted eight years with no problems at all.

Nick Payne

[aeroperf]

I’m going to stick with the spacers going as shown in the exploded view - spacers going between the cup and the BB shell.  
You’re absolutely right on the prep work.  Both bikes were chased, but the Soma was not faced… probably why it gets by with the spacer stack slightly smaller.

[Will Boericke]

I mostly take the number of spacers they recommend and arrange them in whjatever fashion creates the best chainline for the bike. 

Will

[iamkeith]

Nick, as I mentioned in my question, I too only have experience with cranks that take spacers between the arm and the cup.  It makes more sense that way, but this bb definitely seems to require spacers next to the shell.  That's how it was assembled when it arrived.  I just cant figure out why it's asymmetrical.  Ive now learned that, even with a 73mm shell,  it would still require a single 2.5mm spacer on the drive side, UNLESS there was a bb-mounted deraileur or bash gauard plate.  But you'd think the crank arms would be shaped to compensate for that.  

As set up in my photo, I have a 50mm chainline.  Sheldon says a road double should be 47mm, so that's another reason not to add more spacers on that side.

You probably had the best idea though.  I guess I'll take a dremel to the non-drive-side arm or spidel end, so it can slide inward a few more millimeters and at least be symmetrical.  

It'll still be wider than necessary, but I'm not Q- factor sensitive, fortunately.  I guess this is really intended as a mtb crank, even though it doesn't say that on soma's or interloc's website?

[Will Boericke]

Almost all of the external cranks I've installed have multiple spacers.

[Garth]

Keith, road bike double crank chainline spec is always 43.5mm. Are you measuring between the rings ? 

You may hate me for saying so but it sounds like this crank would be better suited for another frame with at least 135mm spacing. With the big ring that far out on a 130mm spaced road frame, your available cogs(without a lot of friction) diminishes by at least one, likely two.

[Max S]

A different, but related question: 

I mistakenly bought the MTN version of a Shimano bottom bracket (external cups) for a recent 105 crank. Comparing to the old (road) BB, it looked to me like the only difference was the internal plastic sleeve length. I swapped in the old sleeve between the new cups, and that seemed to go in fine. Maybe last couple of millimeters were a little tighter than the right hand cup, but went in. 

Did I do something that will come back to bite me?.. 

- Max 

[iamkeith]

To wrap this up, and hopefully make it useful for someone else down the line:

I think Garth is right.  This crank will not work on this bike.

Even if an X-type crankset is advertized as working with both 68 and 73 mm shells, it's not that straight forward.  It can technically "work," but there will be  tradeoffs with chainline or centering or Q-factor not being ideal.   If I had a 73mm shell and used one spacer on the drive side, I'd have a 47.5 mm chainline AND symmetry between the arms.  I'd still have to just  "hope" it cleared the chainstays though. 

This is exactly why I was so slow to adopt X-type cranksets.  After a lifetime of being able to pick a square taper spindle in virtually any length I wanted, the inability to adjust width didn't seem intuitively right.  If my other forrays (on mountain bikes) hadn't been so plug-and-play easy, I would have questioned this more before ordering.  Then again, there was nothing about this on the product description webpage that alluded to the dimensional details or specs. 

For anybody interested, here's the product link.  It's really not bad quality and is attractive and has many chainring options available, with shift aids, AND is available SILVER!  I think Riv was out of everything in the configuration I needed, and this was on sale for a really good price.  For a 10- or 11-speed mountain/hillibike with 73mm shells and 135 or wider rear hub, it's a good option.

https://www.somafabshop.com/shop/ird-vortex-sub-compact-crankset-46-30t-4722#attr=1257,4857

[iamkeith]

Max,  I can't imagine why you'd have a problem but, obviously, someone with more experience should chime in.  I'm going to "try" to use this same bb without the spacers, and get a different crank.  I'm not sure I can find one with the right spindle length, right spindle diameter, and 10 speed compatible rings in suitable sizes, but I know for a fact that the cups install just fine without any spacers at all.  (in this case, they each have an aluminum sleeve that telescopes over each other, but same thing as an accordian plastic sleeve in function.)

On Sunday, January 21, 2024 at 6:28:47 PM UTC-7 Max S wrote:

[aeroperf]

I wanted a “medium” touring bike, so I got an A Homer Hilsen frame.
I was also impressed with how the 24mm, eternal bearing, Hollowtech II drive system felt.  It was, to my feet, noticeably stiffer than the square taper BBs I had been using for years.

So, loath to screw up a Rivendell frame, I bought an ’82 Team Fuji from Craigslist, and used the rebuild of that to determine how to pull a square taper BB and put on the Hollowtech BBR60.  I also put on a few other goodies (Sora 3030 system,105 brakes, Velocity Road rear hub, re-laced the wheels) to learn how it all worked.

I learned that those spacers we’ve been referring to are fragile and don’t want to be between two moving parts.  Also that you put the drive side external cup on, then run the crank through, then screw on the non-drive side cup instead of trying to push the crank through a fully assembled BB (my LBS taught me that).  Also that putting on the crank, then measuring the chainline, then pulling off the crank to change the drive side spacers was an iterative process.

Then I built up the Homer with the MT800 BB and the Shimano FC-M590 triple crank.  It only took one iteration of spacers to get the chainline.  The MT800 worked perfectly on the 68mm BB shell.  I also - wanting to see if this stuff was interchangeable - pulled it all off at one point and tried a Token TK878EX BB. The Token was cheaper then, and came in AHH Blue.  It also worked perfectly.

The Homer was not my longed-for perfect tourer, so I passed it on for a Soma Saga.  But I kept the drive and BB.  The Saga also has a 68mm BB shell and 135mm rear spacing.  Only one iteration to get the spacers/chainline right, and they are a different config from the Homer, but it works great.

Other lessons learned - the 24mm bottom bracket is more fragile than a sealed square-taper BB (also more fragile than the good old 1/4 inch ball bearings in an un-sealed square-taper setup).  I ruined one 24mm BB before I asked my LBS and got a lesson in installation.  I also got told that the 24mm BB has to be replaced more often just from use.  I haven’t killed one yet, but if your crank starts making a hissing sound, check the BB first.

Going back to Keith’s original post, a little asymmetry is the price paid for a correct chainline when your crank shaft is fixed length.  Spacers and assembly iterations are the price paid for a 24mm BB that is supposed to fit both 73mm and 68mm shells - it must be built for the bigger size.  Max, the MTN should work fine, for a few grams extra weight, and might last longer.  And, finally, if you hit a snag ask your LBS.  They've had to deal with this stuff for years.

[Garth]

Keith, I think to get a good road chainline 43.5-45mm for a road frame w/130mm spacing and a 46/30 (or abouts), a basic road triple would work just fine. Be it a 110/74 or 130/74, either way they accommodate the 46t ring in the middle. Even a basic XD2 with a 107mm BB is 45-46mm chainline, and that's as a triple, measured to the middle ring. Since doubles are measured between the rings, in this case the 30 and 46, the chainline will measure as 2.5mm less(the c-c spacing on chainrings). I used a 107 BB on my Bombadil even with it's bowed out stays, so on a road bike without that it'll be no issue either. A 110mm BB is only longer on  the NDS, but I've used 1-2mm spacers on the driveside(between the frame and cup) if desired for a better chainline. I'm referring to cartridges of course, you can't do that with a cup/cone BB !

[Luke Hendrickson]

Everyone’s already made good points (what a great resource this place is!), but always feel free to get ahold of me as I work at Merry Sales. Anyone can email me at luke at merrysales dot com with questions related to stuff you’ve bought from us.