Hi Philipp!
Though I preferred external BB cranks on my Thorns, internal-bearing, square-taper BBs have historically been longer lived. The gap is narrowing, however; a lot of internal-bearing BBs are not as well made as they used to be so this one formerly very reliable part is now a bit less so compared to days gone by.
I chose the HollowTech II for both Sherpa and the Nomad. Primary in my choice was the easy on-road serviceability (allen key, as with a threadless headset), lightweight, and stiffness.
That said, external bearings are a known trouble point and have a much shorter service life than a high quality internal bottom bracket. Though the races are larger in diameter, the bearings are smaller and fewer in number, and there can be sealing and lubrication issues. A friend of mine had a great deal of trouble with his, caused in part by riding it well after it wore out (the bearings seized and scored the spindle to the point where it had to be hammered out, destroying the crank). He replaced it with a similar 4-arm square-taper (internal-bearing) crank and BB and is happy once again.
When I bought the Nomad, I ordered a Phil Wood replacement external bottom bracket at a very good price (USD$99 postpaid) and plan to use the original Shimano unit until it shows initial signs of wear/play, then swap in the Phil unit, which has a much better reputation (I have had exceptional service from their internal BBs, hubs and pedals on my other bikes). Removal and replacement of external bearings is exceptionally quick and easy, and I like the cranks otherwise. On a long, round-the-world trip, it wouldn't be a huge thing to carry a spare bearing set. A number of aftermarket sources also offer replacement cartridge bearings that can be pressed into the original Shimano external cups.
I think poor initial adjustment of the left-arm crank cap is one prime cause of short bearing life in external-bearing BBs. A small plastic (Shimano) or steel (Park) tool is used, and the bearing preload is exceptionally small -- as little as 4
inch-pounds. Nearly everyone gets it wrong by making it too tight. This shortens bearing life greatly. All that is needed is to remove excess play, and that's
it; anything else side-loads the bearings and effectively squashes them into their races.
With the four-arm external-bearing crank and BB comes different chainring compatibility and selection. For single-ring Thorns, that means 104mmBCD chainrings. If you decide to go with a Surly stainless-steel chainring, at present they are available as large as 36T but no larger. Mine works fine with a 17T Rohloff rear cog to provide the gearing I need. This combination has recently been approved by Rohloff for warranty coverage; previously, it fell into the "use-at-own-risk" Forbidden Zone. Thorn offer a wide selection of four-arm, 104BCD aluminum 'rings that are of exceptional quality, thicker than usual, very stiff, very hard, and beautifully finished -- few if any size limitations if you go with a Thorn-branded chainring. Regardless of BCD, they are exceptionally long-lived for alu 'rings.
Five-arm cranks are typically 110mm BCD, and there is a huge selection of 'rings available. Surly's stainless 'rings are available in the popular 38T and 40T sizes for pairing with a Rohloff hub. OF course, Thorn also have a wide selection of their own alu 'rings in this size as well, so you can get pretty much anything you might wish.
I hope this helps, Philipp. I've tried to present the pros and cons fairly. In the end, I went with my preferred crank and also purchased spare bearings to address the one known shortcoming of the HollowTech II's external design. That said, I don't expect to need them for a good while; it was a matter of striking when an exceptional deal on the Phil bearings came my way. I think the original Shimano bearings will last a reasonable amount of time in my use. For reference, all my other bikes have sealed, cartridge-bearing, internal, square-taper cranks and 110/74BCD cranks.
[EDIT: Seeing Andy's well-written post reminded me of another reason why I decided to go with the external-bearing crank. The Thorns were my first new bikes in well over 20 years, and I plan to keep the Nomad for at least the next 20 years. Where each was a new bike, the external-bearing design seemed the way forward -- the square-taper internal-bearing seems generally less popular on current offerings, so I viewed the new bike as an opportunity to modernize. I moved from 5-, 6-, or 7-speed freewheels and cassettes on my other much older bikes to 9-sp on Sherpa and Rohloff on the Nomad. I went from quill stems and lugged frames in standard diameters to threadless stems and TIG-welded oversize frames on the Thorns. Did I choose right with the cranks? Will I prove to be prescient or a hopeless early adopter? Only time will tell...
]
Best,
Dan.
