Hi Andre!
Good points, all.
I've often wondered why we don't see a split-shell eccentric with the split on the upper-rear side, between the rear of the seat tube and the top of the chainstays. Such a design would be self-closing under a rider's weight, unlike the more conventional version, which has the split on the underside...and will stress open against the closure bolts. Having built frames as a hobbyist, I am considering this upper-rear version on a future next effort. Of course a closed shell will always be ultimate stronger, but does the split represent a genuine weak point at the expected level of use for this application?
It would be interesting to instrument frames with each kind of EBB through a number of stress cycles...or review the customer warranty claims for each type. I suppose the economics of excessive claims would ultimately decide if there was a sufficient design margin. Too many warranty claims on a "lifetime" frame would soon result in a change, and we really aren't seeing that. Both kinds of BBs seem to be about equally long-lived, judging by the numbers of manufacturers continuing with each.
There's a couple more points to consider between eccentric BBs and sliding dropouts:
? EBBs tend to change the saddle-to-pedal relationship slightly when adjusted, but leave the rear wheel, brake blocks, and rear mudguard adjustments unaffected. One adjustment instead of two or three.
? Sliding dropouts can alter rear brake block adjustment (though only very slightly in Rohloff's carefully angled design), and if you run your mudguards close to the tire, you'd need to leave a bit of extra room or simply adjust accordingly and the rear wheel has to be squared with the frame. Saddle-to-pedal relationship remains untouched when chain tension is adjusted at the dropouts.
Really, either method and most of the variants work well in practice. You're absolutely correct; it is the Designer's Prerogative and no choice on the part of the buyer except to select a different model, as the cake is already baked by the time the bike is offered for sale.
To be completely fair, while the split EBB can be a weak point, the closed EBB shell has one as well (besides the possibility of seizing) -- it is possible for a ham-handed user to over-torque the grub screws and so ovalize the BB shell. An expensive oops and I have witnessed the results, though it takes some effort to accomplish. And, wedged eccentrics in closed shells can seize as well and require hammering out with every adjustment. Avoiding either sliding drops or EBBs of any sort puts us back to using tensioners -- effectively non-shifting derailleurs with added chain friction and reduced life from the small-diameter pulleys and double-back chain runs as well as greater exposure to road debris and damage.
You pays your money and takes your chances, just as you said.
All the best,
Dan. (...who owns two closed-shelled grub-screwed EBB bikes, but thinks there's more'n one way to skin a cat)
