Originally Posted by
CycleryNorth81
The original iso spindle is a campy copy, 115mm.
Sheldon Brown said an iso crank will go in further by 4.5 mm on a jis spindle per side, thus 9 mm per spindle.
You have that backwards. What Sheldon said was "If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm
farther out than it would on an ISO spindle of the same length." [emphasis mine]
An ISO spindle taper starts smaller than a JIS, so a crankarm made to ISO spec won't press as far onto a JIS spindle.
(Let's also acknowledge that the Super Mighty isn't actually an ISO taper spec crankarm. It is close enough to fit well, but that fact may change the calculations a tiny bit.)
Originally Posted by
CycleryNorth81
New jis length= 115mm-(2x4.5mm)= 106mm
I would use a shimano UN55 (a sealed cartridge) with a spindle length of 107mm.
Even if your math works out, the problem is still the taper size, not the spindle length. On a crankarm taper as short as the Super Mighty, I'd rather have a well-fitted spindle and less unsupported crankarm, which will put more stress on a the unsupported bolt length as well. But that's just me.
I don't go along with Sheldon's reasoning behind why mixing tapers isn't a bad idea. Maybe it worked for him, but it's risky. And I would not take his recommendations on blind faith. He doesn't address the unsupported bolt issue that I mentioned. I've seen broken crank bolts caused by that.
The other, perhaps greater problem is that he divides the BB taper world into ISO and JIS, which might work for BBs and cranks made since about 1990, but ignores the reality previous to that. I'd advise everyone to be aware that that is an oversimplified view. You need not know every possible other taper dimension, but if you're working on C&V bikes, you should be aware they exist. Also be aware that those standards are voluntary, and not every crankarm or BB needs to be made to them.
This Super Mighty is from that previous non-standardized reality. Is that a problem? I can't say. Though useful, these generalizations are guidelines, but any specific implementation has to stand on its own merits of fit, especially when mixing brands or mixing pre- and post-ISO/JIS fitments.
When dry-fitted as tightly as you can get with your hand, and not tightened on, the spindle end should be about 3mm from end of the crank taper. Much more or less than that, and you might have problems. There should also be at least 2mm clearance between the back of the crank and the shoulder of the spindle (or the face of the BB cup) to allow it to tighten on. A little more is better.