Mystery of Shimano Tall Flanges

I really like Shimano hubs. They've served me well for over a decade, through all sorts of bad weather and other abuse. I have looked to them for my relatively recent interest in mountain biking (MTB).

(Well, that Nexus internal hub wasn't very reliable, but the freehubs are all I'm interested in these days, and they've all been very good.)

I built up Bumblebee, my current MTB, using their XTR hubs. I like everything about these except one thing, their low flanges. The XTR flanges look like those of a road hub, which is nice for a lightweight hub. However, I think it's bad for a reliable MTB wheel, because both front and back disc-braked MTB wheels run much higher peak torques than (rim-braked) road wheels ever do. For example, the chain drives at least 50% higher torque in an MTB than a road rear wheel: just compare the low gear ratios. And my vote for highest torque in any bike wheel is a disc front.

Torque is conveyed through the spoke web by differential tension: the pull side spoke tension increases and the push side decreases, both to exactly balance the torque between hub and rim. The taller the flange, the smaller the tension changes because they operate over a larger lever. (The most extreme case of this would be if someone built a fully radial rear wheel. With no lever arm at all, the hub would be pulled apart by any significant torque.) In other words, a low-flange MTB hub requires a very good wheel build, and is still more fragile than a less carefully built wheel with a high-flange hub.

The frustrating thing about the XTR hub flanges is that I have an XT rear hub with tall flanges. It's lower-end, weighing almost twice as much as the XTR. But why wouldn't Shimano offer a tall-flange XTR when there is one in XT?

First I noticed that Shimano's centerlock hubs use low flanges while their ISO 6-bolt hubs have high flanges. I tried to turn this into a demographics situation - maybe ISO users are thought to be poor wheel builders?

It hit me the other day riding in: the receptacles for the 6 ISO bolts would cover the spoke holes of a low-flange hub, making the wheel very difficult to build! So ISO hubs need high flanges for the builder's sake, while centerlock ones do not.

My current plan is to use the XTR hubs and just keep after the wheel tension. I think this will work for me - though I wouldn't recommend it for someone doing big drops or very fast descents. And avoid thin spokes! The DT Revolutions I sprinkled in the first build proved to be a real problem here, and I've since replaced them all in the rear wheel.