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Why is it "not allowed" to use the 14-speed Rohloff in combination with the Schlumpf Mountain Drive?

According to Function-Schlumpf-Mountain-Drive.pdf:

Please notice: the 14-speed Rohloff is not allowed to be combined with mountain-drive.

...but it doesn't say why.

screenshot of a document that reads "Please note: the 14-speed Rohloff is not allowed to be combined with mountain-drive."

Besides the limitations of one's wallet, why does Schlumpf say that you cannot use their Mountain drive along-side the Rohloff 14-speed internal hub?

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If you use the Rohloff Speedhub, then you should use the Schlumpf Speed Drive (not the Schlumpf Mountain Drive).

According to the Manual-Schlumpf-Mountain-Drive.pdf

Please note, that Rohloff only recommends to combine its Speedhub with Schlumpf speed-drive., due to a minimal chainring size required.

Additionally, the Technical-data-Schlumpf-Mountain-Drive.pdf says:

Please do not combine mountain-drive with the Rohloff 14-speed hub. The Rohloff hub has a strict limitation of the entry torque, and may only be combined with speed-drive.

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  • Good find - it might help to add the difference between the speed drive and the mountain drive (afaik both are two-speed gearboxes, but one's an underdrive and the other's an over-drive ?)
    – Criggie ♦
    Commented Apr 7 at 19:26
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    As far as I’m aware the stated limits for internal gear hubs are often quite conservative. Usually they only state a minimum chainring size but don’t take rider mass ~> pedaling force, crankarm length etc. etc. into account.
    – Michael
    Commented Apr 8 at 7:48
  • @Michael yes. And in practice, going to very low gear combinations normally does not mean you're putting more torque through the drivetrain, rather it means you stay in the saddle and spin at high cadence with low pedal torque, where a higher gear would have required high-torque grinding out of the saddle. This is a good thing both for the rider's and drivetrain's health. ...
    – leftaroundabout
    Commented Apr 8 at 9:04
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    @leftaroundabout: Still, all else being equal the Schlumpf Mountain Drive more than doubles the force going through the chain to the rear hub. If you are a 60kg rider that might still be fine (after all, the Rohloff also has to survive a powerful 120kg rider).
    – Michael
    Commented Apr 8 at 9:45
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    The only exception is trials riding, where you actually smash your full force into the pedals instantaneously from a standstill when leaping over a gap between rocks. But this is not something anybody would do on a bike with a heavy Rohloff hub.
    – leftaroundabout
    Commented Apr 8 at 10:13

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