Got no idea about an official primary drive ratio limit. I've ridden my XRK8W for five years with a 25T cog driven by Sturmey's matching FCS80, 30T crankset.
Thanks for the info.
The "
Internal-Gear Hub Torque" article on the Sheldon Brown site went quite above my head, but it does mention this about the SA XRF8(W) IGH:
A higher top drive ratio imposes more forward torque on the frame. The Sturmey-Archer XRF8(W) 8-speed hub's top ratio is 3.24 and so the torque on the frame is 0.69 of that from the chain. With the 38-tooth chainring and 25-tooth sprocket sold with this hub, torque from the chain is 74 pound feet. 69% of this is 51 pound feet, nearly as much as the 60 pound feet which a disk brake can generate with the same rider and bicycle, but taken up at the small radius of the antirotation washer tabs. It is no surprise that antirotation washer failure has been reported with this hub.
[…] As already stated and as the graphs show, the torque to the frame is lower if the hub mostly gears up rather than down.
On the other hand, greatest efficiency of a hub gear is usally at the unity ratio, with the gears idling -- and this is best as the most often used, level-ground gear, which is much closer to the top of the useful gear range than to the bottom. Lower gears will then impose a large torque on the frame.
[…] Yet other hubs minimize torque to the frame by avoiding extreme decrease ratios. The Sachs Elan hub had only two gears below unity ratio, and 9 above. Sturmey-Archer 8-speed hubs have only a unity ratio and increase ratios. Efficiency suffers with these hubs in the most-used upper middle part of the gear range.
I guess I won't have to worry about excessive torque on the D3.