Why do some models have a chain tensioner at the rear and the chain wraper at the front, whilst some just have the wraper?
As Harry has said, some bike frames using vertical spindle dropouts need a tensioner when using a hub gear instead of the derailleur they are designed for. In Kalkhoff's case, many of their e-bikes are based on existing normal bike designs, for example the Agattu and Pro Connect existed long before they modified the frame to take the Panasonic unit to introduce them as e-bikes as well. Maybe the Sahel did also and they haven't duplicated the rear A frame assembly with a slot drop-out version necessitating fitting a tensioner.
However, since the design changes of the new unit have led to problems with the motor sprocket jumping the chain when the rear wheel adjustment has been wrong for the chain tension, they may be trying this alternative approach on hub gear bikes to maintain a correct tension and chain wrap with no chance of the user messing it up. That would make a lot of sense on this new unit, and the fact that they are also doing this on the new Pro Connect with the Alfine hub makes this the much more likely reason. If it proves successful, we may possibly expect to see this rear tensioner on the Agattu models as well in future.
The old Panasonic unit didn't have these same problems with the chain wrapper since it's 14 tooth motor sprocket maintained a good wrap with more variation in the wrapper arm setting, but the new unit's small 9 tooth sprocket with only 4 teeth engaged with the chain at any time to transmit up to 400 watts of power has caused chain jumping problems. That's why the derailleur versions are now forced to have an extended idler system to insulate the chain wrap from the excursions of the derailleur rear mechanism. The old unit's identical basic wrapper worked just as well with hub or derailleur gears, needing no fancy extension arrangement, thanks to the 14 tooth chain drive sprocket it used.
Why the change to the problematic 9 tooth sprocket? It results from the change of a three stage internal gear reduction to the single stage one for efficiency gain reasons, this resulting in a higher output shaft speed, therefore necessitating the smaller sprocket for the same linear chain speed.
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