Mid-drives can be awkward on tandems because of the extra driver chain. You often lose the option to have a front changer even on a regular bike, or get reduced to 2 front sprockets. The mid drive motor probably won't have room to fit a crossover on the left side.

Front drive are easier to fit, and of course don't disturb the gearing at all. One disadvantage of front drive on a regular bike is that it can scrub on steep gravelly hills, With the extra proportionate weight on the front of a tandem this is hardly ever a problem. You do need solid forks, preferably steel, to fit a front drive. Front wheel also has the advantage you get two wheel drive (motor front, riders back). We have often found the rear slipping when riding of wet grass, but the front motor still pulled through.

Rear wheel drive are generally preferred, but means more cabling on a tandem. You should be able to keep the gearing unchanged; just check the motor is right for dropout width and cassette vs freewheel. (or buy a new cassette/freewheel).

I'd recommend looking at what Woosh have to offer. Very helpful pre-sale making sure you get what is appropriate, and also post-sale in case of any questions.   https://wooshbikes.co.uk/

Fitting is very straightforward if you are at all used to bike mechanics, and Woosh may be able to recommend a fitter local to you. For front also consider Cytronex (https://www.cytronex.com/). A bit less powerful and quite expensive, but light. Also Swytch, but their marketing is rather better than their engineering or support.

We had (currently for sale) a Viking tandem with Woosh Mxus XF07 front hub and it  was very  satisfactory for 8 years; though clearly not comparable to a Cannondale.  A little lower powered than we would have liked ideally; there was not much choice when we bought it. We gave it up for an (even more basic) Ecosmo when I found the Viking crossbar too high.

Hi John 

Andy at woosh bike is a good person to speak to he is away till the 29th

they will fit a kit for you or there are many shops over the uk they have contact with that can do it for you for about £100 for fitting

Hi John 

Andy at woosh bike is a good person to speak to he is away till the 29th

they will fit a kit for you or there are many shops over the uk they have contact with that can do it for you for about £100 for fitting

Hi John

we have fitted a Bafang drive to the front B/B of our Bob Jackson built Orbit steel tandem.

we have a straight through drive (all chainrings on the right and a Rohloff IGH for our 14 gears

The biggest issues we faced were keeping adjusting the Bafang chainline inwards to line up with our inner (timing) chainring at the rear. This was achieved with a Lekkie chainring replacing the bafang one.

Rear chain alignment needed to remain unchanged for the hub and we had to fit a Rohloff chain tensioner as we were no longer able to use the front eccentric B/B to adjust timing chainring I tension and had to change to using the rear eeccentric for that and the chain tensioner for the final drive.

Each of the cranks on a Bafang motor is not the same distance from the centre line and the Q factor (distance between the pedal face of each crank is very wide) . We partially corrected the odd pedal offset with Lekkie Cranks but it is still not perfect and the Q factor is further increased. Feels a bit like pedalling like a duck. Some people will not tolerate that and I am not completely comfortable with it as it puts more strain on the knees.

Motor at the front means captain can freewheel while stoker pedals (stops me jolting my partner if I stop pedalling without warning but that is very rare as we are experienced and I used to softpedal to give some warning.

whichever way you fit a postmarket e- assist there will be compromises.

key questions are:

Does your tandem have a crossover drive (photo pf your current setup would help)  i probably does from memory of the Cannondales. I also assume you have derailleur gears rather than IGH (like a Rohloff)

If so you will need to mount the motor at the back unless you are happy to change to a straight through. You will then need to use one of your rear chainrings (inner or outer) as a timing chainring.

If you already have a crossover drive, a rear mounted motor is definitely the simplest and best option.

You can keep your existing chainring setup  but might have some issues with final drive alignment. Chris and Helen Juden would be able to advise on that score.

With a crossover drive you also have the option of going for a slightly cheaper Tongsheng motor which has torque sensing. Bafang is either on or off depending upon pedal rotation at the motor. Various aspects of the Bafang drive are electronically configurable. Not sure about the Tongsheng but I believe they come out of the same factory.

Simpler but less powerful (in terms of Torque) solutions would be front hub motors like the Cytronex.

Hope that helps
Stephen Gray