Sheila

I have no experience of GRIN electric motors on tandems, or on any bike for that matter.   Are you planning a build for only use off road on private land with the owners permission?  If so I wish you luck. 

If not I trust you are aware that whilst the law has not changed the attitudes toward and more importantly capacity for enforcement has started to catch up to the prevalence of illegal use, and the likelihood of serious consequences for misuse of electric powered motorcycles is increasing dramatically.

The Motorcycle Industry Association (MICA) and The Bicycle Association (BA) who represent as trade bodies manufacturers and distributors in the UK have produced a guide which has been circulated to every Police Traffic officer and other specialist officers, to help them identify "illegal ebikes". . 

The industry guide is found here:  https://d2zif723tly95y.cloudfront.net/2144/A-Guide-To-Identifying-Non-Compliant-E-Bikes.pdf

Section 6 points out that if a motor is "rated" at over 250 Watts it cannot be an EAPC.  The evidence for that is often stamped on the plate attached to the motor by the manufacturer.  Note that the common practice of claiming an electronic restriction down to 250W power output does not alter the capacity of the motor, which is how it's legal status for EAPC is defined.

Section 7 states that any switch or software which allows an "off road" or higher power use also classes the machine as non EAPC status.  Software and apps are regarded as switches.  I understand - only from online sources I cannot verify - that GRIN motors are only limited to 250 Watts by software, and even a cursory reading of the guide leads to the conclusion that they could not be classed as EAPC, and are therefore legally motorcycles for "Highway" use.

I have volunteered this year to join TC as Tech Advisor, and will stand at 2027 AGM for the role.  I am a retired Police Traffic officer, previously classed as Expert Witness up to the higher courts and hold C&G in Advanced Accident Investigation and Advanced Vehicle Examination, whilst having ridden tandems since the 1950s and been a lifelong cyclist.  I have some experience and some interest in the matter.  At recent TC rallies we have attended it appears about 50% of members are using Electric assist, it is an area I lack experience in so I am trying to address that.  I have this last week made enquires with leading providers for Cytec and C&G training to seek personal education regarding ebikes and learnt during those conversations that these Industry trainers are now running courses for serving Police Officers - to train them in identifying e motorbikes for enforcement.

I currently have three standard tandems, pedal powered, and a Gepida Thoris tandem EAPC that uses the Bosch CX system (250 Watts).  With two batteries the bike will cover 90 miles daily with full electric assist, why would I need anything more than that?  For riding on roads and cycle paths why would anyone want more assistance than that?  If a rider can pedal 90+ miles a day - and actually want to do so  - I would opine that they do not need electric assist.

All non compliant (not EAPC) electric powered cycles are treated in law as motorcycles with electric motors and need type approval, insurance, driving licence and registration and VEL before use on a road or cycle path (plus wearing a motorcycle helmet).  The sentencing guide - fines dependent on personal income plus minimum 6 penalty points on driving licence for no insurance, and in addition any penalties for the related offences.

From their own online material plus online reviews by users GRIN motors are world leading in power and regenerative braking, and many of their motor options are 1000 to 1500 Watts, from the Industry guide advising Police they therefore cannot be classed as EAPC and would be illegal to use on UK roads and cycle paths.

https://ebikes.ca/resources/theory/power-ratings.html  After reading the webpage from the manufacturer do please let me know which of their products is their 250Watt legal limit motor.

You pay your money and take your choice - at your own risk.

The problem with any wheel hub motor is that the motor can only turn at the speed of the wheel. So, when you are grinding up a steep hill the motor is running very slowly. Think of yourself pedalling up a hill, if you left the bike in top gear, you would be struggling, well the motor struggles too. So, you would change down and your legs spin the pedals faster, the bike goes slower but you do climb the hill. Well, a motor is just like that, it is best when it is revving faster but a wheel hub motor only has one gear, so it has to slow down to the wheel speed from its most efficient RPM.

The answer is to fit a mid-crank mounted motor such as a Bafang BBS02 which drives through your chainset, so as you change down the motor also revs up keeping in its best power band.

We have a Ridgeback Panorama tandem with a mid-mounted motor; a Tongsheng TSDZ2B2 but I would not recommend this one unless you have access to a machine shop. The square drive on the left-hand side did no give sufficient room for the left-hand chain wheel, so I had to redesign the crank spider, not a job to tackle with basic tools.

Our first conversion was with a front wheel hub motor but I was disappointed with its hill climbing assistance.

I previously had a Bafang BBS02 which did fit OK but I wanted to incorporate the torque sensor of the Tongsheng which I have on my solo bike.

I can recommend Woosh bikes 

they can supply a road legal mid drive motor and kit with a 6mph thumb throttle if you ask for one 

it can be done also for a rear wheel drive 

just phone them and ask advice and mention me Peter