I recently bought a recumbent trike for my 12-year old son, who has cerebral palsy.

He can pedal, but his leg strength and stamina are severely limited. Even in lowest gear, he struggles to get up the slightest gradient, and anything more than a kilometre or so is too far for him.
In short, he is the perfect candidate for pedal assist, but there aren’t any readily available children’s electric recumbent trikes.

I am lucky enough to live near KMX Karts, who are super-helpful people who make great value recumbents for kids, so we bought him a Kompact R. They have resellers worldwide.

After some research, I settled on the Tongsheng TSDZ2 motor, as the torque sensor seemed like a good seamless means of control, and it has a wealth of online resources from other hobbyists.
The TSDZ2 kit is intended for a conventional framed bicycle, so I had to make a few modifications in order to get it to work well with the trike - it’s those that I would like to cover here, not the basic motor installation.

Supplies

Main bits:

Motor: I bought the Tongsheng TSDZ2 from PSWPower.
I was intending to buy from Woosh bikes, who are a UK reseller, but they were completely out of stock!
PSWPower are a Chinese firm, but seemed to have good reviews, and had stock in a German warehouse - avoiding import taxes and delays.
I got the 500W 48V version, with a throttle - this one.

Battery: I bought a 13Ah battery to go with it - this one.
The battery I got is perhaps overkill. It will depend on rider weight, terrain, gradient, boost level etc, but we have used one out of 6 bars after 3 rides totalling 20km, on at least 50% steep and rough off road tracks mainly in turbo mode.

Speedo:
You also need a speedo extension cable if you put the sensor on the back wheel, as I did.
I got this one.

Cranks: The cranks supplied with the motor are 170mm, which were way too long for my pilot. I happened to find that cotterless unicycle cranks come in a range of lengths, have the correct square taper fitting, and don’t have the spider for chaninrings. Plus they are cheap!
The spacing between the pedals (q-factor) with the supplied cranks is rather wide, these helped to reduce that, while having decent clearance for the motor. I got these 125mm ones.

Other parts:

Power connectors: https://smile.amazon.co.uk/gp/product/B073QMJMRV

P-clips: https://smile.amazon.co.uk/gp/product/B00X2Y32O2/

Hook-and-loop straps: https://smile.amazon.co.uk/gp/product/B01N1VZLKP/

Heat shrink in a couple of sizes: https://smile.amazon.co.uk/gp/product/B01DXBFSS6/

Zip ties: Lots of zip ties...

Assorted bolts, nuts, washers from the parts bin.

Tools:

As shown in the pic!

For the electrical:

Soldering iron
Wirecutters
Heat gun/lighter

To fit the motor:

Crank puller
Bottom bracket removal tool
15mm pedal spanner

To build the battery mount:

Hacksaw
Drill
Step drill bits
Metal drills bits (4-8mm)
Files (various shapes/sizes)

Step 1: Installation: Motor

The installation manual that Woosh bikes have put together is a great start, it really helped me.

It will take you through mounting the motor itself, and plugging the right cables into the right places.

It's not my intention to re-write what they (and many others) have done here.

Step 2: Installation: Power Cable Extension

You will need to extend the power cable, as the battery is much further from the cranks than in a conventional bike frame.

I didn’t like the connectors that came attached anyway, so I cut them off and replaced them with XT60s, which aren’t waterproof per se, but are pretty solidly water resistant.
I spliced one onto the motor, one onto the battery, and one onto either end of an intermediate cable to run between the two.

I decided that this way I could disconnect and remove either the motor or the battery without pulling out all the wiring.

I used a lineman’s splice, which seemed strong and solid, despite my bad soldering technique (don't look too closely at the photos!)
The connectors I used have pretty fat wires - much fatter than those on the motor or battery - which made splicing it much more tricky - but it seemed to work out OK, just with a lot of solder used!

Make sure to take your time, and get the heat shrink in the right places before you solder!

A note on power cables:
I recycled a power cable from an old microwave for the extension - I know that the 500W/48V I am putting through it amounts to double the current that the 1200W/230V it was specced for, but it features much more beefy conductors than either the motor or battery manufacturers have used, so I figure I'm probably OK.

Please do not take this as a recommendation and ensure that you have correctly-rated power cables!

Step 3: Installation: Battery Mount

This took by far the longest of any of the steps!

The battery is designed to mount on a bottle cage on the down tube - this trike has neither of those, so I fabricated one from a piece of aluminium section rescued from an old roller blind.

I drilled 20mm holes in it to slide over the two struts that run from the seat to the rear axle.
Take care on the spacing for these, the struts are closer to one another at the ends than they are at the seat.
I put masking tape over the struts to avoid scratching them when I was test-fitting.

I placed a pair of 15mm rubber-covered p-clips inside the aluminium section, and these are bolted through it.
It's very fiddly to slide the section and the p-clips over the struts together, but it gives a neater appearance then any other approach I could think of.

I also drilled holes for the bottle cage mounts (to fix the battery mount), and stuck some 3mm rubber matting on the aluminium with double-sided tape to help provide a little damping.

I could then bolt the battery mounting plate into place.

I was concerned about the 3.5kg battery bouncing around, so I have a pair of long (600mm) velcro straps that pull the battery tight against the back of the seat and stop it wobbling about (there’s a bit of foam gaffer taped into the gap) - it’s been rock solid in this arrangement.

There are a few different battery designs, perhaps one of the bottle-shaped ones, or rear rack types would have been easier in retrospect!

Step 4: Installation: Torque Arm

This is really important, and not something I thought about beforehand!

When the motor pulls the chain to turn the wheel, it will want to rotate itself in the opposite direction.
If you’ve placed it under the boom, which is natural, it will rotate forwards and upwards, and it’s got a lot more torque than I suspected.
My first attempt to restrain it - a steel L-bracket - got mangled like a Kraft cheese slice the moment we switched on turbo mode!

I have subsequently used a square-U-bolt to hold the ‘fixing block’ referred to in the Woosh installation manual to the underside of the boom. The U-bolt I bought didn’t have a corresponding plate, so I had to make something, but you can get them with the right plate supplied.

The dimensions of this bolt were spot-on - just need the right plate!

Step 5: Installation: Speedo Sensor

KMX will sell you a little doohickey to mount your sensor to the front wheel, but a) it’s £25, and b) I thought it looked a little exposed, so I used a long bolt, a couple of washers and a few nyloc nuts to mount it on the back wheel, off the seat struts - using the pre-drilled holes.

You need the speedo sensor extension cable for this, the 1m length I bought was just about perfect!

If you have the seat particularly reclined, then this may not be possible.

Step 6: Intallation: Throttle

I bought the kit with the throttle, but intended it ‘just in case’, as I wanted the pilot to actually have to work to get around!

However, I have subsequently fitted it, as he cannot keep a smooth pedal stroke up the hills, and cannot get started on them. I’ve sold it as a ‘hill-start assist’, and he’s using it as that only for now, but it could also come in handy for the return journey with tired legs.

I simply cut about 10mm off the end of the grip and fixed it there with the grub screw.

It is of course, open to abuse, and I believe it technically means it’s an electric motorcycle now rather than a pedal assisted bike, with various legal ramifications, so you have been warned!

It is possible in the controller to limit the top speed the motor assist will cut out at, and the throttle seems to respect that too - I have it set to 15km/h.
The manual has suffered a little in its journey from Chinese to English, but it mainly makes sense - make sure to set the wheel size (I used 17 inches to include the tyre), and of course fit the speedo sensor to make sure that this is effective.

Note: Brake cut-outs:
I had some brake levers supplied with the motor that contain cut-out switches, but as the brake levers already on the trike are specialist, I have not fitted them. The switches are designed to stop you running the motor with the brakes on, it would be best to fit them, but I have not - yet. Perhaps a future upgrade.

(Please ignore the horrible bodge to aid weak hands with the shifter in the pic - need to get a better solution in place!)

Step 7: Installation: Controller/display

I put the controller/display on the bars as pictured.
This is fine for my pilot, but when other, erm, larger testers try it, the turning circle is severely limited by the display hitting my (erm, their) hip.

There are smaller displays available, or maybe you could figure out how to mount it elsewhere!