DIY e-bike conversion - how hard can it be?

You can buy a complete e-bike and there's plenty of choice, from rather ugly 'sit up and beg' models to sleek carbon fibre models with concealed batteries.  But I didn't want straight bars and I wanted a range of at least 70 miles. I'm also very fussy about having a bike that fits me very well, being over 6ft tall, which after a lot of effort I've achieved with two of my bikes. So I started looking for a suitable conversion kit for my winter bike, which I could use all year.

Winter bike before conversion

Types of conversion kit

Conversion kit motors come as crank drive or hub drive, with hub drive being either front hub or rear hub. All UK road legal kits are limited to 250W, they only provide assistance when pedaling and stop giving assistance over 15.5mph. However, many kits come with an optional throttle and brake sensors for off-road use and can have their maximum speed increased.

Crank drive

Crank drives fit to the bottom bracket and drive the pedal crank. They give a low and central centre of gravity, which improves the ride quality. Because you pedal at a fairly constant rate, the motor is geared to run at its most efficient rpm. There is some resistance when pedalling, which can limit the range for a given size of battery. Fitting it means removing the bottom bracket, cranks, chainrings and front derailleur and replacing them with different components from the kit. This has a single large chainring, which limits the range of low gears. The motor is slung between the cranks, where it can be vulnerable to damage if used off-road and to spray from the front wheel.

Hub drive

With hub motors there is little or no resistance when not using electric power. They give a higher torque at low speeds and don't place extra strain on your chain. They are cheaper than crank motors. They let you retain your full set of gears.  Fixing a flat tyre is slower than with crank drives.

Front drive hubs are easy to fit, but can cause the tyre to slip when pulling up steep hills in the wet.

Rear drive hubs are built into a stronger wheel to take the extra torque. They are less prone to slip because the weight is over the rear axle. Fitting the wheel should be straightforward, but may require some adjustment of disk brakes and rear derailleur.

Weighing all this up I opted to go for a rear hub kit.

Rear hub kits

You could buy all the components separately and possibly a bit cheaper, but it's easier to buy a kit to ensure compatibility.  After all, there are a few parts to link up:

• Battery
• Motor wheel
• Hub motor controller
• Pedal Assist Sensor (PAS)
• Display
• Switch
• Throttle (optional)
• Brake sensors (optional) 
• Headlight (if supplied)
• Motor to controller connection cable
• Controller cables to battery, sensor, display, switch, headlight, throttle and brakes

Finding the right kit for your bike

Let's go through each component saying what to consider before buying. I'll also cover methods of fitting and how I fitted them to my bike. To fit a kit you need to be reasonably competent at maintaining a bike (or use a local bike shop) and may need to use these tools:

• Cassette lockring tool
• Chain whip
• Pedal crank removal tool
• Bottom bracket tool (if changing bottom bracket, or fitting a right side sensor)
• Allen keys
• Torx headed wrench (to move the disk rotor)
• Spanners and screwdrivers
• Electric drill with right-angle attachment (if fitting Riv-nuts to down tube)

Battery

• Check the capacity. This is usually stated as Amp Hours or Watt Hours.  A 17Ah (612Wh) 36V battery should be good for a day's ride if used mainly on hills.
• Check the overall size of the battery and the positioning of the fixing points.  Frame fitting batteries are designed to slide into a battery holder, which is fixed onto the down tube in place of the water bottle.  However, it is unlikely that the bottle cage bosses will be in the correct position. Large batteries are heavy and need 3 fixing bolts. so you will probably need to fit riv-nuts.  The alternative is to buy a rear rack battery pack.

My bike: the bottle cage bosses were too near the bottom bracket and the battery holder had a 3rd bolt hole at the bottom end, which was very close to the BB. I didn't really want to start drilling extra holes in the bike frame and I had a rear rack and a large rack pack, so I put the battery in there and bolted the battery holder through to a metal bracket under one of the cross struts of the rack.

Battery holder fixed through base of rack pack

Battery slides onto battery holder

The battery is locked to its holder inside the bag, where it is safe, not subject to extremes of temperature and hidden from view. It's an expanding bag so there's plenty of room around the battery for other things and it came with a USB charging point for the mobile phone.

Motor wheel

• Check the motor power: UK road legal motors are rated at 250W, but you can get motors up to 750W. There are various makes and models so check the suppliers' websites and the discussion forums for recommendations (see list at the end of this article).
• Check the wheel size.  Kits are available with motors built into 20", 26", 700C and 27.5" wheels.
• Check the dropout width. If your dropouts are 135mm apart they could probably spring apart to take up to a 140mm wheel (shown as Over Locknut Dimension (OLD) on motor drawings).
• Check brake and gear fittings. If you have disk brakes pick a motor with disk rotor fittings (usually the standard 6 bolt ISO mounting for 160mm rotors).  Pick one for either a screw-on freewheel fitting or a cassette fitting depending on what you have.

My bike: I picked a 700C wheel with a 250W Bafang motor, with disk and cassette fittings.  The OLD of 138mm fitted easily into my 135mm dropouts. I used a T25 Torx tool to remove the 6 bolts for the disk rotor and fitted it to the new wheel using the shorter, supplied allen bolts. These bolts had a thread locker coating on the threads and no grease should be used. The disk brake needed a minor adjustment to centre over the disk brake rotor by loosening the two bolts arrowed, aligning the brake body and tightening the bolts. The 10 speed cassette moved over to the new wheel, but I had to get hold of a different cassette lockring tool that fitted over the 12mm solid axle. It required an adjustable spanner set at 1".

Lugged washer inside dropout Large washer and 19mm nut outside

My bike has vertical dropouts and the lug on the internal washer fits in the open (down) side of the dropout. The motor cable should also exit downwards to prevent water ingress.

Front derailleur adjusted outwards

The front derailleur had to be adjusted for the wider BB and I needed to cut the end off the spring, which was catching on the inner plate.

Hub motor controller

The controller is the communications hub of the system and is built into the battery holder, or in a separate unit with its own box, which can be mounted on the seat tube or top tube.

My bike: as my battery is in the rack pack I placed the controller next to it, along with the excess wiring. The controller is rated as 12A max. at 36V, well above the 250W max. for the motor.

Pedal Assist Sensor (PAS)

This is fixed to the bottom bracket and has a ring containing magnets that fits over one of the pedal cranks to detect when you are pedalling. The simplest type is for traditional 16mm square taper solid axles, fitted to the left side with the rotor a push fit on the axle and the stator pushed into the 20 slots of the BB cup. Versions are also available for the right side, including a large diameter sensor for external threaded bearings.

Left side sensors are easier to fit, but require a gap of at least 10mm between the inside of the crank and the outside of the BB, which means using a 127mm BB in a standard 68mm bottom bracket shell.

PAS sensor fitted to left side leaving a 3mm gap

My bike: I used my crank removal tool to remove the pedal cranks and replaced my 113mm BB with a 128mm version using my BB removal tool, which uses an adjustable spanner with jaws set at 35mm. It was hard to push the left side sensor into the BB cup slots by hand, so I used a short tube (the core from PTFE tape) and tightened it on using the pedal retention bolt. Note that you need to use an aluminium BB cup as it won't fit properly into the slots in a plastic cup.

Display and Switch

Display, Switch and Headlight

P850C display

These fit on the handlebars to display speed, battery status etc. and to control assistance levels and other functions. There are often a variety of different display units for a kit.

• Check the holders will fit your handlebar diameters.
• Check there is a good range of assistance levels (including level 0).
• Check that you have access to advanced settings.

My bike: I picked the P850C colour display capable of levels 0-9. The holder was suitable for my 31.8mm bars and came with adapters for smaller bars. The switch was designed to fit 22.2mm bars and by using a longer bolt it fitted where my bars tapered down to 24mm.

Throttle (optional)

This is not strictly road legal unless already fitted to an older e-bike before the law changed.

My bike: the kit came with a spring loaded throttle that was designed to slip over 22.2mm straight bars.  I could have fitted it to the end of a drop bar, but decided I didn't need it.

Brake sensors (optional)

These are required for safety if you fit the throttle.  The kit came with brakes for straight bars, which can be swapped for power cut-off sensors for your current hydraulic or drop brakes or combined brake/gear shifters. I didn't fit them.

Headlight (if supplied)

My bike: An excellent 12v LED headlight came with the kit and is controlled by the display switch.  The lamp bracket is designed for a central fork crown bolt. My carbon forks didn't have a hole, so I fitted a long M5 allen bolt with a metal sleeve and washers into the handlebar clamp.

Where to buy from

In the UK there are a few kit suppliers, including:

• Woosh online shop and with a shop in Southend (XF08 CST rear 26" or 27.5" or 700C wheel with 17AH battery: £599)
• Cyclotricity online shop and a warehouse in Glenrothes (SKU 250W500W Rear Drive eKit with 17AH rack battery and 10sp freewheel: £780)
• Electric Bike Conversions - online shop (Rear drive 250W kit with 19AH battery £899 but out of stock)
• e-Bikes Direct online shop (Front drive 250W kit with 17AH rack battery and PAS £650, but out of stock - this is actually Cyclotricity kit)
• Other suppliers are available (Prices taken at 01/04/2020.)

My bike: I nearly bought the Woosh kit, but I didn't want a display computer with only 5 levels of assistance and no level 0, which means the motor is always in use. I opted for a 700C rear wheel kit and battery for £578 from Greenergy in China via Aliexpress:

• Bafang 36V 250W Conversion Kit 850C 700C DC Type with P850C display £298
• Battery Kit 36V 17.4AH with UK charger £265
• Tax free shipping DDP Service £15

You usually get a better exchange rate on Aliexpress if you use their base currency US$ and let your credit card convert it to £. Shipping was delayed because of Chinese New Year, then the virus pandemic, but it all arrived within 2 months.

The completed conversion

So how hard was it?

I had done quite a bit of research before choosing to order from a supplier in China and this led to a long wait for the kit to arrive. It meant I could order what I wanted, whereas UK suppliers were selling older versions they had ordered in bulk.

• The hub motor wheel fitted easily and only needed a minor adjustment of the disk brake - perhaps I was lucky. 
• Fitting the PAS sensor was straightforward but it needed a wider bottom bracket. I also had to cut the end off the front derailleur spring to allow it to be adjusted far enough out.
• Fitting the battery and separate controller to the frame would have been quite tricky. The bottle cage bosses didn't suit the battery holder and riv-nuts would be needed. The controller box (not used) could have fitted on the down tube.  This would leave nowhere to put a water bottle. I was happy to put battery and controller in a large rack pack, but a rear rack battery pack may have been a better choice.

References

The suppliers' web sites and YouTube are useful sources of information. Kits come with installation instructions and often links to YouTube videos.
Other useful sources of information:

Pedelecs forum

Endless Sphere forum

Cycling UK forum

Pedelec Germany forum