The Bafang 750W BBS02 was an instant success as soon as it was introduced. However, almost immediately…some of the enthusiasts asked if they could ditch the chain and swap to a belt drive. Well…I found someone who did exactly that, and here’s how he did it.

The Spider and Chainring

The difference between a sprocket and a chainring is that…a sprocket is one single piece, and “chainring” has a separate part in the center called a spider. A one-piece sprocket has the potential to be a few grams lighter, but…if you are a heavy user that racks up tons of miles, it can be cheaper and easier to replace just the worn outer ring…the part with the teeth.

Because spiders for the BBS02 and BBSHD exist…the Gates pulley-ring system can be easily added to your Bafang mid drive.

This builder in the pic above chose a BBS02 spider from precialps.com, in France

Raised and lowered chainstays, or…a “frame break”

The first consideration when considering the option of a Gates belt-drive is…if the belt doesn’t break in half like a bicycle chain, then…how do I get it onto the frame? If you don’t already have a bicycle to convert to a mid-drive ebike, you might select a frame that has an “elevated stay”, or a “lowered stay”. The “chain stay” is the horizontal tube section that connects the bottom-bracket (where the pedal-spindle is located) onto the rear axle drop-outs. It’s also possible to use a beam-style of swingarm, like the Stealth Bomber, but that would be very rare.

Here is an example of an “elevated” chainstay (in the pic above). By locating the chainstay above the chain, this allows a chain to be removed or replaced without breaking one of the links. By coincidence, it also allows anyone to easily swap the chain for a one-piece belt.

There are frames that come from the factory with a frame-break already built into them. If you have a common frame that doesn’t have a frame-break, but…you like it very much? there are kits that allow a TIG-welder to cut out a small section of the seat-stay and weld the pieces into the frame of your choice. Since the seat-stay is experiencing forces in compression, I’m surprised that someone hasn’t devised a kit that only needs to be epoxied into the frame tube, after a tube-section (of the proper length) has been cut out.

Since the chainstay is experiencing tension when a rider is on the bike (it wants to pull apart), and the seatstay is in compression (it wants to push together), almost all types of frame-breaks have been located in the seatstay. Another reason for the break to be located in the seat-stay is that there are only a few diameters of round tube that are commonly used on seat stays, but…chain stays have a lot more creativity in the shapes that are available in various models of bike frames…

Gates pulley sizes

The acronym “BCD” means “Bolt Circle Diameter”. If you draw a circle, and then locate the centers of bolt-holes on that line, the diameter of the circle is the BCD. The common BCD’s in bicycling chainrings (and their spiders) are 104mm, 110mm, and 130mm…

When it comes to mid-drives, just about any given rider is better off if they use the option of a smaller chainring (or “pulley-ring”, for belts?). However..for the BBS02 and BBSHD, you must use a driving ring that is large enough in diameter to allow the chain or belt to clear the housing, while still having the proper chainline. [when ordering a spider, double check to make sure you are getting the correct one for either the BBS02, or the BBSHD…they are two different BCDs!]

Although chainline is important for bicycles that use the common chain, the “chainline” is vital to be located properly when using a belt. A bicycle chain can tolerate a certain amount of sideways deflection during it’s travel from the bottom bracket to the rear wheel. This is the reason we can use cheap and available sprocket-clusters on the rear wheel (shifted by a derailleur), that allows up to ten speeds on the rear wheel. For this reason, belts are only used on single-speed bikes, or Internally Geared Hubs (IGH’s).

The chainline is always measured in millimeters between the centerline of the frame, and the centerline of the chain or belt. It is vital for the both the front and rear pulley to be as precisely aligned as is possible.

The combination that fits onto the BBS02 housing [and clears all the bumps and shapes in the housing] is the 50T Gates pulley-ring, using the 130-BCD interface. A 50-tooth chainring for a normal bicycle chain is very big, but the Gates belt system has a much smaller distance between the belt-teeth, so…a 50T Gates pulley is actually not very big at all…

The build that is featured below uses the Gates CDX 50T 130-BCD pulley-ring, but Gates also has a 46T, 55T, 60T, and 70T available

You could use spacers to push the pulley farther away from the centerline of the bike (the “chainline”) to allow for the smallest possible diameter of pulley, but…that would also throw off the “Q-factor”. The “Q” in Q-factor comes from “Quadracepts” which is the main thigh muscle on your legs. Each body has a “best” width to place the pedals at, and if your knees are splayed inwards or outwards, it can be bad for you, especially if you log a lot of miles.

The Featured Build

The BBS02 is hugely popular in the EU. Many countries have a 250W power limit for the street-legal ebike kits, which has made the 250W 36V BBS01 very popular. However…the 48V BBS02 version (when used with with 52V and unrestricted amps) can put out 1,000W, and…it looks exactly like the BBS01

That being said, this builder is actually located in Southern California, USA. His internet name is “Rider”, and he’s done a beautiful job putting these parts together.

Another component that proved to be very helpful in getting this combination of parts to work together is the 3-speed Sturmey Archer CS-RK3 IGH. Since it is a freehub with splines on the chain-side (for mounting a cassette), the pulley can be located with spacers, and moved to the left or right until it is aligned as good as is possible with the the chainline of the front pulley.

This may seem like a lot of effort and cost in order to get a small payoff, but…there are some enthusiasts who want to treat themselves to the components that provide the benefits that they want. Here’s a few of those benefits…

Cleaning a belt and IGH combo can be as easy as using a water hose No grease or oil is needed to lube a belt and IGH Belts last three times longer than a chain, so you might only need to replace the belt every three years The pulleys should last decades, since the belt absorbs the majority of any wear Replacing a chain and sprocket set is dirty, replacing just the belt once a year is clean and easy Chains aren’t very loud, but…as quiet as they may be, belts are the absolute quietest drive Belts with carbon fiber cords don’t stretch like a chain, so…you don’t have to adjust the tension once a month

“…For the custom build I selected a Soma B-Side V.2 Belt Drive frame in large. I’m 6″2″ with a 34″ pants inseam…I went with the 18.5” large because it has a lower top tube so others can ride the bike too. I just make up for my size with a higher seat post and long riser stem with some sweet sweep-back bars for a comfortable position…

…One thing I found by riding the BBS02…was that the added power assist allowed me to double shift and even triple shift at times. Most of my riding was done in only three of the 7-speed cassette gears, and the low gear of those three was only used when climbing hills…With this thought in mind, I decided to go with a 3-speed IGH for my belt drive build. In reading the electricbike-blog (great real world ride-it-until-it-breaks information there), he found that 3-speed IGH rear hubs have a very strong 1:1 second gear…

…I took it to my LBS and had it weighed without any battery, rack or water bottles. It came in right at 40lbs. Took her out for a 20-mile test ride over the weekend. Boy is it smooth and quiet! I’m riding it as a pedal bike right now and loving it…

…I am very surprised at how nice it rides with the Bafang installed without any power on. There doesn’t seem to be any noticeable drag…Cracked the 3,000 mile mark with today’s ride to the beach. Definitely a biking weekend here in sunny SoCal. The bike trail was packed and so was the beach boardwalk through Long Beach. Top speed was 29.8-mph…There’s nothing but a big grin on my face so far!”

If you want to read the original build discussion, it can be found here.

Written by Ron/spinningmagnets, June 2018