Soma B-Side V.2 Belt Drive BBS02 750W/48V Custom Build

[Rider]

A few months ago I embarked on a ground up custom eMTB build after six months of riding a BBS01 350W/36V hardtail eMTB. I made many upgrades to that ride and got my feet wet in the E-bike world. Oh what a feeling they can provide!

When I sold that E-bike I kept all the upgrades to put on my custom build that I knew I'd be doing at some point. Some of the upgrades included many 7000 series light weight parts such as: riser stem, handlebars, 42T Lekkie Bling Ring and pedals with titanium spindles. The cockpit got upgraded seat and Thudbuster as well as ergonic grips to fit my body and style of riding. I also invested in a set of Schwalbe Smart Sam Plus 27.5 X 2.25 E-bike Rated tires that really changed the way that eMTB performed off-road, but they are a little noisy on the pavement and like to grab everything in site with their traction.

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 and my old eMTB had a 21" XL frame that fit me very well, but what I didn't like about it was the 71* head tube angle. It worked fine for everything except steep declines. I only had one fall on that bike and I attribute part of it to the non-slack head tube angle; went over the bars on Rattle Snake in Crystal Cove State Park (has some steep rutted sections in it) and got bit by the snake. So I knew my next eMTB would have at least a 68*-69* head tube angle on it. The Soma B-Side met that requirement and a life long desire to own a Gates Carbon Drive bike. 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.


When the frame arrived I was very impressed by the quality of it and the paint job. Note the belt drive frame break circled. That allows the installation of the belt that can't be split like a chain. The frame does require you to face and chase the BB, face and ream the head tube and ream the seat post tube (like most new frames). I took it to one of my well respected LBS shops for that to insure it was done right.

[Rider]

One thing I found by riding the BBS01 350W/36V prior eMTB 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 7spd cassette gears and the low gear of those three was only used when climbing hills. Basically for my riding type and style a 2spd with granny low would be fine. Many of the eMTB manufacturers have found that E-bikes can handle a much wider percentage increase between gears than the typical 15-16% of pedal bikes. In addition, the power level button acts like gears on a BBS02 motor. The levels are actually speed levels.

With this thought in mind, I decided to go with a 3spd IGH for my belt drive build. In reading the electricbike-blog (great real world ride-it-until-it-breaks information there), he found that 3spd IGH rear hubs have a very strong 1:1 2nd gear which is ideal for handling the extra power of electric assist motors that can cause much faster wear on typical bike components.

There are two main 3spd IGH ratios. The Sturmey-Acher has a 177% range with 33% gear gaps and the Nexus/Alfine have a 186% range and 36% gear gaps. I played with my old eMTB gearing front 42T and 46T chainrings and rear 11-34T cassette and found that the 33% gap seemed to be okay with that smaller motor, so I'm sure it will be fine with this 750W version. I ended up going with the Sturmey-Archer CS-RK3 disc brake 3spd IGH 8-9spd cassette rear hub to get the desired 33% gaps (0.75/1.00/1.33) and full cassette width belt cog alignment abilities.

Soma gives a 46T X 26T single speed gearing in their FAQ section; a 1.77 ratio. This gives a 1.33-1st (1.77*0.75), 1.77-2nd (1.77*1.00) and a 2.45-3rd (1.77*1.33). It puts the 1:1 2nd gear right at their recommendation (great if I ever want to remove the motor and make it pedal only at some point without having to reinvest in the expensive Gates sprocket, cog and belt). What I call my "granny low" gear ended up about one gear higher than my 42T X 34T low on the 350W eMTB, but I think the incrase in power to 750W will easily make up for that. 3rd gear ends up between my prior 4th and 5th gears, which is penty fast for off-road use and still provides about a 15mph speed at 80 cadence. This stuff can get confusing and whats on paper and how it feels in the real world can be very different. I like real world testing, but have to rely on gear calculators like the one at BikeCalc.com in addition to my actual testing. It's worth noting that I don't want or need a fast eMTB and the end result is meant to be kept within the California Type-1 classification.

Another consideration with Gates Crabon Drive and the BBS02 motor is sprocket clearance. The BBS02 requires about 153mm diameter to clear the gear reduction hump on the motor. This was determined by measuring my 42T Lekkie Bling Ring. It turns out that a standard 42T sprocket is about equal to a Gates 50T and the I.D. of the lip that turns out on each side is right at the same diameter. I can't test fit it until I get my motor, but from what I can tell the lip should clear. Now all I have to do is cross my fingers that there is enough belt algnment on the rear cassette.

With all that said, a 50T Gates Carbon Drive with a 28T Gates Cog (1.78) gives close to the same ratio as the 42T X 25T recommendation, so that's what I used.

Here is the Gates 50T next to the 42T Lekkie, plus my hubs and blue Halo Vapour rims. That all got laced up DT Swiss triple-butted Alpine III spokes for good strength (I broke a couple spokes on my last ride and didn't what that to be an issue qith this one). Like most things in life, good quality parts last and generally cost a little bit more for that reason.

[Rider]

[Rider]

With the wheels and frame back from the shop I started assembly. Here is the components list and a build photo layout (most parts).

2016 Soma B-Side Parts List:

SOMA B-SIDE Frame (Large - 18.5")
Cane Creek ZS44/28.6 Tall, EC44/40, 52/30 Conversion Race
RockShox 30 TK Gold Solo Air Fork
Single 40mm Fork Stem Spacer
Q2 40* Riser Stem 110mm
Fouriers Trailhead 58* Sweep Bars
ESI Chunky Sylicone Grips 32mm
SRAM X9 Front Hub
SRAM X9 front 3 Speed Trigger Shifter
Strumey-Archer 3spd IGH 8-9spd Cassette Disc 135mm (CS-RK3)
Halo Vapour Rims i21mm
DT Swiss Stainless Steel Triple-butted Alpine III Spokes Black (14/15/13) Silver Brass Nipples
Shimano XT M8000 Hydraulic Brakes
SRAM Centerline 180/160mm Brake Rotors and Adaptors
Schwalbe Smart Sam Plus Tires with Green Guard (Rated 31mph)
Schwalbe 27.5 x 2.25 Tubes
Circus Monkey 27.5 x 18mm Rim Tape
Cane Creek Thudbuster ST 27.2 x 410mm
Soma 29.8mm Seat Post Clamp
WTB Volt Saddle
Aset Titanium Spindle Alloy Flat Pedals
Gates Carbon Drive 50T Front Sprocket
Gates Carbon Drive 28T Rear Cog
Gates Crabon Drive 118T Belt

[Rider]

And this is where she stands today without the BBS02 mounted, rear gates cog (in the mail along with an Origin-8 single speed cassette spacer kit) and missing the gear shift housing.

[Rider]

Next addition will be a Luna BBS02 750W/48V Bafang and hopefully I can get the belt alignment to work with the wide cassette adjustment in the rear.

Hope you like my build. Enjoy!

[ncmired]

Great detail - thank you!

So, the rear hub is one of these, correct?

And then you pad it into position with a fist full of spacers? Pity about the wasted dishing, but great idea otherwise.

[Rider]

Thanks ncrkd. Yes, the dish gets scraficed for the "option" of making it a 21-30spd chain drive at anytime. Because I custom built the wheel set, I wanted all my options open so it will last a long time and can even be used on other bikes down the line. I did the same with the frame having the belt dirve option, but certainly not a requirement of it. The cassette will not quite get a fist full, but this Gates 28T cog and the two large Origin-8 support spacers, plus any others needed. The idea was to have room for adjustment because the BBS02 will only allow a certain inward spacing of the front sprocket (unknown to me at this time). The good thing for others here is, once the Gates front sprocket is mounted I can post up the minimum center belt line measurement. I should have that Gates rear cog soon. It appears to have a dish to it and I believe it is reveresable too. That should help with alignment if it's close and I have to dish it out furthest from the center line in back.

[Rider]

That brings up another interesting feature of 3spd IGH cassette hub. You can use a standard 7-10spd setup on this one (add $3 spacer for 7spd and the new single front 10spd's fit). but here's the interesting twist. With a real cassette on the back you can pick the main gear ratio for the terrain or specific ride you plan to do, then use the 3spd IGH for the ride. So if you are making a run to the beach you select a ratio for speed, then reduce that main drive chain ratio once at the boardwalk for a slower cruise ride. I think you can see what I'm saying. The end result is a pleasant 2spd ratio of your choice while riding with a lower climbing gear handy without the constant shifting that is such a PITA if you have to stop and go.

Personally I really wanted to try a belt drive, but again the "options" are enormous with this setup. I like options! ;-)

[Rodney64]

Looking forward to seeing this complete.

[Rider]

Took my bike to the LBS and weighed it as is. It came in right at 27.0lbs. Areas of extra weight are mainly the Schwalbe Smart Sam Plus Green Guard tires, Cane Creek Thudbuster seat post and the rear hub (1155g/2.55lbs.); all of which add to the bike itself, but at a slight weigh disadvantage. Each worth it IMHO.

I will weigh it again once finished, with and without the battery.

[Rider]

Parts are coming in. Everything except the 130BCD BBS02 spider adapter and 118T Gates Carbon Belt have arrived.

Gates goodies: 50T 130BCD Gates front sprocket, 28T Gates rear cog, Origin-8 cassette single speed spacer kit and a 42T Lekkie Bling Ring for size comparison.



Mounted the 28T Gates rear cog up in what is probably its' ideal location using both large support spacers to show how nice it looks this way. Only one additional spacer is need in this configuration (thin red one on the inside).



Then I mounted the rear cog as far out on the cassette as it will go with the cog offset to the outside and mounted it up. This is the key measurement that can be used to calculate if belt alignment can be achieved. The 28T Gates cog centerline is 10mm inward from the right side of the 135mm rear frame.



Here's a top view of the belt line just for perspective.



I don't have a BBS02 here yet to measure the front Gates 50T belt centerline from the edge or centerline of the bottom bracket yet, but I'll do some quick calculations from schematics of the BBS02 and the CS-RK3 and post them up.

[Rider]

Here are my quick calculations. Half the 135mm rear frame is 67.5mm. The 28T Gates cog is 10mm in from the right side, so it's 57.5mm from the frame centerline. The BBS02 drawings I have say the BBS02 sprocket mounting surface is 50mm from the centerline, so without even moving it inward via a spider adapter and the 50T Gates sprocket offset there is 7.5mm to work with. Not sure there will be enough room to use both Origin-8 wide black support spacers, but it looks like belt alignment won't be a problem if those numbers are correct.

[Rider]

Decided to mount the 28T back on the cassette with both large Origin-8 support spacers, then put it onto the frame to see the sprocket center line measurement. I get 20mm in from the right 135mm frame width. That puts the centerline at 47.5mm in the ideal position (using both support spacers); only 2.5mm inward from the stated BBS02 sprocket mounting surface. 2.5mm seems doable simply using the 130BCD spider and the Gates 50T sprocket off-set. Those calculations make me smile!

[Rider]

Here's a variety of MPH speeds at diffferent cadences using my gearing on this bike:


1st-----2nd----3rd
MPH--MPH--MPH---Cadence

6.5-----8.8----11.7------60
7.6----10.3----13.6-----70
8.6----11.7----15.6-----80
9.7----13.2----17.5-----90
10.8---14.6----19.5----100

Everything I see there is right in the sweat spot of my riding needs. Can't wait to get a ride on this baby!