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

[calfee20]

Early fat bikes used offset frames to clear the big tire and still use 135mm rear hubs. Would that help?

[Rider]

I think that would help chain alignment when using a BBS02 or HD coupled with a 68-73mm BB. Probably easier to get good chain alignment than with a wider BB and normal rear stays. That frame actually looks like it's bend, but I know it isn't. Interesting soluition they used back in the day...

[Rider]

Another piece of the puzzle came in from France. It's a 7050 alloy BBS02 spider with 130BCD. Very nicely made and appears to accept sprockets on both sides with indent edge on the inside and support node on the outside. You can run two front sprockets on this baby, but chain alignment might get a little scetchy for normal frames, so do your calculations if that's your plan. I will just be running the Gates 50T front chainring mounted on the inside. Beautiful spider!

[Rider]

Yeah, I think the Gates 50T is the smallest front belt ring that will clear the gear reduction housing bump. Not to worry though, Gates makes some big rear belt cogs to make up for the extra teeth up front. If you have the money the Rholoff 14spd IGH hubs have a huge offset that works with a BBS02 too. Search for the "Surly ECR BBS02" build on this site.

Here it is:

https://electricbike.com/forum/forum/builds/ebike-building-directions/14961-surly-ecr-expedition-bike-750w-bbs02-mid-drive

[scstusek]

Hi Rider, do you know what the offset on your French spider is? I have a Gates drive that I would like to convert to electric, and it appears that the inside edge of my belt ring is about even with the outer edge of my bottom bracket.I think I read somewhere in your posts that the Bafang motor housing will push that about 20mm to the right. I 'm just looking for ways to get the chainline right. Thanks!

[Rider]

My notes state that a BBS02B spider mounting surface is 22mm from the B.B. edge. The spider I used is not flat, it actual has an inset for the chain/belt ring taking it further outward to the right. A flat spider will get you closer to the B.B. or you can use a 1-2mm spacer on this spider. With a 68mm B.B. the best you are going to get with bolt-on parts is ~52mm; which puts th belt ring in Rohloff chain line territory. If you are using the SA CS-RK3, you align the belt line on the cassette mounting surface. The only way to get a closer belt line is to face the right edge of the B.B. (watching chain stay clearances - both sprocket and reduction gear housing) or a fully custom built/machined spider/beltring combo. It appears the best a custom made setup can achieve is ~46mm, because of the thickness of the beltring (whichout facing the 68mm B.B. edge). You have to do your own individual measurements to assure all is lined up properly. There is really no way around that. Don’t just take stated values and assume everything will line up. Even the Gates belt rings have slight offsets depending which side you mount. Good luck.

[Rider]

The Bafang BBS02-B 750W/48V motor with DPC-14/850C color display arrived and I quickly placed the 130BCD spider from France and Gates 50T CDX belt-ring on it. Everything clears!



It looks like with these parts the Gates belt-ring center line is ~22mm from the large flat surface of the motor facing the BB, but there is a ~2mm indent for the BB to seat into, so I’m getting ~20mm from the 68mm BB edge to the center line on the Gates 50T belt-ring. That gives ~54mm from the frame center line to the Gates belt-ring center line [(68/2) + 20 = 54].

The great news is even the chainring bolts clear and there will be no issue aligning the Gates Carbon Drive CDX belt with the CS-RK3 cassette hub. A Gates belt drive BBS02-B build is easily doable with these store bought parts!

Time to start putting it all together…

[Rider]

Getting down to the nitty-gritty on belt alignment now that the motor, spider and belt-rings are all here. I even bought a digital caliper for better accuracy, so the numbers have changed a little bit.

1) When I measure from the left flat side of the gear reduction housing that goes up against the right BB to the spider mounting surface on the BBS02B I get 24mm, but there is a 2mm indent in the area that sits against the BB. That puts the chainring/spider mounting surface on the BBS02B 22mm from the right edge of the BB.

2) The 130BCD spider from France is flat on the back where it mounts to the BBS02B, but has a 2mm indent into the surface where the belt-ring mounts on the side towards the motor.

3) The Gates 50T belt-ring is 3mm thick at the mounting point and the Gates specifications for this chain-ring state the center line of the CDX is 3.1mm from the mounting surface. So the center line of the CDX ends up virtually the same (0.1mm difference) as the other side of the mounting surface edge.

All that puts the BBS02B chainring/spider mounting surface 56mm from the frame center line [(68/2)+22=56]. The spider indent adds another 2mm, but the Gates 50T CDX center line is 3mm towards the motor, so 3mm gets substracted back, giving a net subtraction of 1mm.

That puts the Gates 50T CDX center line 55mm from the center line of the frame. Boy I hope I have all that right...


Using those digital caliper measurements, I need to position the rear Gates 28T belt cog at 12.5mm inward from the right side of the 135mm frame surface [(135/2)-55=12.5).

Even though the calculations changed slightly using accurate tools, the Gates Carbon Drive CDX belt will align okay. The rear cog ends up 2.5mm inward from its' outer most mountable position.

[Rider]

With Thanksgiving, Christmas and New Years behind me, it's time to update progress on this build. The BBS02B and Gates Carbon Drive got installed without much trouble. A different bracket was made out of aluminum to hold the motor up against the frame down-tube that doesn't dig into the edge of the BB. It uses a torque arm type feature to hold it in place. This new Soma frame was chased, faced and reamed to specifications, so that was my solution to not ruin the face job on the BB. To insure a tight fit inside the BB the part that slides in was wrapped with one layer of aluminum duct tape and each side of the BB got a small bead of tub caulking as well. Here's a few pictures of the install and all mounted up:

[Rider]

She's on the road!




Only thing missing is a battery. Still debating on which one to go with, but leaning heavily towards a Killer Whale on the rear rack with the option to put it in a rear rack bag or in the triangle. Considering an Ibera Mini Commuter rear bag for the 12" JANDD rack (have JANDD Grocery Panniers that fit this rack too).

I've been riding her like a single speed in 2nd gear. Perfect gearing for that and low enough to pedal it easily without power. 3rd gear is easily doable without power too, but takes a constant effort; perfect human power only tailwind gear. I bet I can run this as a single speed starting in 3rd with the motor assist on if desired to eliminate shifting altogether.

Riding this around, I forgot how much I like these tires! And no noise at these speeds either. Everything is whisper quiet except 3rd gear. The S-A 3spd IGH has a light ticking sound in that gear only (normal).

[Rider]

Ordered these two GoPro accessories to adapt for a stem cap display mount for a total of $6. Plan to install the GoPro stem cap mount with the camera tripod mount and use the screw location for a flat "T" piece that will mount to the two threaded inserts on the back of the display. This will get the display out of the way and below the bars in a less obtrusive position.

Also ordered a Selle SMP TRK men's saddle to try out and an Ibera Mini Commuter bag & rack. I'll adapt the bag to the JANDD rack and use the seat post rack on my full suspension MTB when needed.

[spinningmagnets]

Rider, would you mind if I wrote about this ebike for electricbike.com?

[Rider]

Rider, would you mind if I wrote about this ebike for electricbike.com?

Not at all. Let me know if I can be of any help. I'm staying in Anaheim right now caring for an elderly parent, but get out to my ranch house in Canyon Country/Santa Clarita area now and then. I don't get email notifications, but I'm sure you have access to my email that you can use to email me direct. I believe I have that turned on for the site to contact me.

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. Originally I mounted the IGH shifter cable stop on the lower rear stay, but it kept sliding towards the rear because of the tapered tubing. By the end of the ride I had lost first gear. No big deal for a flat beach ride, but on the trail that would have meant walking up steep hills for me. Today I relocated the cable stop bracket to the upper rear stay just above the braze-on derailleur cable stop so it acts as a stop for the clamp-on cable stop that comes with the Strumey-Archer 3spd IGH. Here's how it is routed now.



The other issue that came up on that shakeout ride was the left crank arm came loose. Luckily I found a friendly rider that had a multi-tool with them. For some reason my multi-tool wasn't in my seat bag. I found it in my gear bag in the car upon my return.

The Ibera Mini Commuter bag should be here sometime this week. I'm leaning towards the large Luna 48v 21Ah Panasonic GA block stealthily mounted inside it for killer range. Plus, I can just unclip the bag and battery anytime I want to ride it like a pedal bike without that extra 11lbs.

I very surprised at how nice it rides with the Bafang installed without any power. There doesn't seem to be any noticeable drag either. The gearing is just right for me to pedal. I'm guessing I'll be running this on a very low 750W setting and getting huge range. The bike is agile and flickable as is. That's probably because the heaviest piece is the Bafang motor and that is way down low in the center of the bike.

There's nothing but a big grin on my face so far!

[spinningmagnets]

It may take a while, but there is definitely an interest for info on BBS drives with belt drive...

[Outsidediameter]

Nice build! And Report.I've been looking hard at those Soma frames too!

[Rider]

Thanks. So far I'm very happy with the Soma B-Side V.2 Belt Drive frame. They do require face, chase and ream like most new frames. I see Soma is offering their Juice 29er in with Belt Drive Frame Break now. It can take up to 29" x 2.5" or 27.5" x 2.8".

[Rider]

The Ibera Mini Commuter bag & rack has arrived. The bag happens to have zippers throughout the inside that appear to be used for assembly around the hard components. It has a sturdy hard plastic bottom and as you can see through the inside bottom zipper, the orange mounts are bolted on from the inside. That means those two locations can easily be used for any permanent mounting if desired. In addition, a Luna 8"x6"x4" 21Ah battery block can be installed underneath the inside bottom liner to separate it from anything stored in the bag itself. The two groves in the bottom should allow you to put a couple larger zip-ties around the battery to secure it within the bag by drilling four holes and the zip-ties can sit in that grove.

I'd like to set it up with the quick release system, so I may modify the seat post rack with supports that go down to the rear frame mounts or add something to my JANDD rack to accept the bag mounts.

[Rider]

This is my idea for mounting the Ibera Mini Commuter bag on a modified rack. Cut the two welds holding the square center tube to the quick release rack and slide it out. Save that for a bottle battery mount right to the square tube for a future full suspension build. Add two flat tabs to the front of the rack portion that match the width of the seat post clamp with rack mount pictured, then add two vertical supports from the rear most flat tab on the rack down to the rear most frame rack mount. These two supports end up being virtually vertical, that's how far forward this approach moves the rack.

This eliminates all kinds of extra weight and moves the heavy 21Ah bag battery forward and down as much as possible, plus it looks really good positioned there. The weight ends up under the rider, not hanging out back.

[Rider]

I just put a pair of 3lb and 2.5lb mini hand weights into the bag for a total of 10lbs. and boy does that feel heavy. The 21Ah Panasonic GA block battery is 11lbs. I guess range always cost you in weight, even with the lighter Panasonic GA's. The Killer Whale 17.5Ah is 9lbs. I don't see a weight listed for the 13.5Ah Panasonic GA Shark, but I'd guess about 7lbs.

If only battery technology would improve even faster than it is...

[Rider]

Got started on modifying the Iber Mini Commuter rack. Cut the clamp-on bar out (can use as a bottle battery mount for a full suspension build) and bought a foot of 6061 1-1/2" square tubing. Trimmed the brackets on the bottom of the rack to accept the 1-1/2" square tubing and drilled it for the seatpost rack mount. Slightly grinded the sides of the seatpost rack mount to slide into the square tubing. Still have to get the material for the vertical support mounts that will come off the rear most tabs that point downward on the sides of the rack.