Surly Big Fat Dummy with L-R Big Block Mid Drive

LightningRods · May 28, 2018

Early this year I saw an article for the Surly "Big Fat Dummy" or "BFD" fat cargo bike. I immediately recognized that this was an awesome bike in desperate need of a powerful motor. I decided that as soon as I could I would stuff a Big Block mid bike drive into it.

As luck would have it, A customer in Northern California wanted exactly what I was planning on building. So he shipped his complete brand new BFD with Rohloff Speedhub up to my shop in Oregon.

The Big Fat Dummy differs from the regular Big Dummy in that the dropouts and bottom bracket are wider to accept up to 4.5" wide tires. Also the frame is a complex triangulated lattice instead of the simple single backbone frame that the Big Dummy has. This feature, meant to provide a lot of strength without much added weight, is going to make fitting a square drive a real challenge.

LightningRods · May 28, 2018

Since this was an unusually complicated installation I decided to work out some of the mounting problems in 3D.

I've been using semi-rigid cast urethane to mount drives for some time. In order to locate the drive securely in this spider web of tubes I would need 9 urethane split blocks grabbing the frame in various spots.

LightningRods · May 28, 2018

I have the laser cut aluminum panels back from the laser cutter. I have them propped up in the frame to check the fit and clearance of everything. As you can see cutouts were needed in a couple of places to allow the frame tubes to pass through the mounts.

The primary reduction for the motor is by cog belt. The belt tension is provided by a simple pivot bracket.

LightningRods · May 28, 2018

I have most of the urethane split blocks cast and have the drive securely bolted in place with them. The drive and frame become one unit once everything is tightened down. As a unit they are completely rigid and free from flex.

While I was floating the drive in space I had to get the pedal and final drive chainlines correct, get the drive centered in the frame, and get everything level and oriented straight. It all looks to be dead nuts on.

LightningRods · May 30, 2018

More progress on the Surly Big Fat Dummy drive. I've completed all of the hard urethane rubber frame mounts and the motor is installed in it's brackets. Next will be the two chrome moly jackshafts with their bearings. The primary belt and secondary 219 chain reduction will follow. Things should move along pretty quickly now.

LightningRods · Jun 2, 2018

The biggest challenge in designing this BFD drive was all of the diagonal tubes running everywhere. They weren't where they needed to be and were where they shouldn't be. One of the worst conflicts was the lower return run of pedal chain to the drive. I used a Surly spring loaded tensioner to lift the chain up and over the lower frame tube. As a side benefit we now have really great chain wrap on both the chainwheel and driven track cog.
I put the belt and chain guards in place to see how they'll look. I plan to completely enclose the sides of the guards with tough cast polyurethane. This is the first drive where I'm providing this long requested feature. I plan to retrofit enclosures to my existing drives where possible.

fourbanger · Jun 22, 2018

Oh my God!

LightningRods · Aug 24, 2018

The Big Fat Dummy is completed. To say that it's an awesome bike is an understatement. I would love to have one of these myself. Sadly this bike has already gone to it's new owner in California.

I'd be happy to answer any questions about this build.

Mike

EbikeAus · Aug 24, 2018

Beautiful work Mike. Looks like it was like this from the factory!

Can you tell us about the battery and controller that will be used.
I bet that put a big smile on the owner's face

LightningRods · Aug 24, 2018

Thanks EbikeAus! This drive was months of work to make it fit like it does. The owner of the bike got a screaming deal on the drive because of loaning me the bike for so long. I'm going to send photos of the drive to Surly and see if I can tempt them.

The controller is a PowerVelocity 18 FET sine wave with Bluetooth programming. Vad at PowerVelocity is building a 30Ah triangle pack that will fit in the front frame triangle. 72V, 80A continuous. 5,760 nominal watts running through a 14 speed Rohloff. This bike will go anywhere you can get traction. I'd love to see one with the 4.5" knobby meats on it!

mechanistic · Aug 26, 2018

fourbanger said:
Oh my God!

That's exactly what I said!

Here's how it looked before:

LightningRods · Aug 26, 2018

Hey Erik,
I’m glad that you’ve found this thread. I hope you’ll post some photos of the bike completed with the PowerVelocity triangle pack installed. I’m eager to hear how the bike is working for you.

mechanistic · Aug 27, 2018

Hi Mike,

That pack from Vad @ PV is crazy!

LightningRods · Aug 27, 2018

Vad knows how to bring the power. :twisted:

I typically use EM3EV because their prices and quality are so good. Vad makes 18650 packs that are also top quality and go to much higher amp discharge rates. We wanted an 80 amp pack for your bike. Vad is spec'ing a 200 amp 40Ah pack for a Motoped build that I'm just starting. I prefer not to use LiPo so I'm glad that I found PowerVelocity.

Your bike looks great! A bit of wiring tidying and it looks like you'll be there.

mechanistic · Aug 27, 2018

I completed the Domino throttle install yesterday.

I installed a 470Ω axial:

Inline with the blue wire, like so:

^faces in things!^

Prior to adding the resistor, throttle max output was 4.99V.

Now, with the added resistor, throttle max output is reduced to 4.55V:

FWIW, throttle min output is ~0.34V:

Just like Vad said - nice smooth throttle ramp to controller WOT @ 3.3V.

Current CA settings:

I made a video of baby's first no-load pull: [youtube]LBjCy-N7h50[/youtube]

<EDIT: add microswitch info>

I thought it would be awesome to connect the Domino micro-switch to the CA, in addition to the Magura MT5e switches (CA Active Low or NO).

I terminated the two green wires JST-SM and connected to the ebrake harness.......aaaand the Domino switch is NC. bummer.

I wanted to ID the switch to source the NO version and replace:

But it's potted in there pretty good, so I punted for now.

If anybody knows the MFR/PN of that micro-switch, please LMK.

LightningRods · Aug 27, 2018

I didn't even think about the nearly 5v full throttle signal on the Domino. That much voltage will cause Vad's controllers to go into fault mode and shut down. I'm glad that the inline resistor did the trick. You should have the smoothest, most progressive throttle possible now.

PRW · Aug 27, 2018

such a neat integrated build - well done, Mike, and congratulations, Erik!

How much power is going through the Rohloff?

LightningRods · Aug 27, 2018

5,760 watts with the battery at it's nominal 72V. 6,720 watts HOC.

mechanistic · Aug 27, 2018

yeah, I haven't ridden it yet - but the throttle feels really silky smooth.

Did you watch the video with the sound on? Is that noise normal? It's really loud.

mechanistic · Aug 27, 2018

Here's another example:

[youtube]eCeflGUi5qs[/youtube]

PRW · Aug 27, 2018

LightningRods said:
5,760 watts with the battery at it's nominal 72V. 6,720 watts HOC.

Wow!

LightningRods · Aug 27, 2018

mechanistic said:
yeah, I haven't ridden it yet - but the throttle feels really silky smooth.

Did you watch the video with the sound on? Is that noise normal? It's really loud.

No that's not normal. It sounds like one of the chains is rubbing on a metal part. Look around while you're running the drive and see if you can find the source. Something may have unthreaded or moved. A lot of no load running often does that, especially when the freewheels and sprockets haven't been bedded in by running under load.

mechanistic · Aug 27, 2018

I'm certain there is no chain contact. Slap from the lower pedal run? sure.

Looking around, I do see a potential candidate - the motor mounts are deteriorating quickly:

I hear you about components wearing in, etc. - I'm looking forward to that.

LightningRods · Aug 27, 2018

I've never had a customer break one of these Rohloffs, even with the plastic shear pins in them. I'm pretty sure that Erik stepped it up to the aluminum shear pins. I know that people have run 10kW Astro 3200s through the Rohloff and again it survived. An amazing piece of German engineering. Expensive and worth it.

PRW said:
LightningRods said:

5,760 watts with the battery at it's nominal 72V. 6,720 watts HOC.

Click to expand...

Wow!

LightningRods · Aug 27, 2018

mechanistic said:
Looking around, I do see a potential candidate - the motor mounts are deteriorating quickly:

That broken rubber mount is not making that sound.

It's hard to tell how many rubber isolators you need since you took multiple angles of the same one. I'll send you a full set so that you have extras if you need them.

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