Big Fat Dummy: Make It An Ebike

What is necessary to transform a Surly Big Fat Dummy into an electric bicycle? I thought my BFD series was finished until I realized I left this part out.

The Surly BFD Project Menu
Prologue
Episode 1: 138L (each) Panniers… Seriously?!
Episode 2: Big Fat Dumb Wideloaders
Episode 3: Kickstand Kaos
Episode 4: Add a Flight Deck. And a Hangar.
Episode 5: Leftovers
Episode 6: Electrification (You Are Here)

Oops?

On the surface, it seems I left a hole in my description of my Surly Big Fat Dummy build. I omitted this episode and thought I was done. In my defense, there are zillions of BBSHD installation tutorials out there, and I have described a BBSHD install myself – on a cargo bike no less – right here in this blog.

However, I haven’t done a writeup geared to THIS bike. Since this blog is dedicated to answering questions that I see asked a lot (and I have seen this one more than a few times), I’ll do something a little more focused on the BFD.

Bear in mind the bike has been in near-daily use for months already. I had to dig thru my archives for pics rather than taking them as I went along. So illustrationwise, there’s not much to see. On the flip side of that, there doesn’t need to be as the Big Fat Dummy is an easy (easy!) build.

Since I have covered this ground elsewhere, I’ll be leaving generic details out, and providing links to related content more so than I will be doing in depth step-by-step instructions.

Get On With It Already!

Yeah, yeah sure … here we go.

Step 1: Buy What You Need

This can be very simple or very involved. Especially if you are a first-timer and don’t know what you need and maybe not even what you want. In my opinion the best motor for the job is a Bafang BBSHD. I typically buy my motors from Luna Cycle, and here is their page for their kit. They may or may not be selling a battery along with that kit. I am using a different source for the pack as you will see further below.

If you buy the kit, you don’t have to worry about buying individual bits, with the exception of needing a speed sensor cable extension, and a proper chainring.

But, lets go over the individual bits. Myself personally, I just buy the bare motor from Luna, and add the parts I need to complete the installation. This lets me use exactly what I want, which is not quite possible if buying the packaged kit. Here are all my parts:

  • Bare Motor:
    Purchased from Luna Cycle here. The Surly Big Fat Dummy requires the 100mm sized motor (It will be a perfect fit). if its available you can buy the optional spacer and mounting kit on that page. At the moment, Luna is not selling the 100mm mounting kit – the only difference is some M6 bolts and spacers… you can source that yourself to the correct size if needed. Or just buy the needed parts separately. Go ahead and accept the 46T chainring (aka ‘The Disk of Death’) as its a free throwaway item you would (should) never use.
    Secondary Source: California Ebike is a reliable alternative and one of my go-to sellers for parts, but their motors cost an additional $100 or so. Here is their BBSHD motor page. Note that Cali Ebike offers users who may need it service after sale. Luna is a better choice for the confident do-it-yourselfer who can diagnose and fix most issues themselves, and save a hundred bucks by taking on that risk.
  • Motor Mounting Parts:
    • Triangle Mounting Plate:
      This is what puts the bite on your bottom bracket to firmly affix the motor. You can buy these plates at Luna Cycle, BafangUSA Direct or Amazon.
    • M6 bolts, washers, spacers:
      Needed to affix the Triangle Mounting Plate, these are commonly available. If you work on bikes you probably already have them in the garage. If you buy a mounting kit they may have them but in all cases I recommend you do not use them and instead go out and buy stainless hardware.
    • Lockrings:
      I use two inner rings stacked atop one another. If you like, you can buy the more conventional inner and the aesthetically-pleasing outer. More on the reasoning behind the choices below. Buy the rings at Luna Cycle, Bafang USA Direct or Amazon.
  • Speed Sensor & Cable Extension:
    You will need the sensor (which integrates a length of cable to plug into the motor), the sensor magnet and an extension thanks to the Surly Big Fat Dummy’s long tail. I have seen builders route the sensor to the front fork but by necessity this puts the sensor inside of the tire rim’s width, which makes for issues taking that tire off. Put it on the back where it belongs and forget about it. I described the sensor in a fair bit of detail here. Don’t mess with multiple magnets unless you feel a need to experiment, but I do provide a link to what I think is a lighter weight, superior magnet that you may want to substitute for the Bafang wheel weight that comes with their sensor. The speed sensors themselves are available in a wide variety of places, cheapest at Luna but also at California Ebike and many other sources. You can get the speed sensor extension anywhere you can find the speed sensor. California Ebike or Bafang USA Direct or many other sources, including Amazon with Prime Delivery. Notice all of the options I linked are different lengths. Measure the gap you have when you are routing your cabling and decide which one you want, accordingly.
  • A Proper Chainring:
    I am going to skip most of the detail here and refer you to this blog post on BBSHD chainrings. It was written with the Mongoose Envoy build in mind but the Surly Big Fat Dummy has essentially an identical set of problems and solutions. I will say this: For a combination of mostly street with some mild trail use I settled on the 46T Lekkie Bling Ring, which biases chain line towards the bottom half of the 11 speed cluster. This is the ring that has the most miles on my bike and its the best all-rounder. At the moment I am set up mostly for trails, though, and as such I am running the Luna Eclipse 42T which biases chainline heavily toward the inner half of the rear cluster, giving me the best possible access to the inner cogs.
UPDATE: But I did that anyway.  I put on a very small Lekkie 32T ring on the front in anticipation of some very serious trail rides in the Lower Sierras.  You can go smaller than a 32T but this ring - which is a little smaller than the secondary housing of the motor - is regarded as a very good sweet spot.  A 28T is so small it can contact the motor housing, which is bad.  The 32T in conjunction with the biggest rear gears on my rear cluster (46T) can climb anything possible to climb at all on two wheels, while leaving the motor in a good place, power-wise.

Note with a BBSHD, the stock My Other Brother Darryl rims and the stock Edna tires, you may not be fully able to use all cogs simply because chainline will not be acceptable for a 1x drivetrain pumping out 1000 to 1750w. You can do it, but the chainring teeth and maybe the chain will not last very long if you run at either extreme – say… the smallest cog and the Luna chainring. It all depends on your final component choices, so just be aware of the issue and check for it to make sure you don’t have any issues that you need to compensate for when riding.

Two of the pics below show a 130 BCD adapter which really biases chainline to the lower cogs, and is best for the street. Both of these use 48T, 130 BCD chainrings. Even though most of my mileage on this bike is with the 46T Lekkie ring, it doesn’t appear as if I ever took a picture of the bike with the thing installed.

  • Crankarms:
    In two of the pictures above you can see I used Lekkie Buzz Bars, and with their forged construction and left offset to correct the misalignment under your saddle that will happen with standard crankarms. Luna Cycle sells a less-expensive clone worth looking at if you can’t handle the price of the Lekkies. As a last resort you can also use the standard Bafang crankarms that are cheap and cheaply made, but good enough for many riders. Make sure you buy BBSHD-specific arms or they will not have the left offset.
  • Display:
    I have used many displays and hands down, at the present time the Bafang/Luna 860C is the best out there. It is fully visible in blinding sunlight and can be set to display both real time amp output as well as real time wattage. The Luna version reads battery voltage level accurately up to 60v, meaning it works with 52v batteries. Bafang versions of the 860C may not. There are many other options for a display including a low-visibility-but-clean/low profile EggRider v2. For my money the 860C is worth waiting for if its temporarily out of stock, and its now my go-to for bikes I build.
  • Throttle:
    I like the basic el cheapo Bafang universal thumb throttle. Its an easy fit and unobtrusive. If you follow my lead on BBSHD settings for it, its annoyingly short throw will still be well controllable and allow for fine adjustments while riding. Buy it at Luna, California Ebike or Bafang USA Direct.
  • Main Bus Cable:
    You have options here. the main bus cable is available in short and long lengths, and there are also extensions available. However nothing fancy is required on the Surly Big Fat Dummy. You can buy this standard one from Luna or many, many other sources. If you opt to use Magura MT5e brakes, California Ebike has a specially modified harness to plug in the red Higo/Julet cutoff connectors the MT5e uses. I am using this bus cable and MT5e’s, myself. NOTE: If you opt to keep the SRAM hydraulic brakes you will not have brake-actuated motor cutoffs. This is no big deal. They’re nice but the stock brakes can overpower the motor in a pinch. If you like, you can invest in some hydraulic cutoff conversions that involve gluing on a magnet to your levers and strapping on some wires. the alternative is a brake upgrade (not a bad thing, but not cheap, either).
  • Installation tool(s):
    Using one of the many Bafang-inspired toy wrenches to install a BBSHD is a cruel joke on the inexperienced. You have to use a proper torque wrench and special socket to do the job right, where the motor doesn’t move. I’ll leave the torque wrench choice to you (I use a Wera B2). The socket you need for the inner ring is often out of stock. Buy it here at Luna Cycle or hunt around… its available elsewhere if you look. The tool for the outer ring can either be hand-tightened – if you must – with a stainless steel version of the cheesy Bafang wrench. I bought this one on Amazon so I know it fits. But it is absolutely a sucky solution. Better is to use a 16-notch bottom bracket tool that you can fit onto a torque wrench and do a proper job of applying the manufacturer’s torque spec, written right on that outer ring. Note if you use two inner rings stacked on top of one another like I do you do not need the special outer ring tool.
  • The Battery:
    There are a bunch of different ways you can play this. Among others you can plant one on the bike on the framework just behind the seatpost. At present mine is in the triangle in a bag. My size Medium frame can just barely hold this 21ah, 52v battery from Bicycle Motorworks. ‘Barely’ means after I have added some padding. I also keep the battery in a quick-detach bag inside the triangle. I described the quick-release setup with pics in detail here.

    I’d like to have more room for padding, so I am exploring a shift to that framework as an alternative. We’ll see as its a big change.

Step 2: Remove Stuff

OK so you have all of your parts for the motor… Time to take things off so you are ready to do the installation. Its a simple list: Remove the crankarms, bottom bracket and chainrings.

You’ll also want to pull off one of your handlebar grips in preparation for installing your throttle. Which one depends on how you want to set up the bars. You will also likely want to loosen up and shift around your brake levers and the remaining rear shifter so your throttle is butted up directly against your hand-grip, rather than the brake being there. this is a bridge you should cross when you come to it, disassembly-wise.

Initially I used a Luna Wolf Pack, as shown in this picture. Thats another option for your build.

Thats it. You’re ready to install your motor.

Step 3: Install The Motor

Here again I’m not going to get too deep into the specifics of motor installation. I’ve already covered it myself elsewhere for a similar bike, and God knows there are plenty of video and written tutorials out there on the interwebs. However I will note that the 100mm motor is a perfect fit on the Surly Big Fat Dummy, which requires no spacers of any kind. Just put it in like it belongs there and clamp it down tight.

About that clamping part, I will go into that a bit:

Lockrings

I mentioned above that I like to use two inner (gray) lockrings: I stack them atop one another in jam-nut fashion where each is tightened to 100 ft lbs. Thats quite a lot more than the Bafang specification for using just one inner lockring. I am going off of installation advice provided by Luna Cycle – not in their official installation video linked above. At one time there was a supplemental vid made in their shop that discussed their learning experiences assembling their shop bikes. It went hand in hand with the use of a big 1/2″ torque wrench to apply the necessary force, and that wrench in turn used a specially made Luna tool for the lockrings (that sadly is no longer available, although you can see them on the site still). The use of 100 ft lbs and some additional info on it is in the link to the tool above.

I have stuck to that 100 ft lb specification and it has never let me down. I have also added to it by using a second inner lock ring rather than the ‘beauty’ trim ring that is more typically used. The number of threads needed for another inner ring is about the same. You gain the benefit of a serious jam nut holding down the first ring. Also, something we are not doing here but you can see elsewhere: If you are building an AWD bike, the use of two rings lets you mount the front wheel’s PAS ring in between the two.

I do not use the outer trim / beauty ring at all.

Lastly on the subject of lockrings, here’s a technique to tell at a glance whether the rings are loosening or your motor is shifting (or about to): Make a registration line along the frame and the lockrings. If the line ever breaks apart, something is loosening. You can tell with a simple glance down as you are mounting the bike.

See the registration mark (aka the black Sharpie line)? Also note there are no spacers needed on the locking plate under the M6 bolt on the left. Perfect fit for a 100mm motor.

Next, I’ll make note of how I did the speed sensor installation, both with the factory SRAM brakes and my later MT5e upgrade.

Using the SRAM Brakes / No Helpers

Attaching a speed sensor on a Surly Big Fat Dummy is not as straightforward as it is on a typical bicycle. In addition to the added distance – addressed with an extension cable – there’s no place to put the thing! The frame is different enough that nothing appears to work – on first glance.

Keep looking! The SRAM brakes that come stock with the bike have a weird sort of tail hanging off the caliper, and this is a handy, if unusual, place to mount the speed sensor.

I first wrapped this tail with a length of 3M mastik tape to enlarge its diameter and give the sensor more to grab onto. Then I simply zip tied it on as if it were a chainstay, and aligned the magnet as usual. These pictures show a dusty bike as they were taken just before I uninstalled the sensor and upgraded the brakes to the Magura MT5e’s.

Using Other Brakes – And a Crutch

For the Maguras, there was no such luck as the calipers have no tails or anything else I could glom onto. So I had to add something: I used a simple small handlebar extension, and built up the frame to a proper larger diameter to mount it by wrapping the frame with gorilla tape, which I then faced with silicone tape to provide a grippy surface for the bar mount. Next, I used more zip ties (!) to clamp the new ‘frame tube’ to the upper part of the Big Fat Dummy’s … superstructure. Once this was done, I had a tube close enough to the spokes to re-mount the speed sensor as shown.

What About a Gear Sensor?

Good question. Read this.

Whats With The Heat sinks on the Motor?

I’m glad you asked. Here’s your answer :-).

What Else?

Well, a bunch I suppose if you were looking for a bolt-by-bolt conversion tutorial specific to this one bike. But really, between the other pages already on this site and the links I have given off-world up above, you’ve got everything here that you need to buy – and build – your own.

So have at it!

Author: m@Robertson

I'm responsible for the day-to-day operations at my place of business: Leland-West Insurance Brokers, Inc. We do classic and exotic car insurance all across these United States. I'm also an avid auto enthusiast, a born again cyclist (i.e. an ebiker) and participate in medium and long range CMP and NRA sanctioned rifle competitions.

6 thoughts on “Big Fat Dummy: Make It An Ebike”

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