Dual Motor AWD Electric Bikes – Case Study: Fat Trail and Hill Climber

Built to conquer the weakness of twin hubs in hills – and eliminate the damage a mid drive does to an ebike’s drivetrain. The bike I call 2Fat did all that.

AWD Ebikes Menu
AWD. OMG. WTF!
Case Study – Flatland Fat Bike Commuter. Hub+Hub
Case Study – Alpine Road & Trail. Hub+Mid Drive (you are here)
Case Study – Low-Power Cargo Beast. Hub+Mid Drive

So, my Gen 1, 1.5 and 3 bike layouts are all twin geared hub designs. What was Gen 2?

2Fat

  • 750w, 35a geared Bafang G060 front hub motor
  • 30a BBSHD mid drive
  • 52v, 12.5ah rear motor battery in triangle
  • 52v, 12.5ah front motor battery on rack top
  • Batteries connected in parallel to form a single ‘virtual’ 25ah power source to both motors
  • KT and Bafang displays
  • 160Nm rear… 80 Nm front (do the math on that one!)

I live part-time in two towns: The first, an extended work visa, is in Fresno California, smack in the middle of California’s San Joaquin Valley and flat as a table. I built The Colonel, The Purple Thing and The Great Pumpkin for commuting in Fresno.

While I most often take pics of it in a wooded area, 2Fat is just as happy on paved, hilly roads. Shown here with front panniers, its 4-bag carry capacity makes it a quasi-cargo bike.

After a fashion, I finally was able to get a big enough bike rack on the back of my SUV to bring the Colonel to my actual home in Pacific Grove, California. My house is at the top of the hill there. Unlike in Fresno, nothing in or around the area is flat. You are either going up a steep hill, or down one, or both.

My intention was to be able to use the Colonel like I did in Fresno, as local errand transport and a light duty cargo-shopper. Unfortunately, I found out the first day about the limitations of hub drives: They suck in hills.

Hubs are Single Speed

Hub drives power an ebike via the axle. They don’t – and they can’t – use the gears of the bike. Forcing a single speed hub motor up a hill makes it just as miserable as a human stuck with no gears. So even though I have very powerful hubs, and they were geared hubs that put down the most torque of any on the market… they still struggled. Even with two of them. I could hear the gears groaning inside the motor casings and I could tell that, while I could get up the hills, my motors were not happy about it. I did not want to lug them into an early grave. I had already gone to a lot of trouble to make the bike bulletproof and had no desire to ignore the problem and inevitably kill it.

These twin 80Nm hub motor wheels were on The Colonel at the time of my first hill country ride (above). Now they are on the Great Pumpkin.

The solution?

I Need To Build a New Bike

Everybody knows mid drives are the solution to the hill problem for an ebike. A hub motor is single speed and at least relatively weak on torque, but a mid drive uses the gears in the drivetrain, plus it has double or more the torque output of a hub, and thats before you factor in the multiplier of the gears. Wonderful right? Except mid drive motors – especially DIY builds – are notorious for putting drivetrains into an early grave. Why? Well because they pour a LOT more power through the chainring, chain, rear cluster and cassette body (i.e. “the drivetrain”) than a bicycle was ever meant to withstand.

  • A normal cyclist can pump out maybe 300 watts for a minute or so, but typically normal sustained – strong – output is about 100 watts.
  • A professional sprinter/mutant is capable of pumping out almost 1000 watts, but only for about a minute or two. Thats not enough power to make a slice of toast.
  • EU-market electric motors must peak at 250w of output to stay legal (pssst… they don’t).
  • A 25-amp BBS02 on a 48v system puts out in excess of 1250 watts peak
  • Your garden variety 30-amp BBSHD running under a 52v battery is peaking – and can sustain – about 1750 watts of output.
My rear rack trunk battery. The capped red wire is for charging. The black wire on the rack stay is power output to the front motor. This is the original dual-separate-battery config

That gives some perspective on how much abuse is heaped upon a drivetrain with a mid drive. Coming off the successful builds of The Colonel (v1.0) and the Purple Thing (v1.5), I knew AWD reduced load on the individual motors dramatically when they work together as a team.

Given that, I thought about how I could use a front hub to reduce or eliminate the shock that a mid-drive puts onto the ebike’s drivetrain. Not only would I gain the traction benefits of AWD, and the benefit of reduced load from the team effort – things I already knew were a big positive – the front hub would also, if used in a slightly different way, provide an important added benefit: eliminating all the extra wear and tear that goes with having a mid drive.

If it worked, it would give me a bike with all the original AWD performance benefits, plus the ability to effortlessly climb walls, without tearing the bike up.

SPOILER ALERT: It worked unbelievably well.

New steel on the right, used alloy (1000 miles of use) on the left. And this was WITH AWD to reduce the wear and tear. Ordinary single-motor systems with this mileage would be dramatically worse. Substituting the steel body eliminates all wear.

In fact this bike is a showcase on how AWD can almost eliminate mid-drive wear and tear. Making a bike that climbs hills and bombs trails really well is almost an afterthought.

So that is what I am going to focus on here in this article. What was built and how was it used?

The Build

Briefly, this bike is the Great Pumpkin on the front wheel, and a typical mid-drive installation on the back. In between is the usual extra wiring scattered all over the place to deal with powering two motors, set up dual PAS etc..

The Front Motor

2Fat has another Bafang G060 80Nm front fat motor installed – this time its inside of a custom-built 100mm double-wall Weinmann (branded as an Origin8) rim. Go fat or go home. Attached are the same two torque arms as seen on the Pumpkin’s installation, and once again the controller is sitting in a grommeted handlebar bag that doubles as a wallet/keys/phone holder. It also serves to disguise the uncut steering tube I used to give me a more upright – but not too upright – riding position.

Another big, fat, 80Nm front hub motor. This time laced into a 100mm rim. Because fat bike.

Different from the Pumpkin is the battery setup. This bike was built before the v3.0 Pumpkin came onto this Earth (The Purple Thing had just been born). So, lacking the wherewithal to commission a big custom battery for the triangle, I used two packs, one for each motor. The front motor’s 12ah 52v battery was located in the rack trunk at the back of the bike. I had already learned I did not want to put a battery on the front rack as it made the steering too heavy. The rear motor’s 17.5ah pack was in the triangle.

It could be worse: Tucked in between the panniers in the rack trunk is one of two batteries. The second pack is in the triangle. Whats on the front rack? A weatherproof, adjustable 5a charger.

Additionally, due to the very low standover of 2Fat’s Large sized frame thanks to its top tube (it is a titanium, USA-made Chumba Ursa Major) there is not enough room to plug in a properly big battery in the triangle to handle both motors. So I had to do two batteries and live with the awful choice of putting one on the back rack.

After running the two dissimilar, separate batteries from its initial build in 2017 to March 2021, I switched to two identical, now-parallel’d-together packs in the same locations. Each is 12.5ah (14S6P) with Samsung 25R cells. Each has a 50a continuous BMS. As such, the system has a single 25ah power supply with a BMS capable of handling 100 continuous amps. Considering I can never peak past 65, I’m in great shape. I purchased the packs from Bicycle Motorworks, who builds their packs in the USA and constructs them at time of order.

Sidebar:  
Running battery packs in parallel should only be done by those who have done the research and know exactly what they are getting into.  In this case, both packs are identical, being manufactured to-order together, and have the same charge cycle count.  Their voltages were matched before they were joined and there is some additional babysitting that will be necessary for charging and balancing.  If you can avoid running packs in parallel and use just one battery: Do that instead.  

Using two entirely separate batteries is a kludge and your last resort. Not only will you have to charge them separately, you will also draw down the two batteries separately at different rates which will result in uneven remaining power, lesser range and more frequent recharges.

The Rear Motor

This is pretty much your garden-variety 52v BBSHD installation. It has a couple of nice spiffs in the form of a 42T Lekkie Bling Ring, and Lekkie Buzz Bars, but neither of those things are a requirement of the AWD approach we’re discussing here.

The rear wheel is another matching Weinmann 100mm rim, again with DT Champion 2.0 spokes and 16mm brass nipples. The rear hub is a DT 350 Big Ride, which has sealed cartridge bearings and has been upgraded to a steel cassette body. Additionally a 9-speed Shimano HG400 cluster gives me steel cogs literally welded together into a single cluster, that spreads the enormous torque of the BBSHD across the entire steel cassette body. The DT350’s ratchet engagement mechanism is one of the few known bombproof rear hub mechanisms when faced with the power of the Dark Side 30a BBSHD motor.

A solid rear wheel build with extra strength parts throughout is crucial to a successful mid drive ebike.

205mm front and rear rotors with Magura MT5e brakes

The Special Bits

There is a bit more that went into making these motors work together. The complete integration possible on the Pumpkin thanks to the use of identical motors and controllers wouldn’t work here.

Brake Cutoffs

Trying to share cutoff signals between the motors again – using adapters for the red-to-yellow connections found on the BBSHD – resulted in bricking both motors. I tried everything to share the signals. It can’t be done unless you are willing to install a second, independent set of hydraulic magnet style cutoffs, zip tied to the integrated MT5e cutoffs and connected via a ‘y’ further down the line. That would work but it would look like crap, for very little benefit over what I ended up doing: I set up the front brake to engage the front motor cutoff, and the rear – with a red-to-yellow HIGO adapter – to cut off the rear motor. Since I always use both levers to do my braking I get an effective result.

Pedal Assist

PAS for the BBSHD is built into its motor casing, so it just works. PAS for the front motor was another matter entirely. Ordinarily the assist disc and sensor runs on the right-hand, drive side of the bike and hides behind the front chainring. This is not possible with the BBSHD’s secondary gear housing being there instead. So PAS had to be made to work on the left hand side. The KT controller has left-hand PAS sensor installation settings.

What was needed then was a left-hand install. This was quite a bit trickier, since the 120mm motor running in front of 5″ tire-compatible chainstays had zero extra spindle length to mount the disk. Lacking a bottom bracket cup to mount the PAS sensor ring, I set it behind a second inner lockring – I used two inner lock rings stacked like jam nuts rather than the usual inner+outer ring. These doubled inner rings had the secondary benefit of being a more aggressive, solid mounting for the motor.

With the sensor mounted, next I had the sensor ring to deal with. As noted above… there was no length available period for the sensor disk as the crankarms mounted pretty much flush to the bottom bracket.

The eventual solution involved hogging out the center of the PAS sensor disc so it could sit on the inner flange of the Lekkie crankarm instead of the spindle. I prepped the crankarm with… thin strips of thick duct tape so the disk would sit tightly on the flange. It was a bodge but it worked, and with just one improvement since installation in 2017 it has held perfectly. That improvement is a zip tie to help hold the disk steady in position (of course I used a zip tie. We have duct tape in the mix; all thats missing is a zip tie).

Worth noting: I used this identical PAS ring mounting when I built the Lizzard King AWD cargo bike in 2021. Not only did I have enough extra spindle so I did not have to do this surgery on the ring, I also realized I could unscrew and reverse the sensor in its mounting ring. This eliminated the need to use the reverse settings in the display/controller. Since the inset ring was reversed, it was outset now… and that held the sensor closer to the magnets (they work fine inset as seen here, but closer is better).

The Cockpit

Here’s where the eagle-eyed may spot a preview of how I ride this bike to soften up the mid drive.

Those are Jones SG bars with ESI XL Extra Chunky grips, wrapped in silicone tape

On the right, we have the grip, then the brake lever, followed by the 9-speed shifter. Here again we see a v2.0 feature that v3.0 fixed: A SRAM drivetrain gives us a SRAM, not Shimano shifter. The Shimano shifter needs so much real estate on the grip it is impossible to put a throttle on that side of the handlebars. Look at the cockpit of The Great Pumpkin and the Lizzard King to see how a SRAM shifter solves this and lets me do one throttle per thumb.

Being unable to do that at the time – and believe me I tried EVERY possible combination of throttles. I still have most of them in a box in my garage. I settled on two styles of thumb throttle, side by side on the left, with the innermost throttle being for the hub/front and the outermost for the BBSHD/rear. As for the eagle-eyed part: The front throttle is cocked higher so between that and its longer throw, it is engaged first and when at 100% it follows the natural curve of my thumb. Both throttles can then be at 100% and my hand stays comfortable at WFO.

How To Ride It

At last we get to the point!

I already let the cat out of the bag earlier: The biggest deal associated with this bike is not that it can climb really well (REALLY well). Nor is its ability to handle trails and rotten conditions its star quality.

2Fat was made to get dirty

No, the real point of having a mid drive teamed with a front hub motor is to use that hub motor to take the shock off the drivetrain that mid drives deliver. Do that and you also take away the excess wear and tear on the parts (if we are being fair, a lot of this comes from doofus riders who don’t know what they are doing).

When starting, start with the hub

This is most of the deal right here. Don’t make the mid drive haul the bike up from a dead stop. Its got the torque to do it. But everything takes a beating in the process. The nylon gears inside of the mid drive. The chainring. The poor chain. The suffering cogs. The cassette body being dug into by the cogs. The pawls inside of the cassette body that are straining against the hub. Your ebike hates you for doing this to it.

By using a mid-drive-strong chain, a steel cluster and a steel cassette body with a ratchet engagement mechanism, we harden the drivetrain to be very tolerant of this abuse. Between that and learning how to use a mid drive, wear and tear really isn’t bad at all. Maybe no worse than a quality analog bike used hard. But still… even with the hardened drivetrain it sure would be nice to take things easier.

By using a hub motor to get the bike rolling – even by just a few mph – we accomplish this mission. Using my pedaling-friendly BBSHD programming or something like it, the BBSHD will not kick in on pedaling until the bike crosses about 5 mph. So from a stop, you hit the front throttle for about one full second. That throttle is cocked up a bit higher on 2Fat to make that a natural move. The bike starts up from its dead stop without any strain on the drivetrain since the BBSHD is not even running. Simultaneously, you also start pedaling. This engages the cassette mechanism.

When speed crosses 5 mph, the BBSHD’s pedal assist now kicks in on a drivetrain that is already engaged. There is no longer a risk of having the motor jerk the chain and smash the cluster into engagement. And with this gentle engagement, the motor starts working on a bike that is already moving. So you get the doubled benefit of a lighter effort against all components to get your fat bike off its fat ass. Instead, you get smooth – and strong – acceleration. The lack of lugging the motor has the further benefit of not generating anywhere near as much heat since the motor is no longer running at low rpms for anywhere near as long. And those nylon gears inside the motor are writing you a nice thank-you card.

Or…

Rather than using the throttle, you could also just start pedaling. I have set the KT controller to engage PAS as quickly as possible. Combine that with the BBSHD’s controller being told to hang back for the initial startup, and you have a completely thumbs-off solution that implements at a nice gentle pace.

So… It just works.

Or you can force it! Remember with this setup you have two throttles. There’s no law that says if you need it, you can’t jump the gun and either hit the rear throttle early, or hit it so the motor engages harder than it would have in your designated PAS mode. So if you need a little extra push thanks to an XL load of groceries, or a steep hill, you have options at your disposal.

Downshift? Schmownshift!

One of the mantras associated with smart mid drive riding is that you always Always ALWAYS freaking downshift the bike when you come to a stop. The LAST thing you want is to lug the motor up from a dead stop because of all the brutality it visits on your chain, your cogs blah blah blah. So that means you remember to downshift one or two… maybe even three times before you come to a stop. When the light turns green you upshift in sequence one gear at a time as you get back up to speed again. Thus as we all understand: you row thru the gears.

Another rule mid drive mavens repeat ad nauseum is – if you have an 11T small rear cog… stay the hell off of it. Its too small to use on a mid drive. It bogs the bejesus out of the motor from a stop, its too small to be able to get up to speed before the sun sets and if thats not bad enough, the teeny cog on even the steel Shimano clusters is alloy and it is not attached like all the others are. Its an individual. So not only do they dig into the cassette body harder, they die fast. Like Really Fast. As in a few hundred miles tops.

Well, on AWD mid drives like 2Fat, you can forget about all that. Because of the powered front hub doing its part (either thru a quick dab of throttle or just letting PAS start the bike), there is no longer a need to shift at a light. You can forget about the whole process. Just leave it and it’ll be fine as if it was a hub motor! In fact the front motor allows the bike to increase its speed so it gets up and goes fast just like a hub motor does.

I found this out within a day of building the bike, and since then learned from experience the 11T cog will last about 1500 miles before it typically cracks (two so far). Thats not so bad for a readily available US$7 part. And if I wanted it to live longer, well it wouldn’t kill me to go up a gear at a light once in a while.

At the office, outside my e-garage. There is a trail network along my commute route that 2Fat lets me take

Wrapping It All Up

2Fat itself is not the ideal example of how to execute this concept. It is a product of when it was built and my knowledge level at that time. In the present day, I for sure would not want to build a bike with two batteries, and if I did I would NEVER put one on the back rack. But thats partly the limits of the frame I used. I came within a hair of using a Salsa Blackborow frame kit for this bike until, at the last minute, this titanium beauty fell into my lap for a song… but thats another story.

Next, I wouldn’t use Shimano components thanks to the real estate problems introduced by the shifter. Instead I’d use SRAM components so I could do one throttle per thumb.

Really the Great Pumpkin with its XL size and XL triangle, plus its SRAM shifters – would be ideal here. But them’s the breaks. This is what I’ve got. Boo hoo.

With that said… I did learn as I went along, and in 2021 I returned to dual motor bikes with the Lizzard King, a bike meant to prove a different kind of AWD could be awesome: You don’t need the 80Nm, 35a punch of the Pumpkin’s or 2Fat’s front hub to gain the benefits of AWD. A low power implementation, done a little differently, should be very effective and appeal to a much broader range of everyday, low-speed, low-drama ebike use. And…

SPOILER ALERT: It worked unbelievably well.

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.

4 thoughts on “Dual Motor AWD Electric Bikes – Case Study: Fat Trail and Hill Climber”

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s