Dual Motor AWD Electric Bikes – Case Study: Larry vs Harry Bullitt Cargo

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The Lizzard King

• 500w, 25a geared Bafang G020 front hub motor
• 30a BBSHD mid drive (rear motor)
• 52v, 32ah single battery, skateboard config (box under cargo floor)
• KT and Bafang displays
• 160 Nm rear, 45Nm front

My previous AWD builds all used effectively the same front wheel setup: A 35a controller mated to a Bafang geared fat motor packing an 80 Nm punch. It was so powerful, on my early commuter bikes I needed to turn down acceleration via a slow-start setting. When I graduated to a combination mid drive+hub, I found best results on rough trails came from the same slow start, but also using the front power sparingly: little if any throttle and gentle PAS.

There things stayed for a few years – roughly from the middle of 2017 to early 2021. During this period I concentrated on riding and refining the use and configuration of these AWD bikes. I built other bikes during this time- all more traditional single-motor mid drives. As part of that work I came up with tuning settings that worked very well with pedaling and a cycling mindset. These changes worked great with the 2Fat AWD bike as well.

With regard to tuning, I concentrated on backing off the BBSHD’s power when delivered in ‘pedelec’ mode: limited use of throttle and pedal assist only. The point of this was to have a bike that did not run away from me, still delivered measurable, useful levels of assist, lacked the common complaints against cadence-type assist and did not suffer from any of the weaknesses of torque-sensing.

When 2021 arrived and I wanted to build a bucket-list bike – the Larry vs. Harry Bullitt cargo bike – I decided to go all out and make it AWD. Further, I wanted to prove a concept I had been mulling over for the last few years. For lack of a better term, lets call it Drama-Free AWD: Not a hot rod, but what a normal person who just wanted a reliable automobile replacement would want to ride.

Ingredients

Its a pretty short list:

• Low power
  High power in a front wheel can be fun, but its not necessary to gain the traction benefits that come with AWD. Use a smaller, lighter, relatively low-powered motor (45 Nm vs. the prior 80 Nm) as part of its design. Also use a smaller controller that peaks at 25 amps rather than the previous 35. Continue to use the slow-start setting to ensure … Drama Free AWD. 25 amps on a smaller diameter wheel will still be a strong assist, but those amps will be rolled on slowly so no surprises.
• Fast Wind Front Motor
  The Bullitt has a 20″ front wheel. A ‘fast wind’ motor favors torque off the starting line at the expense of higher top speed. This is normal for a small wheel build and further solidifies the emphasis on slow, strong startup power that melts away on its own as speed increases.
• Toned-Down Rear Motor
  My revised motor settings keep the BBSHD from engaging until speed reaches 6 mph (9.7 km/h) if I rely on pedal assist. I learned how important that is to drivetrain longevity when I built 2Fat. We’ll re-use those identical settings.

What I Expected

On a bike destined to carry heavy loads, the front motor is intended to get the bike off to a painless start. It does this job very nicely. Despite the relatively low power, it still gets the bike rolling from a stop, and effectively takes out the BBSHD’s shock to the drivetrain when that second motor kicks in at 6 mph (9.7 km/h).

That reduced sting will translate into reduced wear and tear, and reduced parts replacement over time. Its too early to pull hubs apart and look inside to verify this assumption, but since I have seen and verified the effect before on similar hardware, there’s no reason to assume different results.

UPDATE: June 2022
Over 1800 miles / 2900 km later, I am still using the original chain. At about 1300 miles / 2100 km, a check on the rear cluster and cassette (upgraded to the steel DT Swiss body) showed barely any discoloration on the cassette surface… never mind any wear.

It was a short list of things to expect and… it all panned out. But there were also some pleasant surprises. This turned out better than I thought it would.

What Surprised Me

I noted above the motor is ‘fast wind’; built for low-speed torque, not high speed rpms, and how this plays into the smaller front wheel size. Intellectually, thats easy to understand. Less obvious was the fact that, in practice, there will be a lot less motor usage than there was before.

With The Great Pumpkin, I usually run both motors at equal levels (usually full blast) all the time. The bike and flat, straight streets just lend themselves to a high speed cruise. Two identical motors and identical controllers gulping juice from one battery mean a big power drain. No surprise.

With 2Fat, while I reduce power to the front motor, I was often giving the bike hard use on trails. More often than not the bike is fighting its way up a hill, thru a bunch of sand etc.

So even though The Lizzard King is not dramatically different than 2Fat in terms of its configuration, the world it lives in is quite different: level and smooth city streets. Easy acceleration and long periods of the motors spinning fast while running at an efficient cruising speed.

More different still: Off the line, the front motor kicks in slowly and then power melts away as wheel revolutions increase. It pulls strong from zero to about 16 mph. But from 17+, it starts scaling back as the motor approaches its rpm limit. By the time 20 mph rolls around, on typical level 2 assist you are down to about 200 watts of output. By the time you hit 23-24 mph on flat ground, wattage to the front wheel has minimized to a steady… 38 watts. Just enough of a dribble to ease the wheel’s free spin.

If you hit an incline, you’ll slow down a tad and see wattage output creep up again. But rolling down the street on the flats, the front motor takes itself out of the picture. Its time for…

… The rear motor to kick in. As noted, pedal assist does not engage the rear motor until it reaches 6 mph. So when the front motor is eating the most juice, the rear motor hasn’t even started. As the mid kicks in and spools up, the hub begins making its graceful exit.

The two motors never really run hard together at the same time, unless climbing a hill (or I force the issue via using the two throttles). Then you can see watts climb on the front rather than fading away. Cruising at an energetic cadence around 24+ mph , you are on the single rear motor, being given a small boost from the front motor (remember those 38 watts?).

With the two motors staying out of each others’ way, this translates to an overall reduction in expected battery drain, consisting of both reduced peak and continuous draws. It gets better though.

Much Better!

The rear BBSHD is also using a lot less power than its siblings in The Pacific Fleet.

At 20 mph, on PAS 2 in the front and maybe PAS 4 or 5 in the back, looking at both displays, I can see 250-300w being output from the rear motor, and another 150-200w being output from the front. 500w or kess are being drawn between the two motors, on a great big cargo bike. All the way up and down the speed curve, watt and amp output for the BBSHD is much less than it is on any of my other bikes.

Not So Fast!

All of this wonderfulness is only true when running under pedal assist. If I mash the rear throttle the BBSHD will, as usual, peg the output gauge until I release. And that means it will burn thru my battery range lickety-split. Not a surprise. There is no free lunch in this world, but if we stay off the throttle we still get a hefty discount.

And I still configured my big single battery (custom-built for this bike) to the usual theoretical limits: A 25a peak front controller and a 30a peak rear controller mean I must have a battery management system with a bare minimum of a 55a continuous rating, and preferably 60 (mine is 70). I would rather not take any chances, but clearly I have a bigger safety margin than I figured on originally.

And despite the capability of the bike, reality is it rides more comfortably around 20 mph. So power consumption is lower still simply because of the type of bike it is. But the big takeaway is its lower power use is lower across the board. It was an unexpected gain in efficiency, but looking back on it, it should not have been. The benefit was hidden by my hard use of the other bikes.

Commercial Manufacturing

Should a commercial bike be made with this Drama Free AWD kind of approach in mind, a thoughtfully designed system could manage power in such a way as to map out the curves on the individual motors. Develop something that never bumps into the limits of a much more conventional BMS. That makes for a battery system less expensive and easier to source in volume. And a street machine is going to have lower power needs than is generally understood to be the case with an AWD bike.

Lower power means safety for the casual rider, lower cost and smaller battery sizes.

Lower power on a street bike could look like – in the USA at least – dual motors fitted to bikes that still remain legal within both federal manufacturing standards and individual state vehicle codes. A 249w front motor and a 500w rear for example. Or even a 250/350.

Whats the Takeaway?

The fact that I can operate a great big bike like The Lizzard King at power levels well below allowed USA ebike power limits is testimony to the fact that viable, useful AWD can operate well within the legal framework of ebikes in this country.

Just because you have two motors does not mean they both have to be running simultaneously at full blast. Turns out… not doing that can be kind of a big deal.