That depends on a lot of factors. I find that a suitably rigid rear rack lets me carry up to about 30 lbs back there without objectionable effects, but my pedaling is impaired if I have something wider than about 2" between my knees. Others here obviously differ, or else they don't bother pedaling.
Rear motor/front frame-mounted battery seems like a good layout, but the opposite is what I've usually used. I'd only tolerate a battery in the front triangle if it was situated down low by the cranks.
Have you tried a triangle battery, or is this a theory?
Water bottles are wider than 2". I can easily put 500 watts pedal power into the system (for a short time) and I can't tell at all that there is a 100mm wide battery compartment there or not. I've never contacted it except when I stop pedaling and specifically tuck in to it. The pedals are considerably wider than 120mm apart on the inside pedal edge. At least for me, 100mm battery width is insufficient to be noticed, and I've never consciously adjusted my pedaling to allow for the width.
Alan B wrote:Weight on the rear rack is very destabilizing and bicycle frames are not stiff in that direction, so they flex and even motorcycles rear racks are not designed for weights over 30 pounds due to the stability problems it causes.
While there is a significant inertial moment between mass on a rear rack and a bike's center or rotation in any axis, I have come to believe that most of the problem lies in the rack itself. Most rear luggage racks are made of small diameter aluminum, with no diagonal bracing at all against lateral forces. Both the materials and the designs tend to make them sway side to side under even small loadings.
The Tubus Cargo rack is an example of a rack that differs in both regards. It's made from tubular chromoly steel, and the middle strut forms a triangle joining the rack sides and top. While I don't use this specific rack-- I have found some less expensive ones that do the job I ask of them-- it's the one I order for touring cyclists who load their bikes heavily.
It's not unusual for a self-supported touring cyclist to carry 80 pounds of gear and supplies among two to four panniers. But unlike the lackadaisical pedalers here, self-propelled cyclists don't seem to want anything wider than a water bottle in the front triangle. If you're curious, check out your legs while you pedal. If they move all in a plane parallel to the bike's frame, then all's well. If your knees splay or flare at all as they rise to the top of your pedal stroke, then something is interfering-- a wide battery pack, a too-low seat, or perhaps even a generous belly. I find it doesn't take much to foul my own pedal stroke.
The weight distribution issue is not about pedaling, it is about stability. Poking along at low speed it may not matter much, but try and execute a quick S-turn and the rear weight will have significant input into the steering and reduce the stability and safety of the vehicle.
As you well know, touring cyclists carry most of the weight low, not above the rack, and only a part of it is on the rear. So in total an 80 pound load is effectively less than 30 pounds on the rear rack. If half was on the front, and the remainder is centered about the rear axle, the effective weight on the rear rack is about 20 pounds. Also a heavy loaded touring bike is hardly a good handling machine, and is not very safe at high speed.
In an attempt to bring this thread back on topic,
The EM3EV triangle bag is about 80mm wide, so even less than 4", and pretty much equal to a water bottle. My Mountain bike battery in the triangle is 100mm wide (side by side Turnigy 6S5AH packs), and the bikeE battery is about 65mm wide with PVC gutter material and single Zippy 6S 8AH packs. So, in my experience a width of 80 to 100mm is narrow enough for a battery compartment. The standard Greyborg compartment is 100mm inside, so a few more mm outside the plastic.