The bike responds quite dramatically to that very slight incline, so that I have to change gear and motor mode when I change from slightly uphill to slightly downhill. That happens a lot, and if I make sure to concentrate on that and make appropriate adjustments then the lurching is under control.
I knew you'd figure it out! Since you clearly ride like a scientist — as do I — you might find this a useful perspective...
What your bike's probably responding to above is a conscious or unconscious increase in pedal force (Fp) on your part. Your torque-sensing Gazelle doles out mechanical motor power (Pm) at least in part on Fp, so the harder you push, the more Pm (assist) you'll get to help you along.
Power balance
Mechanical power at the rear wheel (Pw) ultimately determines your ground speed (Vg) against the total external power loss (Pe) you face from air, rolling, and slope resistance. Among other things, Pe grows rapidly with Vg and headwind speed (Vw) and less strongly with gross weight (bike+rider+cargo), slope, rolling resistance (e.g., on cobble), and the uprightness of the rider.
Ignoring your Gazelle's small drivetrain power losses, Pw is just the sum of Pm and rider power (Pr). This Pr is just the time-averaged mechanical power your legs put into the pedals, and it's proportional to Fp × C, where C is your cadence (pedaling rate) in RPM. So doubling your Fp or C doubles your Pr and doubling both quadruples it.
In steady riding at constant Vg and Pe, the power balance Pe = Pw = Pm + Pr must be met. The ratio of Pm to Pr doesn't matter here — only their sum. You can put in as much or as little Pr as you like as long as you add the Pm to meet the balance. If not, you'll automatically slow to a Vg where the balance is met.
Controlling Pw
On an ebike, you have 4 main controls on Pw: your Fp, your C, your gear ratio (G), and your assist level (L). That's a lot to juggle, but you'll get used to it.
To complicate matters, these 4 controls are all coupled to varing degrees in practice — especially C and G.
Controlling C is ultimately what your gears are for.
NB: There's no one right way to combine Fp, C, G, and L to produce the Pw needed to counter the prevailing Pe at a given Vg. But here's an effective cadence-centric way you might try. It effectively reduces the balls to juggle by one and feels good, too.
Cadence in human power production
Humans have 2 important physical characteristics: (1) a reproducible personal "natural" or "preferred" or "self-selected" cadence Cp, and (2) a roughly parabolic convex-upward rider power vs. cadence (Pr vs. C) curve peaking at a cadence just above Cp. Your own efficiency also peaks near Cp.
You can use this physiology to your advantage — even in daily riding.
Everyone has their own Cp. Yours is just the cadence you find yourself at when pedaling comfortably with some exertion
without thinking about it. Go through the gears on a flat road and settle on the one that
feels best to your legs. You'll be close to your personal Cp at that point.
The Gazelle display might show your cadence. If not, you can easily count it out with a watch.
But most importantly, get to know what your own Cp feels like and learn to reproduce it by feel if nothing else.
Cadence-centric riding
Now,
experiment with various combinations of Vg, Fp, G, and L to stay as close to your Cp as conditions and the desired Vg allow. Simple as that. With Cp as a target, you'll soon figure out the Vg vs. Fp vs. G vs. L game that suits you best.
Just bear in mind that Vg is always proportional to C × G. To change the relationship between Vg and C, you have to change G accordingly.
Surely not for everyone, but this is how I ride when not just poking around. It's just 2nd nature now. I have the benefit of a cadence display on my bike, but that's not essential. If you pay attention, your legs will tell you when you're doing it right.
In return, I get the most speed for my perceived exterion, save my legs, and save some battery, too.
But most importantly, it just feels good.
All of this will make more sense as you gain time in the saddle and observe and experiment along the way. And nothing about it diminishes the pure joy of cycling. Just the opposite, in fact.
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