It's not just to get more range that they don't allow full thrust instantaneously. There's a wheelspin issue, especially since they don't equip even the SR with wide, sticky tires, which could be a liability issue, especially in wet conditions.
There's a belt-snapping issue; I'm pretty convinced that if the belt can stand 106 ft-lbs of torque, it can stand it at any time, but there is the possibility of high jerk loading causing the belt to have issues (jerk is the mathematical derivative of acceleration; look it up).
Thanks for the links above, I just want to say more than a few people have missed the point of that thread. As a builder I'm concerned with the various trade-offs of the types of motors you can run, and it seemed to my butt that all of the AC/brushless motors have less of a feel of a jolt right off the line than the PMDC motors I've run. It's pretty clear that every motor type and design has slightly different "stall torque" characteristics. That's basic motor design and physics, as I learned. And it's backed up by the graphs.
Softer stall torque in a PMAC motor I'd say even applies to the Energica EGO I rode. Peg the throttle off the line, and it feels soft (and no, I couldn't pull the front end up or spin the tire even though I tried, but it gets 0-60 in under 3 sec.). Once it spins up, it's all hell breaking loose. The Zero SR I rode is what got my attention on this issue, because it was so quick 0-60, but my PMDC would lift the front end where the Zero SR won't.
Regarding the quote above, I can't imagine wheelspin being an issue under any normal riding conditions for a bike like this. Nor is belt-snapping. The only bikes I've ever ridden that spin the wheel as opposed to lifting the front end are heavy pigs - Harleys, Leadwings, etc. Anything that has a decent power to weight ratio will lift the front long, long before it spins the tires. My bike, very similar to the Harley Livewire in all performance specs, and not far off the SR in everything but range, would never spin the tire, though the front end comes up all the time. It's running a very narrow, '70s style width on a 1971 stock lace wheel.
As far as belt-snapping due to off-the-line power application, it ain't gonna happen.
And to be clear, I'm talking about torque from start - that is, 0 RPM to the 500 RPM range. The torque a motor has to start itself spinning, which is not the same as other measurements of torque and power.
I think it's pretty clear (to me, anyway) that first, the characteristics of the Zero motor being what they are, and second, the throttle mapping they've decided on, gives the SR a slightly soft start. That's good for 95% of riders.
