Hello Chris, you've presented me with an easy batch to sort, since they are very different.
The Forza is by far the most powerful and best hill climber, and is also very fast, so it can leave the others standing both on the flat and on any hill.
The Nano motor in the Brompton is quite powerful for it's size, but it's important to read between the lines in it's recent test. A to B reported that it assisted up to 14 mph, and 13 mph when the battery was low. But they reported the average speed of their test as being over 16 mph, making it clear that much of the time, it was the rider's performance that was being reported, not the motor's! Since that 14 mph indicates it's quite low geared in the 16" Brompton wheel, it's obviously climbs well for it's size, but many other e-bikes carry on to 17 mph or so still helping. If you don't need a folder, I don't think it's worth entertaining, as a normal size bike is almost always a far better ride, certainly better than a 16" wheel one.
The Gazelle Easy Glider will climb just about anything, but very slowly, since it's not powerful. The Extra Energy test picked up on this and highlighted it. On a steep hill, you'd slog up at about 5 or 6 mph, where the Forza would be doing more than double that with less assistance from you.
The Sparta Ion is not a good hill climber, mainly because direct drive motors like the one it uses aren't good at that, though very refined in their running. When tested on steep hills by Velovision, it was reported as only able to carry it's own weight up 1 in 5, the rider left to do as much as if they were on a normal bike. Therefore I think you can rule that straight out as hill climbing is what you want.
Electric bikes are usually either Pedelec, meaning you pedal to get any power, or throttle control to get power whether you pedal or not. A few have throttles as well as being pedelec only, meaning the throttle affects the power, but you still have to pedal to get it. None of these is exactly that last type, though the Forza is close.
The latest Forza is a pedelec on which the level of power is dictated by a control called EAF. This has nine positions on which you dial up the power level you want, so you can choose between economy and power in varying degrees. It also has a derestrictor that enables you to do speeds over 20 mph assisted on private land, this being illegal on public roads, but it's always a fast bike due to it's high power.
The Brompton Nano is twistgrip throttle controlled whether you pedal or not.
The Gazelle and Sparta are both pedelec bikes, needing pedalling to get power, this being the law anyway in mainland Europe. For the moment in Britain we are able to have twistgrip only power. Our law may change to be in line with Europe, but that won't affect existing bikes.
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