Best Powered, Internal Gear Hub for an Electric Bike?
There Aren't Any
Perhaps the biggest drawback of hub motors for electric bikes is that they do not use the gears. The gears on bikes have evolved over 150 years to be exceptionally efficient. The motor not using them is inefficient, meaning it:
does not perform as well as an identical mid-drive motor;
wastes power meaning your ebike has less range with the same battery; and
overheats and either burns out or shuts down to protect itself.
Given that internal gear hubs exist, a theoretical solution to these problems is to have a powered, internal gear hub. Unfortunately, the single example of this combined technology that we could find is of no use.
In this article, we:
explain what a powered rear hub motor is;
explain what a internal gear hub is;
touch on the difficult in combining these technologies; and then
look at the single example we could find of this combined technology.
Powered Rear Hubs
The motor on electric bikes provides power via the front hub, rear hub, or crank (called a mid-drive motor). A mid-drive motor adds power to the chain; hub motors power the relevant wheel directly.
A separate article discusses why mid-drive motors are better than hub motors for electric bikes.
There are two basic types of rear hub motors for electric bikes: direct drive and geared. All hub motors power that wheel directly (i.e. not via the chain). In this case, "direct drive" has a different meaning.
Direct drive motors have no moving parts. The hub is the motor. The advantage of no moving parts is that there is not much that can break. The disadvantage of direct drives is that they are heavier and are much less efficient because motors prefer to operate at higher revs.
Geared hub motors use a planetary gear system that allows a smaller motor to rev faster. The gears then step down the revolutions to propel the wheel.
Geared hub motors are lighter and more efficient, thus extending the range of your bike given a specific battery size. Unfortunately, the moving parts do wear out, and on cheaper models, the planetary gears are made of plastic, with a decidedly lower life expectancy.
The basic principle of motors is they are most efficient around a given speed (which is spinning pretty fast). Direct drive motors are never able to run at these revs as they directly power the wheel and the bicycle wheel is never going to spin that fast - the bike is never going to move that fast.
Geared hub motors are able to operate at their most efficient revs, but the manufacturer has to choose the speed at which the bike is most efficient (say 15 mph). Going slower than this optimal speed is less efficient.
Direct drive motors have no moving parts and thus it would be difficult to integrate internal hub gears.
Geared hub motors use planetary gears to operate more efficiently. The planetary gears do not require a lot of hub width, but the motor itself does. This makes it difficult to integrate internal hub gears, as discussed below.
Internal Gear Hubs
Hub gears are not a new invention. The first patent for a compact epicyclic hub gear was granted in 1895. The full details of implementation are not important. This basic design supports 3 gear ratios:
low gear – where rotations are reduced, meaning that one rotation of the sprocket (which is driven by pedaling) results in less than one rotation of the wheel;
direct drive – where one rotation of the sprocket results in precisely one rotation of the wheel; and
high gear – where rotations are increased, meaning one rotation of the sprocket results in more than one rotation of the wheel.
In this design, the low gear and high gear ratios are the same but reversed – e.g. 2:1 vs 1:2
Over the past 100 years, more sophisticated internal gear hubs have been created.
Rohloff, for example, has a 14-speed Speedhub with an impressive 5.25 to 1 ratio between the highest and lowest gears. Often this is referred to as a 525% gear ratio range.
Brompton Cycles uses a 3-speed rear hub for their folding bikes, specially designed with Sturmey-Archer. Combined with up to 2 external gears, their bikes can achieve a 302% gear ratio range.
Although perfectly functional (indeed, the Rohloff is superior to the vast majority of exterior gears) - internal gear hubs add weight and require hub width. The hub on the rear wheel can only be so wide before it starts to lose structural integrity.
CONCLUSION: Combining the Two Technologies
Although you will find internal gear hubs on electric bikes, in nearly all cases power is delivered via a mid-drive electric motor.
Advantages and Disadvantages of
Internal vs External rear gears
If you are considering an internal gear hub for your electric bike, here is a good video explaining the advantages of:
external gears with a derailler;
advantages at 04:18
disadvantages at 07:31 and 10:55
the Rohloff 14-speed Speedhub (at 05:19); and
the Enviolo internal geared hub (at 06:30).