The design of the mid drive motor is intended to improve many of the shortcomings of the hub motor. The greatest advantage of the mid motor over the hub motor is its gear ratio. They allow cyclists to power the rear wheels through the same chains and gears as the pedals, which means they can choose lower gears to power steep hills or to stop accelerating from high-torque stops. Low-speed mid-drive motors can climb steeper hills than similar-power hub motors, and can climb longer than hub motors, which can overheat on steep hills for longer periods.
A mid-range drive motor is also usually smaller and lighter than a similarly powered hub motor. Smaller and lighter mid-range drive motors are usually more concealed because they can be mounted directly on a bicycle frame. Many people don’t even realize that midsize motorcycles have become electric bikes just by watching. It’s much easier to replace the tires on a medium-sized electric bike with a drive motor, because you don’t have a heavy-duty hub motor to deal with. You can change it like a regular pedal bike. Also, since you can use regular bicycle wheels, you are free to use any wheels, tires and tapes you want.
Finally, the middle drive motor allows the use of a true torque sensor for the pedal assist system, which adjusts the motor power according to the pedal force you measure on the crank. The hub motor usually relies on the pedal frequency sensor for pedal assistance, only adjusting the motor speed according to the pedal speed, and may lead to the motor timing instability or clumsiness, especially in climbing or moving the bicycle around obstacles.
Of course, the central drive motor also has disadvantages. Mid-drive motors can be brutal on your drive system, which can be one of their biggest flaws. A healthy human can produce 100 watts over a long period of time, and 250 watts during a hard sprint is reasonable. But the middle drive motor can continuously output 250-750 W power. It’s like having a professional cyclist hitting your pedals all day. Cheap bike chains have no chance. Retail e-bikes often come with upgraded bike chains, which is why fast chains can be a top maintenance issue for midsize e-bikes. And because both motors and pedals require chains to drive the wheels, riding a mid-sized electric bike with a broken chain is a strict business.
One way to mitigate the chain problem is to choose a mid-drive e-bike with belt drive, but also consider the added price. More failure points because the number of moving parts in the middle drive motor increases. If the motor fails and the motor is mounted directly on the bike frame, replacing it may be more expensive than replacing the hub motor. In fact, mid-drive motors are also generally better than the proven, mass-produced hub motors found on many electric bikes.
The middle drive motor has another disadvantage that many people don’t think about before they stop for the first time. You can’t shift gears unless the bike is moving (except for a few bikes with internal gears and a rear hub). This means that if you are flying down the street at the top end but are forced to stop at a red light, you will want to remember to lower gear before stopping. Otherwise when you try to pull out the light you get trapped at the top end and your acceleration is affected.
Do not even consider shifting gears at motor power. This is the best way to tear your chain in half. Motor torque is too large, when the extra stress between the gears is easy to break the chain. Better mid-drive e-bikes have shift breakers that briefly cut off the throttle when you switch gears. But many e-bikes don’t have this feature, so remember to ease up on the gas when you switch gears.