Why Hub Motors are Better than Mid-Drives

by Henry Kellogg November 21, 2018

Why Hub Motors are Better than Mid-Drives

One of the most common questions that we get at Bike Swift is some variation of, "Can you install a mid-drive electric assist system?" And our answer is simply, "No."

I have installed dozens of the popular BBS mid-drive systems from Bafang (as well as stokemonkey kits, the TSDZ, and other systems), and serviced the Bosch, Shimano, and Yamaha mid-drive systems. I chose not to offer mid-drives through Bike Swift for the following reasons:

  • Mid-drives are less efficient than hub motors in most situations (that's right!)
  • Mid-drives are more expensive than a comparable hub motor setup
  • Mid-drives increase the wear on the chain and cassette, doubling or even tripling maintenance of the drivetrain
  • Mid-drives cannot perform regenerative braking on a standard bike
  • Mid-drives tend to revise the "Q-Factor", or crank arm spacing, of a bike in an uncomfortable way
  • Mid-drives are often limited to a single chainring

I'll take a moment now to elaborate on each of these claims:

First, efficiency. Strictly speaking, motor efficiency is defined as

(Mechanical Power Output, Watts) / (Electrical Power Input, Watts)

The idea here is that the more of the power you're drawing from the battery that goes to propelling you forward, the more efficient the motor. 

Hub motors have an inherent efficiency advantage because there is no meaningful intermediary between the motor and the road. The mechanical power from the motor goes directly into the tire-road interface, and there are no real efficiency losses there. However, a typical bicycle chain and gear system, which is what a mid-drive system uses to drive a bicycle forward, reduces the efficiency of a motor by about 5% due to frictional losses. 

This chart shows the efficiency of our rear hub motor system compared to a typical Bafang BBS system. Assumptions for mid-drive: 8 speed 11-32T cassette, 83% peak motor efficiency at chainring, perfect shifting. Waviness of red line due to interpolation error, but accurate to 2% efficiency

Notice that in the above chart, the efficiency of our rear hub motor is actually higher from 15 mph to 27 mph - and that's the speed you'll ride at most of the time! This chart shows full throttle, and at less than full throttle our rear hub motor actually wins out above 12 mph. 

Second,  expensive - the raw cost of BBS systems is roughly 50% more than a hub motor counterpart. Because many of these systems are sold direct to consumer online, it may seem to some that the prices aren't higher - but keep in mind that this is consumer direct pricing, and generally doesn't include a warranty or US based service.

Third, wear. Bafang BBS units have a 3-stage gear reduction system, which is prone to failure, while other mid drives have internal gear reduction systems of their own. In addition to potential failure of the unit itself, mid-drives place the chain under 2-3 times the tension it would otherwise experience during normal, human-powered riding. This means 2-3 times the chain and cassette replacements!
Get ready to replace a bunch of chains, or settle for poor shifting, if you choose a mid-drive!

Regenerative braking can increase your range by 10-15% and greatly reduce wear on the brake pads, and that simply isn't possible with a mid-drive motor. 

Q Factor is the distance between the outside of one crank arm to the outside of the opposite crank arm. Mid-drives, and especially mid-drive retrofit systems like the BBS systems, space the crank arms further out, and often asymmetrically. The result can leave you feeling as if you're riding a lopsided horse!
 
If you're looking for a bike that can give peak torque and efficiency at very low or high speeds, don't mind reliability concerns, and don't care about the other points above, a mid-drive is a good solution for you. Mountain biking, for example, can benefit from a mid-drive motor delivering peak efficiency at low speeds. However, if you're looking for a reliable bike for regular riding on roads, look no further than a hub motor!




Henry Kellogg
Henry Kellogg

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