In this post I’d like to discuss the differences between hub drive and mid drive systems used on e-bikes. Almost everything in this post, I got from somewhere on the internet, and of course, you could find it all yourself, but I’ve just tried to consolidate some of it and comment on it a little from an old electronic technician/mechanic’s point of view. This post is in no way intended to be a “sales pitch” for either the hub drive or the mid drive system. I don’t even own an e-bike, but the purpose of this post is just to get a little “technical” about their workings, advantages and disadvantages.
We’ll start with the mid drive. It is named accordingly because the thing that does the driving (the motor) is mounted somewhere toward the center of the e-bike, instead of being contained in the rear (or front) hub of the wheel. However, it is much more than just a “location thing”. Since the energy of the motor is applied to a different place, it makes the system quite different is several ways than a hub drive.
Almost all mid drive systems add their energy at the crank assembly of the e-bike. That means that they add torque to the pedal assembly just as the rider does. Now if you stop and think about it, that’s cool in a lot of ways. For one reason, if you have a flat tire, you can change it just as you would have on the e-bike if it weren’t an e-bike. It doesn’t affect any of the drive components. The system simply adds torque to the crank (the part that the pedals are attached to). That torque is then delivered to the drive wheel through the chain and its associated gearing system. So, the system takes advantage of the gear ratios that are available to assist the rider in his pedaling or even if the rider chooses not to pedal at all, the gear ratios are still available. That’s neat, to me. You still have the advantage of a 10 speed, 7 speed, 6 speed or whatever the derailing system offers to choose from, to select the right gear ratio for the circumstances that you are encountering. However, that means that the chain is under a whole bunch more stress that with a normal bike. Not only does it “feel” the torque delivered by the rider, but it “feels” the additional torque put out by the motor. Some of the mid drive systems can add up to as much as 90 Newton meters (Nm) of torque (or more) to the drive system. That translates to about 66 ft. lbs. That is a lot of force, and translates into a big increase in the demand put on the chain. Let’s say you have a rider that weighs about 150 lbs. and he puts all of the weight that he has on a pedal that sticks out 6 inches from the center of the spindle. That means he is putting 75 ft. lb. of force on the crank (.5 ft. multiplied by 150 lbs.). In talking about e-bikes, almost everything is discussed in metric terms, so we’ll call that 67 Nm of force. Now this is assuming that he is just standing on the pedal and NOT doing any pushing with his muscles. This only accounts for gravitational pull, not his ability to do “leg pushes”. So, that 75 ft. lbs. of force goes directly through the spindle to the chain pulley on the crank assembly and then to the chain. Now to figure out how much force that would apply to the drive wheel you would have to get into calculating gear ratios and all of that, but for our example, we just know the force goes to the chain. Now let’s say that our motor is able to generate an additional 90 Nm of force on the crank. That means that we have actually more than doubled the Nm’s to the chain. Just for kicks, I looked up some numbers on how much force a rider can put on the crank (and chain) when he is really working at it with all of the muscle he can conjure up, and I was surprised to learn that they can put up to at least 400 Nm combined weight and muscle. That’s around 295 ft. lb. I concluded that the chain already gets a good workout without the motor’s help, so when you add a motor and put it all together and the chain becomes an important part of keeping things going.
That’s an issue with mid drive e-bikes, because if the chain breaks, it’s all over for the rider until he can repair or replace the chain. Since all of the power from the both rider and the motor are delivered through the chain, the rider would probably want to carry some extra chain lengths or an extra chain if he is going to get a long way from home.
Mid drive systems can also use “torque sensors” to monitor how much work the rider is doing when pedaling so that the controller can do a better job of assisting the rider with the pedaling in the portion of help that best matches what the rider is doing. The torque sensor is usually mounted in the e-bike’s lower bracket area or can even measure the tension of the chain (depending on the system you have) and very accurately determine how hard the rider is working and offer a reasonable amount of assistance, no matter what the speed is. The rider just feels like he has “very strong” legs while the system is working.
Mid drive systems can provide more power going up hills because they can use the bike’s gearing system to their advantage. They also tend to be lighter than their hub drive counterparts.
Jumping over to the hub drive e-bike for a minute, we see several things in contrast to the mid drive systems. These always have the outer rotor type motors in them, with the electromagnets remaining stationary in the center of the hub, while the rotor (with the permanent magnets) is attached to the wheel. One of the neat things about the hub drive system is that if the chain breaks, you can just use the throttle and cruise on home. Since the power is supplied directly to the hub, who needs a chain? Of course, you won’t be able to pedal in this situation, but at least you can get where you need to go. The down side to this is that you lose being able to take advantage of the bicycle’s gear ratio selections that come with the crank’s multiple chain rings and the cassette (cluster of different sized sprockets on the back end). Some hub drive systems do have gear selections within the systems themselves (inside the hub), but that is totally different than the chain ring and cassette system.
Some kits are available that will allow you to convert a traditional bike into a hub drive e-bike. Some of them even add the drive system to the front wheel instead of the back wheel.
The hub drive models are generally quite a bit less expensive than the mid drive e-bikes. They are cheaper to produce and are by far, the most common e-bikes on the road (at least at present).
How “hard” the rider is pedaling is not known to the hub drive system. It can monitor the “speed” that the pedals are moving at with what they call “cadence sensors” and try to match the rider’s needs as well as possible, but it isn’t nearly as effective as the torque sensors used in the mid drive systems. The cadence sensors (there can be as many as 12 or more) are mounted to the spokes or rim of the wheel, and when they pass by their counter-part, which is a stationary device that is mounted to the frame of the bike, they create an electrical signal that is relayed to the controller. The controller is smart enough to translate this signal into something it can use to calculate the speed of the e-bike. After all, it knows what the circumference of the tire is, so it knows how far the e-bike has gone with each revolution of the wheels. The controller reacts by offering more assistance at slow speeds (you might be trying to get up to speed or going up a hill) and by offering less assistance at higher speeds. Riders often complain about situations where the assistance is not offered in exactly the amount it should be. Let’s say you are working to get up over a hill. Because the speed is slow, the system is trying to help you as much as possible, but then you hit the top of the hill and before the controller can recognize the speed difference and reduce its assistance, it surges just as you hit the top of the hill. Remember, it only responds to speed and doesn’t know how hard you are working to go that speed.
Another thing that is a little different about a hub drive e-bike has to do with having a flat tire. If you have to change the tire that has the hub drive motor at its end, you’ve got a bit more of a project than with a conventional bike or a hub drive e-bike. Since there are connections from the hub motor to the controller and sensors, all of the wires, cables or whatever must be disconnected before removing the hub. Some of those cables are tied into place with wire ties, so you’ll want to carry a few of them also. You also probably want to have your owner’s manual with you (and your bible) for your first outing.
Once again, from what I learned, the majority of e-bikes sold have the hub drive system. They are quite a bit cheaper to produce and to buy, and in most cases are adequate for a rider’s needs. They do however, take a little getting used to. I think I would prefer the mid drive system for the reasons above, but they are more expensive. Happy riding, Ron.
For information about books written by Ron Staley about both electric and gas driven golf carts and their repair, visit the following links.
Electric Golf Cart Repair, both as an eBook and in Hardcopy:
Book: Ronald L Staley: 9780578560557: Amazon.com: Books
Gas Golf Cart Repair, both as an eBook and in Hardcopy:Gas Golf Cart Repair Book: Ron Staley: 9798987911303: Amazon.com: Books