What's best, front hub motor, mid-drive motor or rear hub motor?
Posted on May 30 2016
Where's the best place for the motor on an e-bike?
With more and more electric bicycles on the market in Australia, we're seeing more examples of the various places a motor can be added to a bicycle. This article is about the advantages or otherwise of the three main motor placements on an electric bicycle. It is about 'complete electric bicycles', as opposed to electric bike conversion kits, which have slightly different criteria though a similar conclusion. Friction drives are ignored here, as are kickstarter/crowd-funded projects.
Spoiler alert - there's no particular 'absolute best' place for a motor and there's no ground-breaking technical information in this article. The usual common sense approach applies for anyone considering buying any given electric bicycle and it applies equally for battery placement, battery size, type of gears, frame type etc:
1 -Do you feel confident when leaving the house on this e-bike, that you'll have an enjoyable and safe ride that will make you want to use it next time you leave the house?
2 - Do you feel comfortable riding this electric bicycle?
3 - Can the electric bicycle go up hills with an amount of assistance sufficient to your needs/desires?
4 - Can someone service this electric bicycle for you, are spare parts available now and will spare batteries be available in 5 years?
If the e-bike you're looking at satisfies those 4 points above, then who really cares where the motor is? However, if you can't happily answer yes to those questions above, then there's probably a reason for that and motor placement might be one of them. Let's dive into the different motor types. They all have their pros and cons, so the way to make this article useful is to consider each pro and con and ask yourself "how well does this benefit match my riding/lifestyle" and "how much will this 'con' affect me".
Front Hub Motor - the sensible option
This is one of the simplest implementations of the electric bicycle. The motor is the hub of the front wheel, it turns the wheel over which then makes the bicycle move.
Advantages: Simple is good. This is very low maintenance as it shares the strain on the bike between the front wheel (motor power) and rear wheel (human power and rider weight). It also physically separates the motor system from the rest of the bicycle parts, making your bike easier to service and spare parts more aligned with conventional bike parts. Allows any type of gearing in the rear wheel, such as internal hub gears. No matter where the battery goes, you'll end up with a well balanced and very stable bike.
Disadvantages: Front end of the bicycle becomes heavier, reducing choice of front forks and making it a little harder to lift up a curb. Traction is an issue on very steep tracks with loose surface. Quick release front wheel not an option. Above a certain power level, front hub motor becomes unviable for various reasons. However, this article is not interested in e-bikes of that level of power and speed, it is focused on legal electric bikes.
Best suits City Riders. So what kind of rider or what kind of bike best matches up with the advantages on offer? A low maintenance and simple e-bike with good balance and internal hub gears is going to best suit 'everyday riders' who ride to get around the city with the occasional rail trail and just want a comfortable, safe and reliable e-bike. These riders don't suffer much from the disadvantages either, being mainly on paved roads. For this person, some of the disadvantages are actually advantages - no quick release front wheel means less theft risk. And a heavy front end on a bike with 'upright' seating position isn't a problem while riding and actually adds to stability. Another 'sweet spot' for these e-bikes is for commercial and fleet use, when predictable maintenance requirements and costs are paramount to the fleet's success.
Least Suits MTB Riders. Which kind of rider would not benefit from these advantages and would suffer most from its disadvantages? A mountain-biker. They are always riding on loose surfaces, their bikes are not 'upright', choice of high tech front forks is essential and a heavy front end when jumping over tree-roots etc isn't going to work. Low maintenance doesn't appeal to them either, they won't want internal hub gears and with the rates of crashing their bikes, a trip to the shop every now and then is normal.
On the fence - Touring Riders. Electric bicycle touring riders are in between. They will encounter a variety of riding conditions where a heavy front end might be inconvenient. On the other hand, low maintenance and simple is paramount for long distance riders. The decision will be based on the likely terrain and will be some sort of compromise. I used an eZee Torq with front hub motor for the Perth-Sydney ride with mixed terrain and the eZee Forza with rear wheel drive for the Cape York-Sydney ride which was more heavily off road.
Example of a perfect fit - the eZee Sprint 7 step through.
Rear Hub Motor - the Fun option
In reality there isn't a huge difference between a front hub or rear hub motor electric bicycle. Both have similar performance, the motor has similar design constraints and both are very efficient in propelling the bicycle without a drive chain (direct to the spokes). The real consideration for the buyer is a trade-off in maintenance vs the feel of the ride.
Above a certain power level, a rear hub motor is the easiest way to pack in a lot of power relatively safely without damaging the frame or causing the bike to slide out around corners. For this reason, electric motor bikes like the Stealth Bikes use rear hub motors. This is not particularly relevant to the kind of electric bicycles we sell (road legal ones).
Advantages - Light and maneuverable front end, allowing any type of front fork. All your weight is on the motor, giving it maximum traction even in adverse conditions. Aesthetic advantage of being able to conceal the motor among the gears, panniers bags etc. Even when the same hub is used, noise levels are lower on a rear hub, due to the extra weight on the wheel minimising vibrations and resonance through other parts.
Disadvantages - Choice of pedalling gears are restricted. In general, derailleur gears are the only option, and not internal hub gears. Rear wheel and spokes under enormous stress - your weight, your pedalling power, the motor's power, the motor's weight and even the dimension of the motor make for a more difficult wheel build than a conventional bicycle hub. It is not unusual for electric bicycle with rear hub motors to break spokes regularly.
Best Suits: Riders who will often go off-road on steep/loose tracks. Also city riders who are happy to prioritise the feel of the ride and accept higher maintenance on the rear wheel and weren't going to choose internal hub gears in any case. It is best suited on high-end e-bikes where the rear wheels are built with high quality components and to a high standard of manufacture.
Least Suits: Any owner for whom maintenance is a priority including fleet operators.The combination of more motor wheel maintenance, plus no option for internal hub gears will mean overall increased maintenance. Note that rear wheel drive bikes are easier to sell in general, so you see more of them. Unfortunately this means there are a lot of cheaper electric bikes with rear hub motors where it's only a matter of time before the spokes start breaking and expensive and lengthy work is required. The heavier the motor and the heavier the rider, the more of a problem it will be.
Mid-Drive Motors - the 'have it all (almost)' option
Mid-drive motor systems on electric bicycles from German manufacturers Bosch and Impulse have started to become more common in the Australian and NZ market. This option is quite different to the hub motor setup. The mid-drive motor pulls on the chain, just as your pedalling power does. This then turns the rear wheel and away you go. The motor's drive goes through the bicycles gears, so the motor is effectively geared too.
Advantages: Allows both wheels to be 'normal' bicycle wheels, giving all the available options of quick release, internal hub gears etc. Front forks can be any time and both front and rear end of bike is kept light. Centre of gravity is low and the whole e-bike is usually a little lighter too. The drive is via the rear wheel, so traction is not an issue. Rear wheel spokes are not stressed as much as on a rear hub motor setup because the motor's weight is not back there and the wheel build is not affected. Motor is geared, so going slow and steady up very steep hills is achievable.
Disadvantages: More complicated system, so motors are usually more expensive and the proliferation of sensors in the bike adds to possible failure modes. The drive-chain is under increased stress (human power + motor power through the one chain), so the chain, gears and rear hub will need to be maintained and replaced more frequently.
Grey Area: There's a lot of aspects on a mid-drive e-bike that are either advantages or disadvantages, depending on what you want from an electric bicycle. Make sure you try the bikes out and think about the 4 questions at the beginning of this article and make your choice. Are the following good or bad things for you?
- Motor replaces bottom bracket, providing an opportunity to measure the rider's effort on the pedals and tuning the electric drive system to respond accordingly, making for a ride that feels more like riding a normal bike. This also means it forces you to do more work
- The motor is going through the gears, so you can change gear to keep the motor in its most efficient mode. But this also means you have to be in the right gear for the motor whenever you change speed/hills. More to do/learn
- At the moment (and this will change), most of these bikes are coming from Europe. This gives access to e-bikes of a higher standard of manufacture and finish than what we are often used to down under. This also means more scarce or expensive spare parts and usually the bikes are speed limited to 25kph and they usually don't have a throttle.
- Longer range (km) per unit of battery capacity. For example a Focus Aventura e-bike with 17Ah battery will likely go further than a hub motor bike of the same battery capacity. But this also means that you're doing more of the work which might be good or might be a drag.
Best Suits: Hands down the best system for expert MTB riding. Rear wheel traction but with good centre of gravity, lightweight wheels and any suspension option is possible. Also a good compromise for electric bicycle touring. Has rear drive traction, not too many rear spoke problems, often easier to ride unassisted than a hub motor e-bike. Not as low maintenance as a front hub motor but by carrying some spare chains with you and careful cleaning/maintenance it is completely manageable.
Also great for riders who want an electric bicycle to feel like their normal bicycle but with less hills, or perhaps less a few decades - still an effort to get around, but not painful. With this they also get the benefits of longer range and a lighter e-bike, often with a higher standard of finish/manufacture.
Least Suits: Riders who want as much help on the hills as possible and a throttle to get off at a start. This is particularly an issue for electric cargo bikes where the total weight of the bike + gear is higher and the size of the bike means that taking off in a straight line is so much easier with a throttle. Anyone living with M.S., Polio, Chronic Fatigue, Prosthetics and other conditions that affect balance and muscular control in the legs and core. Applications where budget for maintaining and replacing chains, cassettes, rear hubs etc is limited.