I bought a new bike specifically for my conversion, so there was a lot to choose from and no repairs or alterations were needed. Most electric bike shops are willing to talk about the process of electric conversion. East Coast Alpine, in Danvers, Massachusetts, helped us find a bike that would be a good fit. Many bike shops are beginning to add electric bikes to their product offerings, so it’s something they’re embracing.
Motors. The most common type of bike conversion motor is the hub-integrated motor, built into the wheel—there’s no drive train to deal with and no radical modification of the frame required. Generally, you buy the motor and wheel as a prebuilt unit. They are built to accommodate the drive gears of your pedal-bike, and its braking system. You simply place the order and specify what size and type of wheel you need, and the motor/wheel comes complete. Installing it is simply a matter of mounting the tube and tire on your new wheel, and replacing the original wheel with the powered one.
There are two basic types of integrated hub motors: geared, and direct drive. The geared motors are smaller and lighter, and they offer less resistance when you’re pedaling. The direct drive motors are bigger, and can be more powerful, but are heavy and have more pedaling resistance. If you’re going for a power-assist strategy, where the motor is there to help you out, the slightly more expensive geared motors are the way to go. If you’re looking for a high-powered ride that you’re probably not going to pedal much, one (or two) of the larger direct-drive motors will give your bike more of a motor-scooter/moped feel.
There are some motors that ride “outboard” of the wheel, but they’re fading from the market. These also require a special wheel, with a separate drive gear for the motor on the bike hub, and are generally cheaper than a hub motor. They’re bulkier, not as powerful, and add weight on the left side of the bike, not the center. With the progress being made in hub-motor development, they’re not as attractive an option for a daily ride.
Motors’ maximum speed is determined by the voltage—the higher the voltage, the faster the motor will spin. A 48-volt motor will have a higher top speed than a 36 V motor (typical voltages for electric bike kits), but a 48 V motor needs more battery cells to produce that speed. If you’re looking for a simple assist, 36 V is fine; if you’re looking for scooter-like performance, you’re going to want a 48 V kit.
The second critical rating is the motor’s power capacity, rated in watts. This will tell you how much weight it can pull and give you a pretty good idea of top speed and acceleration. The smallest motors you’ll see on an electric bike are around 250 W, and will give you a gentle assist. More typically, you’ll see a 300 to 350 W motor. The biggest hub motors are in the 1,000 W (1 kW) range, giving them a performance more like a true moped or motor-scooter.
When making the motor choice, you can choose rear or front mount. Typically, you want to mount the motor on the rear wheel to maintain the balance of the bike. The only time you’d stray from that is if you’re running a very small motor, or your bike won’t accept a rear wheel replacement. For instance, some bikes are equipped with 3- or 5-speed rear hub gears. In that case, if you put in a rear hub motor, you’d lose your gearing.
Brakes can also be a complication. Most hub kits have options for wheel-caliper or disk brakes, but keep this in mind when you’re making your decision—make sure your brakes are compatible. Almost all hub motors will fit in the axle space on a standard frame (100 mm front, 135 mm rear).
Batteries. Batteries are the oomph behind the motor, possibly the single-most important factor in the bike’s performance and cost. Two basic choices are available—lead-acid (L-A) and lithium-ion (Li-ion), a term covering a range of lithium-based chemistry. For the L-A choice, sealed batteries are usually used—they don’t need maintenance and they won’t leak if mounted on their side or tipped over. They’re low-cost but heavy for the amount of energy they provide. NiMH (nickel metal-hydride) is a choice you may see on occasion. Their weight-to-capacity ratio is somewhere between L-A and Li-ion; they were a common option a few years ago before Li-ion technology was readily available.
For the same capacity (rated in amp-hours, Ah), Li-ion battery technology is usually half the weight of a L-A battery—but they’re typically about twice the cost. Li batteries have greater longevity, so they won’t need to be replaced as often, and the lithium is not as toxic as lead, although a little more difficult to recycle. You’ll want a battery that provides 10+ Ah, no matter which type you choose.
When weight-saving is important, Li-ion is clearly the best choice if it fits your budget. The good news is that the system is completely “battery agnostic”—that is, you can switch battery types at any point without having to make major changes in the system or design. You can add more batteries, you can change battery type, you can even switch between several types of packs as easily as you can remove and replace the pack, as long as the voltage is the same—for most systems, either 36 V or 48 V. It’s not at all unusual for people to start off with L-A packs and then upgrade to Li-ion when it is time to replace the pack.