How we picked
Fitment first, because it is what returns a kit: wheel diameter, rear dropout spacing and brake type have to match your donor bike exactly. Motor typesecond — a geared hub freewheels when you stop pedaling, which matters enormously on a bike you will sometimes ride with the power off. Support third: Bafang parts are the most widely available in this category, which is worth something in three years when a controller dies. We have not ridden every one of these; this is exactly what we do instead.
The real cost of a conversion
Here is the arithmetic nobody puts on the product page. A kit at roughly $165–$300 buys you a motor in a wheel, a controller, a display and usually a pedal-assist sensor and brake cutoff levers. It does not buy you the battery.
A 48 V pack with meaningful capacity is a substantial second purchase, and it is the part where cutting corners is genuinely dangerous rather than merely disappointing — see our battery roundup for what to insist on and our charger guide for matching voltage and connector safely. Add a suitable charger, possibly a new chain, and your own labor.
Once you total that, a conversion often lands close to the price of a complete budget e-bike like the ones in our under $1,000 roundup. The conversion still wins when the donor bike is genuinely better than a budget e-bike frame — a decent hybrid with good brakes and a fitted geometry — and loses when the donor is a supermarket bike that was never worth much to begin with.
Will it fit your bike?
Four checks, in order. Get these wrong and the kit goes back.
- Wheel size. The kit includes a wheel, so it must match your tire and your brake type. 26 in, 27.5 in and 700C are the common ones; fat bikes at 20 x 4.0 need a fat-specific kit.
- Rear dropout spacing.Measure between the inside faces of the rear dropouts. Standard bikes are typically 135 mm; fat bikes are often 170 or 175 mm. A motor built for the wrong spacing will not go in without spreading the frame, which you should not do.
- Brakes. Disc brake mounts have to line up, and rim brakes on a converted bike are marginal for the extra weight and speed. Read our brakes guide before converting a rim-brake bike.
- Frame material and dropouts.Aluminum dropouts with a powerful hub motor need a torque arm — a bracket that stops the axle spinning in the dropout. Fitting one is not optional on a 750 W kit.
Geared or direct drive?
Geared hub motors(like the Varstrom kit here) contain a reduction gearset and a freewheel. They are lighter, they produce more low-speed torque per watt, and crucially they spin freely when the power is off — so a flat battery leaves you with a bicycle rather than an anchor.
Direct-drive motors have no gears and no freewheel. They are quieter, essentially maintenance-free, and better at sustained high speed, but heavier and with noticeable drag unpowered. For a conversion, where riding unpowered is a real scenario, geared is usually the right answer.
The legal part
A converted bike is still subject to e-bike law, and the three-class system cares about assisted speed and throttle, not about who built it. A 750 W kit that assists to 28 mph makes your bicycle a Class 3 e-bike where that class exists, with the path restrictions and, in some states, age and helmet requirements that come with it.
Two practical consequences. Set the controller’s speed limit to the class you intend to ride, rather than to the maximum the kit allows — our classes and laws guide explains what each class permits. And check with your insurer before you convert: a home-built e-bike can sit outside the wording that would have covered a factory-built one, which our insurance guide goes into.
