The Best Ebike Conversion Kits, and How to Pick One

A conversion kit is the only way to get an electric bike in a shape nobody manufactures. If you own a steel tourer set up exactly right, a cargo bike you have already fitted to your life, or a frame with geometry no ebike brand offers, converting it is a genuinely good idea.

What it is not is a cheap route to an ebike. By the time a decent kit and a battery with real cells are on the bike, you have spent complete-ebike money on a machine with no warranty and no system-level safety certification. That trade is worth making for the right reasons, and the reasons are about the bike you keep, not the money you save.

Kit selection is also mostly decided by your frame rather than your preferences. Three measurements and a look at your dropouts will eliminate most of the catalogue before you have compared a single spec sheet, so start there.

Fit checks come first, and they eliminate most kits

Every disappointing conversion story starts the same way: the kit arrived, and it did not fit. These checks take fifteen minutes with a ruler and save the restocking fee.

For a mid-drive kit

• Bottom bracket type. You need a threaded BSA shell. Press-fit standards, BB30, and most modern carbon frames are out entirely. Look for a shell with visible threads once the cranks and cups are removed.
• Shell width. Measure across the shell face to face. 68 mm and 73 mm are the standard road and mountain widths and are what the common kits target. Fat bike frames run 100 mm or 120 mm and need the specific wide variant.
• Chainstay clearance. The motor body sits to the drive side of the shell and needs room. Frames with chainstays that flare early, or with a heavily shaped yoke, can foul the motor housing. This is the check that catches people after the kit has already shipped.
• Front derailleur. You will lose it. The kit brings its own chainring on its own spindle.

For a hub kit

• Dropout spacing. 100 mm front and 135 or 142 mm rear covers most bikes. Thru-axle frames generally will not take a standard nutted hub motor axle.
• Dropout material. Steel tolerates motor reaction torque. Thin aluminum can round out, letting the axle spin. Aluminum is workable with a proper torque arm and correct torque on the axle nuts; carbon forks are not workable at all.
• Freewheel or cassette. Budget rear kits often ship with a threaded freewheel body limited to six or seven speeds, which may not match your shifter.
• Brakes. Adding twenty pounds and eight miles per hour to a bike with rim brakes teaches an expensive lesson about fade. Our guide to ebike brakes covers what the extra mass demands.

Mid-drive kit or hub kit

The mechanics of this choice are covered properly in mid-drive vs hub motor, and the short version applies to kits exactly as it applies to complete bikes: a mid-drive sends power through your gears, so the bike multiplies its torque on climbs and lets the motor spin near its efficient rpm. A hub motor drives the wheel directly at a fixed ratio and heats up when it is forced to turn slowly.

What differs with kits is the installation gap. A hub kit is genuinely easier. The motor arrives already laced into a wheel, so the mechanical work is swapping a wheel, clamping a cadence sensor, and routing wires. A competent home mechanic can do it in an afternoon.

A mid-drive kit involves removing your cranks and bottom bracket, which needs specific tools, and then living with the consequences: your chain and cassette now carry motor torque as well as leg torque, and wear roughly doubles. Budget for chains at somewhere around 1,000 to 1,500 miles rather than 2,000 to 3,000.

The kits worth shortlisting

The BBS02B is the default recommendation because of its ecosystem rather than its specification. It has been sold in volume for over a decade, which means every failure mode is documented, replacement nylon reduction gears and controllers are available from multiple sellers, and a programming cable lets you adjust current limits and assist levels yourself. Bafang rates it at 750W nominal with a controller in the region of 25A. Through a low gear that is more than enough for loaded climbing.

Its weakness is the assist feel. It reads cadence, not effort, so power arrives at whatever level you selected regardless of how hard you are pushing, with a noticeable lag off the line. See torque sensor vs cadence sensor for why that matters more than most spec comparisons suggest.

The BBSHD is the heavier-duty sibling: more current, sturdier internals, and a better tolerance for sustained abuse. It earns its place on cargo bikes, on genuinely steep sustained climbs, and under heavy riders. On an ordinary commuter it mostly adds weight and cost, and in most US states a 1000W motor puts the bike outside the legal definition of an electric bicycle entirely.

The TSDZ2 is the only widely available kit that measures how hard you are pedalling rather than whether you are pedalling. That single difference makes the bike feel like a bicycle with strong legs instead of a scooter with pedals, and it improves range, because you are not commanding full assist while soft-pedalling. The trade is build quality: the internals are less robust than Bafang's and the stock firmware is mediocre, which is why a community-maintained open-source firmware exists and is widely used. Choose it if ride feel is the point and you are comfortable being your own support.

Generic geared rear hub kits from established sellers are the cheapest credible way in, and on flat to rolling terrain they are entirely sufficient. The motor is sealed, wears nothing, and leaves your drivetrain alone. Buy the version with a cassette body rather than a freewheel if your bike has more than seven speeds, and fit a torque arm.

Systems in this category, of which Swytch is the best known, trade power and range for weight and simplicity. Assist is modest and range is short, but the bike stays close to its original weight and handling, and the battery comes off in seconds. On a folding bike or a light commuter where the whole point is portability, this is the only kit category that does not undermine the bike. Newer designs that drive the rear disc rotor rather than the hub, like Skarper's, target the same audience with even faster removal.

The battery is half the budget, and all of the risk

Kit listings advertise motors, because watts are easy to compare. The battery is the part that determines range, whether the motor can actually draw its rated current, and whether the bike is safe in your hallway.

The specification that matters most is not capacity. It is the continuous discharge rating of the battery management system. A BBSHD asking for 30A from a pack whose BMS trips at 25A gives you a bike that cuts out at the bottom of every hill. Match the BMS to the controller's peak draw with headroom, then worry about watt hours. This is covered in detail in choosing a battery for a conversion kit, and the underlying voltage and capacity arithmetic is in 36V vs 48V vs 52V.

Buy packs built with cells from Samsung, LG, Panasonic, or Molicel from a seller who names the cell. Unnamed cells at half the price are the single worst economy in this hobby, and battery fires in converted bikes are overwhelmingly a cell-quality and charging-practice problem rather than a motor problem.

What the cheapest kits cut

Sub-$250 complete kits exist and they are cheap for identifiable reasons, not because someone found an efficiency.

• The controller. Undersized FETs, no thermal rollback, and current limits that are optimistic rather than measured. Failures are usually the controller rather than the motor, which is at least the cheaper part to replace. See what an ebike controller does.
• The wheel build. A hub motor laced by machine into a low-end rim with inconsistent spoke tension will go out of true quickly under motor torque.
• The connectors. Unbranded bullet connectors that corrode, and a loom with no strain relief where it exits the axle.
• The brake cutoffs. Sometimes omitted entirely. A kit that keeps driving while you brake is a genuine hazard, not an inconvenience.
• The battery. Recovered or unbranded cells with a BMS that reports capacity it cannot deliver.

Where a kit bike stands legally

Fitting a kit does not create a new legal category. A converted bike is judged by the same motor output and assisted speed limits as a factory ebike, and in most states that means 750W and 20 or 28 miles per hour depending on class. A 1000W kit takes the bike outside the definition of an electric bicycle in most of the country, whatever the seller's listing implies.

Two practical consequences follow. Trail and path access is decided by class, and an uncertified kit bike has no class label to point at, so assume the strictest interpretation where it is enforced. Insurance and liability also get harder to argue if the bike exceeds the statutory limits. Ebike classes explained covers the framework, and street legal electric bikes covers what happens above it.

Which kit for which bike

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If you are still weighing this against buying finished, price the whole build honestly, battery included, and compare it against complete ebikes under $1,000 and under $2,000. Then, once the kit is chosen, work through how to convert a bike to electric before you order, because the tool list is longer than most kit listings admit.

Frequently asked questions

Sources and further reading

• Bafang drive systemsBafang
• Electric bike laws by statePeopleForBikes
• 15 U.S.C. 2085: Low-speed electric bicyclesLegal Information Institute, Cornell Law School
• Lithium-ion battery safetyNational Fire Protection Association