1. Rear dropout spacing: the measurement that decides everything
Take the rear wheel out and measure between the inside faces of the rear dropouts, where the hub sits. Nearly every rear hub kit sold for conversions is built for 135 mm quick-release spacing, which is what mountain bikes used for roughly two decades and what makes an older hardtail such a good donor.
If you measure something else, note it before you shop. Modern frames use a thru-axle rear end — the axle threads into the frame rather than being clamped by a skewer — and the common widths are 142 mm and the wider 148 mm “Boost” standard. A 135 mm quick-release motor does not drop into either without an adapter, and the adapters that exist are not something we would trust under a 750 W motor on a trail. Fat bikes are wider again, and the fat kit here is the 175 mm answer.
Do not spread or squeeze an aluminium frame to make a motor fit. Steel frames tolerate a small amount of cold-setting in a frame shop; aluminium does not, and a cracked dropout on a 70 lb bike at 20 mph is not a theoretical risk. If the spacing does not match, the honest options are a different kit, a different donor, or a mid-drive.
2. Dropout material and axle type: where the torque arm comes in
A hub motor does not just spin the wheel; it tries to spin itself inside the dropout, and the reaction force wants to rotate the axle out of the slot. Steel dropouts resist that reasonably well. Aluminium dropouts chew out, and when they do, the axle climbs out of the dropout under power.
That is what a torque armprevents: a steel bracket that clamps to the axle and bolts or clamps to the frame, so the reaction load goes into the frame instead of into the dropout faces. On a 750 W kit, fitting one is not optional, and on an aluminium frame we would fit two — one each side — because a single arm still lets the axle work against one dropout. Most kits include something nominally for this purpose; the thin stamped ones are the first thing we would upgrade.
The related warning: do not put a hub motor in a suspension fork. The dropouts on a suspension fork are not designed for drive torque, they are thinner than a rear dropout, and the failure mode is the wheel leaving the bike. A rigid steel fork can take a front hub with proper torque arms. On a mountain bike, which almost certainly has a suspension fork, the answer is a rear hub or a mid-drive.
3. Brakes: disc, rotor pattern and whether rim brakes are enough
The kit includes a laced wheel, so its brake interface has to match yours. Check two things: whether your bike uses rim or disc brakes, and if disc, whether the rotor mounts with six bolts or a centerlock ring. Six-bolt is the pattern conversion wheels overwhelmingly use, and a centerlock rotor will not bolt to one — you would need a new rotor rather than a new wheel.
Rim brakes deserve a straight answer: they work, and they are marginal for a converted bike. You are adding roughly 20 lb of motor and battery, raising sustained speed, and asking a brake pad to clamp a wet aluminium rim rather than a steel rotor. A mid-1990s cantilever-brake mountain bike is the donor we would most strongly steer people away from. Our brakes guide explains what the different systems actually deliver in a panic stop, and it is worth reading before you convert anything with rim brakes.
4. Wheel size and what the freehub takes
Mountain bikes come in 26-inch, 27.5-inch and 29-inch wheels, and the kit’s wheel must match your tire and your frame. The Bafang kit here covers 20, 26, 27.5, 700C, 28 and 29-inch fitments, which is most of what you will encounter; the fat kit is 20 x 4 only.
The detail that catches people is the drivetrain interface. Older bikes use a freewheel— the ratchet is part of the sprocket cluster — and newer ones use a cassette on a freehub body. A motor built for one will not take the other. The fat kit here specifically states cassette compatibility, which is why your existing gearing carries over on that one. If your donor has an 8-, 9- or 10-speed cassette, check the kit states a cassette freehub before ordering, and if it has a 7-speed freewheel, check for that instead. And measure your chainline: a motor hub is wider than a normal hub, and a badly-dished wheel makes the small cogs unusable.
5. Bottom-bracket shell, if you are thinking mid-drive
A mid-drive motor replaces the bottom bracket and drives the cranks, which means it uses your existing gears. On a mountain bike that is genuinely appealing: the torque goes through the chainring, so a low gear multiplies motor torque the way it multiplies yours, which is exactly what you want on a steep loose climb. Hub motors cannot do that — their torque is fixed at the wheel no matter what gear you are in. Our hub versus mid-drive guide covers the trade-off properly.
The measurement is the shell width — typically 68 or 73 mm on a threaded mountain bike bottom bracket — plus whether the shell is threaded at all, because press-fit standards exist and complicate things. Be aware of the honest downsides before you go this way: mid-drives cost more, they put all the motor’s torque through a bicycle chain and cassette that will wear visibly faster, and the kits we currently list and can price are rear hub kits, not mid-drives. We are not going to recommend a mid-drive we have not researched.
6. Frame triangle: where the battery actually goes
This is the measurement people take last and should take first. A 48 V Hailong-style pack is a physically large object that bolts to a rail on the down tube, or straps to it. Measure the inside of your main triangle, and check whether you have bottle-cage bosses on the down tube — many 1990s frames have them, many modern ones put them somewhere awkward, and full-suspension frames often have no usable space at all because the shock is in it.
That is the real reason full-suspension conversions are a bad idea more often than not: not that the motor cannot work, but that the only place left for a 10 lb battery is a rear rack, which puts the heaviest single component high and behind the rear axle. The bike handles like a pendulum, and on a mountain bike that is being ridden off pavement it is worse than it sounds. A hardtail with a big open triangle is the frame you want.
What the whole job costs and takes
Three purchases, not one: the kit, the battery, and a charger if the pack does not come with one. The battery is normally the most expensive line, which is why kit-only listings look so cheap — our kits-with-a-battery page is the bundle version of this and covers what has to match between the two. Add a torque arm pair, a connector adapter you will probably need, and consumables: new brake pads, a chain, and cable ties.
Time: an afternoon if the measurements were right, a weekend if you are also replacing brake pads and re-routing cables, and considerably longer if you discover the spacing problem with the wheel in your hand. The fiddly parts are not the motor — that is a wheel swap — they are the pedal-assist sensor on the bottom bracket, the brake cut-off sensors, and tidying 2 m of wiring so nothing rubs. Our assembly guide covers the torque checks that apply to any newly-built e-bike, and they matter more on one you built yourself.
Is it legal once it is done?
It can be, and the law does not care who assembled it. The federal definition used in the National Park Service rule is a cycle with fully operable pedals and a motor of not more than 750 watts, inside a three-class system based on assisted speed. Both kits here switch between 500 W and 750 W, so a conversion can sit legitimately inside that definition. Configure the controller for more power or more speed and you have built something the three-class system does not cover, which is where trail access and insurance problems begin. Our classes and laws guide covers the state-level detail.
One thing a conversion can never have: a UL 2849 listing. That standard certifies a complete electrical system as built by a manufacturer, and on your bike that manufacturer is you. You can still buy a pack that publishes UL 2271, and you should prefer one that does. The CPSC’s charging guidance applies with extra force to a self-assembled system: charge on a hard surface, never unattended, never in the path you would use to get out of a bedroom.
When we would tell you not to bother
If the donor is a supermarket mountain bike, stop. A conversion costs the same whether the frame underneath is good or bad, and spending it on a bike with stamped dropouts, rim brakes and a bent derailleur hanger buys you an unsafe heavy bicycle. If the donor is a modern Boost thru-axle trail bike you actually ride, also stop — you will spend more than a complete value e-bike costs to make a worse mountain bike.
The conversion is worth it when the frame is genuinely good and you want that bike electrified: a steel tourer, a beloved hardtail, a cargo bike, a bike that fits you unusually well. Our is-a-kit-worth-it guide does the arithmetic against buying a complete bike, and our under-$1,000 roundup is the honest comparison point.
