How to Choose the Right E-Bike Conversion Kit for Your Bike

Byhenry

In this hub: Conversion Kits & DIY Hub — browse the recommended reading order.

Buying a conversion kit before measuring your bike is how people end up with an expensive box of parts that “almost fits.” This guide is a practical checklist + decision tool so you can pick a kit that fits your frame, matches your riding, and stays on the right side of local rules.

Quick decision rules

1) Mostly flat + easiest install? Choose a hub kit.

2) Hills/cargo + best ride feel? Choose a mid‑drive.

3) Want the smoothest pedal assist? Look for torque sensing (often mid‑drive).

Step 1: Measure your bike (5 minutes, saves hours)

For hub motor kits (wheel-based)

Measure:

  • Wheel size: 26 / 27.5 / 29 / 700C
  • Dropout spacing: rear usually 135mm (common), but bikes vary
  • Axle type: quick release vs thru-axle (many hub kits prefer QR)
  • Brake type: rim vs disc; rotor standard (6‑bolt vs centerlock)

For mid‑drive kits (bottom bracket-based)

Measure:

  • Bottom bracket shell width: commonly 68mm or 73mm
  • Bottom bracket type: threaded vs press-fit
  • Chainstay clearance: fat chainstays can affect chainline

If you’re not sure, snap a photo of your BB area and look up your bike’s spec sheet—it’s faster than guessing.

Step 2: Choose hub vs mid‑drive (honest pros/cons)

| Feature | Hub kit | Mid‑drive kit |

|—|—|—|

| Install difficulty | Easier | More involved |

| Hill climbing | Good (rear hub) | Excellent |

| Ride feel | “Motor pushes wheel” | “Legs feel stronger” |

| Drivetrain wear | Low | Higher |

| Best for cargo/steep terrain | OK | Strong |

Shortcut:

  • Flat commuters → hub (rear hub if you want better traction)
  • Hills/cargo → mid‑drive

Step 3: Pick power + speed that fits your use (US context)

Many US discussions reference the Class 1/2/3 framework and motors under 750W. PeopleForBikes class overview

Practical guidance:

  • 250–500W: typical commuting and errands
  • 750W: steep hills/heavier riders (but double-check local rules)

If you want to avoid legal headaches, aim for “normal e‑bike behavior” (20 mph assist for many situations; 28 mph pedal-assist if your area allows Class 3). Class definitions

Step 4: Battery selection (range math you can actually use)

Estimate the watt-hours you need

  • Light assist: 10–20 Wh/mile
  • Heavy/hilly/faster: 20–30 Wh/mile

Then:

Target Wh = miles × Wh/mile × 1.3–1.5 buffer

Example:

  • 20 miles × 20 Wh/mile × 1.5 ≈ 600Wh

Safety and charging

Follow CPSC’s micromobility charging recommendations: be present while charging, use the supplied charger, and avoid modified or “reworked” packs. CPSC charging safety

Also pay attention to certification language and consensus standards in the market (UL references are common). UL standards overview

Step 5: Match the kit to your bike’s hardware

Drivetrain compatibility (hub kits)

  • Freewheel vs cassette: verify what your bike uses and what the hub motor accepts.
  • Gear count: don’t assume 7/8/9/10/11-speed compatibility.

Brake compatibility

  • Rim brake bikes: hub kits are often simplest.
  • Disc brake bikes: verify rotor mount and caliper clearance.

Frame + mounting

Battery mounting is where conversions get “real”:

  • Best: triangle-mounted frame battery (stable handling)
  • OK: rear rack battery (more wobble, check rack rating)
  • Last resort: frame bag battery (watch heat + strap pressure)

Step 6: Decide DIY vs shop install (and be realistic)

DIY is totally doable if you can:

  • remove/install wheels,
  • route cables neatly,
  • and follow torque specs.

Mid‑drives often justify a shop check if:

  • you’ve never removed a bottom bracket,
  • your bike is expensive,
  • or you’re not sure about chainline/brake setup.

“Buy this / avoid this” checklist

Usually a good sign

  • Clear specs: wheel/BB sizes, voltage, controller limits
  • Sealed connectors + decent strain relief
  • Real documentation + wiring diagrams
  • Support for replacement sensors/displays

Usually a bad sign

  • Battery with vague specs or no credible safety information
  • No torque arm included for front hub kits
  • Listings that hide the hub spacing or BB type
  • “Too good to be true” pricing

CPSC has issued warnings about certain battery products associated with fire hazards—so battery sourcing matters. CPSC warning example

A more detailed “fit check” (so you don’t miss the annoying stuff)

Wheel size (hub kits): measure, don’t guess

  • Look at the tire sidewall (e.g., 26×2.1, 700×35C).
  • If it’s worn off, measure rim diameter or search your bike model.

Why it matters: the motor is laced into a specific rim size. Wrong size = wrong geometry and brake alignment.

Dropout spacing (rear hub): quick measuring trick

Use a ruler or calipers:

  • Measure the inside distance between rear dropouts.
  • Common is 135mm, but bikes vary.

Why it matters: too wide/too narrow and the wheel won’t seat correctly.

Axle style: QR vs thru-axle

A lot of mainstream hub kits are designed around quick-release-style frames.

If you have a thru-axle bike, you’ll need a kit specifically designed for it (or adaptors that actually fit).

Bottom bracket (mid‑drive): the three things you must know

  1. Width (68/73mm common)
  2. Type (threaded vs press-fit)
  3. Clearance (chainstay shape affects chainline)

If you’re unsure, a bike shop can identify your BB type in minutes.

36V vs 48V (and why most commuters pick 48V)

36V can be great for gentle assistance and lower speeds.

48V is a common “sweet spot” for responsive power and better headroom on hills.

What matters is system matching:

  • battery voltage must match controller and motor limits
  • charger must match battery

If you’re building for hills, 48V often feels less “strained.”

Cadence sensor vs torque sensor (how it feels)

Cadence sensor (common on cheaper kits)

  • Feels like a “switch”: you start pedaling and power comes on
  • Can surge at low speed if not tuned well
  • Works fine for many commuters

Torque sensor (more natural, often pricier)

  • Feels like “your legs got stronger”
  • Better control on slippery climbs and in traffic
  • Often improves real-world range because it’s smoother

If you care about ride feel, torque sensing is one of the best upgrades.

Controller details that matter (but listings hide)

When comparing kits, look for:

  • Current limit (amps): affects acceleration and heat
  • Water resistance: sealed connectors are a big plus
  • Brake cutoff support: safer on higher-power setups
  • Display settings: wheel size input, assist levels, speed limit behavior

A good controller setup is the difference between “reliable commuter” and “random cut-outs.”

Compatibility matrix (quick reference)

| Your situation | Best starting point | Notes |

|—|—|—|

| Rim brakes + flat commute | Front or rear hub kit | Simplest install |

| Disc brakes + mixed hills | Rear hub kit | Check rotor standard + clearance |

| Steep hills + cargo | Mid‑drive | Budget for drivetrain wear |

| Thru-axle frame | Specific kit required | Don’t assume QR hub kits fit |

| Full suspension | Mid‑drive often easier | Battery mounting can be tricky |

After you buy: the 3 “first week” tasks

  • Re-torque hardware after 10–20 miles
  • Inspect cable rub points and protect with wrap
  • Confirm safe charging habits (don’t charge while sleeping; use correct charger) {md_link(‘CPSC’, sources[‘CPSC charging safety’])}

Doing these prevents the most common early failures.

FAQ

Hub or mid‑drive for hills?

Mid‑drive, almost every time—because it uses your gears.

What’s a beginner-friendly kit?

A pre‑built wheel rear hub kit with clear instructions and a reputable battery.

How do I avoid buying the wrong kit?

Measure first (wheel/BB), confirm brake/drivetrain type, and read the compatibility notes twice.

Related guides (same hub)

Official resources (worth bookmarking)

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