Bafang Motor Component Sizing Checklist: Hub Motors, Spiral Bevel Gears, and Linear Bearings

This checklist is for anyone who needs to order drivetrain or motion components and wants to avoid the classic 'but it said compatible' problem. I'm a quality and brand compliance manager at a motion components company. I review every motor, gear set, and bearing batch before it ships—roughly 200 unique items per quarter. In 2024, I rejected more first deliveries from unapproved suppliers than I'd like to admit, usually because the stamped code didn't match the purchase order.

This is not a 'best motor' article. It's a five-step pre-order checklist. You'll need a caliper, a flashlight, and about 20 minutes.

1. Pull the full motor model, not the marketing name

If you're buying a Bafang ebike hub motor, the marketing title can be misleading. The product page might say 'Bafang rear motor' or Bafang G020 hub motor, but the part number stamped on the side cover is what matters. The difference between one generation and the next often shows up as a suffix letter or a slightly different connector. I've seen that with a G020 unit: same color, same size, different phase wire order.

Before you order, do this:

• Find the stamped code on the side cover. It's often partly covered by paint or stickers. Clean it and take a photo.
• Send that photo to the vendor and ask, 'Does this version match your controller and torque arm?'
• If the listing only says 'Bafang compatible,' don't assume it means every Bafang hub motor.

2. Measure the frame dropouts before you measure the motor

People skip this because it isn't as exciting as the motor. It's also the fastest way to turn a $300 purchase into a return. For a hub motor, the axle has to fit the dropout slot. The key numbers are the axle thread size, the flat width, and the flat length. A motor can be listed as 'universal' and still not fit a particular frame.

Measure the inside distance of your dropouts and the slot width. If you're buying a Bafang G020 hub motor or a similar unit, ask for the axle diameter and flat length. Most sellers don't know your frame, so tell them the numbers you measured and ask them to confirm before shipping.

If the dropouts are aluminum and you're installing a hub motor with torque above the 'casual commuter' range, plan on a torque arm. That's not an optional upgrade.

3. Know your linear bearing sizes before you search for a rail

The search that often lands here is what size is lm8luu linear bearing. The answer: 8 mm bore, 15 mm outside diameter, 35 mm long. The standard LM8UU is 8 x 15 x 24 mm. The extra 11 mm in the L version gives more surface area and more load capacity, but it also means it needs a housing that can accept that length.

Some people order a set of LM8LUU bearings because the specs look better, then find they don't fit the carriage. Measure the housing pocket length first. If it's 24 mm, you need LM8UU. If it's 35 mm, you can use the long one.

A note on rail types: if someone says linear guide rail, ask whether they mean a round shaft or a profiled rail. A round shaft is measured by diameter. An 8 mm shaft matches LM8LUU. A profiled rail with a carriage doesn't use LM bearings at all. The carriage and rail are sold as a set or by a different code. Don't try to swap an LM bearing into a profiled rail.

4. Match spiral bevel gears as a pair

Spiral bevel gears are common in right-angle motor drives. They're quieter than straight bevel gears, but they're also more sensitive to setup. The contact pattern is ground into the pair, not into a single gear.

When you replace a spiral bevel gear, the safest move is to buy the pair. If you can't, request the spiral hand, tooth count, module or pitch, pressure angle, and mounting distance. I rejected a replacement gear last year because the contact pattern was off. On paper it was the same module and tooth count. In the test rig, it sounded like a small chainsaw.

5. Confirm the connector and controller protocol before you wire anything

Motor manufacturers change connectors without changing the motor's silhouette. I've handled Bafang hub motor units with at least three different connector generations. The first time, the vendor had sent a photo that looked like the same connector. It wasn't. We bench-tested it and caught it before the customer did.

The most frustrating part is that this is fully avoidable. You'd think a part number would be enough, but it isn't. Ask for the wiring pinout. If you're using a third-party controller, match the hall and phase sequence. If you get the order wrong, the motor will stutter or refuse to move.

At this point, connect the motor to the controller on the bench and run it through the full throttle range before installing it on the bike.

When this checklist is not for you

This level of checking makes sense in our shop because we order for customers and we can't afford a batch that doesn't work. If you're a hobbyist building one bike with a known good controller, you can cut some of the paperwork. If you're ordering a bare Bafang G020 hub motor for a customer that expects it to bolt on, you need the same diligence.

I can only speak to the small-batch and repair side of this business. A manufacturer doing 50,000 units a year needs a formal first-article inspection process. This checklist is not a substitute for that.

Common mistakes I check for on every order

• Relying on 'Bafang compatible' in a title. Always ask for the stamped code.
• Ordering an LM8LUU without measuring the housing length or shaft diameter. It is 8x15x35, but only if the rail is actually 8 mm.
• Confusing a round-rail linear guide with a profiled rail. They use different bearings.
• Replacing one gear in a spiral bevel set without a contact pattern check.
• Skipping the bench test. Connector problems show up immediately when the motor is unloaded.

As of January 2025, if the final e-bike is being sold in the EU, the whole e-bike needs to comply with EN 15194. That's not exactly a component spec, but it's a good reason to keep your model codes and drawings in the file.

Most of this checklist is just looking at physical parts and asking for documents. It's not elegant, but it's what keeps a purchase from the bafang-motor catalog from turning into a delay. If you're going through that catalog for a G020 unit, use the stamped code, not the thumbnail.