Measuring the rear dropout fork on an eBike means finding the inside spacing between the two frame slots where your rear wheel sits.
This measurement tells you whether a new wheel, hub motor, or axle will fit correctly. Getting it right prevents safety issues and makes upgrades smooth.
Key Summary:
To measure the rear dropout fork on an eBike, remove the rear wheel and measure the inside-to-inside distance between the frame slots with calipers or a tape measure. Record the spacing in millimeters and compare it to standard sizes like 130–135mm for commuter eBikes or 170–190mm for fat tire models.
What is a Rear Dropout?
A rear dropout is the open slot at the end of the bike frame where the rear wheel’s axle fits. The dropout spacing is measured in millimeters and determines what size hub or wheel can be installed.
On eBikes, this measurement is even more important because hub motors often come in fixed widths like 135mm or 190mm. If the spacing is off, the wheel won’t fit securely.
When I first started working on eBikes, I underestimated how big of a difference a few millimeters could make. I once tried to fit a 135mm hub into a 130mm frame and cracked the dropout after a week of riding.
That mistake taught me that measuring properly saves money and keeps your bike safe.
Tools You’ll Need
You don’t need many tools, but accuracy matters.
- Tape measure: Works if you’re careful, but not as precise.
- Digital calipers: Best choice for accurate millimeter readings.
- Bike stand (optional): Keeps the bike steady while you work.
- Basic wrenches or hex keys: To loosen axle nuts or bolts.
- Gloves: Protect your hands when handling the chain and wheel.
How to Measure Rear Dropout Fork on an eBike: Step-by-Step Guide
Here’s how to measure your dropout spacing the right way:
- Prepare the bike.
- Remove the rear wheel.
- Locate the dropouts.
- Measure inside-to-inside.
- Record the measurement.
Now let’s walk through each step in detail.
Step 1: Prepare the Bike
Put your eBike on a flat surface or in a bike stand if you have one. Shift the gears so the chain moves onto the smallest cog.
This reduces chain tension and makes removing the wheel easier. Stability is important here; you don’t want the frame rocking while you’re measuring.
Step 2: Remove the Rear Wheel
If your eBike has a hub motor, disconnect the motor cable first. Then loosen the axle nuts or release the quick-release skewer.
Gently pull the derailleur back so the wheel can drop out cleanly. Take your time. Forcing it can bend the derailleur hanger.
Step 3: Locate the Dropouts
Look at the rear end of your frame where the axle used to sit. You’ll see two slots, one on each side of the frame. These are the dropouts.
Your measurement should only be taken from the inside faces of these slots.
Step 4: Measure Inside-to-Inside
Use your tape measure or calipers across the inside faces of the dropouts. Make sure you’re holding the tool level and not angled.
A slight tilt can throw off the measurement by a couple of millimeters, which matters when choosing a hub motor.
Step 5: Record the Measurement
Write down the spacing in millimeters. Common eBike sizes are:
- 130mm or 135mm for most commuter and road eBikes.
- 170mm or 190mm for fat tire eBikes.
- Some cargo or specialty eBikes may use wider dropouts.
This note becomes your reference when shopping for new wheels or parts.
Common Dropout Widths for eBikes
Dropout widths on eBikes vary depending on the style of bike. Most commuter and city eBikes use standard widths, while fat tire and cargo bikes require much wider spacing. Knowing the typical sizes helps you quickly compare your measurement to what’s considered normal.
Tips for Getting the Most Accurate Reading
Getting dropout measurements wrong by even a couple of millimeters can cause big problems later. Here are some ways to keep it precise:
- Use calipers if you can: They provide the most exact millimeter reading compared to a tape measure.
- Measure twice: Taking two or three readings ensures consistency.
- Keep the tool straight: Even a slight angle between the dropouts can add extra millimeters.
- Clean the dropouts first: Dirt or paint buildup can interfere with a snug fit.
- Check manufacturer specs: Your frame’s manual may list the exact dropout size for confirmation.
Why Accurate Dropout Measurement Matters
Accurate dropout measurement ensures your new wheel, hub motor, or axle will fit your eBike securely without stressing the frame. A wrong measurement can lead to poor alignment, unsafe riding, or even damage to your bike.
From experience, I’ve seen riders try to “force fit” wheels that were just a few millimeters off. In one case, it caused uneven pressure on the frame and eventually cracked the dropout. The cost of repairing or replacing a frame far outweighs the few minutes it takes to measure properly.
Getting it right also means your drivetrain, brakes, and motor all stay in alignment. That translates to smoother shifting, better handling, and peace of mind knowing the rear wheel is locked in safely.
Final Words
Measuring the rear dropout fork on your eBike isn’t complicated, but it’s one of those small details that makes a big difference. With the right tools and a careful approach, you’ll know exactly what wheel or hub motor fits your frame.
Remember the basics: measure from the inside faces, write it down in millimeters, and compare against standard sizes. It’s a simple step that saves you from headaches, wasted money, and safety issues down the road.
