How to Measure Rear Dropout Fork on an eBike?

Key Takeaways

Getting your ebike rear dropout measurement right comes down to these simple rules.

• Measure Inside-to-Inside: The single most important rule. The measurement must be taken from the inner flat surface of one dropout to the inner flat surface of the other.
• Accuracy is Paramount: Use digital calipers if possible. A few millimeters can be the difference between a perfect fit and a part that won't install. When it comes to how to measure ebike dropouts, precision is your best friend.
• Know Your Axle Type: Look at your frame and identify if you have open U-shaped slots (Quick Release) or enclosed holes (Thru-Axle). This determines the type of hub you can use.
• Disconnect Power First: On any electric bike, always unplug the motor connections before you remove the rear wheel to prevent any electrical damage.

Planning to upgrade your electric bike with a powerful hub motor or a new wheelset? Before you click "buy," there's one critical measurement you can't afford to get wrong. Getting this measurement right is the key to ensuring your new parts fit perfectly, keeping your e-bike safe and performing at its best. A wrong fit can lead to wasted money, a damaged frame, or an unsafe ride.

This text explains what a rear dropout is, why its measurement is so important, and gives you a step-by-step process for measuring it accurately. You'll gain the confidence to select the right components for your ebike.

Understanding the Basics of Your E-Bike's Rear Dropout

To get an accurate ebike rear dropout measurement, you first need to know exactly what you're measuring. The terms can seem technical, but they are actually quite simple once you see them.

What is a Rear Dropout?

A rear dropout is the pair of slots on the very back of your bike's frame that holds the rear wheel's axle in place. Think of it as the designated parking spot for your wheel's axle. It's the point where your wheel connects to the rest of the bike, making it a super important part of the frame structure.

What is Dropout Spacing (O.L.D.)?

This is the main measurement you need. Dropout spacing is the inside distance between the two rear dropouts. In technical circles, you'll hear it called O.L.D., which stands for Over-Locknut-Dimension. It dictates the width of the wheel hub that will fit in your frame. An ebike hub motor has to match this dimension exactly.

Why is the term "Fork" sometimes used?

You'll often hear the term "fork" used for the front of the bike that holds the front wheel. Sometimes, people refer to the rear part of the frame holding the wheel as the "rear fork" or frame ends. For clarity, this text focuses only on the rear of the e-bike where the motor and drivetrain are.

Axle Types Matter: Quick Release vs. Thru-Axle

The style of your dropout depends on the type of axle your wheel uses. There are two main types, and they look quite different. Knowing which one you have is important for measuring ebike rear wheel hub spacing.

• Quick Release (QR): This is a classic design with an open, U-shaped slot. The wheel is held in place by a skewer with a cam lever on one end that clamps it tight. It's designed for fast wheel changes without tools.You might find this design on commute ebikes, where convenience is a priority.
• Thru-Axle: This is a more modern, robust system. The dropout is an enclosed hole. A thick axle slides through one dropout, through the wheel hub, and screws directly into the threads on the other dropout. This creates a much stiffer and more secure connection.

A Step-by-Step Guide to Your E-Bike Rear Dropout Measurement

Now that you know the parts involved, you can take a precise measurement. The process is straightforward if you follow these steps carefully. Accuracy here will save you a lot of headaches later.

Step 1: Gather Your Tools

To do this job right, you'll need the proper equipment. The best tools for measuring ebike rear dropout are ones that give you a precise reading in millimeters.

• Primary Tool: A digital caliper is highly recommended. It's the most accurate tool for the job and will give you a perfect reading. You can find inexpensive ones online or at a hardware store.
• Alternative: A tape measure or a metal ruler can work if you're very careful, but there's a higher chance of a small error.
• Other items: You'll want a pen and paper or your phone to write down the measurement. A bike repair stand is also very helpful, but you can get by without one.

Step 2: Prepare Your Electric Bike

Set up your workspace to make the job easy and safe.

• Place the bike in a repair stand if you have one. If not, carefully flip the bike upside down so it rests on its saddle and handlebars. Lay down a towel to prevent scratches.
• Critical E-Bike Step: Before you do anything else, safely disconnect any hub motor wires. Look for a connection point a few inches away from the motor. Gently pull it apart; it should be a simple plug. This prevents any damage to the electronics.

Step 3: Remove the Rear Wheel

You need clear access to the dropouts, so the wheel has to come off.

• Shift the chain to the smallest cog on the rear cassette. This will make it easier to remove and reinstall the wheel.
• If you have rim brakes, you might need to release the cable tension to open them up.
• For a quick-release axle, just open the lever and unscrew the other side a few turns. For a thru-axle, unscrew it completely and pull it out.
• Gently pull the derailleur back and guide the wheel down and out of the dropouts. Set the wheel aside.

Step 4: Take the Measurement

This is the moment of truth. An accurate ebike rear dropout measurement is the goal.

• Take your digital caliper and turn it on. Make sure it's set to millimeters (mm).
• Place the jaws of the caliper on the inner flat surfaces of the two dropouts. Do not measure from the outside edges. You want the exact width of the space the hub fits into.
• Gently close the jaws until they touch the frame. Don't squeeze them hard. Read the number on the screen.
• Take the measurement two or three times to be certain your reading is consistent.

Step 5: Record Your Finding

Don't rely on your memory. Write down the number immediately. This simple action prevents mistakes when you are shopping for your new motor or wheel. For example, write down "Rear Dropout: 135mm."

Common E-Bike Dropout Standards You Need to Know

You have a number, but what does it tell you? Your measurement should match up with one of the common ebike rear dropout standards. This helps confirm you measured correctly and tells you what size parts to look for.

Here's a quick list of standard ebike rear dropout dimensions you're likely to see:

• 135 mm: This is a very common standard for bikes with quick-release axles, including many hybrids, older mountain bikes, and a large number of conversion-friendly ebikes.
• 142 mm: A common standard for bikes with thru-axles that are not "Boost" spaced.
• 148 mm ("Boost"): This is the current popular standard for most modern mountain bikes and eMTBs with thru-axles. The wider spacing creates a stronger wheel.
• 150 mm / 157 mm: These wider thru-axle standards are typically found on downhill or other aggressive long-travel bikes.
• 177 mm / 197 mm: These extra-wide spacings are for fat tire electric bikes to accommodate their massive tires.

The Important Secondary Measurement: Dropout Slot Width

For those installing a powerful hub motor, there's one more quick measurement. Check the width of the dropout slot itself (the U-shaped opening).

• A standard quick-release dropout slot is 10 mm wide.
• Many hub motor axles are thicker (like 12mm or 14mm), but they have machined flat sections that are 10mm wide so they can slide into a standard dropout. You just need to confirm your dropout slot is the standard 10mm. Use your calipers to measure it.

FAQs About E-Bike Dropouts

Q1: Can I just use a regular ruler instead of calipers?

You can, but it's not ideal. A ruler or tape measure can easily be angled slightly, throwing off your reading by a few crucial millimeters. For an expensive component like a hub motor, spending a few dollars on a cheap set of digital calipers is great insurance against a costly mistake.

Q2: My frame is 135mm, but the hub motor I want is 142mm. Can I just spread the frame?

This is strongly discouraged. Spreading an aluminum frame will likely cause it to crack, ruining it permanently. Spreading a steel frame (a process called "cold setting") is possible but should only be done by an experienced bike mechanic. It's always safer and better to buy a hub that matches your frame's intended size.

Q3: What if my measurement is 137mm? Is it 135mm or something else?

Bike frames can have tiny manufacturing variances, and paint can add a little thickness. If your measurement is within a millimeter or two of a common standard (like 135mm, 142mm, or 148mm), it is almost certainly that standard. In this case, your frame is 135mm.

Get Ready for Your E-Bike Upgrade

Getting an accurate dropout measurement is a fundamental skill for any e-bike owner looking to do their own upgrades. Follow these steps, you've learned how to measure ebike rear dropout width correctly, ensuring your next project is a success from the very first step.