How to Measure Rear Dropout Fork on an Ebike: Step-by-Step Guide

Understanding the Rear Dropout Fork on an Ebike

Let's clear up a common mistake first. When people talk about the rear wheel of an e-bike, they mean the rear dropouts. These are part of the main frame, not a fork.

The term "fork" only refers to the front part of the bike that holds the front wheel. There is no such thing as a "rear fork." People searching for how to measure rear drop out fork on ebike usually want to measure their frame's rear dropouts. This measurement helps ensure your new wheel, hub, or motor kit will fit properly. Getting this measurement right is the most important step for any rear wheel project.

An incorrect measurement can cause serious problems. It might lead to buying parts that don't fit, unsafe installation, or damage to your e-bike frame. This guide will show you the exact process to get it right the first time.

Tools You Need for an Accurate Ebike Dropout Measurement

You need precise tools for this job. Don't use a simple ruler or tape measure because bicycle parts require exact measurements.

• Digital Caliper: This is the best tool for the job. It gives you precise measurements in millimeters and eliminates guesswork. You'll use it to measure both the spacing and the width of the dropout slots.
• Bike Repair Stand: This tool makes the job much easier and safer. It holds your e-bike securely at a comfortable working height. You can also flip the bike upside down to rest on its handlebars and saddle if you don't have a stand.
• Wheel Removal Tools: You need the right wrenches or hex keys to remove your rear wheel. Most bikes use 15mm or 18mm wrenches for nutted axles or hex keys for thru-axles.

How to Measure Rear Dropout Spacing on an Ebike

This measurement has a technical name called the Over Locknut Dimension (O.L.D.). It tells you what width hub your frame can accept.

Step 1: Set Up Your Bike
Put your e-bike in a repair stand for stability. If you don't have one, lay a protective cloth on the ground and flip the bike upside down. This gives you stability and clear access to the rear wheel.

Step 2: Take Off the Rear Wheel
First, safely disconnect the hub motor cable if your bike has one. These connectors are usually found along the chainstay. Then use your wrench or hex key to loosen the axle nuts or thru-axle and carefully remove the rear wheel. Be careful around the derailleur and chain.

Step 3: Make the Critical Measurement
Open the jaws of your digital caliper. Place the inner jaws firmly against the inside flat surfaces of both dropouts. You're measuring the empty space where the hub's axle ends would sit.

Don't measure from the outside edges of the frame because this gives you a wrong, larger number. Make sure the caliper is straight and not angled. The reading on your caliper is your rear dropout spacing. Take the measurement several times to make sure it's consistent.

Common Rear Dropout Spacing Standards for Ebikes

Bicycle and ebike frames use several standard dropout spacings. Knowing these standards helps you check your measurement and understand what parts will work. Your measurement should be very close to one of these common sizes. There are two main axle systems: Quick Release (QR) uses an open slot, and Thru-Axle (TA) uses a fully enclosed hole.

Standard Rear Dropout Spacing Measurements

Measuring the Rear Dropout Slot Width on an Ebike

After you know the spacing, you need to measure the width of the dropout slot itself. This measurement works for QR-style dropouts and tells you what axle diameter can fit. Use your caliper to measure the opening of the slot. Most standard non-electric bikes have slots around 9.5-10mm wide to fit a standard 9mm QR or 10mm nutted axle.

E-bike hub motors often have larger axles like 12mm or 14mm. But these axles are made with flat sections that are typically 10mm wide, so the larger axle can fit securely into a standard 10mm dropout slot. You must make sure the motor axle's flats will fit your dropout width. An axle that's too large won't fit, and one that's too loose can be dangerous.

Front Fork Dropout Measurement for Ebikes

Let's cover the ebike front fork part since that was in the original search. Measuring front dropout spacing works exactly like the rear: remove the front wheel and measure the inside-to-inside distance between the two fork dropouts.

Standard Front Dropout Spacing

Special Considerations When Measuring an Ebike Rear Dropout Fork

When you measure rear drop out fork on ebike for a hub motor upgrade, you need to think about more than just spacing. Hub motors create huge amounts of torque that put special stress on your frame.

Hub Motor Axles and Wires
Hub motor axles are built differently than regular bike axles. They're thicker for strength and have flats to fit standard dropouts. They also have a motor cable coming out of one end. When you install the motor, make sure this cable has a clear path and isn't pinched by the frame or other parts.

Why Torque Arms Matter
A hub motor spins the wheel, but it also applies an equal twisting force to the axle. This force tries to rip the axle right out of the dropouts. The small flat surfaces of a dropout weren't designed to handle this kind of continuous torque. Torque arms solve this problem by transferring the motor's rotational force from the axle to a stronger part of the frame or fork.

• Front Hub Motors: You almost always need a torque arm for front hub motor installation. Forks, especially suspension forks with aluminum parts, aren't strong enough to resist motor torque and can fail suddenly.
• Rear Hub Motors: For powerful rear motors (over 750W) or any motor in an aluminum frame, you should use a torque arm. Steel dropouts are stronger, but repeated stress from a powerful motor can still make them stretch or fail over time. Aluminum dropouts are much more brittle and can easily crack without torque arm support.

What to Do When Rear Dropout Parts Don’t Fit Your Ebike

You've measured carefully, but your new hub motor is 140mm and your frame is 135mm. Here are your options.

Minor Mismatch (Steel Frames Only)
Steel frames have a small amount of natural flex. You can spread a steel frame by a few millimeters (like from 135mm to 138mm) to fit a slightly wider hub. You can usually pull the stays apart by hand to insert the wheel. For larger gaps, you can use a process called "cold setting."

Cold Setting Steel Frames
Cold setting means permanently bending a steel frame to a new dropout spacing. Only do this on steel frames and be careful to keep the dropouts parallel. It's an advanced technique that can misalign your frame if done wrong. For those comfortable with advanced mechanics, this DIY guide offers good information: Change Dropout Spacing using DIY Tools.

remove the front wheel
Never try to bend or spread an aluminum frame. Aluminum is rigid and doesn't flex like steel. Trying to spread aluminum dropouts will likely cause tiny cracks that lead to sudden frame failure. If the hub doesn't fit your aluminum frame, you need a different hub.

Other Solutions for Size Problems
If there's a big mismatch or you have an aluminum frame, look for other solutions. Some hub motor kits like the Grin All-Axle motor use adapters and spacers to fit various dropout standards. This gives you a safe and reliable solution for non-standard fits.

Other Important Ebike Fork Measurements to Know

While dropout spacing and width are most important, a few other measurements matter when changing a front fork.

Axle-to-Crown Length: This is the distance from the center of the front axle to the top of the fork crown. It affects your bike's head tube angle and how it handles. When replacing a fork, match this length as closely as possible.

Steerer Tube Diameter: This tube passes through the frame's head tube. Common standards are straight 1 1/8" or tapered (1 1/8" at the top to 1.5" at the bottom). Your new fork must match your frame's head tube.

Fork Offset (Rake): This is the horizontal distance between the steering axis and the front axle. It slightly affects how stable and quick your steering feels.

Final Checklist Before Buying

Before you buy a new hub, motor, or wheel, go through this checklist.

• Rear Dropout Spacing: __ mm (Measured inside-to-inside)
• Rear Dropout Width: ____ mm (Measured slot opening)
• Axle Type: Does your frame use Quick Release or Thru-Axle?
• Frame Material: Is your frame Steel or Aluminum? This determines if minor spreading is possible and tells you if you need a torque arm.
• Brake Type: Do you have disc brakes or rim brakes? Your new hub must work with your braking system. A hub for disc brakes will have a 6-bolt or Centerlock mounting point.

Frequently Asked Questions

1. Q: Can I use a regular tape measure to measure my rear dropouts?
A: No, you shouldn't use a tape measure for this job. Bicycle parts require very precise measurements, and even small errors can cause compatibility problems. A digital caliper is the best tool because it gives you exact measurements in millimeters.

2. Q: What's the difference between Quick Release and Thru-Axle systems?
A: Quick Release systems use an open slot in the dropout and a lever mechanism to secure the wheel. Thru-Axle systems use a closed hole and a threaded axle that passes completely through the hub and frame. Thru-Axle systems are generally stronger and more precise.

3. Q: Do I always need a torque arm when installing a hub motor?
A: It depends on your motor power and frame material. Front hub motors almost always need a torque arm. For rear motors, you should use one if you have a powerful motor (over 750W) or an aluminum frame. Steel frames with lower-power motors might not need one, but it's still good insurance.

4. Q: Can I make my aluminum frame wider to fit a larger hub motor?
A: Never try to spread or bend an aluminum frame. Unlike steel, aluminum doesn't flex and will crack if you try to force it. If your hub motor doesn't fit your aluminum frame's dropout spacing, you need to find a different motor or use adapters if available.

5. Q: What should I do if my measurement doesn't match any standard spacing?
A: First, double-check your measurement technique and take the measurement again. If it still doesn't match a standard, you might have an unusual frame or you might be measuring incorrectly. Consider taking your bike to a local bike shop for a professional measurement, or look for hub motors with adjustable spacing systems.