How to Measure Rear Drop Out Fork on eBike: Complete Guide

Finding the Right Parts for Your eBike

You found the perfect hub motor for your eBike conversion. Maybe you got a great deal on a high-performance suspension fork too. But now comes the big question: will it fit your bike?

The difference between a successful upgrade and wasting money comes down to a few key measurements. In the world of bike parts, making sure everything works together is the most important thing. Getting a measurement wrong can lead to a part that won't install at all, or even worse, one that makes your bike unsafe to ride.

This guide will make the whole process simple and clear. We will show you step-by-step how to measure your eBike's rear dropout and all the important parts of a front fork. By the end, you'll feel confident picking the right parts for your project.

Here's what we'll cover:
* Measuring Your Rear Dropout: The first step you need to take.
* Understanding Front Fork Specs: It's about more than just width.
* Picking the Right Fork: Making sure the numbers and your riding style match up.
* Pro Tips and Checklists: How to avoid making common mistakes.

Part 1: Your eBike's Rear Dropout

What is a Dropout?

A dropout is the pair of slots at the very back of your bike's frame. These slots hold the axle of your rear wheel in place securely. The distance between these two slots is called dropout spacing, and it's one of the most important measurements on your bike.

This number tells you exactly what width hub (and hub motor) your frame can accept. Getting this measurement right is the foundation of any rear-wheel upgrade you want to do.

Tools You Need for Measuring

Being precise is very important here. You can use a ruler, but we strongly recommend using a caliper for the most exact measurement possible.

• Digital or Vernier Caliper: This gives you the best accuracy, down to tiny fractions of a millimeter.
• Metric Ruler or Tape Measure: A high-quality metal ruler works if you don't have a caliper available.
• Pen and Paper: Write your measurements down right away instead of trying to remember them.

How to Measure Rear Dropout Spacing

You must remove the rear wheel to get an accurate reading. This makes sure there's no pressure on the frame and gives you a clear view of what you're measuring.

Step 1: Get Your Bike Ready
Put your eBike on a repair stand or flip it upside down so it rests on the handlebars and saddle. This keeps everything stable while you work. Remove the rear wheel completely.

Step 2: Measure from Inside to Inside
This step is the most important one. Put your caliper jaws on the inner surfaces of the two dropouts. You're measuring the empty space where the hub sits. Don't measure from the outside edges of the frame because that will give you the wrong number.

Step 3: Read and Write Down Your Number
Read the measurement in millimeters, which is what all modern bike parts use. Write it down right away so you don't forget.

Standard Rear Dropout Sizes

Your measurement will probably match one of several standard sizes that the bike industry uses. Knowing these helps you confirm your measurement is right.

• 135mm: This has been the most common standard for many years on bikes with quick-release axles.
• 142mm / 148mm (Boost): These are newer standards found on mountain bikes and eMTBs that use thru-axle systems for extra strength.
• 177mm / 197mm: These much wider standards are only used on fat bikes to fit their huge tires.

Different Types of Axles

Dropout spacing is only half of what you need to know. You also need to figure out what kind of axle your frame uses.

• Quick Release (QR): This system uses a thin skewer that goes through a hollow axle in the hub.
• Thru-Axle: This is a stronger system where a large axle passes through one dropout, through the hub, and screws into the other dropout.

A hub made for a QR axle won't work in a frame made for a thru-axle, and the other way around. You must match both the spacing and the axle type perfectly.

Part 2: Understanding Your Front Fork

Whether you're upgrading to a suspension fork or replacing a broken one, these measurements are just as important as the rear ones. Making a mistake here can change how your bike handles and even make it unsafe to ride.

Measuring Front Dropout Spacing

The process is exactly the same as measuring the rear: take off the front wheel and measure inside-to-inside between the front dropouts.

Here are the common front standards:

• 100mm: This is the standard that most bikes have used for many years, for both quick-release and many thru-axle forks.
• 110mm (Boost): Just like the rear, Boost spacing on the front gives you a wider hub for a stronger wheel.
• 135mm / 150mm: These wider spacings are found on fat bike forks to fit their big hubs and tires.

The Most Important Part: Steerer Tube

Beyond the dropout width, the steerer tube is the most critical part of a fork. This tube goes up from the fork and passes through your bike frame's head tube, connecting to your stem and handlebars. Get this wrong, and the fork won't work at all.

Steerer Tube Size and Type

Your new fork's steerer tube must match your frame's head tube. There are two main types you'll see:

• Straight (1-1/8"): The steerer tube has the same width from top to bottom.
• Tapered (1-1/8" to 1.5"): The steerer tube is narrow at the top but gets wider at the bottom where it meets the fork.

A tapered fork will not fit in a frame made for a straight steerer tube. However, you can sometimes fit a straight steerer fork into a tapered frame using a special adapter.

Steerer Tube Length

This measurement can make or break your fork choice. A steerer tube that's too short is completely useless and can't be fixed.

Use your current fork to measure this. Measure from the bottom of the fork crown all the way to the top of the steerer tube. Your new fork's steerer tube must be at least this long, or longer is even better. New forks come with very long steerer tubes that get cut down to the right length when installed.

Other Important Fork Measurements

Two more measurements affect how your bike feels when you ride: Axle-to-Crown and Offset. When you replace a fork, it's best to match these as closely as possible to your original fork.

• Axle-to-Crown (A2C): This is the distance from the center of the wheel axle to the top of the fork crown.
• Offset (Rake): This is how far forward the front axle sits compared to the steering axis.

Pro Tips for Success

Getting the right numbers is only part of what you need to do. Here are some expert tips based on mistakes we see people make all the time.

Mistake #1: The "Close Enough" Problem

We've seen people measure 133mm and round it up to 135mm thinking it's close enough. That 2mm difference can mean you have to bend your frame dangerously to install the wheel, which puts constant stress on the frame joints. Be exact with your measurements.

Mistake #2: Forgetting About Brake Mounts

Your new fork must be able to hold your brake caliper properly. There are two main standards for disc brakes that you need to know about.

• IS (International Standard): Has two mounting tabs that run parallel to the wheel axle.
• Post Mount (PM): Has two threaded posts that the caliper bolts screw directly into.

Post Mount is newer and more common, but you must check what your bike has. You can get adapters to change from one to the other, but it's always better to get a fork with the right mounts.

Mistake #3: Not Thinking About Frame Material

Let's say your steel frame has 130mm spacing, but your new hub is 135mm wide. With a steel frame, you might be able to carefully bend the frame to fit the new spacing in a process called cold setting.

Important Warning: Never try to bend an aluminum or carbon fiber frame to make parts fit. These materials will crack and break suddenly, which is extremely dangerous.

How to Choose the Right Front Fork

You have your measurements now. It's time to pick the right fork for your eBike and riding style.

Match All the Numbers First

Before you look at any performance features, make a list of specs that must match exactly. The new fork must have:

• Right Wheel Size: 26", 27.5", or 29"
• Correct Dropout Spacing: 100mm, 110mm, 135mm, etc.
• Right Axle Type: Quick Release or Thru-Axle with the correct diameter
• Matching Steerer Tube Type: Straight or Tapered
• Long Enough Steerer Tube: Equal to or longer than your current one
• Right Brake Mount Type: Post Mount or IS Mount

Only look at forks that check every single one of these boxes.

Think About How You Ride

Next, consider suspension travel, which is how much the fork can compress. This depends on what kind of riding you do most.

• Commuting and Cruising (60mm - 100mm): For city streets and bike paths, shorter travel is perfect for smoothing out bumps without adding weight.
• Trail and eMTB (120mm - 160mm): For off-road riding, more travel helps you stay in control on rough terrain.
• Fat Bikes: These forks are built specifically for wide hubs and often have 100mm-120mm of travel.

Special eBike Forks

Can you put any mountain bike fork on an eBike if the specs match? Yes, you can. Should you always do this? Maybe not, depending on your bike and riding style.

Major brands like FOX and RockShox now make forks specifically for eBikes. These aren't just marketing gimmicks but are built with stronger parts to handle the extra weight and forces that eBikes create. If you ride aggressively or have a heavy full-power eBike, getting an eBike-rated fork is a smart choice for long-term reliability.

Measure Twice, Buy Once

Taking time to measure your eBike accurately is the most important thing you can do for a successful upgrade. Following this guide gives you the knowledge to identify your bike's standards, understand what they mean, and pick parts that fit perfectly. With a caliper and this checklist, you're ready to build a better ride.

FAQ

Q: Can I use a regular ruler instead of calipers to measure my eBike dropouts?
A: While you can use a ruler, calipers give much more accurate measurements. Since even 1-2mm can make the difference between a part fitting or not, we strongly recommend using digital or vernier calipers for the most precise results.

Q: What happens if I install a hub that's slightly wider than my dropout spacing?
A: This depends on your frame material. Steel frames can sometimes be carefully "cold set" to accommodate slightly wider spacing, but aluminum and carbon frames should never be bent as they can crack and fail catastrophically. Always match the spacing exactly for non-steel frames.

Q: How do I know if my eBike needs a straight or tapered steerer tube fork?
A: Look at your current fork's steerer tube where it enters the frame. A straight steerer tube maintains the same diameter throughout, while a tapered one starts narrow at the top (1-1/8") and widens at the bottom (1.5"). You can also check your bike's specifications or measure the head tube opening.

Q: Are eBike-specific forks really necessary, or can I use any mountain bike fork?
A: Any fork that matches your specifications will physically fit, but eBike-specific forks are built stronger to handle the extra weight and forces from electric motors. For casual riding, a regular fork may be fine, but aggressive riders or heavy eBikes benefit from the enhanced durability of eBike-rated forks.

Q: What's the difference between Post Mount and IS Mount disc brake standards?
A: IS (International Standard) mounts have tabs parallel to the axle where bolts thread in from the side, while Post Mount has threaded posts where the caliper bolts screw directly down. Post Mount is more modern and common, but you must match your existing brake caliper or use an adapter.