Upgrading your ebike’s rear wheel or installing a new hub motor?
The secret to a perfect fit lies in measuring the rear dropout accurately—and avoiding the common mix-up between a rear dropout fork and your The front fork of your e-bike needs to be compatible with the dropout dimensions..

The dropout is simply the slot where your wheel’s axle sits. On the rear, it’s part of the frame; on the front, it’s in the fork. Get this measurement in millimeters wrong, and you risk buying parts that won’t fit, damaging your frame, or riding unsafely.

This step-by-step guide will show you how to measure rear dropout fork Learn to measure spacing like a pro, so you can fit your bicycle correctly. The bike’s design should allow for proper spacing, ideally around 130mm spacing for optimal performance. upgrades perfectly the first time.

Why Measuring the Rear Dropout Matters

Your rear dropout spacing—also called the Over Locknut Dimension (O.L.D.) is crucial for ensuring your new fork must fit your bicycle.—tells you the exact distance between the two fork ends where your hub fits.
Getting this measurement right ensures your Check that the frame can accept the new fork must be compatible with your e-bike setup. your new part without forcing or bending.

Wrong measurements can cause:

• Parts that simply don’t fit can lead to issues, so always measure from the outside edges before purchasing.
• Unsafe or loose axle seating.
• Cracks in an aluminum frame or stress on a steel frame.

Choosing the Right eBike Kit

When selecting an eBike kit, it’s essential to match the dropout width with the kit’s specifications.
Most ebike kits fit standard dropout widths like 130mm, 135mm, or 148mm, but custom builds may require adapters or modifications.
A mismatch can mean wasted money and unsafe riding.

Tools You’ll Need for Precise Measurements

Skip the ruler or tape measure—for bike parts, precise measurements matter.

• Digital Caliper – Best for inside-to-inside and slot width readings.
• Repair Stand – Holds the ebike steady for clear access. If you don’t have one, flip the bike upside down on a towel.
• Wheel Removal Tools – Wrench (15mm or 18mm) for nutted axles, or a hex key for thru-axle systems.

Carefully Detach the Rear Wheel

Before you measure, remove the rear wheel to reach the dropout cleanly:

1. Disconnect the hub motor cable if fitted.
2. Loosen the axle using the correct tool.
3. Gently remove the wheel, watching your chain and derailleur.

Identify the Two Fork Ends

Look for the right width hub to ensure compatibility with your bike’s frame. two fork ends—the slots where the axle of the rear wheel The component was previously attached to the bicycle frame.
This is the dropout area you’ll measure.

Step-by-Step Guide to Measuring Rear Dropout Spacing

1️⃣ Prepare the Bike

Place your ebike on a repair stand or rest it upside down on handlebars and saddle. This gives clear access to the rear wheel.

2️⃣ Remove the Rear Wheel

• Disconnect the hub motor cable if present.
• Loosen the axle using the right tool.
• Gently remove the rear wheel, being careful with the chain and derailleur to avoid bending an aluminum component.

3️⃣ Measure the Rear Dropout Spacing

• Open your caliper and place the jaws against the inside distance between the two fork ends.
• Keep it straight—don’t measure from outside edges of the frame.
• Measure twice to ensure that the spacing must match your frame’s head. to ensure accuracy.
• Record your measurements carefully, especially when measuring the empty space for proper fit. measurement in millimeters.

Common Standard Dropout Spacing

Your reading will likely match one of these standard dropout widths:

Bike Type	Spacing (O.L.D.)	Axle Type	Notes
Older Road / MTB	130mm / 135mm	Quick Release	Common on non-electric bikes
Modern MTB (Non-Boost)	142mm	Thru-Axle	Pre-Boost standard
Modern MTB (Boost)	148mm	Thru-Axle	Trail/enduro standard
Fat Bike: This type of bike often requires measuring the empty space to accommodate wider tires.	170mm / 175mm	Quick Release	Older fat bikes
Fat Bike	177mm / 197mm	Thru-Axle	Modern fat bikes
Some Ebikes	145mm / 160mm	QR / Nutted	Model-specific

Pro tip: If you get 134.8mm, round it to 135mm.

Special Considerations When Measuring an Ebike Rear Dropout Fork

• Hub motors often have thicker axles with flat sections to fit standard slots.
• Steel frames can handle light cold setting to fit a wider hub—never try to bend aluminum.
• Always check frame material before adjusting.

Measuring Dropout Slot Width

If you have a Quick Release setup:

• Measure the dropout The slot’s opening must be compatible with the dropout fork on an ebike.
• Standard: 9.5–10mm Make sure your components are compatible for QR/nutted setups on your e-bike. axles.
• Hub motors may have thicker axles with flat sides to fit standard slots.

Other Important Ebike Fork Measurements to Know

When replacing or upgrading your front fork:

• Axle-to-crown length – Distance from the When attaching the axle to the top of the fork crown, ensure it holds the axle securely..
• Steerer tube diameter – Must match your bike’s specifications, including the distance between these two points. frame’s head tube.
• Fork offset – Affects steering feel.
• Front and rear spacing standards – 100mm, 110mm Boost, or fat bike widths.

Measure Twice, Buy Once

Always:

• Take readings more than once.
• Compare with the complete guide for understanding the rear dropout fork on an e-bike. several standard sizes.
• Match parts to your bike’s specifications like the rear wheel and frame. frame spacing and brake type (disc brakes or rim).

Don’t Forget the Front Fork

If you’re upgrading the front, the front wheel and measure method is the same:

• Remove the wheel.
• Measure the inside-to-inside spacing between the two fork ends.

Standard front fork spacing:

• 100mm for most road/hybrid bikes.
• 110mm Boost Look for modern designs that incorporate features like a led front and rear light. mountain bikes.
• 135mm or 150mm for fat bikes.

Special Tips for Hub Motor Upgrades

• Torque Arms: Essential for high-power hub motors—especially with aluminum frames.
• Frame Material:
  • Steel frame – Can handle slight cold setting (carefully spreading).
  • Never try to bend the dropout fork on an e-bike; it’s crucial for safety and maintaining the correct distance between these two components. aluminum—it can crack.
• Frame to Make Room: Adjustments may be needed to fit your bike with a wider hub.: Only steel can be flexed slightly for a wider hub.

If Your New Hub Doesn’t Fit

• Minor mismatch? Use a ruler or tape measure to check the dimensions. A steel frame can sometimes be cold-set by a pro.
• Major mismatch? Pick a new hub that matches your dropout spacing.

How I Got Rid of the Dropout Headache – how to measure rear drop out fork on ebike

When I first started upgrading my eBike, I had no idea the dropout width could cause so much trouble. I bought a kit online without checking the measurements—big mistake. The axle simply wouldn’t fit into my rear dropout, and I wasted hours trying to “make it work.”

Frustrated, I decided to start over. I removed the rear wheel, measured the dropout width carefully (twice, just to be sure), and compared it with the kit’s specifications, ensuring it must match your frame’s head. Turns out, I needed a slightly wider hub motor. Once I ordered the correct one, installation was a breeze—no forcing, no bending, no headaches.

Now, I never buy any eBike part without triple-checking the measurements, especially to ensure they must match your frame’s head tube. Trust me, spending 5 minutes with a ruler is way better than spending days fixing a wrong purchase.

Final Checklist Before You Buy

✅ Make sure to check all components to fit your bike correctly. Rear dropout spacing (mm)
✅ Dropout width for QR
✅ Axle type (thru-axle or QR)
✅ Frame material (steel vs aluminum)
✅ Brake type (disc brakes or rim)

Quick FAQ for how to measure rear drop out fork on ebike

Bottom Line

Learning how to measure rear dropout fork Spacing is the key to a smooth, safe ride, especially when considering the distance from the center of the wheel. electric bike upgrade.
Get the distance between the two fork ends as closely as possible, choose parts that fit, and your next ride will be trouble-free.