To check the rear dropout on your e-bike, take off the back wheel and grab a ruler or caliper. Measure the inside distance between the two slots where the axle sits. That number, in millimeters, is your frame’s rear dropout width.
It’s a key measurement to know before you buy a new wheel or hub motor, since it makes sure everything fits the way it should. The process is quick and simple, and it saves you from running into fit issues while keeping your ride safe.
What is a Rear Dropout Fork on an E-Bike?
The rear dropout, sometimes called the rear fork end, is the spot where your e-bike’s back wheel attaches to the frame. You’ll see two little slots on each side where the axle slides in to hold the wheel steady. The distance between those slots is known as the dropout spacing, or frame spacing.
When people bring up the “rear dropout fork” on an e-bike, they’re really talking about that spacing at the back of the frame, not the front fork. It’s an important detail because it decides what hub or wheel size will actually fit your bike.
If the spacing doesn’t match the hub or the fat tire electric road bike wheel you want to put on, the parts won’t fit right or work safely. That’s why knowing your dropout width is one of the first things every rider should check before replacing or upgrading a rear wheel.
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Why Does Rear Dropout Measurement Matter?
Measuring your rear dropout fork accurately is more than just a numbers exercise – it directly impacts your e-bike’s safety and performance. Here’s why it’s so important:
Ensures Compatibility
The rear dropout width dictates what wheel hubs or hub motors will fit your bike. For example, most hub motor kits for standard bikes are made for 135 mm dropouts. If your frame is narrower or wider, that kit won’t install properly.
Knowing your bike’s width beforehand ensures you buy the right sized parts (axle, motor, wheel), saving you time and money.
Safety and Frame Integrity
Forcing a wheel or motor that’s too wide into a smaller frame can put enormous stress on the dropouts. The metal could bend or even crack over time – a dangerous situation that could lead to accidents.
Conversely, a hub that’s too narrow might rattle or shift, potentially slipping out of place. Proper fit keeps the wheel secure and the frame structurally sound during rides.
In one case I encountered, a rider tried to jam a 142 mm axle into a 135 mm frame – the frame split at the dropout after a short period of riding, ruining the bike. This kind of damage is avoidable by simply matching the correct sizes.
Optimal Drivetrain and Brake Alignment
Modern e-bikes often have disc brakes and precisely indexed gearing. The hub needs to sit at the correct width for the brake rotor to line up with the caliper, and for the derailleur to shift through gears smoothly.
If the spacing is off, you could experience brake rub, misaligned gears, or premature wear on components. An accurate dropout measurement ensures that when you install a new wheel or motor, the brake rotor and cogset line up exactly as intended.
Peace of Mind
Finally, knowing your e-bike’s critical measurements (like dropout width) gives you peace of mind. Whether you’re upgrading to a fat tire setup or just replacing a broken axle, you’ll have confidence that the parts will fit. You can ride knowing the rear wheel is locked in correctly. It’s a small detail that makes a big difference in the long run.
In summary, measuring the rear dropout is a quick task that can prevent big problems. It ensures all the components in your e-bike’s rear end work in harmony without undue stress.
A few minutes with a measuring tool can save you from potential mechanical failure and keep your e-bike running safely and efficiently.
Tools Needed to Measure the Rear Dropout
Measuring the rear dropout width doesn’t require an expensive toolkit, but accuracy is key. Here are the tools and items that will make the job easy:
Metric Ruler or Tape Measure: A standard metal ruler or tape can work if used carefully. Make sure it has millimeter markings for precision.
Digital or Vernier Caliper: Highly recommended for accuracy. A caliper lets you measure the inside distance very precisely (to the nearest millimeter or better). This is the best tool to get an exact reading, especially if you’re dealing with tight tolerances on hub motors.
Bike Stand or Stable Support (optional): Holding the bike upright (on a repair stand or by flipping it upside down onto the saddle and handlebars) will keep it stable while you work. This isn’t mandatory, but it helps.
Wrench or Hex Keys: Needed to loosen and remove the rear wheel. Depending on your e-bike, you might have axle nuts (use an appropriate wrench) or a quick-release lever or thru-axle (might need hex/Allen key to unscrew).
Pen and Paper: Jot down the measurement immediately so you don’t forget it. Writing it down prevents any mix-ups later.
Using a caliper is the gold standard because it ensures the most exact measurement. However, many riders successfully use a tape measure; just double-check alignment and read it at eye level for accuracy. Gather these few items, and you’re ready to measure.
How to Measure the Rear Dropout Fork on an E-Bike (Step by Step)
Measuring the dropout spacing on your e-bike is pretty simple once you know what to look for. Here’s how to do it:
Step 1: Get the Bike Ready
Safety first. If your e-bike has a power button or a removable battery, switch it off or take the battery out. Place the bike on a flat surface. A repair stand makes things easier, but if you don’t have one, you can carefully flip the bike upside down so it rests on the handlebars and seat.
That way the bike stays stable and the wheels are easy to reach. It also helps to shift the chain onto the smallest cog in the back to reduce tension, which makes removing the wheel smoother.
Step 2: Take Off the Rear Wheel
You’ll need the wheel out of the way to measure correctly. If your e-bike has a hub motor, unplug the motor cable near the axle before doing anything else. Then loosen the axle.
Depending on your bike, this might mean opening a quick-release lever, using a wrench on the axle nuts, or unscrewing a thru-axle. Once it’s loose, pull the wheel straight out of the frame.
You may need to move the derailleur back a bit and slip the chain off the sprocket to clear the wheel. Set the wheel aside so the dropouts are fully visible.
Step 3: Find the Dropouts and Line Up Your Tool
Look for the two U-shaped slots at the back of the frame where the axle was sitting. These are the dropouts. You want to measure the inside faces, the parts that actually touch the axle.
Use a ruler, tape measure, or digital caliper. Hold it straight across from one side to the other, keeping it level. If you’re using a ruler or tape, having a friend hold one side while you read the other can make it easier.
Step 4: Take the Measurement
A digital caliper gives the most accurate reading, but a ruler will also work. Place your tool so it measures the distance between the inside edges of the dropouts, not the outer edges of the frame.
Keep it straight across, not on an angle. Most e-bikes will measure somewhere around 135 mm, 142 mm, 170 mm, or another common size. Always read the number in millimeters since that’s the standard for bike parts.
Step 5: Double-Check and Write It Down
Once you’ve got your measurement, jot it down right away. Measure a second time to confirm, and if the numbers don’t match, measure again until you get a consistent result. Your measurement should line up with one of the common standards.
If you’re off by a millimeter or two, it’s usually from holding the tool slightly crooked or small variations in the frame. The key is figuring out which standard size your bike matches. This number is what you’ll need when buying a new wheel, hub motor, or axle.
Common Rear Dropout Widths on E-Bikes
E-bikes use the same frame standards as regular bicycles, so their rear dropout widths tend to follow a few standard sizes. Knowing these common widths will help you verify your measurement and understand what components are compatible:
| Bike Type | Typical Rear Dropout Width |
| Road & Hybrid E-Bikes (older or rim-brake) | ~130 mm (rare on e-bikes, mostly older road bikes) |
| City/Commuter E-Bikes (quick-release axle) | 135 mm (very common standard) |
| Mountain E-Bikes (MTB, enduro e-bikes) | 142 mm (older thru-axle) or 148 mm Boost (newer MTB standard) |
| Fat Tire Electric Road Bike / Fat E-Bike | ~170 mm (quick-release) or 177 mm (through-axle) for 4″ tire models |
| Fat Tire Electric Bike (wide 5″ tires) | ~190 mm (quick-release) or 197 mm (through-axle) for extreme fat bikes |
Most standard e-bikes designed for city or road use have dropouts around 135 mm wide, especially if they use older style hubs or hub motors. Modern electric mountain bikes that have thru-axles often use 142 mm or 148 mm spacing (148 mm is known as “Boost” spacing, offering a stiffer wheel).
At the large end, fat tire e-bikes (the kind with very wide tires) require much wider rear dropouts – often in the 170–190 mm range – to accommodate the bigger hubs and tires.
For instance, a fat tire electric road bike built for sand or snow riding might have a 190 mm rear dropout so it can fit a 4-inch or 5-inch wide tire. On the other hand, specialty cargo e-bikes or tandems may also use wider spacing (sometimes 145 mm or more) to increase strength for heavy loads.
If your measurement matches one of these common sizes, you can be confident you did it right. Any significant deviation (e.g., a reading of 139 mm or 150 mm) could mean either a measuring mistake or a very unusual frame.
In such cases, double-check your method, and consult your e-bike manufacturer’s specs if possible. Manufacturers often list the rear dropout width (frame spacing) in the bike’s documentation or website specs, which can serve as a confirmation.
Tips for Getting an Accurate Measurement
Measuring your dropout width is pretty straightforward, but it’s easy to be off by a millimeter or two if you’re not careful. Here are a few tricks to keep your numbers reliable:
Use a Caliper if You Can
A digital caliper makes life easier and gives you a direct inside measurement with great accuracy. They’re not expensive, and most will measure within half a millimeter, which is plenty precise for this job.
Keep the Tool Straight
When you line up your ruler or caliper, make sure it’s perfectly across the dropouts. If it’s even slightly on an angle, the reading can come out bigger than it really is. Take a second to check that it’s level before you read the number.
Measure More Than Once
Don’t stop after a single reading. Take at least two, ideally three, and compare them. If they match, you know you’re good. If they’re off, try again and go with the value that repeats the most.
Clean the Dropouts First
Wipe out any dirt, grease, or extra paint inside the slots before you measure. Even a thin layer can stop the tool from sitting flat and throw the number off.
Double-Check with the Manufacturer
If you’re unsure, look up your bike’s specs. Most brands list the dropout width on their site or in the manual, sometimes under terms like “hub spacing” or “O.L.D.” It’s a quick way to confirm what you measured.
Using a Tape Measure
If a tape is all you’ve got, be extra careful to hook it squarely against the inside face of one dropout. Try to use the metric side so you can read millimeters directly. If you measure in inches and convert, make sure you don’t round too much.
Go Easy with the Caliper
When you’re closing the caliper on the dropouts, let the jaws touch lightly. Don’t squeeze the frame, since that can flex the gap and make it seem narrower than it really is.
Follow these simple tips, and you’ll get a solid reading. Once you know the true dropout width, you can shop for wheels, hubs, or axles with confidence and avoid headaches later.
How to Choose the Right Front Fork
Just as knowing how to measure rear drop out fork on e-bike is essential for the back end of your bike, selecting the right front fork matters just as much. The fork determines how stable your ride feels, how your brakes align, and even whether your chosen wheel or tire will fit.
The first step is to measure the front dropout spacing. Place a ruler or caliper inside the fork ends and check the internal distance.
Standard commuter and road e-bikes typically measure around 100 mm, mountain e-bikes often use 110 mm “Boost” spacing, while a fat tire electric road bike may require as much as 135–150 mm to clear wide tires. Once you know your spacing, confirm your axle type.
Quick release skewers are still common on entry-level or older e-bikes, while thru-axles (12 mm, 15 mm, or even 20 mm) dominate modern e-MTBs and performance bikes for stiffness and security. Many hub-motor e-bikes rely on bolt-on axles, which use nuts to keep everything firmly in place.
Next comes tire clearance. Standard forks only accept tires up to about 2 inches, while mountain forks usually handle up to 2.5–2.8 inches.
If you’re upgrading to a fat tire fork, you’ll need far more width, often 135 mm or more, to fit a 4–5 inch tire without rubbing.
At this stage, you’ll also need to decide between a suspension fork and a rigid fork. Suspension forks absorb shock and are ideal for rough terrain, but they add weight and require regular service.
Rigid forks are lighter and simpler, great for city commuters and cargo bikes that prioritize efficiency and low maintenance.
Finally, double-check your brake mounts and steerer tube. Most new e-bikes use disc brakes, so your fork will need post-mount fittings for calipers. And your steerer tube must match your frame’s head tube diameter, usually a straight 1 1/8″ or a tapered design.
What to Do When Rear Dropout Parts Don’t Fit Your E-Bike
Even after carefully measuring your rear drop out fork on e-bike, it’s possible to buy a new hub, motor, or wheel only to discover it doesn’t fit. When this happens, resist the temptation to force things—it’s better to diagnose the issue calmly.
Start by re-checking your measurement. A digital caliper is best, but even with a ruler you should be able to get within a millimeter. Clean any dirt, paint, or burrs inside the dropouts, as these can throw off your reading.
Many riders accidentally measure outside-to-outside instead of inside-to-inside, which leads to incorrect numbers.
Next, consider your frame material. Steel frames are the most forgiving and can sometimes be “cold set”, carefully bent a few millimeters wider or narrower by a professional.
For example, a 132 mm steel frame can often be spread to accept a 135 mm hub. Aluminum or carbon frames, on the other hand, should never be bent; these materials are brittle and can crack, compromising your safety.
If the mismatch is small and your hub is slightly narrower, spacers or washers can fill the extra gap safely.
For instance, a 132 mm hub in a 135 mm frame can be shimmed. If the hub is slightly wider, some steel frames may flex enough, but this should only be done within a tolerance of about 2–3 mm. Beyond that, it’s not worth the risk.
Also check brake and drivetrain alignment. Even if you manage to wedge a hub into the frame, the brake rotor must line up perfectly with the caliper, and the cassette or freewheel should sit in line with the derailleur. If either is off, the wheel won’t function correctly, and you may notice brake rub or poor shifting.
When the difference is significant, say, trying to fit a 150 mm hub into a 135 mm frame, the safest choice is to replace the component with one that matches your frame’s official dropout width. Always check your bike’s manual or manufacturer specifications to confirm what that spacing should be.
If you’re still unsure, a visit to your local e-bike shop is a wise move. A technician can re-measure with precision tools, advise whether minor adjustments are safe, and help align your drivetrain and brakes after installation.
FAQs
Do I have to remove the wheel to measure the rear dropout?
Yes. Taking the rear wheel off exposes the true inside faces of the dropouts. If you measure with the wheel still in place, the axle or hardware blocks an accurate reading.
What if I don’t have a caliper?
A caliper is best, but you can still use a tape measure or ruler marked in millimeters. Place it inside the dropouts and measure straight across from inner face to inner face. Double-check by measuring twice to avoid errors.
What are common rear dropout widths on e-bikes?
The most typical widths are 135 mm on commuter and city e-bikes, 142–148 mm on mountain e-bikes with thru-axles, and 170–190 mm on fat tire e-bikes. Cargo or tandem e-bikes may use wider spacing.
What is the rear dropout width of a fat tire electric road bike?
Most fat tire e-bikes use between 170 mm and 190 mm rear dropout spacing to handle wide hubs and 4–5 inch tires. Always measure your frame since different brands may vary slightly.
Can I adjust my frame to fit a different dropout width?
If you have a steel frame, a bike shop can sometimes “cold set” it a few millimeters wider or narrower. Aluminum and carbon frames should not be bent—they may crack. For larger mismatches, the safest solution is to use components that exactly match your frame’s spacing.
How can I tell if my e-bike uses a quick release or a thru-axle?
A quick release has a lever and a thin skewer that slides through the hub, usually on 135 mm frames. A thru-axle is thicker, threads directly into the frame, and often requires a hex wrench. Thru-axles are common at 142 mm and 148 mm widths.
