“You know your front dropout is loose?” the mechanic asked me when I dropped my mountain bike off for a shifter adjustment. This was back in the days when I didn’t know a bike tube from those rubber floaties you ride down the river. I didn’t know, and I didn’t care.

“I just need a derailleur adjustment. What’s a ‘dropout,’ anyway?”

The mechanic was stunned. His face literally paled, and his mouth fell open. “Your front wheel is about to fall out.”

Rear axle drop out on an MTB
Rear axle drop out on an MTB

So, I figured I better learn: What’s a mountain bike dropout (anyway)?

A mountain bike dropout is a slot in the bicycle where the rear axle sits and allows the wheel to be removed without removing or derailing the chain. In casual lingo, the term also refers to the slots on the fork where the front axle is inserted.

The dropout was introduced in the 1930s and literally means what it says: it allows the wheel to “drop out” of the bicycle. This meant faster, easier rear wheel changes without derailing the chain. Mountain bike manufacturers did this by making the slot face forward or even straight down, in the case of vertical dropouts.

Technically, the term “dropout” refers to the slots in the frame where the rear wheel is inserted. However, most people use the term for the front wheel as well, where the front axle fits into the fork. There are different types of mountain bike dropouts, but most rely on a quick release skewer to hold the axle – and thus, your wheel – securely in the bike frame.

Front Fork Dropouts

Front Bike Fork Dropout
Front Bike Fork Dropout

Regardless of the front suspension system on your mountain bike, the front fork dropout functions the same. The slots at the bottom of the front fork where the axle fits is the “dropout.” A quick release skewer, through the front axle, holds the wheel in your mountain bike.

Rear Derailleur Hangers with Dropouts

Rear Bike Frame Dropout
Rear Bike Frame Dropout

As you see in the video for replacing the derailleur hanger in a rear dropout, the design of this mountain bike dropout allows you to remove the rear wheel easily, without derailing the chain. This is great for fast tube or tire replacement or repairs on the rear end of the bike.

Another benefit is that derailleur hangers are cheap to replace – and it’s an easy mountain bike repair you can do yourself. You might be surprised how easily a derailleur hanger can be bent if you, or your bike (or both!) take a tumble. A bent mountain bike derailleur hanger is much easier to repair than a bent frame. 

MTB Tools I Love and Recommend

Bike Hand Repair Stand
Bike Hand Repair Stand
Bike Hand 37 pcs Tool Box
Bike Hand 37 pcs Tool Box
Topeak Smartgauge D2 Air Pressure
Topeak Smartgauge D2 Air Pressure

I own each of these tools and only recommend things I own and use.

  • Bike Hand Bike Repair Stand.  Nice mountain bikes don’t have a kick stand so keeping your MTB safe but conveniently stored is essential.  I keep my bike on my stand whenever I’m not riding it.  This makes it easy to lube the chain, inflate the tires and adjust the derailleur.  Highly recommended – Bike Hand Bike Repair Stand (👈 Link to Amazon to see what thousands of others have said)
  • A basic MTB toolbox for replacing a chain, adjusting brakes and dialing in the fit.  Bike Hand has a 37-piece box that has most of the specialty bike tools to keep your MTB properly maintained.  The Bike Hand brand is value packed for the avid rider.  Check out the competitive prices with this link to Amazon 👉 Bike Hand 37 pcs Bike Repair Tool Kit
  • Get a good air pressure gauge, if you get just a tiny bit serious about MTBing you’re going to start playing with tire pressure.  A couple psi can make your tires sticking or not.  Get a good gauge, I highly recommend the Topeak Smartgauge D2, it’s accurate, flexible and easy to use.  An Amazon best seller, here’s a link 👉 Topeak Smartgauge D2
  • Carry a multitool with you on every ride.  I’m serious, most of the time you can MacGyver something to get back to the trailhead if you have a multitool.  I’ve got the Crank Brothers M19, it’s worn, rubbed and abused – but it still works.   Thousands sold on Amazon – check it out with this link 👉 Crank Brothers M19

Horizontal Rear Dropouts

Horizontal rear dropouts are not as common on mountain bikes as a rear derailleur hanger with a dropout. In this system, the slot for the rear axle is horizontal, either opening to the rear of the bike or the front.

I may be wrong on this one (and let me know if I am), but I think the primary benefit of a horizontal rear dropout is when converting a mountain bike into a single speed or fixed-gear. Rear-facing horizontal dropouts are primarily used on track bikes, while forward-facing horizontal dropouts are more often used on singles-speed mountain bikes.

One difficulty with rear-facing horizontal rear dropouts is setting and maintaining chain tension. Meanwhile, the downfall of forward-facing rear dropouts is that axle bolts can interfere with disc brake systems. Neither style is recommended for standard, recreational mountain bikes.

How Do You Replace a Rear Derailleur Hanger with Dropout

If a bike crash damages your derailleur hanger, there’s a good chance it’ll be a fairly easy fix. First, you need to determine whether your derailleur hanger needs to be realigned, bent back into position or replaced. For a more detailed look at rear derailleur repairs for your mountain bike, check out DIY Mountain Bike’s article, “What is a Mountain Bike Rear Derailleur Hanger and How to Fix It.”

Rear derailleur hangers are usually lightweight aluminum, and it is possible to bend the derailleur hanger back to its original shape – or at least close enough to be functional. But, a note of caution: you can only bend your derailleur hanger a few times before you start damaging the integrity of the part.

Rear Derailleur Hanger with Dropout
Rear Derailleur Hanger with Dropout

If you’ve already straightened your derailleur a few times by bending it – or the latest crash bent it so badly you can’t bend it back – you probably need to replace it. The bad news is: it’s pretty hard to straighten a twisted derailleur hanger. But, the good news is: it’s easy to replace to a derailleur hanger.

Based on the premise that it’s easier to replace a derailleur hanger than fix a bent frame, the derailleur hanger is made to be, in essence, a sacrificial part. Read our step-by-step directions on how to replace a derailleur hanger, or watch our video.

Remember when I dropped off my mountain bike for service and was stunned to realize my front wheel was about to fall off? At the risk of exposing my age, that was twenty years ago. Interestingly, dropouts and quick release mechanisms haven’t changed significantly since then. Indeed, since the design was first invented. There is, however, a newer, better design that is becoming the dominant axle system for high-end mountain bikes and may eventually render the mountain bike dropout obsolete. It’s the thru axle.

What’s a Thru Axle?

Just like a mountain bike “dropout,” a “thru axle” is exactly what its name indicates: an axle that goes through. In this case, the axle is a hollow tube in the wheel hub. It uses a cam system – like a quick release skewer – to hold it in the frame or fork. In fact, it’s easy to mistake it for a quick release with one glance. But, unlike a quick release, the skewer only serves to hold the wheel on, not provide the axle.

Benefits of a Mountain Bike Thru Axle

The thru axle is superior to a mountain bike dropout with a quick release for many reasons:

  • It’s more rigid, reducing energy loss in climbing.
  • It’s more structurally sound. (Quick release skewers have been known to snap under the intense stress and weight of mountain biking, particularly downhill riding.) Through axles are beefier, and I’ve never heard of one breaking during normal bike use.
  • It’s virtually impossible for the wheel to detach. (Remember by near-brush with one-wheeled mountain bike riding?) This is because the axle threads directly into the fork or frame guaranteeing a secure bike connection. With a thru axle, the entire axle would have to come out of the wheel before the wheel would detach from the frame. 
  • With a disc brake system, the force of the disc pushes forward and down, which can push the wheel right out of the dropouts. Not so with a thru axle.
  • Alignment of disc brake rotors is more precise and repeatable than with quick release system.

If you’d like to learn more about thru axles and how they differ from quick release axles, check out this article: “What Is the Difference Between a Quick Release and Thru Axle?”

Can Mountain Bike Dropouts Be Repaired?

The answer to this is a very solid: maybe. It depends on the degree and location of the damage, if the structural integrity of the repaired fame will be safe, and the material of the bike frame (titanium can be welded; carbon cannot). It’s not a repair I’d suggest attempting yourself. Additionally, repairing dropouts usually requires specialized tools that most folks don’t have in their garage. And no, you can’t borrow my blowtorch. 

Check Your Mountain Bike Dropout Quick Release Before Every Ride

To summarize: mountain bike dropouts will very likely become obsolete in the near future, due to the superiority of the thru axle. They’re already nonexistent on high-end mountain bikes. However, many of us still have a quick release attachment on standard mountain bike dropouts, so it’s good to know the basics for repair and safe riding.

To the casual biker, the mountain bike dropout is arguably the most unnoticed or unknown part of the bike with the most potentially disastrous repercussions. I say this because it is incredibly easy to inadvertently snag a quick release – or forget to close it securely after removing a wheel – and risk having your wheel fall off.

More than once, I have seen experienced, aggressive mountain bikers shoot off a jump and have their front tire fall off in midair. What happened? The quick release on their front dropout wasn’t closed properly. And then what? I can summarize these accidents by saying: they’re not pretty, and they’ve always entailed a trip to the emergency room.

If you have a mountain bike dropout, always check your quick releases before you ride. The quick release lever should be:

  • Hard to close. (Quick tip: closing it should leave an indentation in your palm.)
  • Closed securely. (Lever-to-skewer angle greater than 90-degrees.)
  • Closed so that the lever points to the rear of the bike (this limits the chances of snagging it – and having it pulled open – while riding.)

Kat Jahnigen Writer

Professional writer Kat Jahnigen was 2 miles from the nearest village – and roughly 2,310 miles – from the nearest English-speaking town – when her bike tire burst. At that time, she was a college student on a bike trip across the desolate, rocky island of Crete. It suddenly occurred to her that it would’ve been good to learn some basic bike repairs before setting off on a solo bike trip.

Check out Kat’s website WriteHire at writehire.net.