Dropouts

Dropouts are the slotted plates at the back of a BMX frame and the tips of the fork where the wheel axles sit. The rear dropouts are horizontal so the wheel can slide back to tension the chain. The width of the slot decides which hub axles fit, and the plate's thickness and shape affect strength and peg fit.

What it does

Dropouts are where the wheels meet the frame. The axle sits in the slot and the axle nuts or bolts clamp it against the dropout faces. Every landing, grind and pedal stroke passes through this joint, so the dropouts have to be strong and the axle has to be tight.

On the rear, the dropouts do a second job. Because a BMX has one gear, the chain cannot be tensioned by a derailleur. Instead, the rear wheel slides backwards in the horizontal slots until the chain is tight, then the axle is locked in place. See how to fix a BMX chain for setting tension.

Types and standards

Rear dropouts

Almost all BMX frames use horizontal rear dropouts that open towards the back of the bike. The slot is long enough to give some adjustment, which also changes the effective chainstay length. A frame listed with a chainstay range, such as a short and a long setting, is describing the ends of that slot. See frame geometry.

Other details vary by frame:

• Plate dropouts. Flat plates cut from steel sheet and welded into the stays. The most common design.
• Tapered dropouts. Thinner towards the slot or shaped to save weight and leave room for pegs and hub guards.
• Integrated chain tensioners. Some frames have threaded holes or built-in tensioner bolts in the dropout.
• Brake mounts. Frames for U-brakes carry the mounts on the seatstays, not the dropouts, but some brakeless-specific frames leave them off entirely. See brakes.

Front dropouts

The front dropouts are part of the fork. They are usually short, downward-facing slots. Most freestyle forks have 10 mm slots, which take 3/8 inch (about 9.5 mm) or 10 mm axles. Some forks are built for other sizes, so check the maker's spec.

Axle sizes

Axle	Where it is common	Notes
14 mm	Rear wheels on most freestyle frames	Strong and stiff. The current standard for rear hubs.
10 mm	Front wheels, and rear wheels on some budget bikes	Common on female front axles.
3/8 inch (about 9.5 mm)	Entry level and older bikes	Lighter, but bends more easily. Fits a 10 mm slot.

See cassette hub, freecoaster hub and front hub for the hub side of this.

Sizing and compatibility

1. Slot width against axle. The axle must fit the slot without forcing. A 14 mm axle will not go into a 10 mm slot.
2. Do not file slots. Opening a slot to take a bigger axle weakens the dropout and usually voids the frame warranty.
3. Rear spacing. BMX rear hubs are commonly 110 mm wide between the dropouts. Check the frame spec.
4. Thickness against axle length. Thick dropouts use more of the axle. Add chain tensioners, hub guards and pegs, and there may not be enough thread left for the nut. The nut or bolt needs full thread engagement.
5. Male and female axles. Male axles have threaded ends and nuts. Female axles are hollow and take bolts. Dropout thickness affects how much of each sticks out.
6. Peg and guard clearance. The outside face of the dropout must sit flat for pegs and guards to seat properly.

Chain tensioners

Chain tensioners are small plates or blocks that sit on the axle in the dropout. A bolt pushes against the frame or the axle to stop the wheel being pulled forward. They help heavy riders and strong pedallers, and bikes where pegs and guards leave less room to clamp the axle hard. They add thickness on the axle, so check the axle length.

Hub guards

Hub guards sit on the axle against the outside of the dropout. They protect the axle ends and the hub driver during grinds. Like tensioners, they use up axle length.

Materials

Material	Where	Notes
4130 chromoly	Most aftermarket frames and forks	Strong and tough. Often heat treated for extra strength.
Hi-ten steel	Budget frames	Heavier and more likely to bend. See chromoly vs hi-ten.
Aluminium	Some race frames	Light. Usually thicker to make up for lower strength.

Freestyle frames and forks commonly use dropouts around 5 to 6 mm thick, and some frames go up to 8 mm, often with chain tensioners built in. Thicker dropouts resist bending from pegs and grinds, but add weight and use up axle length.

How to choose

You rarely buy dropouts on their own. They come with the frame or fork. When comparing frames, look at:

1. Rear slot size. 14 mm is the standard for freestyle and works with almost all modern rear hubs.
2. Thickness and material. Heat treated chromoly is the norm on good frames.
3. Slot length. A longer slot gives more chainstay adjustment and more room for gearing changes.
4. Tensioner provision. Built-in tensioners save a part and keep things neat.
5. Clearance. If you run pegs or guards, check that the dropout shape suits them.

Maintenance

• Keep axle nuts and bolts tight. A loose rear wheel can pull forward and throw the chain. Follow the hub maker's torque figures.
• Check the dropouts for cracks around the welds and at the back of the slot, especially after heavy landings.
• Look for spread or bent dropouts. If the wheel will not sit straight, the dropouts may be out of line.
• Clean grit off the dropout faces when you remove the wheel so the axle seats flat.
• Re-check chain tension after a crash, since the wheel can shift.
• See BMX maintenance basics for a regular routine.

Related parts

Frame geometry, forks, cassette hub, front hub, hub guards, pegs and chain.