So You Want to Build a Single Speed?

So you want to build a single speed.

Congratulations. You have looked at a perfectly functional bicycle and decided that it has too many useful parts.

The good news is that a single-speed drivetrain is mechanically simple. You need one chainring, one rear cog, a chain, and some way to keep that chain properly tensioned.

The bad news is that removing nine, eleven, or twelve gears does not eliminate compatibility problems. It merely concentrates all of them into one extremely important gear.

A good single-speed build is quiet, direct, dependable, and oddly satisfying. A bad one throws its chain into the woods, slips under load, loosens its rear wheel, and makes noises normally associated with farm equipment.

Let us try to build the first kind.

Build a Single Speed or Convert a Bike?

You have two basic options:

Build around a frame designed for single speeding.
Convert a geared bike you already own.

A dedicated single-speed frame will usually provide a built-in way to adjust chain tension. That may take the form of rear-facing horizontal dropouts, sliding dropouts, rocker dropouts, or an eccentric bottom bracket.

A conventional geared frame will usually have vertical dropouts. Those make wheel installation easy and hold the axle in one fixed position, but they provide no useful way to adjust chain tension.

That does not mean a vertical-dropout bike cannot become a single speed. It just means you will need another piece of equipment to take up the slack left behind by the derailleur you triumphantly removed five minutes earlier.

Rigid bikes and hardtails are generally the easiest conversion candidates.

Full-suspension bikes require more caution. Many suspension designs change the effective distance between the crank and rear axle as the suspension moves. That movement changes chain tension throughout the bike's travel. Unless the frame was designed for single-speed use or you have a tensioner that can accommodate that chain growth, the chain may go from properly tensioned to frighteningly tight as soon as the suspension compresses.

Do not discover this while landing a jump.

First, Look at Your Dropouts

The dropout is the part of the frame that holds the rear axle.

For single speeding, its shape determines whether the rear wheel can move forward and backward to adjust chain tension.

This is the first thing to inspect before buying anything.

Rear-Facing Horizontal Dropouts

Rear-facing horizontal dropouts are the classic single-speed arrangement.

The axle enters the frame from behind and can slide forward or backward inside a horizontal slot.

Moving the wheel backward tightens the chain.

Moving it forward loosens the chain.

This is ideal because the frame itself provides the chain adjustment. The chain runs directly from the chainring to the rear cog without a derailleur or sprung tensioner hanging underneath it.

The advantages include:

Simple and mechanically direct chain tension
Excellent chain wrap around the rear cog
No tensioner pulley
No tensioner spring
Fewer moving parts
A clean drivetrain
The ability to adjust for chain wear
A very convincing reason to spend twenty minutes making sure the wheel is straight

The disadvantage is that the rear wheel must be aligned manually.

You need to make sure:

The chain tension is correct.
The wheel is centered in the frame.
The brake rotor is positioned correctly.
The axle is tight enough that the wheel cannot pull forward under pedaling force.

A chain tug or axle tensioner can make this much easier. It gives you a small adjustment screw that pulls the axle backward and helps prevent the wheel from creeping forward when you stand on the pedals and attempt to relocate the bottom bracket with your legs.

Chain tugs are especially helpful with quick-release or bolt-on wheels. Some even include a bottle opener, because single-speed component designers understand their audience.

Disc brakes add another consideration. On some horizontal-dropout frames, moving the wheel changes the rotor's position relative to the brake caliper. A properly designed single-speed frame may provide an adjustable brake mount or another method of keeping the caliper aligned.

Check the frame's actual design rather than assuming that anything with a horizontal slot will work perfectly.

Sliding and Rocker Dropouts

Sliding dropouts are the modern mountain-bike version of the same idea.

Instead of sliding the axle by itself inside the frame, an entire dropout assembly moves forward or backward. On many designs, the brake mount moves with it.

That makes sliding dropouts particularly well suited to:

Disc brakes
Thru-axles
Modern mountain-bike hubs
Switching between geared and single-speed configurations

Loosen the dropout hardware, move both sides evenly, set the chain tension, align the wheel, and tighten everything to the frame manufacturer's specification.

Rocker dropouts accomplish a similar job by rotating through an adjustment range rather than sliding in a straight line.

Both systems are excellent when properly designed. They retain the clean drivetrain of horizontal dropouts while working with modern axles and brakes.

Their only real drawback is that they contain more bolts for you to forget to tighten.

Semi-Horizontal and Forward-Facing Dropouts

Some older road, touring, and cyclocross frames use semi-horizontal dropouts.

These may provide enough fore-and-aft axle movement to tension a single-speed chain, but compatibility varies.

They can work well, particularly for road or gravel conversions, but watch for:

Limited adjustment range
Wheel-removal difficulty
Brake alignment
Axle slippage
Fender interference
The wheel pulling in an inconvenient direction under chain load

They are usable, but inspect the frame carefully before ordering parts.

Vertical Dropouts

Vertical dropouts hold the axle in one fixed location.

The wheel moves upward into the frame and stops. This is convenient for reinstalling a wheel because its position is automatically established.

It is less convenient when you remove the derailleur and discover that your chain is either one link too long or one link too short.

With vertical dropouts, the rear wheel cannot be moved to tension the chain. You therefore need another adjustment method.

Your main options are:

A chain tensioner
An eccentric bottom bracket
An eccentric rear hub
A miraculous gear combination that works until the chain wears slightly

Use one of the first three.

Option One: Use a Chain Tensioner

A chain tensioner such as a Surly Singleator mounts to the derailleur hanger and uses a sprung pulley to take up chain slack.

This is often the simplest way to convert a geared hardtail with vertical dropouts.

The result is technically no longer the absolute minimum number of moving parts, but it works.

A good tensioner should:

Hold the chain securely
Align with the chainring and cog
Provide enough spring force
Accommodate the selected gear combination
Avoid contacting the chainstay
Preserve as much chain wrap as possible

Some tensioners can push the lower chain upward toward the cog or pull it downward.

Pushing upward is generally preferable when the frame provides enough clearance because it wraps more chain around the rear cog. More engaged teeth means less chance of the chain skipping under heavy pedaling force.

If the frame prevents an upward setup, a downward setup can still work. Remove as much unnecessary chain length as possible so the tensioner does not have to swing excessively far away from the cog.

Follow the tensioner manufacturer's instructions. Some products use different springs or spring orientations depending on whether they push up or pull down.

Installing the spring backward and cranking harder is not an adjustment technique.

A sprung tensioner is appropriate for a coasting single speed.

Do not use a sprung chain tensioner on a fixed-gear drivetrain. A fixed drivetrain transmits force through both sides of the chain. A tensioner can fold, bind, or be pulled into the drivetrain.

This article is primarily about freewheeling single speeds: pedal forward, coast when desired, and retain the ability to stop moving your legs before they achieve atmospheric reentry.

Option Two: Use an Eccentric Bottom Bracket

An eccentric bottom bracket, commonly called an EBB, adjusts chain tension at the crank instead of the rear wheel.

The crank spindle sits off-center inside a larger rotating assembly. Rotating that assembly moves the crank slightly forward or backward relative to the rear axle.

That movement tensions the chain while the rear wheel remains fixed in the vertical dropouts.

An EBB provides several advantages:

No external chain tensioner
A clean, direct chain path
Good chain wrap
Consistent rear-wheel placement
Easy wheel removal
No need to realign the rear brake after fixing a flat

The catch is compatibility.

Some frames are built around an oversized shell specifically designed for an eccentric bottom bracket. Conversion EBBs also exist for certain threaded, BB30, and PF30 frames, but they are not universal.

Before buying one, verify:

Frame bottom-bracket standard
Shell width
Shell diameter
Crank spindle diameter
Crank compatibility
Available adjustment range
Chainring and frame clearance
Manufacturer installation requirements

An EBB may also require occasional cleaning or maintenance. Depending on the design, a dirty or improperly tightened eccentric can creak, slip, or become permanently committed to its current position through corrosion.

It is still a wonderfully clean solution when the frame and parts cooperate.

Option Three: Use an Eccentric Rear Hub

An eccentric rear hub places the axle off-center inside the hub hardware.

Rotating the axle changes the hub's position within otherwise fixed vertical dropouts, providing enough fore-and-aft adjustment to tension the chain.

The White Industries ENO eccentric hub is the best-known example.

This can create an elegant conversion because it provides a straight chain path without a derailleur-mounted tensioner or special frame bottom bracket.

The limitations are mostly compatibility and cost.

Eccentric hubs are available only for certain:

Axle standards
Dropout types
Hub widths
Brake configurations

They are much less universal in the era of Boost spacing and modern thru-axles.

Still, for the correct older hardtail, road bike, or cyclocross frame, an eccentric hub can turn a vertical-dropout bike into a very convincing dedicated single speed.

What About a Half-Link or a "Magic Gear?"

A half-link changes the chain length in smaller increments than removing a normal pair of inner and outer links.

It can help fine-tune chain length when you already have a legitimate adjustment system.

It is not a replacement for one.

A "magic gear" is a chainring-and-cog combination that happens to produce nearly perfect chain tension in a frame with vertical dropouts.

This may work briefly.

Then the chain wears.

Or the chainring is not perfectly round.

Or you replace the cog.

Or the weather changes.

Or Mercury leaves retrograde.

Without a proper adjustment mechanism, a magic gear is less of an engineering solution and more of a temporary alignment of the planets.

Cog on a Freehub Versus a Threaded Freewheel

This terminology confuses almost everyone at first.

The thing commonly called a "freewheel hub body" is usually a freehub body.

A freehub and a threaded freewheel provide the same basic riding behavior (you can stop pedaling while the bike continues moving), but they put the ratcheting mechanism in different places.

Freehub Body With a Single Cog

Most modern geared mountain bikes use a freehub.

The ratcheting mechanism is built into the rear hub. A cassette slides onto splines on the freehub body and is held in place with a lockring.

For a single-speed conversion, you remove the cassette and install:

One single-speed cog
A set of spacers
A cassette lockring

The spacers position the cog laterally so it lines up with the front chainring.

This is one of the easiest and most flexible conversion methods because you can rearrange the spacers to adjust chainline.

Use a dedicated single-speed cog when possible.

A good single-speed cog has:

Full-height teeth
No shift ramps
No shift pins
A wider base where it contacts the freehub
Enough material to withstand repeated high-torque pedaling

A thin individual cassette sprocket may technically fit, but a narrow steel cog can dig into a soft aluminum freehub body. It may also have tooth shaping intended to help a chain leave the cog during a shift.

On a single speed, the chain leaving the cog is not a feature.

It is an incident report.

Also verify the freehub standard. Many conversion cogs and spacer kits are made for Shimano HG-style freehubs. XD, XDR, and Micro Spline drivers require compatible parts and may offer fewer straightforward conversion choices.

Threaded Single-Speed Freewheel

A traditional single-speed hub may have a threaded section instead of a splined freehub body.

A complete freewheel screws onto those threads.

The freewheel includes:

The cog
Bearings
Pawls
Ratchet mechanism
Threaded mounting body

Pedaling tightens it onto the hub.

Removing it later requires the correct freewheel-removal tool and sometimes a persuasive amount of leverage, because every climb you have ridden has spent time tightening it.

Many traditional single-speed and BMX freewheels use a common thread standard, but confirm the hub and freewheel specifications before ordering.

A threaded freewheel gives you a simple, dedicated arrangement with no cassette spacers or lockring. Chainline adjustment, however, may be less flexible because the freewheel sits where the hub manufacturer intended it to sit.

You may need to adjust the front chainring position, crank, bottom-bracket spacing, or hub spacing to align everything properly.

A Freewheel Is Not a Fixed Cog

A threaded freewheel allows coasting.

A fixed cog does not.

A proper fixed-gear hub generally uses a separate reverse-threaded lockring to prevent the cog from unscrewing when resisting the pedals.

Do not thread a fixed cog onto an ordinary freewheel hub, skip the lockring, add a chain tensioner, and declare victory.

That is not single-speed minimalism.

That is a request for dental work.

Pick a Gear Ratio

This is where the single-speed build becomes personal.

Your gear ratio is:

front chainring teeth ÷ rear cog teeth

Examples:

32x16 = 2.00:1
32x18 = 1.78:1
32x20 = 1.60:1
30x20 = 1.50:1

A larger front chainring makes the gear harder.

A larger rear cog makes the gear easier.

Unfortunately, a simple ratio does not tell the entire story. The same 32x18 combination feels different on a 26-inch bike, a 27.5-inch bike, and a 29er because the larger wheel travels farther with each revolution.

Tire diameter matters too. A 29x3 tire creates a taller effective gear than a smaller 29-inch tire.

Use the single-speed gear calculator on this site to compare:

Chainring size
Cog size
Wheel and tire diameter
Gear ratio
Gear inches
Speed at different cadences

The goal is to find a gear you can climb without spinning out too quickly on flat or descending terrain.

Those goals are in direct opposition.

Welcome to single speeding.

Choose for the Climb You Actually Ride

Do not choose a gear based on the one climb you once cleared while fully rested, being chased by weather, and temporarily possessed by the spirit of Ned Overend.

Choose a gear you can turn repeatedly during a normal ride.

For a mountain bike, it is generally better to begin slightly easier than slightly harder. You can learn to spin faster. You can carry more momentum. You can become stronger.

You cannot negotiate with a 20-percent grade halfway through a three-hour ride.

A climb-friendly 29er setup might begin around 32x20. A stronger rider, flatter terrain, or a race-oriented build might use 32x18 or something taller.

These are examples, not commandments.

Ask local single-speed riders what they use. Their answers will account for local terrain far better than a generic internet chart.

Then remember to ask:

What wheel size?
What crank length?
What tire size?
How strong are you?
Are you lying?

Spinning Out Is Normal

At some point, you will run out of cadence.

Your feet will be moving quickly, the bike will stop accelerating, and someone on a geared bike will shift three times and ride away.

That is fine.

Coast.

Single speeding has three speeds:

Pedal
Coast
Push

The correct ratio is not the one that eliminates all three. It is the one that creates an acceptable balance among them.

Bigger Cogs Are Usually Your Friend

Several combinations can produce nearly the same ratio.

For example:

32x16
34x17
36x18

All are 2:1.

They do not behave identically.

Larger chainrings and cogs generally provide:

More teeth engaged with the chain
Better chain wrap
Lower load on each individual tooth
Slower drivetrain wear
Less chance of skipping under heavy torque

A smaller chainring provides more ground clearance, which is valuable on a mountain bike.

The practical compromise is usually to avoid extremely small rear cogs, especially when using a sprung tensioner. A cog with 18 or more teeth is often a good place to begin for an off-road conversion.

If you need an easier gear, increasing the rear cog is generally preferable to shrinking the front chainring into something that belongs on a child's unicycle.

Choose the Right Chainring, Cog, and Chain

A single-speed drivetrain does not need shifting features.

Use a chainring without:

Shift ramps
Shift pins
Heavily sculpted teeth intended for front shifting

Use a dedicated single-speed cog without shifting ramps.

The chain, chainring, and cog must all be width-compatible.

A 3/32-inch drivetrain is a practical choice for many mountain-bike conversions because it works with common multi-speed chains and many single-speed components.

A wider 1/8-inch drivetrain is common on some BMX, track, and urban bikes, but wider does not automatically mean stronger. Use it only when the chainring, cog or freewheel, and tensioner are designed for it.

Do not mix widths and hope the chain will become more open-minded after the first ride.

Whenever possible, begin with parts that have compatible wear:

New chain
New or lightly worn cog
New or lightly worn chainring

A new chain on a badly worn cog may skip under load. A worn chain on a new cog may run noisily and accelerate wear.

Single-speed parts live in the same gear for every pedal stroke. They do not share the work across a cassette. Check them regularly.

Chainline: Important, but Not Mystical

Chainline is the alignment between the front chainring and rear cog.

Viewed from above, the chain should run in a straight line.

It does not need to be aligned with laboratory-instrument precision, but it should be close.

Poor chainline can cause:

Noise
Accelerated wear
Chain derailment
Cog skipping
Tensioner wear
Unpleasant mechanical feelings in your soul

On a freehub conversion, adjust chainline by moving cassette spacers from one side of the cog to the other.

At the front, chainline may be adjusted through:

Chainring position
Chainring spacers
Direct-mount chainring offset
Bottom-bracket spacers
Bottom-bracket spindle length
Crank selection

Do not stack a ridiculous quantity of improvised washers under the chainring bolts.

Use the correct parts.

And What About Q-Factor?

Q-factor is the lateral distance between the pedals.

It influences stance width and bike fit.

You should know it exists, but you do not need to redesign the entire bicycle around it for your first single-speed build.

Do not move the crank so far outward to fix chainline that the pedals feel like they belong on separate bicycles.

If achieving acceptable chainline requires dramatically compromising crank fit, frame clearance, or pedal position, choose a different crank, chainring offset, bottom bracket, hub, or cog arrangement.

Crank Length Matters, Just Not as Much as the Gear Ratio

Single-speed riding exaggerates both ends of the cadence range.

On a climb, you may be turning the cranks slowly while producing a great deal of force.

On a descent or flat section, you may be spinning much faster than you would choose on a geared bike.

Crank length affects how those situations feel.

Longer Cranks

Longer crank arms provide slightly more leverage for the same force applied at the pedal.

They also:

Reduce ground clearance
Increase the circle traveled by your feet
Increase hip and knee movement
Make very high cadences feel more dramatic
Increase the opportunity for pedal strikes

Shorter Cranks

Shorter crank arms provide slightly less mechanical leverage for the same pedal force, but they can:

Improve ground clearance
Make high cadence more comfortable
Reduce pedal strikes
Reduce hip and knee range of motion
Work well with modern low-bottom-bracket mountain bikes

This does not mean you should install extremely long cranks to climb better or tiny cranks to spin indefinitely.

The difference among common mountain-bike lengths is usually smaller than the difference created by changing the rear cog by one or two teeth.

For most conversions, keep the crank length that already fits you comfortably.

If you are choosing new cranks, consider:

Rider fit
Leg length
Bike geometry
Bottom-bracket height
Pedal-strike risk
Preferred cadence
Terrain

Adjust the gear ratio before using crank length as a substitute for appropriate gearing.

A 180 mm crank will not rescue a terrible ratio.

It will simply allow you to hit more rocks with greater authority.

Set the Chain Tension Correctly

The chain should be tight enough that it cannot bounce off, but loose enough that it does not bind the drivetrain.

Chainrings and cogs are rarely perfectly round. Cranks, freewheels, and hub bodies may also have slight variations.

This means chain tension often changes as the cranks rotate.

Before final adjustment:

Rotate the cranks through several complete revolutions.
Find the tightest point.
Set the tension at that point.
Confirm the cranks still rotate freely.
Check that the chain is not excessively loose at the loosest point.

The chain should have a small amount of movement.

It should not hang like a clothesline.

It should also not sound like a guitar string tuned by someone preparing for a very angry concert.

A chain that is too tight can:

Add pedaling resistance
Accelerate bearing wear
Damage the freewheel or freehub
Wear the chain and sprockets
Make the drivetrain feel rough

A chain that is too loose can:

Fall off
Skip under load
Slap the frame
Turn a pleasant descent into an unscheduled chain-management workshop

Recheck chain tension after the first few rides. New drivetrain parts settle and wear, and a chain that was perfect in the garage may become loose after several hard climbs.

A Sensible Build Order

A single-speed conversion goes more smoothly when performed in this order.

1. Inspect the Frame

Identify:

Dropout type
Rear axle standard
Hub spacing
Brake configuration
Bottom-bracket standard
Suspension chain growth
Derailleur-hanger condition

2. Choose the Tensioning Method

Decide among:

Horizontal dropouts
Sliding or rocker dropouts
Chain tensioner
Eccentric bottom bracket
Eccentric rear hub

Do this before buying the drivetrain.

3. Choose a Starting Ratio

Use the calculator.

Ask local riders.

Err slightly easier for mountain biking.

Remember that pushing is a gear, but it should not be the only gear you use uphill.

4. Choose the Rear System

Determine whether you have:

HG-style freehub
Another splined freehub standard
Threaded single-speed hub
Eccentric hub
Dedicated single-speed hub

Buy the correct cog, spacers, lockring, or threaded freewheel.

5. Choose the Front Chainring

Verify:

Tooth count
Crank compatibility
Bolt-circle diameter or direct-mount interface
Offset
Chain width
Frame clearance

6. Establish Chainline

Install the rear cog and front chainring.

Align them as closely as practical before shortening the chain.

7. Size the Chain

Route the chain around the chainring and cog.

Remove as many links as practical while retaining sufficient adjustment range.

Do not make the tensioner absorb six inches of unnecessary chain because using the chain tool felt inconvenient.

8. Set Tension and Wheel Alignment

For adjustable dropouts:

Center the wheel.
Align the brake.
Set chain tension.
Tighten each side gradually.
Torque the hardware correctly.
Install chain tugs if appropriate.

For a tensioner:

Set the pulley alignment.
Use the correct spring orientation.
Maximize chain wrap when possible.
Confirm chainstay clearance.

For an EBB or eccentric hub:

Follow the manufacturer's procedure.
Set tension at the tightest point.
Tighten all retention hardware correctly.

9. Test Under Load

A drivetrain that works in a repair stand has passed the least demanding test imaginable.

Ride somewhere safe.

Pedal hard while seated.

Pedal hard while standing.

Compress the suspension if applicable.

Listen for:

Clicking
Popping
Skipping
Grinding
Chain derailment
Wheel movement
Your own immediate regret

Recheck all hardware after the first ride.

Common Problems

The Chain Falls Off

Likely causes:

Chain too loose
Poor chainline
Worn cog or chainring
Shift-ramped cog
Tensioner misalignment
Frame flex
Insufficient chain wrap

The Chain Is Tight in One Spot and Loose in Another

This is common.

Find the tightest point and set tension there.

If the variation is extreme, inspect:

Chainring installation
Bent chainring
Damaged cog
Freewheel alignment
Crank or spider damage
Hub condition

The Rear Wheel Pulls Forward

Likely causes:

Axle not tight enough
Dirty or ungreased axle hardware
Dropout hardware not properly torqued
No chain tug
Insufficiently secure quick release
Heroic amounts of torque

Use the correct torque and a suitable chain tug or axle-retention system.

The Drivetrain Skips Under Load

Likely causes:

Worn chain and cog
Insufficient chain wrap
Rear cog too small
Poor chainline
Weak tensioner spring
Freewheel or freehub engagement problem
Shift-ramped cassette cog
Chain tension too loose

Do not continue repeatedly stomping on it to see whether the problem has developed respect for you.

The Bike Is Too Hard to Climb

Install a larger rear cog or smaller front chainring.

This is not surrender.

It is component selection.

The Bike Spins Out Too Soon

Install a smaller rear cog or larger front chainring.

Alternatively, coast.

Coasting is free, lightweight, and already installed.

What You Actually Need

For a basic freehub conversion, you will generally need:

A compatible frame
One front chainring
One dedicated single-speed cog
Freehub spacers
Cassette lockring
Compatible chain
Chain-tensioning solution
Cassette tool
Chain whip
Chain tool
Appropriate axle and dropout tools
Torque wrench where specified
Patience

For a threaded single-speed hub, replace the cog, spacers, and cassette lockring with:

A compatible threaded freewheel
The correct freewheel-removal tool

You may also need:

Chainring spacers
Different chainring bolts
Chain tug
Eccentric bottom bracket
Eccentric rear hub
Different bottom bracket
Different crank
A conversation with your local bike shop
Another bicycle

The Final Test

A successful single speed should be quiet.

The chain should remain in place.

The rear wheel should remain where you put it.

The cranks should turn without binding.

The selected gear should make local climbs difficult but possible and flat terrain fast enough to remain entertaining.

It will not be the perfect gear everywhere.

That is the point.

A geared bike tries to provide the correct ratio for every part of the trail.

A single speed asks you to become the correct rider for the ratio you chose.

Sometimes that means standing and pedaling harder.

Sometimes that means carrying more speed into the next climb.

Sometimes that means coasting.

Sometimes that means getting off and pushing while insisting that walking is an alternate drivetrain mode.

Build it correctly, choose a reasonable gear, and go ride it.

You can overthink the second single speed.

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