Drivetrain
The drivetrain is where most compatibility pain lives. Shimano, SRAM, and Campagnolo all use slightly different indexing, and 9-speed parts will not play nicely with 11-speed parts without deliberate engineering.
Cogs, Freewheels & Cassettes
If you want a rear cog larger than 11t, it must be splined (i.e., it mounts to a splined freehub body). A 12t or larger threaded freewheel cog does not exist in practical production. Threaded track cogs max out at 11t.
There are two entirely different systems here that beginners constantly confuse:
| Type | How it mounts | Max cog size | Hub required |
|---|---|---|---|
| Track cog / freewheel cog | Threaded onto hub shell | ~11t | Threaded track hub (1.375" × 24tpi) |
| Cassette cog | Splined onto freehub body | 46t+ (with right derailleur) | Freehub (Shimano HG, SRAM XD, Micro Spline) |
| Freewheel (BMX style) | Threaded onto hub shell | 22t+ common | Threaded BMX hub (1.375" × 24tpi — same thread!) |
Track hubs and BMX threaded hubs use the same thread pitch (1.375" × 24tpi), but a BMX freewheel will not work correctly on a track hub because the freewheel needs the correct dish and lockring clearance. Also: track hubs are double-threaded — left side is reverse-thread for the lockring.
Chain Width & Speed Compatibility
Chains get narrower as you add more speeds to the cassette. A 9-speed chain is wider than an 11-speed chain. Running the wrong width causes poor shifting at best, dropped chains and damage at worst.
| Speeds | Outer width (approx) | Shimano chain codes |
|---|---|---|
| 1s (single speed) | ~8.7mm outer | any 1/8" chain |
| 8s | 7.1mm | HG40, HG71 |
| 9s | 6.7mm | HG53, HG73 |
| 10s | 6.2mm | HG54, HG95 |
| 11s | 5.62mm | HG701, CN-LG500 |
| 12s | 5.26mm | CN-M9100 |
1/8" chains (single speed) are much wider and will NOT fit between cassette sprockets. Geared bikes use 3/32" chains. If you're building single speed with a cassette cog and freehub, you still use a 3/32" narrow chain.
Bottom Bracket
Bottom bracket standards are one of the most fragmented areas in cycling. You need to know: (1) your frame's shell type, (2) your crankset's spindle type, and (3) pick a BB that bridges the two.
Frame Shell Standards
| Shell Name | Width | Bore diameter | Thread? |
|---|---|---|---|
| BSA / English | 68mm or 73mm | 33.8mm | Yes (1.37" × 24tpi) |
| Italian | 70mm | 35mm | Yes (36mm × 24tpi) |
| BB30 | 68mm or 73mm | 42mm | No (press fit) |
| PF30 | 68mm or 73mm | 46mm | No (press fit) |
| BB86 / BB92 | 86.5mm or 92mm | 41mm | No (press fit) |
| T47 | 68mm or 79mm | 47mm | Yes (47mm × 1mm) |
BSA shells are reverse-threaded on the drive side (left-hand thread). You install it counter-clockwise. Italian BB is same thread both sides. Getting these confused will strip your frame.
Press-fit BBs (BB30, PF30, BB86) require a press tool for installation. Hammering them in damages the bearing seats. They also require a facing tool on cheaper frames or they'll creak forever.
Crankset Spindle Types
The other half of the equation is what kind of spindle your crank uses:
- Square taper — old standard, simple, still common on entry-level builds
- Shimano Hollowtech II — 24mm spindle, threaded to the crank arm
- SRAM GXP — 24mm drive side, 22mm non-drive side (this asymmetry trips people up)
- 30mm spindle (BB30, PF30 native) — Cannondale, some SRAM cranks
- ISIS — dead standard, but still floating around on old/cheap cranks
Which BB Do I Need?
Headset
The headset connects the fork steerer tube to the head tube. Get any one of three dimensions wrong and nothing fits.
Three things you must know about your frame
1. Head tube inner diameter — This determines the cup size.
| Standard | Head tube ID | Cup OD |
|---|---|---|
| EC34 (old 1") | 34mm | 30.2mm cups |
| EC44 (standard 1-1/8") | 44mm | 41mm cups |
| EC49 / OS | 49mm | 46mm cups |
| IS42 (integrated) | 42mm — cups press straight in | N/A |
| IS52 (tapered lower) | 52mm lower | N/A |
2. Steerer tube diameter — Must match the bearing inner race.
- 1" (25.4mm) — Old bikes, some BMX/cruiser. Threaded or threadless.
- 1-1/8" (28.6mm) — The dominant modern standard for flat-top forks.
- Tapered (1-1/8" top, 1.5" bottom) — Modern MTB and many road/gravel forks.
A tapered fork CANNOT go into a straight 1-1/8" head tube without special conversion cups (and even then it's sketchy for load-bearing use). The fork taper is there to stiffen the lower bearing — the frame must be designed for it.
3. Head tube length — Affects stack height, spacer needs, and whether you'll have enough steerer above the headset to fit your stem.
When cutting a carbon steerer, you typically need to leave at least 3–5mm of steerer above your top spacer before cutting. Cut it too short and the cap can't preload the bearings. You cannot add material back.
Threadless headsets need a stem and star nut (or expander plug for carbon) to apply preload. The stem clamp does the clamping. The preload cap sets bearing tension. These are two separate functions — don't overtighten the preload cap thinking it holds the fork in.
Axles & Dropouts
Hub and fork/frame dropout standards have multiplied aggressively since the thru-axle takeover. Quick release still works fine for many builds, but if you're mixing wheels and frames, know your numbers.
Quick Release vs. Thru-Axle
QR (quick release) is a 5mm skewer passing through a hollow 9mm (front) or 10mm (rear) axle. Thru-axles are solid threaded bolts of specific diameter and length.
| Standard | Diameter | Common length | Typical use |
|---|---|---|---|
| QR Front | 9mm | 100mm spacing | Road, old MTB |
| QR Rear | 10mm | 130mm (road) / 135mm (MTB) | Road/MTB |
| Thru-Axle Front | 12mm or 15mm or 20mm | 100mm spacing | MTB / road disc |
| Thru-Axle Rear | 12mm | 142mm or 148mm (Boost) | MTB / road disc |
"Boost" spacing (148mm rear, 110mm front) adds 6mm to each side compared to standard. A Boost hub will NOT fit a non-Boost frame without chainline issues. A non-Boost wheel can go into a Boost frame with spacers but the chainline shifts.
Thru-axle thread pitches vary — M12×1.0, M12×1.5, M15×1.5 are all out there. An axle with the wrong pitch will cross-thread and damage your fork. Measure or look up your fork's spec before buying a replacement axle.
Single Speed
Single speed and fixed gear builds look simple. They're not, because there's no derailleur to take up chain slack — everything about your chainline, spacing, and chain tension has to be manually engineered.
Chainline
Chainline is the distance from the centerline of the frame to the center of the sprocket (front and rear). For single speed, front and rear chainline must match — ideally within 1–2mm.
Standard road chainline is 43.5mm. Standard MTB (triple) is 47.5mm. A rear hub designed for geared bikes puts the cog at roughly 42–47mm depending on the spacer stack. You often need to experiment with spacers to align chainline.
Spacers, Lockrings & the Spacer Game
When running single speed on a geared freehub, you fill the extra space with spacers to position the cog correctly and tension the lockring.
A common single-speed spacer kit approach: cog + spacers on the freehub body, then a lockring (threaded, usually Shimano cassette lockring). The total stack needs to fill the freehub so the lockring engages the last threads and clamps everything. If you have too much stack, the lockring can't reach the thread. Too little, and it can't clamp.
A Shimano cassette lockring requires a cassette lockring tool to tighten — the same one used for removing cassettes (FR-5 or equivalent). It's not something you can tighten by hand. It needs ~400 in-lbs of torque.
Dropout Types & Tension
On a single speed bike, tension is everything. How you achieve it depends on your frame's dropout style:
| Dropout type | Tension method | Notes |
|---|---|---|
| Horizontal / track dropout | Slide wheel back in slot | Best for fixed/SS. Simple and classic. |
| Vertical dropout | Chain tensioner device | Tends to creak. Tensioner can derail under hard load. |
| Vertical with EBB (eccentric BB) | Rotate BB shell to tension | Elegant but frame must have EBB shell. Complex. |
| Sliding dropout | Bolts allow fore/aft adjustment | Common on modern SS-ready frames. Good. |
Most modern road and MTB frames use vertical dropouts — they're optimized for quick wheel changes with a derailleur. Running single speed requires a chain tensioner, which adds mechanical complexity and a point of failure. If you're committed to single speed, get a frame with horizontal or sliding dropouts.
Brakes
Brake compatibility breaks down into cable pull ratios, mount standards, and (for disc) rotor sizes. Mixing incompatible levers and calipers is dangerous — the brakes may still work but have dramatically reduced power or poor modulation.
Cable Pull — The Silent Killer of Brake Builds
Road brake levers and MTB brake calipers are not compatible. Road levers pull ~18mm of cable per actuation. MTB levers pull ~22mm. MTB-ratio calipers (like V-brakes) need that extra pull. A road lever on V-brakes gives about 30% of the braking power you'd expect — barely enough to stop at all.
| Caliper Type | Required pull | Compatible with |
|---|---|---|
| Road dual-pivot caliper | Short pull (~18mm) | Road drop bar levers |
| V-brake (linear pull) | Long pull (~22mm) | MTB levers, not road levers |
| Cantilever | Short pull (~18mm) | Road or MTB levers (check spec) |
| Mechanical disc caliper | Varies by design | Check specific caliper's pull spec |
Travel agents / Travel adjuster are small inline cable devices that convert long-pull to short-pull, letting you run V-brakes with road levers. They work well. Brand: Tektro makes a common one.
Disc Brake Mount Standards
| Mount Type | Description | Notes |
|---|---|---|
| IS (International Standard) | Two parallel M6 bolts, bolt-on tabs | Older MTB, some forks still use it |
| Post Mount (PM) | Two M6 bolts facing forward | Current standard for most calipers |
| Flat Mount (FM) | Slim low-profile design | Road/gravel standard; needs specific calipers |
Adapters exist to convert IS↔PM and to run larger rotors, but they add weight and can introduce flex. Always get the correct mount for your rotor size. A 160mm rotor on an IS mount needs a different adapter than a 180mm rotor.