Which Cassette Fits My Bike? | Fit Rules And Shortcuts

Pick the right cassette once, and shifting feels crisp, quiet, and predictable. The wrong match brings chain rub, ghost shifts, or a wheel that won’t accept the cogs at all. This guide gets you from “which cassette fits my bike?” to a confident choice with quick checks, plain terms, and zero guesswork.

Which Cassette Fits My Bike? Step-By-Step Check

Work through four simple questions: cassette or freewheel, freehub style, number of speeds, and derailleur capacity. If those line up, the new cassette will fit and shift well.

Step 1: Cassette Or Freewheel?

Look at the hub when the wheel is off the bike. If the cogs slide onto a splined body and are clamped by a lockring, you have a cassette. If the cogs spin freely as one piece and thread onto the hub like a big nut, that’s a freewheel. This matters because a freewheel and a cassette are not interchangeable on the same hub, and they use different tools and standards. When in doubt, compare your hub to a clear reference photo set from a trusted workshop resource such as the Park Tool guide.

Step 2: Identify The Freehub Body

Cassette-compatible hubs have splines that define what will fit. The common patterns are Shimano/SRAM HG (often called Hyperglide), Shimano Micro Spline, SRAM XD/XDR, and Campagnolo bodies. Match the cassette to the body and you’re halfway there.

Freehub Body	Cassettes That Fit	Notes
Shimano/SRAM HG 8–10	8–10 speed HG; many 11-speed MTB	Widest legacy standard; most 11-speed MTB fit HG 8–10
Shimano HG 11-Road	11-speed road; many 12-speed road	Shorter cassettes (8/9/10) need a 1.85 mm spacer
Shimano Micro Spline	12-speed Shimano MTB	Allows 10T small cog on Shimano MTB
SRAM XD	MTB 11/12-speed XD	Allows 10T small cog; fits XDR with 1.85 mm spacer
SRAM XDR	Road 11/12-speed XDR	Road version; longer than XD by 1.85 mm
Campagnolo Classic	Campagnolo 9–12-speed	Distinct spline; Campy-only
Campagnolo N3W	Campy 13-speed and many 10–12	Shorter body; ring adapter makes it backward-friendly
Threaded Freewheel	Freewheels only	Not a cassette; threads onto the hub shell

Step 3: Count Your Speeds

Stand behind the bike and count the rear cogs. That’s your “speed.” Match the new cassette to that number unless you’re upgrading the shifter, derailleur, chain, and in some cases the freehub. Mixing speeds mismatches cog spacing and shift ramps, so the chain won’t index cleanly.

Step 4: Check Derailleur Limits

Every rear derailleur has a maximum largest cog and a total capacity. Max largest cog tells you the biggest sprocket tooth count it clears. Capacity tells you how much chain slack it can wrap from big-big to small-small. If you jump from a 28T to a 36T, you may need a longer cage or a hanger-link to keep tension tidy.

Cassette That Fits My Bike — Quick Compatibility Keys

HG Freehubs: The Everyday Workhorse

HG is the most common spline. Most 8–10-speed cassettes slide on any HG 8–10 body. Many 11-speed mountain cassettes also fit the same body width. For an HG 11-road hub, 8/9/10-speed cassettes need a 1.85 mm spacer behind the stack. Many 12-speed road cassettes mount on the same 11-road body with no spacer. A concise spacer rundown appears in trainer setup notes that say 8/9/10-speed cassettes need the 1.85 mm ring on an 11-road body, while 11- and many 12-road cassettes do not.

Micro Spline: Shimano’s 12-Speed MTB Interface

Micro Spline is for Shimano 12-speed mountain and some gravel setups. It enables a 10-tooth top cog and spreads torque across more, smaller splines so lightweight freehubs don’t get notched by the cogs. If your hub is Micro Spline, choose a 12-speed Shimano MTB cassette. Shimano lists the intended use on its Micro Spline page.

XD And XDR: SRAM’s 10-Tooth Path

XD is the mountain interface that accepts 10-tooth small cogs. XDR is the road version, 1.85 mm longer. An XD cassette can sit on an XDR body with a 1.85 mm spacer; an XDR cassette won’t sit on an XD body. If your hub says XD, buy an XD cassette. If it says XDR, pick an XDR cassette, or use a spacer with an XD model. SRAM explains the length difference and spacer rule in its XD/XDR driver article.

Campagnolo: Classic And N3W

Older and many current Campagnolo cassettes use the classic Campy spline. The newer N3W body is shorter to accept 13-speed road and gravel cassettes, with an adapter ring that lets many 10-, 11-, and 12-speed Campagnolo cassettes fit as well. If you’re unsure which ring you need, see the official N3W compatibility table.

Fast ID: What Freehub Do I Have?

Remove the rear wheel and look at the bare body. HG has tall, even splines with one narrow relief. Micro Spline has many fine splines. XD/XDR has a smooth, conical start and a notched end; the cassette slides on as one piece. Campagnolo splines are taller with a different pattern. If you’re stuck, check your wheel maker’s spec sheet by hub model name.

Spacer Rules That Keep You Sane

HG 11-road hubs are slightly longer than HG 8–10 bodies. That’s why shorter cassettes sit loose unless you add a 1.85 mm ring behind them. Many 12-speed road cassettes seat on HG 11-road with no extra ring. XD to XDR needs the same 1.85 mm spacer when using an XD cassette on an XDR body. Keep one in the toolbox; it solves common mismatch headaches.

Chain, Freehub, And Shifter Must Match The Speed

Chains get narrower as speeds go up. A 12-speed chain on an 8-speed cassette rides too deep. An 8-speed chain on a 12-speed cassette won’t settle between cogs. Also, shifter indexing must match the cassette’s cog spacing. When you change the number of speeds, plan to change the shifter and chain, and confirm the derailleur is designed for that range.

Real-World Combos That Work

These pairings are proven on many bikes. Use them as patterns when you shop or upgrade. They answer the nagging “which cassette fits my bike?” with simple, repeatable logic.

Road With HG 11-Road Hub

Run an 11-speed road cassette as intended. If you’re reusing a 9- or 10-speed cassette on that wheel, install the 1.85 mm spacer first. Many riders running current 12-speed road groups mount the matching 12-speed cassette on the same body without extra rings.

Gravel And XC With Micro Spline

Pick a 12-speed Shimano MTB cassette in the range your derailleur supports. The 10-tooth top gives tall gears without extreme chainring sizes, and the fine splines help prevent notching on alloy bodies.

Trail And Enduro With XD

Choose an XD cassette to access a 10-tooth small cog for pace on fast descents. If you later move that wheel under a road-spaced bike with XDR, add the 1.85 mm ring to reuse the XD cassette.

Capacity, Largest Cog, And B-Screw Setup

Match the cassette’s biggest sprocket to the derailleur’s max and set the B-screw so the upper pulley clears the largest cog. If you push past the listed max sprocket, shifting will suffer. Some hangers accept a link extender that re-positions the derailleur for larger cassettes, but expect slower shifts at the top of the range.

Capacity Math You Can Do At Home

Add the tooth difference of your chainrings to the difference across the cassette. That sum can’t exceed the derailleur’s stated capacity. A compact 50/34 with an 11–34 cassette needs 16 + 23 = 39T of wrap. If your derailleur lists 35T capacity, downsize the cassette or swap to a long cage.

Common Pitfalls And Easy Fixes

• Mismatched spline: Cassette won’t slide on. Recheck HG vs Micro Spline vs XD/XDR vs Campy.
• Wrong speed: Indexing is off across the board. The shifter and chain must match the cassette speed.
• Loose cassette: You forgot the spacer on an HG 11-road hub with a 9/10-speed cassette.
• Chain too short: After a bigger cassette, size a new chain and reset B-tension.
• Derailleur out of spec: Big cog exceeds the printed limit. Use a cassette within range or change the derailleur.

Quick Reference: Hub To Cassette Match

Your Hub Says	Buy This	Add This
HG 8–10	8–10 speed HG or many 11-speed MTB	No spacer
HG 11-Road	11-speed road, many 12-road	1.85 mm ring for 8/9/10-speed
Micro Spline	Shimano 12-speed MTB	No spacer
XD	XD cassette	To use on XDR: 1.85 mm ring
XDR	XDR cassette	Or XD + 1.85 mm ring
Campagnolo	Campagnolo-splined cassette	Check classic vs N3W ring
Threaded	Freewheel, not a cassette	None

How To Read Gear Ranges And Pick Ratios

Two numbers define a cassette: the smallest and largest cogs. An 11–28 suits flat to rolling rides with tight steps between gears. An 11–34 widens the low end for hills. Mountain cassettes reaching 50–52T pair best with a long-cage derailleur and a narrow-wide single chainring up front. Pick the spread that keeps you in the middle of the cassette on your usual terrain.

Tools, Parts, And A No-Stress Swap

You’ll need a chain whip, a lockring tool matched to your lockring, and a torque wrench. Add a fresh chain sized on the large chainring and the largest rear sprocket without routing through the derailleur, then add two links. Grease the freehub threads or light oil if the maker specifies, seat spacers if required, slide the cassette on with the ramps facing out, and torque the lockring. Set cable tension, fine-tune limit screws, and dial B-tension so the upper pulley clears the big cog.

Trusted Sources For The Finer Points

Need pictures and spec tables? Check a respected repair guide for a freewheel vs cassette ID walk-through, Shimano’s Micro Spline page for what fits that body, and the SRAM XD/XDR driver article for driver length and spacer details. Those three cover nearly every fit question you’ll meet.

Final Fit Checklist

Match freehub spline, match speed, respect derailleur limits, and add spacers only when listed. Do that, and the new cassette will fit your bike cleanly and shift the way it should.