Putting together a rear axle isn't rocket science, but it's also not something you should wing it.
The Mountain Bike Bicycle Rear Axle Assembly Diagram shows how the dropout, quick release or thru-axle, hub, brake rotor, and spacer stack together. I've been pulling and reinstalling these for long enough that I can do it with my eyes closed, but I still take my time because messing it up means a wheel that wobbles at 30 kilometers per hour on a descent. That's not fun. Start with the diagram itself. Most manufacturers put one in the manual, but a lot of people don't read it. The diagram breaks down into three main zones. The left side shows the non-drive side with the freehub body, the center shows the axle path through the hub shell, and the right side shows the cassette, lockring, and chainline. Understanding these zones matters more than memorizing part names. The critical detail everyone misses is the orientation of the lockring on the freehub. It has a specific threading direction. If you force it the wrong way you'll cross-thread it in about thirty seconds and then you're buying a new freehub body instead of tightening a ring.
The actual assembly process
Here's how I actually do it, in order that makes sense in practice. First, remove the wheel. For a quick release, flip the lever to the open position and pull it out. For a thru-axle, unscrew it fully and set it aside. Don't lose that little O-ring or washer that usually sits on the axle itself. I learned that the hard way on a trail ride when my thru-axle fell apart somewhere between second gear and nowhere. Had to duct-tape the lever back on and ride four kilometers to my car. Clean the dropout surfaces. This sounds obvious but a lot of people skip it. Brake dust, trail gunk, and old grease on the dropout faces will make the wheel sit at a slightly different angle every time you remove it. That changes your chainline and can cause the chain to rub or drop. Wipe both dropout faces with a rag and some degreaser. Takes about ten seconds.
Install the brake rotor if you're using a post-mount system. Thread the bolts by hand first. Then torque them to spec. Don't overtighten. The rotor is aluminum. The bolts are steel. Force them too hard and you'll strip the threads inside the rotor carrier. I typically use eight to ten Newton-meters. If you don't have a torque wrench, finger tight plus a quarter turn with a hex key is usually safe. Now the wheel goes into the frame. Slide it into the dropouts from the non-drive side. Make sure the cassette clears the chain. For disc brakes, align the rotor with the caliper gap before you seat the wheel all the way. If you force it, you'll bend the rotor and then you're dealing with a different problem entirely. Tighten the axle. Quick release tension should be firm enough that you can barely move the lever with your thumb. If you need a hammer, it's too loose. If you can't close it at all, it's too tight. Thru-axles get torqued to whatever the manufacturer specifies, usually five to six Newton-meters. Mark the axle with a paint pen after you torque it so you can see if it backed off during a ride.
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Things that go wrong
The most common issue I see is chain line problems after reassembly. This happens when the wheel isn't seated evenly in the dropouts. With modern thru-axle systems it's less of an issue than with quick release, but it still happens. If your derailleur indexing is off after putting the wheel back on, check that the wheel is fully seated. Squeeze the brake, wiggle the wheel side to side, and make sure it's centered before you tighten. Another issue is rotor rub. Usually this means the caliper shifted while you were working on the wheel. Loosen the two caliper mounting bolts just enough to let it move, squeeze the brake lever, and retighten. The caliper self-centers on the rotor when you do this. The edge case I remember clearly involved a Specialized Epic that I was rebuilding after a crash. The frame had internal cable routing and the rear derailleur hanger was slightly bent. When I put the wheel back in, the chain wouldn't sit on the largest cog no matter how I adjusted the B-screw. Turned out the dropout itself was pushed inward by about two millimeters from the impact. I used a derailleur hanger alignment gauge to check it and confirmed the misalignment. The fix was to gently bend the dropout back with the alignment tool over about twenty minutes. Not something most people know how to do, but it saved me from replacing the entire frame.
What the diagram won't tell you
Manufacturers assume you have a clean workspace and the right tools. They don't mention that you sometimes need to compress the chain tension when installing a worn cassette. Or that on certain hub models, the spacer stack between the cassette and the lockring varies depending on whether you're running an 11-speed or 12-speed cogset on the same freehub body. Put the wrong spacer and your chainline shifts enough to cause premature wear. Also, the diagram doesn't show torque specs for everything. The axle nuts on some older bike models are torqued differently than the quick release mechanism itself. Axle nuts go to about forty Newton-meters. Quick release skewers rely on leverage and spring tension, not threaded torque. Mixing up these two concepts will get you a loose wheel or a cracked dropout. For those looking for a visual reference, most replacement hub manufacturers like Hope, DT Swiss, and Shimano publish their own diagrams online. Search for your specific hub model plus "assembly diagram PDF." Those tend to be more accurate than the generic ones floating around forums because they're drawn by the actual engineering team.
When this approach fails
There are situations where the standard diagram and procedure don't apply. Boost spacing is one. A 148-millimeter rear hub doesn't fit a standard 135-millimeter frame without adapters or modification. Some aftermarket frames have dropouts that are machined to accept both standards, but if yours isn't, you're looking at a different kind of problem. Another limitation is with magnetic freehub bodies like those found on some newer hubs. The magnet can attract metal shavings from your drivetrain over time, and cleaning it requires disassembly that the basic diagram doesn't cover. If you're riding in sandy conditions regularly, plan to service these more often than the manufacturer recommends. Otherwise the magnetic drag increases and your coasting feels sluggish. If you're dealing with a hub that doesn't have publicly available documentation, contacting the manufacturer directly usually gets you something within a week. Third-party diagrams for obscure brands are often inaccurate or missing critical details like spacer thickness. Don't trust a forum photo of a parts layout. It's easy to misidentify a two-millimeter spacer as a one-millimeter one, and that difference shows up as chain noise after you've already ridden home.
