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GT: Bike Anatomy 101

Riding bikes has never been more popular, which is pretty awesome if you ask us. More people on bikes means more bike lanes, more trails—and more humans enjoying themselves more often. All of which are good things. However, the combination of the recent bike boom and supply-chain issues within the bike industry has made many bike parts difficult—if not impossible—to come by.

If you’re looking to maintain or upgrade your current ride, picking up some pre-owned parts somebody else isn’t using anymore might be your best option. And if you’ve got lightly used bike parts sitting in a garage or storage bin, you can turn them into extra cash while getting a fellow rider back out on the trail or the road. Win-win. To read our article about buying and selling UnNew Bike Parts click here.

Bikes can seem complicated at times, with lots of different sizes, specs, and standards to keep track of. Whether you’re shopping for deals on UnNew bikes or bike parts, or selling your own stuff, this quick refresher on bike anatomy will remind you of a few things to keep in mind when you’re trying to figure out what parts are compatible with one another.



When planning out parts changes or a new build, some of the important aspects of your frame to keep in mind are tire clearance, headset and bottom bracket shell size, and hub spacing. It’s super helpful to track down a webpage or PDF with your bike’s frame specs and geometry, so you can be sure your fancy new upgrades will work without any issues. Your frame manufacturer’s website is a great place to start.


If you’re looking to switch out your fork, make sure to match the steer tube to the spacing of your frame’s head tube. It’s also a good idea to know how the axle-to-crown measurement compares to the original geometry from the frame manufacturer.

For suspension forks, it’s recommended to stay within 10mm or so of the original travel amount to avoid changing your bike’s geometry too much, or exposing your frame’s head tube to forces it wasn’t designed to handle.

Rear Shock

When upgrading a rear shock on a full-suspension bike, you’ll need to know the eye-to-eye length and stroke length, as well as the spacing of the frame you plan to put it on. Bear in mind that not all rear shocks with the same amount of travel will have the same stroke lengths, due to variances in frame geometry.


For seatposts, it’s important to know the diameter to make sure it matches your frame. You’ll also want to pay attention to the offset, meaning how far back the saddle clamp is from the seatpost collar. If you’re installing a dropper post, you’ll also want to double-check the travel and insertion length to make sure the full dropper assembly will fit inside your frame, and that you can get your saddle at the proper height.


Nearly all saddles use standard 7mm rails to attach to your seatpost. The biggest thing to keep in mind here is finding the right saddle width for your sit bones. To measure your sit bone width, put a piece of corrugated cardboard on a flat, hard surface and sit on it for a few minutes, then measure the distance between the centers of the two indentations. Add 20-25mm to that measurement for your ideal saddle width.



The number and range of cogs in a cassette determines your available gears. Make sure the cassette you’re choosing matches the mounting standard on your rear wheel’s freehub body, and the number of gears on your shifter/derailleur combo.


Chains stretch and wear out after a few thousand miles. Replacing your chain regularly will avoid excess wear on your cassette, chainring, and other components. Chain checkers don’t cost much, and are worth adding to any home mechanic’s kit.


Rear derailleurs move your chain between cogs on your cassette. It’s important to know the minimum and maximum tooth counts your derailleur is designed to handle to get the best performance out of your shifting. (Manufacturer websites are a great resource.)

While 1x drivetrains have become extremely popular, some riders still enjoy the range and gear spacing of using multiple chainrings on their crankset, meaning they’ll need a front derailleur. For those, you’ll need to know the mounting style, and whether it’s compatible with your bike’s frame.


Cranksets need to be compatible with your bottom bracket and frame spacing, in order to maintain a proper chainline and deliver the best shifting performance. Make sure you’re paying attention to the mounting standard for your chainring(s) as well.


Some chainrings mount directly to the center of your crankset, while others attach to spindles. Mixing manufacturers is totally fine, just be sure you can attach your chainring to the crankset you’re planning to pair it with, and that it’s within spec for your frame’s minimum/maximum chainring sizes.

Bottom Bracket

Bottom brackets are the bearing around which your cranks rotate, and need to be compatible with your frame’s bottom bracket shell size and your crankset. Many different standards exist here depending on the age of the bike, so don’t be afraid to ask a mechanic if you’re not quite sure.


Most modern pedals have a 9/16” spindle size. Flat pedals are pretty hard to mess up (just remember the left one is reverse-threaded!) If you’re dealing in clipless pedals, make sure you know what cleats you need and that they’ll be compatible with your preferred cycling shoes.



The width, angles, and rise (or drop) of a bar can make a big difference in bike fit and handling. You’ll also need to know the diameter of the clamping area to make sure it matches your stem. If you’re using carbon bars, make sure to use a torque wrench during installation and never exceed the manufacturer’s recommended tension.

Brake Levers

For hydraulic disc brakes in particular, you’ll generally need to pick a manufacturer for your lever/caliper combo. Mechanical and rim brakes give you a little more flexibility in terms of pairing components together.


Your stem clamp diameter needs to match your handlebars, as well as the diameter of your fork’s steer tube. Shortening or lengthening your stem can have a significant effect on your bike’s fit and handling, so it’s generally recommended to change maybe 10-20 mm at a time.


Your headset needs to match your bike’s frame spacing, as well as the steer diameter of your fork. (A frame spec sheet really comes in handy here.) Headset installation can be a bit tricky and requires specialized tools, so it’s one of those things that might be best left to a trained mechanic.


Your shifters need to match your derailleurs in terms of manufacturer and number of gears, but mixing component ranges within a manufacturer is 100% fine. While most flat-bar shifters have a standard 22.2mm handlebar clamp, keep an eye out for manufacturer-specific mounting standards that attach directly to a certain type of brake lever, for example.



While knowing your wheel size is critical, knowing your hub spacing is also important to make sure you’ve got a match with your frame. (This refers to the inside distance between the dropouts on your frame and fork—many people get confused when they measure their axles instead.)


Experimenting with different tire widths to see what works best for you and your riding style can be a lot of fun. However, be sure that whatever tires you’re choosing are within the recommended width for your rims, as PSI ratings are dependent on an appropriate rim/tire match. Similarly, make sure you know your frame’s maximum tire size—meaty tires are great, but not when they’re introducing clearance issues.


For disc brakes, you’ll generally need to stay in the same manufacturer ecosystem when matching up levers and calipers. Keep an eye on the mounting standards—flat-mount vs. post mount—and be aware you may need a bolt-on adapter to make sure your calipers line up with a new rotor size. Rim brakes give you a little more flexibility, though be absolutely certain you’re using wheels designed for rim brakes!


If you’re running disc brakes, you’ll want to keep in mind the way the disc rotors mount to your wheel hubs—centerlock vs. standard six-bolt. You’ll also want to make sure your rotor size is compatible with your calipers, frame, and fork. Often, the same calipers can be compatible with multiple rotor sizes. Generally speaking, bigger rotors offer more stopping power, at the expense of additional weight.

Is your head spinning yet? We’ve barely scratched the surface with this stuff, but nerding out on parts research is part of the fun. Now go forth and upgrade—or turn the unused stuff in your parts bin into a savings fund for your next cycle-centric getaway.

TJ Parsons is a semi-reformed snowboard bum who now has a semi-adult career as a professional writer and creative. He's a self-proclaimed perpetual intermediate who thinks the outdoors are for everyone, and who wants to help dismantle gatekeeping and elitism in outdoor sports. When he's not squeezing brain juice into a keyboard, you'll find him riding boards or bikes throughout the Intermountain West.

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