How BuildMyMTB Checks Your Build

The idea

BuildMyMTB lets you assemble a mountain bike from individual parts — frame, fork, wheels, drivetrain, brakes, cockpit, and so on — and checks, as you go, whether the parts you've picked actually fit together. It runs roughly twenty distinct compatibility checks covering things like wheel size, hub and axle spacing, drivetrain system/speed/actuation compatibility, UDH and SRAM Transmission requirements, cassette range and freehub type, brake mounts, rotor interface and size, headset/steerer matching, fork travel, dropper post sizing, tire clearance, handlebar/stem clamp diameter, rear-shock fit, and frame-and-shock bundling rules.

Three tiers of verdict

Every check that finds something produces one of three kinds of result:

• Errors — the parts won't physically fit together.
• Warnings — the parts do work, but there's something worth knowing first (maybe you'll need an adapter, a shim, or you're accepting reduced travel).
• Infos — a note that isn't a fit problem at all, just useful context (like a reminder to pick a headset, or a pointer to a manufacturer's insertion-depth calculator).

Four dot colors, and what each one actually promises

As you shop, each part shows a small compatibility dot with one of four states:

• Green — adding this part introduces no new error and no new warning.
• Yellow — the part fits, but it adds a new warning; hovering shows you the specific reason.
• Red — the part adds a new error; it conflicts with something already in your build.
• Grey — nothing relevant is selected yet, so there's nothing to compare against.

The guiding principle: silence over false confidence

The engine is built around one rule that shapes everything else: a wrong verdict is worse than a missing rule. A false "this won't fit" can talk someone out of a build that would have worked fine. A false "this fits" can sell someone a part that physically won't mount. Both are worse than the tool simply not having an opinion yet on some narrow dimension it doesn't have solid data for. So where the underlying facts aren't solid enough to be confident, the engine stays silent on that dimension rather than guessing.

That's also why the all-clear message in the app reads "No conflicts found" rather than "All compatible." That phrasing is deliberate and honest: it means your build triggered no conflict among the dimensions the tool currently checks — it is not a blanket guarantee that every conceivable aspect of fit has been verified. Many of the checks are also direction-aware, meaning they can fire one way but not the other — for instance, a rotor larger than a fork's rated maximum produces a warning, while a rotor below a fork's native minimum mount size is treated as a firmer error, because those two situations carry different real-world risk. Some checks are adapter-aware too, naming the specific adapter or shim that resolves a warning rather than just flagging a mismatch and leaving you to research it.

Sample data vs. verified data

Not every spec in the catalog carries the same weight of confidence. Most catalog entries are sample data — reasonable, but not independently confirmed against the manufacturer. A part that has actually been checked against the manufacturer's own published page (or a credible measured source, with a link) is marked with a green "Verified" badge. If a part doesn't carry that badge, treat its listed specs, price, and weight as approximate rather than gospel — worth confirming yourself before you buy, especially for anything the compatibility checker is leaning on to clear your build.

What a green build tells you — and doesn't

A "No conflicts found" result means your picks passed every check the tool currently runs, using the data currently in the catalog. It's a genuinely useful signal, but it's not a substitute for double-checking specs on anything unverified, and it's not a claim that every possible fit dimension in mountain bike parts has been accounted for. Understanding both the tiers and the dot states should make it clear when the tool is confidently clearing something versus quietly declining to make a call.