Gap Calculator

Plan your MTB jump before you build — or ride — it.

Enter the jump's dimensions and the run-up speed, see the flight path drawn to scale and get a warning about an under-shoot, an over-the-top landing or a bone-rattling impact.

Kotlin Multiplatform · Android · iOS · Web · Русская версия

   

Warning

Jumping is dangerous and can injure or kill you. The results are a rough preliminary estimate: air resistance, wind, rider behaviour, friction and many other factors are not taken into account. Never use them as the only basis for a decision — the responsibility for the risk is entirely yours.

What it does

• Flight path to scale – one scale for both axes, so the jump looks like the jump. Grid, filled take-off ramp, landing and table.
• Tolerance paths – besides the main (green) path two thin ones show a ±5° deviation of the take-off angle.
• Warnings – big jump, looks like an under-shoot (landing before the landing slope starts), hard landing.
• Numbers – speed at the end of the ramp V0, take-off radius and the minimum recommended radius (2 g), the height needed to build up the speed and the landing hardness G, expressed as the height of an equivalent drop to flat ground.
• Drops and vertical take-offs – a negative take-off angle is a drop, 90° is a straight-up launch.
• Several jumps are stored on the device, can be renamed, edited and shared as text.
• English and Russian, light and dark theme.
• Zoom and pan on the chart (pinch / wheel, double tap or double click to reset).

Screenshots

	List	Chart	Parameters
Android
iOS

Web

How the calculation works

The ballistic equations of motion with the speed lost to climbing to the take-off height (V0 = √(v² − 2·g·h)); the landing point is the intersection of the parabola with the landing slope. The take-off ramp is a circular arc fitted to the entered height and angle. The minimum radius is the one that keeps the load on the ramp below 2 g. The code is in usecases/calculator and is covered by tests.

Input	Meaning
Gap	distance from the end of the take-off to the start of the landing, m
Table	length of the table (flat top), m
Takeoff height / angle	height above zero and angle at the lip; an angle ≤ 0 is a drop
Landing height / angle	height and slope of the landing
Run-up speed	speed at the start of the take-off, km/h

Architecture

All the logic – calculation, storage, view-models – is shared; every platform only draws screens.

flowchart LR
    subgraph shared["sharedGapCalc (Kotlin Multiplatform)"]
        VM["ViewModels<br/>(androidx.lifecycle, StateFlow)"] --> Repo[Repositories]
        Repo --> Calc[Calculator]
        Repo --> DB[(SQLDelight)]
    end
    Android["androidGapCalc<br/>Jetpack Compose"] --> VM
    iOS["iosGapCalc<br/>SwiftUI"] --> VM
    Web["webGapCalc<br/>Kotlin/JS + DOM"] --> VM

Loading

Target	Status
Android	app (androidGapCalc)
iOS	app (iosGapCalc)
Web (JS)	app (webGapCalc), the database lives in memory
JVM, macOS	the shared module compiles and is tested; platform dependencies are ready for a UI

Details for contributors (and for AI assistants) are in CLAUDE.md.

Build and run

You need JDK 17+, the Android SDK (ANDROID_HOME or local.properties) and, for iOS, Xcode.

./gradlew :androidGapCalc:installDebug          # Android app on a device / emulator
./gradlew :webGapCalc:jsBrowserDevelopmentRun   # web app on http://localhost:8080 (add ?lang=en or ?lang=ru)
open iosGapCalc/iosGapCalc.xcodeproj            # iOS app: Run in Xcode (it builds the Kotlin framework itself)

Tests and lint

./gradlew :sharedGapCalc:jvmTest                 # shared tests (calculator, view-models, database)
./gradlew :sharedGapCalc:testAndroidHostTest :sharedGapCalc:macosArm64Test :sharedGapCalc:jsNodeTest
./gradlew detekt                                 # Kotlin lint + formatting

Release

Android release builds are shrunk with R8 and signed from a git-ignored keystore.properties – see docs/RELEASE.md.

Tech

Kotlin 2.4 · Compose · SwiftUI · SQLDelight · kotlinx.coroutines · androidx.lifecycle (multiplatform ViewModel) · detekt

License

GPL-3.0