Cycling Simulation from a GPX File: How It Works

A GPX file (GPS Exchange Format) is an open XML format holding a list of points: latitude, longitude and, usually, elevation. There is no road surface in it, no width, no scenery and no traffic — just a line through space with heights attached. Everything any simulator shows you or tells you is derived from that line.

From those points a program can compute the two quantities that matter: the distance between consecutive points, and the gradient between them, which is rise over run. Once it has distance and gradient it faces a fork. It can build something for you to look at and ride through, or it can run a physics model over the profile and hand you a number. Ride simulation and performance simulation are those two branches, and the confusion between them is why the same search phrase returns 3D cycling apps and pacing calculators side by side.

A ride simulator reconstructs the course as something you move through. The gradient from the GPX drives two things at once: what you see, and how hard the pedals are. On a smart trainer the app sends a target resistance over Bluetooth FTMS or ANT+ FE-C as the slope changes, so a 9% ramp in the file becomes a 9% ramp in your legs. Your speed comes back out of a physics model fed by your power, your weight and that same gradient.

The visual half is where platforms diverge most. Some replay filmed video of the real road and overlay your avatar. Some render a 3D world from map and elevation data. Some show little more than a moving dot on a map. All three get called route simulation, and the gap between them in practice is enormous.

Cycloscape sits in the third camp and takes it further: it procedurally generates a full 3D world from your GPX — the road, the terrain, the climbs, the roadside — in the browser, in seconds, with nothing installed. Chrome or Edge on desktop or Android is the whole software requirement, and it talks to trainers and heart-rate straps directly through Web Bluetooth and WebUSB.

A performance simulator never renders anything. It walks the course profile segment by segment and solves the cyclist power equation at each step: the power needed to overcome gravity on the gradient, rolling resistance from tyres and surface, and aerodynamic drag, which grows with the cube of speed and dominates above roughly 25 km/h on the flat. Give it your FTP, your weight, your bike and an estimate of your drag, and it returns a predicted time and a target power for every segment.

That is genuinely useful for a specific job: pacing a time trial, a gran fondo or a long climb you have never ridden. It tells you not to burn matches on the first ramp, what wattage the flat section is worth, and how much a headwind costs. Best Bike Split and RaceYourTrack are two well-known tools in this category; RaceYourTrack will also estimate a power profile from a GPX that only contains speed, for riders without a power meter.

Its limits are equally specific. The prediction is only as good as its inputs — an optimistic FTP or a guessed drag coefficient moves the answer by minutes — and it assumes you execute the plan. It is a spreadsheet with physics in it, not a training session.

Almost no product does both well, and most do not attempt it. The table below covers ride simulation only, because that is what people usually mean when they say they want to ride a GPX indoors. Every figure is the provider's own published statement, checked on the date in the caption.

Three things decide whether a simulated climb feels like the climb, and only one of them is graphics.

The first is gradient fidelity. Consumer GPS elevation is noisy — barometric drift and sampling error put spikes in the data that become impossible 30% ramps and phantom descents in the file. Apps handle this differently: some smooth aggressively, some snap the line to known roads, some flag implausible gradients and let you fix them. Over-smoothing flattens a real wall; under-smoothing gives you a resistance unit that lurches. Both are wrong, in opposite directions.

The second is the physics model. If speed is not an honest function of power, weight and gradient, the effort you feel is fiction. On Cycloscape those three inputs are all there is: nothing you buy or unlock changes your speed, so two riders at the same watts and the same weight go the same speed. That is not universal — on Zwift, frames and wheelsets carry weight and aerodynamic values that measurably change in-game speed.

The third is trainer control. Simulation mode (SIM) means the app sends the road gradient and the trainer computes resistance from your speed and weight; ERG mode holds a fixed wattage regardless of the road. For riding a route you want SIM — and you want a trainer that can actually deliver the slope, since many cap somewhere between 10 and 20%, which is why a genuine 15% pitch can feel muted on hardware rated to 10%.

Any route planner or head unit will give you one, and the export is a couple of clicks. Two things are worth checking before you ride it.

If the goal is to train, to see a road you love again, or to preview a climb you are riding in three months, you want ride simulation — and the cheapest honest test is a browser-based one that costs nothing to try. If the goal is to arrive at a start line with a pacing plan, you want a performance simulator, and no amount of 3D scenery substitutes for it.

Riders preparing for a specific event often use both, in that order: a pacing calculator to work out the numbers, then a ride simulator to rehearse the course with those numbers on the screen. They answer different questions about the same file.