Flat vs. Climb: How Terrain Changes Your RoboPacer Watts

There's a moment every Zwifter knows. You're comfortably tucked behind Maria, maybe 30 minutes into a steady ride, when the road tilts upward. Within seconds the group stretches, your watts creep up, and you're suddenly working a lot harder than the 2.2 W/kg on the label suggested.

That's not a bug. It's physics — and it's worth understanding before you pick your next route.

Why Pace Partners don't ride at a constant speed

Zwift's RoboPacers hold a fixed W/kg, not a fixed speed. Maria always rides at 2.2 W/kg. On flat ground that produces one speed; on a 7% climb it produces a much slower one. Their power output doesn't change. The terrain just determines where that power gets them.

On flat roads, the main resistance is aerodynamic drag, which scales with the square of your speed. Absolute watts matter here — heavier riders can produce more force relative to their frontal area, so they tend to find flat Pace Partner riding a little more forgiving than the W/kg number implies.

On a climb, gravity takes over. Now it's W/kg that counts, because every extra kilogram costs the same energy per metre gained. A heavier rider needs proportionally more absolute watts to match a lighter rider's climbing speed. The W/kg figure becomes the real equaliser on gradients — you can't hide behind mass when you're going uphill.

What changes for you? Your required watts shift with the terrain, even though the Pace Partner's don't.

How much more do climbs actually cost?

A moderate Zwift climb — 5–8% gradient, the kind you'll find on Watopia's Hilly or the Innsbruck loop — adds around 25–35% to your watt requirement versus flat ground. The calculator uses a +30% modifier for climbing as a reasonable estimate for that kind of terrain.

In real numbers: a 75 kg rider targeting Maria at 2.2 W/kg needs 165 W on the flat. Tap Climbing and that becomes around 215 W. A 90 kg rider goes from 198 W to 257 W.

Descents work the other way. The descent modifier is approximately −40%, which means you can ease off significantly, let your heart rate drop, and the bunch will naturally regroup before the next climb. Use that time — don't squander it sprinting to "bank" advantage you don't need.

The bunch effect — and why terrain changes that too

Terrain isn't the only variable. The draft matters enormously on flat roads and far less on climbs.

Tucked into a large bunch on a flat Zwift route, the slipstream from the riders ahead cuts your power demand by roughly 25%. This is why experienced riders often find their actual watts running well below the Pace Partner's listed figure on flat sections — the group is doing a significant chunk of the work for them. It's not a glitch; it's how Zwift's physics is supposed to work.

On a climb, the bunch spreads out. Riders sort themselves by ability, gaps open, and the aerodynamic benefit mostly disappears. You're essentially climbing on your own watts even if you're technically within touching distance of the group.

Put those two variables together and you get a much wider range of actual effort across a single Pace Partner session than the W/kg number suggests. Flat road in a full bunch can feel genuinely manageable. A short punchy climb where the group fragments can feel like a completely different sport.

Using the scenario buttons on IndoorWatts.com

The calculator now has four scenarios you can toggle through to model different riding conditions:

• Flat (baseline) — the standard watt target based on the Pace Partner's W/kg on flat terrain
• In the Bunch (−25%) — approximate power when properly drafted in the group on flat ground
• Climbing (+30%) — approximate power for a moderate 5–8% gradient
• Descending (−40%) — approximate power on a downhill where you can ease right off

The most useful thing to do before a ride: enter your weight, pick the Pace Partner you're targeting, and check the Climbing number. That's the one that catches riders out. If it's significantly above your threshold watts, a hilly route with that pacer is going to push you into hard intervals whether you planned that or not.

That's a 112-watt swing between a tucked-in flat section and a moderate climb with the group spread out. Same Pace Partner. Same ride. Very different legs required at different points.

Route choice matters more than most riders think

The effective difficulty of a Pace Partner session depends heavily on the route, not just the W/kg. Miguel at 1.8 W/kg on Tempus Fugit — Watopia's famously flat desert route — is a genuinely different workout to Miguel at 1.8 W/kg on a route with multiple climbs, despite being the same pacer on paper.

If you're working on building fitness, or trying a harder Pace Partner for the first time, a flatter route is a reasonable starting point. It lets you dial in pacing and learn what the group feels like before gradients start adding variation you're not ready for.

If you want to use climbing specifically as a training tool, the Climbing scenario gives you an honest preview. If the number is above your sustainable output, the session will be interval-shaped whether you intended it that way or not.

A few things that actually help on hilly routes

Get ahead of the climbs

The standard mistake is waiting until you're already losing ground before lifting your power. By then, closing a gap on a gradient costs far more than simply not letting it open. If you can see the slope coming on the mini-map, lift your watts slightly before you reach it — not after.

Use descents as actual recovery

Ease off properly on the downhills. Let your heart rate drop. The bunch will regroup naturally, and you'll arrive at the next climb in better shape. Trying to sprint on a descent to "bank" time is a good way to blow up halfway through a session.

Judge the session by average watts, not peaks

A Pace Partner ride on a hilly route won't produce a steady power graph — it'll spike on climbs and drop on descents. That's normal. The meaningful numbers are average power and time in zone, not what you saw on screen for 20 seconds going over a KOM.