PACESCIENCE ENGINE
Physics, not guesswork.
The course, race-day weather and your own numbers go into one power equation, which is solved segment by segment for how many watts to ride and how long it takes. This page explains how it works, how accurate it is and what it cannot do.
Demo: FTP 250 W · 68 kg · tri bike. Colours = the six power levels, line = target watts per segment, arrows below = wind on each segment; move over the chart and the readout follows you.
WHAT GOES IN
ENGINE
WHAT COMES OUT
01
One calculation needs just four groups of data.
Each group can be filled in with just a few fields; anything left blank uses a sensible default you can refine later.
COURSE
Course
- 207 real courses: the full IRONMAN series plus Challenge, both 226 and 113; your own GPX works too
- Segment grades are computed from the track; enter the official total ascent and the inflated elevation is scaled back proportionally
- Corners only count when you really have to slow down (a change of direction over 25°); a turnaround counts once as 180°
RIDER
You
- FTP, weight, height, bike weight: those four fields are the minimum
- CdA: looked up by bike type and position, or solved from your own ride file with power
- Tyres and road surface set rolling resistance; your lowest gear sets how slowly you can climb
WEATHER
Weather
- Pick the race date and start time and temperature, humidity, pressure, wind direction and speed are fetched for you
- Air density is computed per segment: the hotter, wetter and higher, the thinner the air
- Wind is projected onto each segment's heading, so headwind, tailwind and crosswind are each handled
TARGET
Target
- Race type sets how hard to ride and how many kilometres before T2 to ease off
- Intensity is not read from a fixed table; it follows how long you will roughly ride on this course: the longer, the lower the sustainable intensity
- Strategy conservative / standard / aggressive only changes how much harder than the flat you push on climbs
02
One equation, and every number has a source.
This is the textbook cycling power equation, not a black box. The engine's job is to get every term right on every segment, then turn the question around: how many watts on this segment for the shortest total time.
At 37 km/h on the flat, 86% of your effort goes into pushing air aside, so CdA is the term that matters most and is easiest to get wrong.
Drag the sliders: at the same 35 km/h, change the grade or wind and the three resistances trade places. Demo values 68 kg · 9 kg bike · CdA 0.245 · Crr 0.004 · air density 1.18.
03
Six levels, and each segment gets just one.
Each level is a fixed multiple of your target NP. While riding, just follow the level and watt range your bike computer shows; no numbers to memorise, and the race card uses the same colours.
A little more on the climbs and easing off on the descents saves the most time in physics terms. The bigger the difference, the more you save, at the cost of tired legs early in the run, so strategy adjusts only this one thing.
You can stand and power over a short steep ramp; if a long climb cannot be held at that effort, the engine drops it back to a level you can sustain rather than sending you over the top blown.
Headwind sections automatically step up a level and tailwind sections step down. The same climb can be a different level into the wind and with it.
The last kilometres deliberately drop a level to save your legs for the run; how far out depends on the race type.
04
Two weather sources, depending on how far off the race is.
Within 9 days of the race
Hourly forecast
An hourly forecast from a national weather service, matched to the hours you will be riding. Recalculate before the race and the plan updates with that day's wind.
Further out
Multi-year same-period average
Measured records from several past years for the same period and time of day give the typical temperature, humidity and wind. The heat and wind penalties in the rankings use the same data.
Wind direction follows the meteorological convention of where the wind comes from, and averages are speed-weighted vector averages, so 350° and 10° never average to 180°. When the nearest weather station is too far away, the engine assumes no wind and says so on the results page; that is the common cause of the largest backtest errors.
05
CdA solved from your own races.
A lookup table only gives a range. Upload a race file with power and the engine runs the same equation backwards: with power, speed, grade and that day's wind known, it solves for your drag area.
Illustration: every uploaded race narrows the error band of the converged value a little
Every estimate comes with an error band and a count of usable segments; several estimates merge into one converged value, weighted towards the more reliable ones, with each bike type and position kept separate.What is CdA →
06
Sixty real races, recomputed, to see how accurate it is.
Method: power files from nine athletes across 60 real races, 2019–2026, in more than 16 countries were fed back through the engine. To keep it honest, each race was predicted using only the athlete's CdA from other races, never that race's own result.
Each dot is one race; the axis is finish-time error. Dark = within ±2%, mid tone = within ±5%.
The bias is conservative: predictions usually come out slightly slower than reality. The two cases with larger errors are windy days with no nearby weather station, and athletes who had just changed equipment or position before enough CdA data had built up.How the backtest was done →
07
What are its limits?
Where the course data comes from
Routes from official race pages, official GPX files and public community routes; ascent is computed from the track, and every course shows its verification tier.
08
Terms
- FTP
- The wattage you could hold for a full hour of all-out riding. Every wattage in this plan is worked out from it.
- CdA
- How big you and the bike look head on to the wind. The lower and narrower you are, the smaller the number and the less power you need. On the flat, nine tenths of the resistance comes from it.
- Crr
- The effort the road takes back as your tires roll over it. The better the tire and the smoother the road, the smaller the number.
- NP
- Turns your up-and-down watts into the steady wattage that would feel just as hard.
- IF
- NP divided by FTP, in other words how hard this ride is for you.
- TSS
- How much the whole ride takes out of you, counting intensity and time together. 100 = one hour all out.
- VI
- NP divided by AP. The closer to 1.00, the more evenly you rode.
- Converged CdA
- All your estimates combined into one representative value; this is what planning uses.
- Error band
- The true value is somewhere in this range (plus or minus). The narrower it is, the more certain you can be.
- Usable stretches
- How many stretches in this file were ridden steadily enough to be used. The more there are, the more reliable it is.
- Speed score
- A 0-100 score for how easy it is to post a good time on this course. Lots of climbing, lots of corners, too much heat and strong wind all cost points.
- What segment pacing buys
- At the same intensity, how many seconds a section-by-section plan saves over holding one fixed wattage the whole way.
Now run it on your own course.
A free account builds the six-level plan; race-day weather, export files and your own CdA are bought per race.
Build my free race plan