Bolton lets you ride a harder bike than most 70.3 courses — if you keep your power steady. That's the whole race in one sentence, and it's not obvious from the course map.
Here's how we know. Keiro simulates the entire race — swim, bike, and run together — to find the execution that produces the fastest finish, not the fastest bike split. A fast bike that wrecks your run is a slower race, so the plan only ever pushes the bike as hard as the run can absorb. We ran that simulation for three athletes on the Bolton course. The answers came back different for each of them — and the reasons why are the most useful thing this post can give you.
First, the course, because it isn't what it looks like. The bike is 783 m of climbing over 89.8 km — rolling, not flat (third-party GPX files range 757–840 m). It comes in two halves: a genuinely flat opening third on two loops around Pennington Flash, then a rolling back two-thirds on two loops through Horwich and Bolton, where the day's one real climb — Victoria Road into Old Kiln Lane — gets ridden twice. The run is 216 m of climbing, not the 100 m some guides list. The 1.9 km lake swim is short, freshwater, and wetsuit-legal — uncomplicated, and not where this race is decided.
Athlete comparison
| Tilda | Mark | John | |
|---|---|---|---|
| FTP | 220 W (3.55 W/kg) | 260 W (3.33 W/kg) | 210 W (2.50 W/kg) |
| CSS | 1:35/100 m | 1:40/100 m | 2:05/100 m |
| Run threshold | 4:15/km | 4:25/km | 5:15/km |
| Bike NP | 181 W | 206 W | 144 W |
| Bike IF | 0.82 | 0.79 | 0.69 |
| Bike VI | 1.016 | 1.027 | 1.027 |
| Bike TSS (% of cap) | 180 (90 %) | 170 (85 %) | 160 (80 %) |
| Bike split | 2:40:19 | 2:42:05 | 3:24:06 |
| Avg speed | 33.6 km/h | 33.2 km/h | 26.4 km/h |
| CdA (aero) | 0.274 | 0.293 | 0.331 |
| Run intensity | 94 %T | 91 %T | 88 %T |
| Run pace | 4:33/km | 4:53/km | 6:02/km |
| Run split | 1:38:59 | 1:46:29 | 2:11:20 |
| Finish | 4:58:45 | 5:09:34 | 6:24:18 |
CdA values are aero-position estimations — the simulation separately models time spent upright for cornering and the speed-bump section out of T1 using grade-based position switching.
Look at the bike IF row: Tilda 0.82, Mark 0.79, John 0.69. The first two are high for a 70.3. That's deliberate, and it comes down to what isn't limiting them at Bolton.
On most courses, something external caps the bike — heat compresses the ceiling at Texas, sustained climbing eats the TSS budget at Mallorca. Bolton has neither. June in Lancashire averages 13 °C — too cool to cap anyone — and the one climb is long enough to ride at a steady wattage rather than punch over. With nothing forcing the bike down, the only real limit is the TSS budget the run can tolerate. So Tilda and Mark can spend close to it.
John can't, and the reason is duration. TSS scales with intensity and time together, so the same IF accumulates more TSS the longer the bike takes. John's 3:24 bike split is 44 minutes longer than Tilda's 2:40 — ride Tilda's 0.82 IF for that long and the TSS maths land near 230 by T2, well past anything John's run could carry. At 0.69 he arrives at a similar TSS to the faster riders, just by spending longer to get there. Same course, same conditions, three different right answers, because the limit isn't the course — it's how much bike each athlete's run can absorb. That's the calculation that's genuinely hard to do in your head on race morning, and it's the one Keiro does for you.
The catch is in the VI column. Tilda's 1.016 and Mark's 1.027 both sit under Friel's VI ≤ 1.05 standard. A high-IF plan only works if it's also a low-VI plan — high intensity and steady delivery. Bolton makes the second part hard.
The flat opening is the first and easiest trap. Out of T1, the road is flat, often wind-assisted on a leg of each loop, and a rider holding ten watts over plan won't feel it — the legs are fresh and the gradient is zero. But ten watts over a flat third turns a 90 % TSS day into a 95 % one before the climb is even reached. Nothing tells you it happened. The first 30 km is meant to feel easy; riding it at target, not above, is the hardest discipline of the day.
The cost lands at Victoria Road, where the climb is hit — twice. Each athlete's plan asks for roughly 6 % over flat target on the climb and 12–14 % under on the descent that follows:
Flat / climb / descent target watts
| Flat | Climb | Descent | |
|---|---|---|---|
| Tilda | 181 W | 192 W | 159 W |
| Mark | 206 W | 219 W | 180 W |
| John | 144 W | 155 W | 123 W |
Plan-Target Power Over the Bike Course
None of it is dramatic, and that's the point: the climb is meant to be ridden as a controlled step up, not a surge, both times. Five TSS over the cap from pushing the climbs harder than this won't show in the bike split — it shows as pace drift in the second lap of the run.1 At matched average power, riders with higher variability ran the following 9.3 km about 42 seconds slower. The course doesn't create that variability — the reaction to it does.
The descents (averaging around −5.7 % near km 58, −5.8 % near km 85, sharp left at the foot of the first) are where you protect the rest. Ride them at the low descent target and you arrive at the next climb with VI still under control. Push them and you've banked nothing and tightened the budget. Treat them as recovery — the plan does.
Course Gradient — Where the Bike Climbs and Descends
Each 500 m coloured by its length-weighted mean gradient. Climb 1 / Climb 2 are the event-average grades of the kicker climbs in each Horwich/Bolton loop; Steepest Pitch is the maximum point pitch from the raw 1503-point gradient series. Dashed line at km 19 marks the platform's prologue / lap-core boundary; zone labels are the brief's editorial Pennington / Horwich–Bolton sections.
We covered the VI–run relationship in more detail in the IF / TSS / VI explainer, and course intelligence changes everything explains why energetic shape — not gradient charts — is what the simulation is actually optimising against.
This is a typical-June characterisation from 2024–25 data, not a race-week forecast — plans refresh as the forecast firms up. Expect around 13 °C (9–16 °C), SSE wind near 20 km/h, often overcast, water about 18 °C and wetsuit-legal. Heat isn't a factor here, which is part of why the bike call runs high. Wind is the one variable that moves the plan: the exposed Pennington loops are where a stronger-than-typical wind would shift the bike targets, redistributing power toward the sheltered Horwich loops to hold the same total. Plan for a cold start, possible wet, and exposure on the flats.
Bike Wind Profile — historical SSE wind
Headwind / tailwind by km if the historical typical-June SSE wind (≈20 km/h, 154°) blew uniformly across the course. Per-km bearings derived from the stored route geojson; wind decomposed from the stored historical wind speed and direction. This is a derivation from historical inputs — not a race-week forecast. Race-day wind will vary across the course; plans refresh as the forecast firms up.
Most of what matters about Bolton has already been decided. FTP is not changing this close to the day, neither is CdA. A few things still are.
Position discipline through the Pennington loops. The flat opening is where the high-IF plan most often gets broken. Going through that section consciously — power meter visible, deliberate stay-in-aero, no excursions above target — is the single most important decision the plan asks the rider to make. The first 30 km is meant to feel under-powered. That is the plan working.
Descent restraint at km 58 and km 85. Neither descent is where the race is decided. The plan asks for low watts there — that is where the VI budget gets earned back for the next climb. The sharp left at the foot of the km-58 descent is a useful enforcer: sitting up before the corner makes the descent target easier to hit by default.
Run-pace adherence on Chorley New Road. The two-lap run climbs through Queen's Park onto Chorley New Road. The plan has Tilda at 4:33/km, Mark at 4:53/km, John at 6:02/km — 94 %, 91 %, and 88 % of their respective run thresholds. None of those targets allow for the extra watts spent on the bike that were not in the plan. Hold the pace target on the first lap, and the fatigue accounting holds for the second. Push the first lap, and there is no separate plan for the second.
These numbers come from Keiro — a physics-based race planning tool that models all three disciplines together, so the bike plan accounts for the run and the run plan accounts for the bike. The full IRONMAN 70.3 Bolton course breakdown covers the swim entry, the 23 climbs and 26 descents, and the cut-off structure in more detail. See a plan for your fitness, your equipment, and the day's conditions.