Ironman bike pacing is where age-group triathletes lose the most time. Not by going too slow — by going too fast. The riders who post the fastest overall times almost never have the fastest bike split. They have the smartest bike split, followed by a run where they don't fall apart.
I've had enough conversations on this podcast to know that pacing discipline is the single hardest thing in triathlon. Every guest we've spoken to — from Alistair Brownlee to Ben Hoffman to Olav Bu — says the same thing in different words: the bike is where you exercise restraint so the run is where you exercise dominance.
If you've got a power meter on your bike, you have the single best pacing tool available. But only if you know what numbers to ride at — and more importantly, have the discipline to stick to them when you feel good at kilometre 30.
The Cost of Going Too Hard
Here's what happens physiologically when you ride at 85% FTP instead of 72% FTP on an Ironman bike leg:
Your glycogen burn rate roughly doubles. Your lactate accumulation increases significantly. The muscular damage from sustained higher-force pedalling compounds over 180 kilometres. And the metabolic cost of that effort doesn't just affect the bike — it cripples your run.
Ben Hoffman, who's raced more Ironmans than most people have had hot dinners, put it to us with zero ambiguity: every watt you ride above your sustainable ceiling costs you three times as much on the marathon. A 5-minute gain on the bike becomes a 20-minute loss on the run. He told us: "A happy athlete is a fast athlete, and the only times I performed at my potential were when I had everything in my life kind of vibing." That includes not red-lining the bike and arriving at T2 in a state of controlled readiness, not survival mode.
That's not theory. The data from race after race confirms it.
Your FTP Power Targets
First, you need an accurate FTP. If you haven't tested recently, sort that out before race day. Our FTP zones guide covers the testing protocols. An inaccurate FTP means inaccurate pacing, which means you're racing blind.
Full Ironman (180km bike):
- Flat course: 68-73% FTP
- Rolling course: 70-75% FTP (slightly higher to account for variable terrain)
- Hilly course: 72-78% FTP (but manage surges on climbs carefully)
70.3 (90km bike):
- Flat course: 75-80% FTP
- Rolling course: 78-82% FTP
- Hilly course: 80-85% FTP
For context: if your FTP is 250 watts, a full Ironman on a flat course means riding at 170-183 watts. That will feel easy for the first two hours. That's entirely the point.
Dan Lorang told us something that reframed how I think about this: "The race is basically our best performance diagnostic. We train for racing. In training there could be some good days, some bad days, but I'm not a fan to create new best results in training. We want to have the best new results in competition." The same logic applies to pacing. You're not trying to set a bike PB in an Ironman. You're trying to set an overall PB, and that means the bike has to be conservative.
The Negative Split Strategy
The smartest Ironman bike pacing strategy is a subtle negative split. Start conservative, finish at or slightly above your target.
First third (0-60km): 2-3% below target power. Your legs are fresh, your glycogen stores are full, and the temptation to push is enormous. Resist it. This discipline pays dividends at kilometre 150.
Middle third (60-120km): Target power. Settle into your rhythm. This is where you should feel controlled, sustainable, and almost frustratingly comfortable.
Final third (120-180km): If you've paced correctly, you can push to target power or 1-2% above. You should still feel in control. If you're suffering here, you went too hard earlier.
The key insight: you should get off the bike feeling like you could have ridden harder. If you dismount at T2 thinking "that was easy," you've paced it perfectly. The run will reveal whether you got it right.
Olav Bu's planning philosophy applies perfectly here. He works backwards from the finish line, not forwards from the start. For Ironman pacing, that means: what state do I need to be in at T2 to run the marathon I'm capable of? Work backwards from that, and your bike power targets become obvious.
Why Heart Rate Lies to You
Heart rate is unreliable for Ironman pacing. Over 4-6 hours of racing in the heat, cardiac drift pushes your heart rate higher even at the same power output. Dehydration, rising core temperature, and accumulated fatigue all inflate heart rate.
Professor Seiler explained this mechanism clearly on the podcast: "Cardiac drift means you're recruiting more muscle because you're fatiguing; your brain is calling in reinforcements and turning up heart rate." Your heart rate at kilometre 150 tells you almost nothing useful about your effort compared to what it told you at kilometre 20.
A rider pacing by heart rate will unconsciously slow down as the race progresses — because maintaining the same heart rate requires lower and lower power output as drift occurs. You end up riding slower than you should in the second half.
Power doesn't drift. 180 watts at kilometre 20 is 180 watts at kilometre 160. That's why it's the gold standard for triathlon pacing.
If you don't have a power meter yet, read our power meter guide. For Ironman racing, it's the single best investment you can make.
Managing Surges and Climbs
Flat courses are easier to pace because power output stays consistent. Rolling and hilly courses require more discipline because climbs force power surges.
The rule for climbs: don't exceed 85% FTP, even on steep gradients. Let faster riders go. You'll catch many of them on the run when their legs have paid the price for those surges.
On descents, soft-pedal or coast. Your average power should still hit your target over the full bike leg. The goal is to minimise power variability — steady efforts cost less metabolically than an equivalent average produced through surges and recoveries.
Use Normalised Power (NP) on your bike computer to track this. If your NP is significantly higher than your average power, you're surging too much. Seiler's point about TSS is relevant here too: "TSS measures load, not actual stress — cardiac drift and breathing frequency are more accurate indicators of the physiological cost of a workout." A steady 180 watts for five hours costs your body far less than an average of 180 watts achieved through constant surging.
Practise in Training
Race pacing needs to be rehearsed. You can't develop the discipline to ride at 72% FTP on race day if you've never practised it. Include race-simulation rides in your training block — 3-4 hour rides at target race power, ideally followed by a short run.
Brownlee told us something that I think every triathlete needs tattooed somewhere visible: "Don't be distracted by the minor sometimes — make sure you fundamentally do the training because that's the massive metaphorical boulders, the 95% of it." Pacing rides aren't sexy. They're not Instagram-worthy. They're the sessions that win races.
These sessions also help you improve your FTP by teaching your body to sustain sub-threshold efforts efficiently.
Key Takeaways
- The fastest overall Ironman times come from smart bike pacing, not fast bike splits
- Target 68-75% FTP for a full Ironman, 75-82% FTP for a 70.3
- Use a negative split: start 2-3% below target, finish at or slightly above
- Power meters are essential — heart rate drifts and becomes unreliable over 4-6 hours
- Minimise power surges on climbs — keep it under 85% FTP even on steep grades
- Know your FTP zones and trust the numbers — use our FTP Zone Calculator to set them
- Rehearse race pacing in training — discipline at 72% FTP is a skill you need to build
Frequently Asked Questions
What percentage of FTP should I ride at for an Ironman?
For a full Ironman, target 68-75% of FTP on flat to rolling courses. For a 70.3, you can push to 75-82% of FTP. These ranges account for the fact that you still need to run a marathon or half marathon after the bike. Going above these ranges significantly increases the metabolic cost and risks blowing up on the run.
Should I use a power meter for triathlon pacing?
Absolutely. A power meter is the single most valuable pacing tool for triathlon. Heart rate drifts upward with heat, dehydration, and fatigue — making it unreliable over 4-6 hours. Power gives you an objective, real-time measure of effort that doesn't lie. It's the difference between racing by feel and racing by data.
What happens if I ride too hard on the Ironman bike leg?
Riding above your sustainable power burns through glycogen faster, accumulates more lactate, and causes greater muscular damage. The result is a dramatically slower run — often 20-40 minutes slower than your standalone marathon pace. A 5-minute gain on the bike can cost you 25+ minutes on the run.
