Every Ironman bike leg gets wrecked the same way. Not by a flat tire, not by a bad nutrition plan, but by an athlete who feels great at mile 20 and rides like it. Three hours later they're walking the back half of the marathon wondering what happened. What happened is simple: they spent bike matches they needed for the run, and they didn't know it until it was too late.
Intensity Factor is the tool that keeps you from spending those matches too early. Training Stress Score tells you what that pace is actually going to cost you before the run even starts. Get both dialed in and you've done the hard part. Getting the exact number right for your course, your fitness, and the conditions on the day is where most athletes are still just guessing.
What Intensity Factor Actually Measures
Intensity Factor is your Normalized Power divided by your Functional Threshold Power (IF = NP / FTP). It's a single number that answers one question: what fraction of your threshold effort did you hold over the ride. An IF of 0.70 means you rode at 70% of the power you could theoretically sustain for an hour, across four, five, or six hours of racing.
That's the whole point of pacing an Ironman bike leg. You're not trying to ride your fastest bike split. You're trying to ride the fastest bike split that still leaves you a marathon.
How to Pick Your IF Range
For the full Ironman distance, most age group athletes should be targeting an IF between 0.65 and 0.75, with 0.68 to 0.72 as the sweet spot for the majority of the field. That range isn't fixed. It moves with how long you're actually going to be out there. Sustainable intensity drops as duration climbs, so the longer your bike split, the lower your IF ceiling needs to be. Two riders with identical FTP but a two-hour gap between their expected bike times shouldn't be riding the same percentage of it. Where you land depends on a few things:
Time on the bike. This is the biggest lever of the group. An athlete on pace for a 4:45 split can hold a higher IF than one on pace for 6:30, even at equal fitness. If your expected split is on the longer end, plan from the bottom of the range, not the middle.
Course and conditions. Hot, hilly, exposed courses punish anyone riding above 0.70. Flat, cool, sheltered courses give you a little more room.
Your bike-to-run strength ratio. If you're a stronger cyclist than runner, you can push toward 0.72 to 0.75 and hold your run pace. If running is your strength, ride conservatively at 0.65 to 0.68 and let your legs do the racing later.
Experience. First-time Ironman athletes are almost always better served riding conservatively, partly because pacing discipline takes practice and partly because a first-timer's pace usually means more time on course. The single most common mistake I see, in my own athletes and in the data, is going 3 to 5 percent too hard on the bike. That small overage doesn't cost you much on the bike split. It costs 20 to 30 minutes on the run, and it costs more the longer your day already is.
At the extreme end, pro men racing Kona typically hold an IF around 0.80, with smooth, steady power delivery and almost no variability. That's not a target for age groupers. It's a reminder of how much fitness, and how much margin for error, it takes to ride harder than 0.75 and still run well off the bike.
Translating IF Into a TSS Target
IF tells you how hard you rode. Training Stress Score tells you how much that effort actually cost you, because it accounts for how long you were out there too. The formula is TSS = IF² x hours x 100.
As a target, a good range for most age group athletes is roughly 250 to 285 TSS for the full Ironman bike leg. That number isn't a guess. It comes from coach Rick Ashburn's widely used IF-and-time reference chart, which marks 250 to 285 TSS as the "good range for most age group athletes with good preparation" across a range of realistic bike splits, with lower numbers reading as safe but conservative and anything meaningfully higher pushing into "proven, strong Ironman runners only" territory or worse.
Pull up the chart itself and the picture gets specific fast:
The Relationship of Bike Intensity and Bike Time in an Ironman Triathlon (Expressed as TSS)
Note: Intensity Factor (IF) = Normalized Power (NP) divided by Functional Threshold Power (FTP). Training Stress Score (TSS) = IF2 x Time (in hours) x 100. Copyright 2008 Rich Ashburn
The medium gray band, roughly 250 to 285 TSS depending on your bike split, is marked as the "good range for most age group athletes with good preparation."
Read across any row and the same story shows up every time. Hold 70% IF for a 5:00 bike split and you're at 245 TSS, comfortably in the good range. Hold that same 70% IF for a 6:30 split and you're at 319 TSS, deep into "run a few miles, then walk it in" territory. The intensity didn't change. The clock did.
The chart also confirms the pro data point from earlier: a 4:30 bike split at 80% IF, roughly what pro men hold at Kona, lands at 288 TSS, just outside the age-group good range and squarely in "proven, strong Ironman runners only" territory, which is exactly what it should be.
The good range also gets harder to reach as your bike split stretches out. By a 6:20 split, only the very bottom of the chart, 67% IF, still lands in it, at 284 TSS. By 6:30, even 67% pushes you to 292 TSS, above the good range entirely. If your honest bike split is heading toward 6:30 or slower, that's the chart telling you something worth listening to: there's no realistic IF left that keeps you in the 250 to 285 window, so ride as conservatively as the conditions allow and plan on the run to make up more of the day than the bike will.
That's the same duration principle from the IF ranges above, just showing up as a number. The longer you're going to be out there, the narrower and lower your IF window gets if you want to land in that same TSS band, not the same intensity held for longer. If your longest training rides haven't put a comparable TSS number in your legs, that's a signal to ride the bottom of your range on race day, not the top.
Dialing Your Race Plan In with Best Bike Split
Picking an IF range from an article is a reasonable starting point. It is not a race plan. Your actual target depends on your specific course profile, wind, expected temperature, position, equipment, and current fitness, and it should change segment by segment rather than sit at one flat number for four hours.
That's what Best Bike Split is built for. It takes your fitness data and the real course, elevation, wind exposure and all, and builds a physics-based, segment-by-segment power plan instead of a single average target. You can load that plan straight to your Garmin or Wahoo for race day, test how equipment or fitness changes shift your projected split, and compare your actual file against the plan afterward to see exactly where you drifted off pace.
An IF range and a TSS target get you in the right neighborhood. Best Bike Split gets you to the exact address, built for your course and your conditions instead of averaged across every Ironman bike leg in existence.
Dave Schell / Kaizen Endurance
Dave Schell is the Founder and Head Coach of Kaizen Endurance coaching based out of Boulder, Colorado, where he works with off-road cyclists of all abilities. Dave is the former director of coach education at TrainingPeaks where he traveled the world teaching coaches how to better use training software with their athletes.
Read more about Dave Schell at Kaizen Endurance.
