IF is calculated as NP divided by FTP, or in other words, it's the percentage of your individual FTP that you maintained, on a normalized basis. The pros will usually target an IF between 0.75 to 0.80 in an Ironman bike, meaning that they will try to hold about 75% to 80% of their FTP. For a 70.3 race, this figure rises to a range of 0.85 to 0.90 IF for the pros.
Most age groupers can and should not race as intensely. Age group Ironman bike IF generally ranges from 0.6 to 0.7 for most, higher for more competitive age groupers, lower for novice or slower athletes.
So where should you fall on that spectrum? The second piece of this puzzle is your Training Stress Score (TSS). TSS is an estimate of the workload of a session, based on the intensity and the duration of the session. It's calculated:
IF^2 x 100 x [duration in hours] = TSS
A 1-hour, all-out time trial effort at FTP results in 100 TSS.
(1.0 x 1.0) x 100 x 1 hour = 100 TSS
Another example that's more illustrative, a 5.5-hour effort at an IF of 0.7:
(0.7 x 0.7) x 100 x 5.5 hours = 270 TSS
According to Friel's guidance, a bike that results in more than 300 TSS will probably lead to a blow-up on the run, but anything much lower than 250 to 270 TSS may mean you left something on the table. Because TSS is so largely determined by duration, the slower you are on the bike, the lower an intensity you will need to ride to stay under that 300 TSS mark.
This is explained in an excellent piece by Friel, accompanied by a table credited to Rick Ashburn that lays out a range of target Ironman bike IF depending on your experience level. Use the table to back into the IF and thus the target power you need to hold for your Ironman bike based upon your estimated bike split and target TSS. Did you blow up on the run in your last Ironman? Check your IF and TSS, and whether you need to dial down the intensity. See the full article on target IF and TSS here
You'll note that IF and thus TSS are both functions of your FTP. To get an accurate IF and TSS, you have to have your FTP entered accurately into your analysis software, which also means you need to be testing your FTP regularly and making sure your power zones are up to date.First vs. Second Half Pacing
Another key analysis that is often performed is to assess overall pacing by comparing average or normalized power for the first half of the race vs. the second. For age groupers, Friel says, "Assuming the rider is doing a time trial or triathlon on a flat course I'd like to see the second half have a power that is about 1-2% greater than the first half. That would mean the athlete held back in the first quarter of the race and remained focus on the third quarter which is when the greatest drop off usually occurs when going out too fast." For the pros, first-half power is often higher as they jockey for position at the start of the bike. Generally, when we saw a drop off in power of 10% or less between the first and second half of a pro Ironman power file, we considered it a pretty well-paced race.
There are some caveats to this. Consider the terrain, as a course that is primarily uphill in the first half and downhill in the second, for example, may result in a larger drop off in power that has more to do with the course than a pacing strategy. Secondly, consider the effects of changing conditions like wind. In Kona 2013, for example, though conditions were historically calm, analyst AJ Johnson noted in an article written for Ironman.com that competitors still had a light tailwind on the way out and a headwind on the way back. This, along with the competitive dynamic of the race, may have contributed in part to lower average power in the first half of the race for Luke McKenzie, who held 277 watts on the way out and 285 watts on the way back (his goal for the race was 290 watts). The fact that he was able to maintain a higher output on the way back, however, still indicates excellent pacing and ability to hold himself back a bit during the first half of the bike.