Fulfil your Ironman potential by learning how to pace yourself on the bike, says Garth Fox

Ironman bike training: pacingApproximately 50 per cent of your Ironman will be spent on the bike. How you pace yourself will not only influence how fast you get round the bike course, but also how well you run the marathon.

Ironman performance is limited not just by fitness but also by fuel. The harder we exercise the faster we use our precious but limited glycogen stores. When we run out, we get that dreaded feeling of hitting the wall. The answer is to ride at an even intensity, which allows the body to use more fat as fuel and spare glycogen for the run. Here’s how:


A power meter is expensive, but it’s the best way to ensure your effort levels are even and precise. Once you have calculated your optimal power output for the Ironman distance you ride as close to the number as possible. If you split the distance into two halves, an optimal power profile would mean that each half was within about five per cent of the other.

When riding up hills you should allow your power output to drift about five per cent and when descending you can let the power drop by five per cent. This is because the power required to overcome aerodynamic drag rises with speed.

Heart Rate

If you have calculated your heart-rate training zones on the bike then you should ride the entire distance in zone 2. Don’t make the mistake of using heart-rate zones derived from running, because they will typically be 8-10bpm higher. Also it’s better to target a heart-rate range than a specific number, due to variables such as heat, fatigue, hydration, anxiety and sleep.

You will also find that heart rate drifts higher for the same effort over the course of an Ironman due to cardiac drift – the result of decreased hydration and increased fatigue.


Maintaining a given speed for the duration of the bike leg is a precarious means of pace judgment due to external factors such as wind and gradient. However, over a flat course such as Ironman Florida or Melbourne on a windless day, it can be used in conjunction with other methods of pacing.


Often also called perceived exertion, feel has been shown in research to correlate well with other measures such as a heart rate, lactate and power. On a scale of 1-10 where one is resting on the sofa and 10 is the point of maximal effort, the first half of the Ironman bike leg should feel like a three and the second half like a four.

The problem with feel is that it’s so subjective. A well-trained athlete is likely to feel good on the bike for the first few hours, so runs the risk of pushing too hard. In Ironman that means trouble on the run. As an independent measure of pacing, perceived exertion is flawed but not redundant.


If you don’t own a power meter you can target a heart-rate range and an approximate average rolling speed. Knowing how those feel in the first half versus the second half of an Ironman bike distance, you can work out how to pace your race. The key lies in doing your homework. Have you researched the route? Considered the changes in gradient? Looked at typical wind speeds? Done the distance in training? Have you tried running off 180km? Pacing is an acquired skill and, like all skills, it’s improved by homework and hard work.

Make it work for you: Rolling loop race practice

If you’d like to keep things simple on your Ironman bike leg – or are new to tri and need a trick that will work in shorter races – learn to pace by feel with a rolling, twisty training loop that’s just a couple of miles long. All you need is a watch to learn to pace your laps evenly and experiment with how hard you can push yourself over different numbers of laps, up the gradients, and round corners. It’s good practice for multi-loop short races, too.