Should you trust your bike power meter?

Know what your power output should be for an Ironman bike leg, but worried this doesn't allow for fitness/injury issues and wondering whether to trust the power meter on race-day or should you should back off if you are not feeling comfortable? Joe Beer has some advice


With an accurate power meter you have, in my opinion, the best race-day tool to ensure your best complete race. A power meter gives you an informed metric that can help you execute the most efficient bike split, allowing you to make the most of your running ability at the end of the race.


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Hopefully your “pretty good idea” has come from having good runs off the bike as a result of your own number-crunching. But  there are only two actual options: going on feeling, average speed and your ego, and hoping you don’t do yourself too much damage for the run; or sticking to a power plan that will allow you to reach T2 with good legs, not necessarily in your fastest time. Hopefully it’s obvious to you which one I prefer!

I give all coaching clients and bike optimisation customers a sweet spot of target power, based on recent maximum bike power test data and, ideally, time-trial best performances. This enforced race-day limit will rarely increase, unless other data such as recent peak power and/or TT power data shows significant real gains.

Bear in mind that just a small increase in effort, say 20 watts, will get you to T2 around seven minutes faster. However, don’t forget the impact of mechanical strain and carbohydrate use that six hours of overexertion causes. But how do you adjust to declines in fitness due to injury or training inconsistency?

 Ramp testing – Using a series of wattages for three minutes each (100, 150, 200, 250w) to regularly see both heart rate and perceived exertion. Your fatigue levels and drops in fitness can be gauged from this, as can your improvements.

Race data – Races and time trials in the weeks before your Ironman can still give a ‘trajectory of form’, revealing whether your numbers are good, bad or indifferent.

Training outcomes – When someone is in good form, training in Zone 1 is easy and when they hit top-end efforts they may even get close to PBs. However, when it’s a struggle, alarm bells should ring.

All three metrics above can help you choose what your power should be for your iron-distance race. If you feel you have to drop 20 watts to be safe, do so – perhaps a slower bike split will still allow you to run well and complete a solid Ironman race. Remember that the aim is to ride the best bike split to achieve your maximum race-day potential. Too many post-race reports blame factors that could be honestly explained as: ‘I went too hard on the bike and paid for it!’



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