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Ironman Pacing

By Youngun: I’ve been asked several times about my take on bike pacing in IM – a typical question being “if I ride faster, how much time will I lose on the run?”. I did a sketch for AndyS in the “IMUK with limited running training” thread. I thought I’d tart up that sketch, here it is:


The diagram shows how a hypothetical athlete’s run split and overall time vary with your bike pace. I calculated the bike split from the bike pace (mph on the x axis) and added my opinion of a typical effect on the athlete’s run split. I then totalled the times using a hypothetical 1:15 swim.

I drew on three bike paces, A B and C :

A is the bike pace that gives you the best overall race time. This feels very slow (especially in the first half of the ride), but allows you to run a strong tempo marathon. In fact this pace is probably not far off your fresh marathon on that day (say 10-20 minutes slower). But don’t think it would feel easy like this – the second half of the marathon is still mighty tough, but you’re still managing to run a decent pace. I estimate 15% of an IM field race like this.

B is a common pace employed by many athletes and gives a reasonable overall result. The bike feels hard in the final third and the run feels very tough too – but they battle it out. Most athletes perceive this is how an Ironman “is”. I estimate 55% of an IM field race like this.

C is what happens if you ride too aggressively. Disaster on the run (and possibly second half of the bike – can’t show that on the diagram). When athletes consider riding more aggressively (and thinking they won’t lose much time on the run) this is what happens. The truth is they lose loads of time on the run. I estimate about 30% of an IM field race like this.

I get the impression from people that they think there’s some kind of “wall” or “step” out there – if they exceed that pace on the bike then their run times fly up. I don’t quite see it like that, I think there’s a long trough in the middle. Problem is, most people miss that trough as it is a lot slower bike pace than they think. The good news is that if you ride conservatively (in that trough) then exact bike pace isn’t that critical, you’re going to get very close to your best time.

This is a pretty contentious subject, and this is only my take… hopefully it’s a good thought starter.

Category:Ironman Training


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