Variability Index or VI is a cycling power meter metric that can help you pace your triathlon bike leg to perfection. Here’s how it works.
VI, or Variability Index, is a hidden gem within the list of power meter terms for triathletes, giving you data on how good your pacing is for a ride. A very simple formula, VI is calculated by dividing your normalized power by average power.
If you’re not up to date on NP, check out our post on normalized power versus average power. But in short, it’s average power taking into account the peaks of high power such as climbing and the troughs of low power such as freewheeling downhill.
When it comes to Variability Index, the magic number you’re aiming for is 1 to 1.05. Hit figures within that range and you can pat yourself on the back for having paced your effort evenly and efficiently, burning minimal matches along the way so you’re ready for the run.
But how do you get your VI into that range in the first place? It’s all about staying controlled and avoiding big, match-burning efforts. The aim is to stick as closely to your prescribed power range as possible – usually a percentage of FTP that’s suitable for the distance or duration of your race. This could be prescribed by your coach or honed through race-pace simulation rides.
How To Maintain A Low Variability Index
So, let’s say you’ve calculated a range of 180 to 190 Watts for an Ironman 70.3 or 50-mile time trial. If you consistently stick within this range for the majority of the ride, you’ll have that nice low VI. It sounds simple, but there are several spanners that can be thrown into the mix to ruin your pacing.
Lack of vigilance can ruin your pacing. We’ve all done it: the excitement of racing gets to us and we go out way too hard. Before you know it, you’ve been holding power well above your range for minutes at a time, leading to the inevitable crumbling of legs later on. Keep an eye on your data – 10-second power is ideal to smooth the figure out – and stay in your zone.
The next issue is ego. It’s a common tale no matter your experience: another athlete goes past at crazy speed and we up our own pace to match. Perhaps you play cat-and-mouse with another rider for a few miles and the frustration gets to you, so you burn it up the road to finally drop them. Or maybe you just get obsessed with a high average speed at any cost. All these out-of-range peaks will hurt you in the end.
Another easily solved problem is gear ratio selection. While it’s important to have enough hard gears so you don’t spin out on flats or downhills, it’s even more important to ensure you’re prepared for the climbs. Running out of gears on an uphill section so that you’re grinding away well above goal power is a sure-fire leg sapper so experiment on different gradients and different gear setups.
Depending on the severity of the climbs you’ll face on the race course, you might want a compact crankset with a 36 or 34t inner ring or perhaps a 30 or 32t sprocket on your rear cassette.
This will allow your legs to spin at a lower power, helping you stay in your zone even with a mountain to climb. All you’ve got to do then is stick to the plan and don’t sweat it when other riders go tearing up the road past you – either they’re in their zone and super strong or they’re going to pay for it later!
Master the discipline and technical aspects of keeping a low VI and you’ll know you couldn’t have paced the rider any better, keeping your legs as fresh as possible and giving a massive confidence boost for the run.