Using FTP as a guide for Ironman bike pacing

Using FTP as a guide for Ironman bike pacing

In recent years the use of power meters for cycle training has become commonplace. Power meters both on the bike and measurements of power using smart turbos are now widely used by both elite and amateur cyclists to measure and monitor training load and performance. Power meters can be great tools for both coaches and athletes, but here's some current thoughts which are based on a recent conversation with a coached athlete who was asking "what percentage of FTP is my Ironman pace?"

Is it actually your FTP?

Here's the first issue I have with FTP scores. Your FTP is technically what you can sustain, riding hard for a 1 hour period. However, it's very rare that anyone actually tests themselves over 1 hour. Instead they'll do a ramp test, or a 20 minute test, to 'predict' their FTP . 

The 20 minute test simply involves people riding as hard as possible for 20 minutes, taking the average power over that 20 minute period and then calculating FTP as 95% of your 20 minute average. For example, if you average 300 watts for 20 minutes, FTP would be 285. 

Very few people actually do a full hour of riding to calculate their 1 hour average. The simple reason why they don't is because it's too hard. Instead, they do a 20 minute test and for those who find the 20 minute test too hard, they do a ramp test instead. 

From my experience, very few athletes can actually ride for 1 hour at their 'predicted' FTP, the calculation is generally too high. If a rider holds 300 watts for 20 minutes, to then expect them to hold 285 for a full hour is rarely realistic, certainly in amateur age group riders, as they simply don't have the conditioning. 

My second issue, is that some people will purposefully opt for a ramp test as they know it will give them a higher 'predicted' score, but in reality, it's just a made up number that is not achievable in a real world situation.

FTP has recently become the badge of honour by which we judge people's ability. Riders will therefore select the test which gives them the highest predicted score. I'm getting bored of dull YouTube videos with click bait titles of 'FTP 300 Watts' where people use a manipulated ramp test to achieve a pointless figure.

Let me be very clear... I don't have an issue with either ramp testing, 20 minute testing or the use of FTP itself and I have used this kind of testing frequently with athletes. But I do think it's important to call it what it is... It's your 20 minute average power on an indoor trainer, or a maximal aerobic power score on a ramp test. It's not your 1 hour power output. 

Does it correlate to road riding?

Whether 20 minute power scores on an indoor trainer correlate to outdoor riding is an interesting debate. For some people they do (some even score higher outside) but for a lot of people, it's easier to generate power indoors than outdoors. There's a few possible reasons for this, but most likely one of the biggest factors is the ability to maintain a smooth and constant pressure on the pedals. If you're a cyclist, you'll know how it feels when you find that piece of road where the perfectly smooth tarmac allows you to fly at speed with no effort. Admittedly you probably need to go abroad to fulfil that fantasy, but you know what I'm talking about. An indoor trainer is like riding the smoothest road in the world, in fact, manufacturers pride themselves on being the smoothest. It's much easier to maintain pressure during the pedal stroke on a smooth surface and the UK roads ain't very smooth... 

If you train indoors all winter and your power scores go up, then certainly you've made improvements in your fitness. But don't expect them to automatically correlate to outdoors riding and don't be upset when the score is lower... indoors and outdoors are not the same. 

Can I use it to predict my race pace?

If you're riding Ironman, you can use your FTP as a bike intensity predictor. As an example, general recommendations are that Ironman should be ridden at 70-80% of your FTP, depending upon your experience level. This is where, in my mind, the whole 'prediction' thing starts to become a bit ridiculous. 

We've already said that most people won't do an actual FTP test (60 mins of work) because it's too hard. Instead they do a 20 minute test or a ramp test, to give themselves a higher predicted score. That means that all other 'predicted paces' such as the 70-80% for Ironman, will also be too high. 

"We'll use a ramp test to predict FTP because doing a full hour is just to hard... but we'll then use that test to predict power output for a 6-8 hour ride, yeah that makes sense"

By focusing on gaining a higher FTP score and manipulating testing protocols to do so, you are simply setting unrealistic targets for race day. The outcome of this will be you 'blowing up' half way through the bike and then mentally collapsing because 'you really thought you could hold that power'. 

What other factors do we need to consider for accuracy?

Ok... so putting the factors above to one side, if you're an athlete who wants to use FTP to predict Ironman bike power, that's cool. But you should ensure that it's 'race specific' to get the best results. So, the night before your test, you should have a broken 4 hours sleep, then wake up anxious before swimming a hard 3.8k swim. Then you should immediately jump on your turbo trainer and do the test. You know... just to make it accurate. 

On top of that, you might also want to consider that your power output will vary whether you're sat upright (as most people do when testing) or in the aero position. It'll also vary depending on whether you are riding on the flat or going uphill, and whether you're climbing in or out of the saddle. With all these variables thrown into the mix, estimating your power output on a hilly Ironman route, by doing a 20 minute test (which effectively is a flat road) is tenuous at best.

Look at the big picture

There is absolutely nothing wrong with developing your 20 minute power, it can be an important part of your training plan which will potentially make you a faster rider. However... as there's so much hype around the FTP figure, do not get drawn into a narrow minded focus. This is very simple... if you spend all your time trying to improve your 20 minute power output, on Ironman day, you will fall apart in the latter stages of the bike and walk the marathon. By over-predicting FTP, the whole situation becomes worse, because you will ride 2 hard for the first 2 hours (based on your calculated paces) and then be unable to back this up, due to your lack of endurance. Look at the big picture, ensure that your training also enables you to cycle 112 miles and get off the bike in a good physical and mental state, ready to run the marathon. 


Power meters can be a great tool when used in the correct manner, but they'll never be as accurate or intelligent as your own brain. If you're training for Ironman this year, you need to go out, ride long and learn, by feel, what you can sustain for the full distance without being exhausted at the end. Perceived effort is the most accurate gauge of intensity you will ever have available to you and experience is the only way to fine tune it. 


Marc Laithwaite is a level 3 qualified coach, who has been coaching endurance sports for the last 22 years. He is a former sports science lecturer of 12 years and spent 2 years with the British Cycling team as a bloods analyst. He has worked with British Triathlon Coach Education as a coach educator and spent 5 years as head coach of the NW Regional Triathlon Talent Squad. He's also a former national age group triathlon champion, European duathlon champion and Ironman age group winner. Despite his success at Ironman, his 6 year old daughter is faster than him over a 50m sprint (can't win em all). 

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