Time Trialing and Power Meters: When the ‘Race of Truth’ is False

BY IN Exercise Institute News On May 18, 2015

For many athletes acquiring a predetermined averaged speed for a time trial (TT), like averaging 40km/hr for a 40km TT, is an internal or performance goal. It is after all the ‘race of truth’ and of their own doing exclusively, right? Well it may be a little more complex than this.

The beautiful thing about TT’ing is that it can be either: a race against oneself; a race against others; or just a hard ride. A time trial allows individuals to create an internal goal based on their effort, unlike road racing where most goals are result or outcome based (external goals or goals of one’s own doing relative to others). The importance of internal or performance based goals is well documented (Weinberg & Gould, Foundations of Sport Psychology) and of great importance to athletes and coach’s alike.

Internal goals have been associated with higher levels of motivation and lower athlete attrition rates (athletes leaving a sport) compared to athletes who select only external goals as a point of reference. Time trialing has been called a ‘race of truth’ as all momentum is created and sustained by the individual and their aerodynamic apparatuses. It is of their own doing thus the goals are often internalized.

There is, however, a mediating factor: wind direction and speed. A case in point was a TT completed on two separate occasions where the same equipment on exactly the same course averaging the exact same power output (the power meter had been calibrated on both occasions), with racing days just a month apart. Here one would think that with all these variables constant the average speed, or time taken to complete the course, would be the same, but this was not the case.

A 45 second time difference was observed between measures. The course was of a rectangular shape where on the faster occasion the wind blew along the length of the course (mainly a head or a tailwind) whereas on the slower occasion the wind blew across the width of the course (mainly a crosswind). So here I was able to infer that the aerodynamics of a TT bike, helmet, wheels etc was compromised by the ‘yaw’ or ‘heading of the resistant wind’.

As demonstrated in the image below, when there is no wind the ‘Airspeed due to rider motion’ is the critical factor. The front of a bike has a small cross sectional area and can move quickly for a given power output. If the wind is coming from the left of the bike then the ‘airspeed due to wind’ is impacting on the bike from the side. The cross sectional area of the side of the bike (and cyclist) is much greater then the front alone. Therefore the ‘Total airspeed’ means wind is flowing off the bike at an angle and is having to get around a greater cross sectional area, slowing the object down for the same given power output. A cross wind means a lower speed for the same given power output.

Yaw_A

Trek Speed Concept

On the 2nd particular testing day the wind direction was not fore and aft (head or tail wind) as is the greatest aerodynamic effect when time trialing, but featured a larger ‘crosswind component’. It is in this instant that using km/hr or time to completion as a guide to exercise performance may not be truly reflective of individual performance or ‘form’ across measures.

So in concluding we see that even in a race of truth one cannot compare their performance with previous outings when km/hr is used as the indicative measure. Rather a case for wattage or relative wattage (Watts/Kg) is made as this clearly defines the individuals’ effort relative to themselves across multiple testing occasions. Now all one has to do is come up with the $1500+ required to purchase a power meter outright!?

Hall Cycling supplies and installs Verve: Infocrank power meters

Copyright Brad Hall 20/11/11

-Written by Brad Hall, Brad has a degree in Exercise Science and Psychology (Hons.) AMAPS


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5 years ago / 1 Comment

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  • Luke dawson

    4 YEARS AGO

    Nice work with this article Brad. I am very supportive of goal setting as an internal affect. It has also been shown the pre determined goals (what you set before the event) can give a benefit in mindset. Something to think over is the stepped process of motivational interview, a counselling technique. For those interested just google.

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