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Time Trial Pacing and the Wind

I’ve given much thought to time trial pacing and generally battling the wind during windy races. Some people get it right and seem to minimise the impact of a headwind, whilst others seem to struggle and have poorer results in comparison. Having a power meter helps with the mental side of battling the wind. After all, you tend to know what you can do power-wise and a bit of wind wouldn’t alter that. Despite that, it can be difficult to keep yourself in check when you feel like you are getting nowhere fast.

Time Trial Pacing on Windy Days

Time trialists are judged on ground speed, limited by air speed.

When pacing your time trial what is quickest? Should we push harder into the headwind or harder with the tailwind? It’s a debate I often hear and everyone seems to have their own perspective. Instinctively I want to push harder into the wind because it feels like I am losing time by going slower. I am! However it is what it is and simply getting frustrated and pushing harder than you can sustain for the entire time trial can often lead to a blow up. After all you can not break through the headwind by beating it into submission. Whilst in a tailwind the temptation is also to push harder because it feels like we are getting more out of it, making hay while the sun shines. But which makes for the fastest time trial time?

The effect of a headwind

Into a headwind your airspeed is high but your ground speed is low. For example, a cyclist travelling at 20km/h into a 20km/h headwind is actually riding with an air speed of 40km/h. They could only ride faster if, on a still day, they would be averaging more than 40km/h. Whilst isn’t 100% correct as rolling resistance, mechanical efficiency and gravity aren’t in this equation it serves to demonstrate my point.

Ground speed Wind speed Air speed
20km/h 20km/h 40km/h

Flying with a tailwind

Now with a tail wind we can reverse the headwind scenario. The same rider is in that beautiful place when the world goes silent – with only the crisp sound of the tyres rolling effortlessly along the asphalt. An experience only slightly dulled by the squeak and clicking of his poorly maintained drive train. His airspeed is at 0, ground speed 20km/h. The only thing holding him back is mechanical efficiency, gravity and rolling resistance.

Ground speed Wind speed Air speed
20km/h -20km/h 0km/h

My theory

So how would a simple “headwind out, tailwind back” scenario be best paced? My theory is a small increase of power during the tailwind is quicker, why? Power required to move air out of the way is power cubed, which means that for every km/h faster you go you will require 4 times the power. So accelerate from 30 to 31km/h is perhaps 3 watts more. Going 31-32km/h is 12 watts more. Now add a headwind component and you can see how our efforts at increasing speed are diminished. However with a tailwind the other parts of the equation become a large proportion of the total forces against us, mechanical efficiency,gravity and rolling resistance. Overcoming these negative forces require your power to increase less for the same speed increase. Therefore we should be able to get more speed from the additional power we make.

Testing my theory in the Race Comparison Tool

Let’s put the following into the race comparison tool. Using the same stretch of road as an out and back course we’ll have three riders. One rider riding a steady 300 watts out and back. The second rider rides harder into the headwind at 310 watts but now needs to take it easy back at 290 watts. The third rider rides easier into the head wind at 290 watts but harder back at 310 watts. The 14km stretch of road is slightly down hill (0.004% grade) out and as such slightly uphill back as seen in typical time trial courses.

Head wind out 5m/s favouring slight down hill

Out (Headwind) Back (Tailwind) Total
Even 12:32 08:05 20:37
Pushing into Headwind 12:21 08:11 20:32
Pushing with Tailwind 12:44 07:59 20:43

View the experiments: Out leg | back leg

Head wind back 5m/s favouring slight up hill

Out (Tailwind) Back (Headwind) Total
Even 07:36 13:12 20:48
Pushing into Headwind 07:41 12:59 20:40
Pushing with Tailwind 07:31 13:25 20:56

View the experiments: Out leg | back leg

Aggregate result

Race 1 Race 2
Total
Even 20:37 20:48 41:25
Pushing into Headwind 20:32 20:40 41:12
Pushing with Tailwind 20:43 20:56 41:39

I’m wrong and I’m glad I found out now

The numbers above disprove my theory that pushing into a headwind is wrong. The opposite appears to be true! I think I know why this is. Because you are travelling slower, the overall time spent as a portion of the ride is at this slower pace. Therefore increasing the pace during the slow part even by just a little will increase the overall speed for the larger proportion of the ride. There is on average 4 minutes between the tail and headwind sections. That’s an extra 4 minutes of going slower.

However if in doubt pace it evenly as it produced the middle time in the above scenarios. One observation on the above experiment: the rider pushing into the headwind is in fact averaging more watts that the other two riders as his increased effort is taking him longer, an extra 4 minutes at 310 watts. So this leads me further to think that even pacing is the best bet of the three scenarios.

Give me shelter

Aside from time trial pacing there are ways to minimise the impact of a headwind on our time trial. Rarely is the wind directly head on to us during a race, this gives us an opportunity to get out of its way. Hedgerows and buildings lining our route all provide shelter and it is our job to maximise these opportunities and limit the cost of the wind. Being lower than the hedgerow has a massive advantage. Smaller riders get a better shelter from cross winds because of this. If riding in a team or peloton, ride to the side of the rider in front, away from the wind, echelon.

On public highways we can’t do much about choosing our position on the road. If we are in the situation where we can choose, always ride on the side of the road closest to the wind. Similarly in a tailwind the reverse is preferred. This is why many courses have ‘float’ days where super quick times are posted. The direction of the course enables a headwind to be minimised by being closest to banking or hedgerows, protecting the rider against the wind with the return leg being more open allowing more of the tailwind to reach the rider.

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