Aerodynamics Questions

Training Concepts
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Let’s get aero here -

We had a couple great questions from one of our listeners recently on some aerodynamics topics. This is a great one to get insights from @colbypearce with his knowledge on pedaling a bike and positioning. Here they are:

When riders are tested in a wind tunnel they are always on their own but when they are on the road they are usually in a peloton or small group, taking advantage of drafting. Has any wind tunnel testing been done on how aerodynamics are affected by nearby riders and would this lead to changes in, say helmet design, to take advantage of any gain?

Thinking of team pursuit and watching the riders in line on the track the following riders shelter in the draft of the leader. This is fine on the straights but when on the banking the “wind” would not be coming from directly ahead. Do the riders stay in line or do they naturally fan out a bit to stay in the draft? (a bit like how echelons form with a cross wind) As I’ve never been to a Velodrome I can’t really tell from looking at a TV.

Lots of information to unpack here, so let’s see where we end up taking this one!

Coach Ryan

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Great questions.

I am sure tunnel testing has been done on the effects of drafting, this is where we get the “30% less energy when another rider breaks the wind” tag line in the older Tour commentary. I am not aware of any data that suggests that helmets or other things should be modified to take advantage of riding in a group. I think the logic is that anything that is aero when a rider is solo will also be the best choice in a group. Also, fluid dynamics become really complicated in a pack so trying to optimize aero for a rider in a group might be fruitless.

The other problem is that if you were to optimize a rider’s aero choices for riding in a group, as soon as they were solo, they might be less aero. You could theoretically make an argument that a rider who is GC focused and only has to climb fast would be better off riding in a suit that minimizes side turbulence, assuming they would only have wind exposure when on the side of the peloton, if their teammates were setting pace all day to set them up for an effort on a steep climb. The problem is, this plan would all fall to pieces as soon as they had a flat and had to chase on through the valley, or when there is a headwind on the climb and they are now in a slower kit than would normally be chosen. I can’t imagine a scenario where you could pick kit that would give you an advantage in a group but maybe I am missing a key idea here.

In team pursuit, you might think the draft would be coming off the lead rider at an angle, and it probably is, but the riders follow so much more closely on the track than on the road this gets factored out. After racing about 99 million laps on the track, I can tell you from experience that the best place to be on someone’s wheel in a corner is directly on it. The lead rider is turning, but they are also leaning and accelerating, and you are too. The physics of track riding is surprisingly complex. maybe the acceleration offsets the slight change in draft angle, I don’t know honestly, but I can tell you I have never had the inclination to ride on someone’s hip to get a better draft. The only reason you do that is if you are setting up to pass them. Also, as the lead rider pulls off in the turn. the second rider cannot be overlapped or he /she will go down. One more consideration is that for every cm you are above the black line, you are doing extra distance per lap; so riding high comes at a penalty. Only by riding exactly on the black line do you ride 250M/ lap. When you ride high, you are going farther than required …

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@colbypearce thanks for all your comments on that. This got me thinking back to the track bike that Felt Bicycles built where they moved the crankset to the opposite side of the bottom bracket. If memory serves, this was to help with aerodynamics on the banked track.

What are your thoughts on that type of setup when we’re talking about aerodynamics on the track? Was that a reasonable change that is worth the improvements, or is that type of adjustment to the usual design of bikes just too much work for the added benefit?

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Yes, that is how it was marketed. I haven’t seen the data to know what the difference is, but you figure that if the crank smooths the airflow over BB/ rear wheel when the bike is at a slight yaw, there could be an advantage. I have no idea how much the advantage would be though. Also, part of the marketing splash on that bike was the L side drive train, and at $26K / bike they needed all the splash they could get, so even if it was a draw aerodynamically I could see them justifying the L side drive train to make the bike more exotic and cool to talk about. I am not saying marking was all of that decision, but I wouldn’t be surprised if it factored in to the final design.

1 Like
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Being aero is about trying to get the air to flow smoothly past you in laminar fashion

laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. It’s the turbulence, mixing of layers,that causes the drag

The ideal for this is the droplet shape , with the head of the drop facing forward, no sharp edges, with a long smooth tail,. It’s why if you ever look at velo mobiles or stream liners they try and get as close to that shape as possible.

To give how effective that is , some records

2016:

LAND - MEN’S 1 HOUR STANDING START SPEED TRIAL (Single Rider): 92.43km

LAND - MEN’S 6 HOUR STANDING START SPEED TRIAL (Single Rider): 383.73 km

I’ll let you work out the average speed over 6 hours to show how far aero dynamics can go when a bike is fully faired.

For more mundane every day velo mobiles you can average 30 km/h on the flat at 79 watts of effort, and 50 km/h at 250 watts.

So back to the question of riding in a group. Aero helmets, wheels are designed to smooth air flow to make it more laminar. If you’re riding in a group and not at the front, you are already needing to push less air out the way. But the need to smooth air flow is still necessary if you wish to minimise drag. If the air in the slip stream is already turbulent then it’s unlikely anything you wear is easily going to smooth it.

If you have ever watched air craft disasters. There are the cases where a smaller airplane took off too soon after a larger airplane. They run into the wake / turbulence and that just makes it impossible to control the smaller plane. If anything is aero it’s aeroplanes.

What would be interesting would be to see riders on a closed road to ride in a teardrop formation all with HR and power meters. Then to see how things differed over that formation in terms of internal effort and external power. Not as easy to swap round as a pace line though!

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Sailors use two wind directions when underway in a sail boat. There is “true wind” and “apparent wind.” True wind is what you feel when you are standing still - on a boat or on a bike. Apparent wind is a sum of the vectors (a little algebra) between the two. If the true wind is basically zero - like within an indoor velodrome, then the apparent wind is the only thing that matters. Hence pursuit riders stay in line. If you are outdoors, say riding in Belgium in the wind, the sum of the vectors is often not straight ahead and off at an angle. Hence those classic scenes of riders in an echelon.

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there actually has been wind tunnel work done and academic papers written on this topic. Also some of the newer mobile wind sensors might provide some insights into the effect of nearby riders. It is well established that when using the Chung method for aerodynamic testing that disturbances such as a vehicle passing on the other side of the road will be seen in the virtual elevation plot. So as noted above anything that changes the flow of air will impact the aerodynamic drag of the rider. I have read the paper on the group dynamics, I believe the did scale modelling but also I think maybe Specialized did something on this as well. Also in an indoor velodrome the person going around actually does create an air flow eventually. In the real world the effects are harder to actually figure out because the wind is not often constant in both direction and force, I think the word is stochastic.