# CDA - AERODYNAMICS - DRAG: influence of motorbikes, bike position etc

#### Logic-is-your-friend

BANNED
I posted this some weeks ago in the "general news thread". Since there is now a follow up, i've decided to make a new topic for it.

Red Rick said:
Logic-is-your-friend said:
Belgian professor Bert Blocken from universities Leuven (Belgium) and Eindhoven (Netherlands) has done more research regarding the advantage of drafting behind motorcycles. It turns out the advantage is much bigger than expected. In perfect conditions at an average speed of 54km/u, even at 30 meters (!) still an advantage of up to 2.6 seconds per kilometer is gained! Due to 12% less resistance.

At 50 meters behind a motorcycle, the advantage is still up to 7% less resistance.

https://sporza.be/nl/2019/06/27/motoren-voordeel-renners/
Should be much bigger news.
Logic-is-your-friend said:
Sestriere said:
Perfect conditions would imply no wind at all and the cyclist following the motorbike the best possible way? Does the study also make implications about real world scenarios, i.e. cross or tail wind, changing speed and distance to the motorcycle due to undulating roads or the leading rider of the group taking the shortest possible way instead of perfectly following? It would also be interesting to know whether the results only apply to individual riders and single line groups or pelotons as well.
I can imagine the advantage being even bigger with a head wind. With cross wind, the advantage gets a lot smaller. I assume the actual study does go into that further, but the news report didn't.
This guy (Bert Blocken) was in the NOS Avondetappe on Dutch national TV. Those who understand Dutch and want to see it, you can view all episodes of the Avondetappe, via "NPO Start".

https://www.npostart.nl/nos-de-avondetappe/12-07-2019/POW_04304525

It's an interesting watch. I recommend it even if you don't speak Dutch (just for the visuals). He also compared descending/downhill positions on the bike (Alaphilippe vs Froome). Ewan's sprinting position. The fact that a car/bike riding BEHIND a rider also has a positive influence on air resistance for the rider (the car effectively pushes air forward, and creates a "bubble" wich also protects the rider). And that a car/bike riding NEXT to the rider impacts the aerodynamics in a negative way (i guess unless the wind is coming from that direction).

Apparently, ASO has briefed the motorbikes (/pilots) in light of this research, to keep their distance. Other organisations have asked UCI for years, to do this research, but UCI never committed.

#### ClassicomanoLuigi

Caleb Ewan sprint position is smart, aero matters a lot because power requirement to overcome air resistance increases exponentially with speed. But he gets away with that position by being diminutive, not for everyone, and Ewan makes himself more likely to crash by putting the center of gravity forward.
Chris Froome is not nearly as far forward on the top tube in reality as the rider figure in the simulation of 'Position Froome' . But certainly it's believable that 'his' method is less effective, because of having the head far away from the hands compared to the tight tuck, and also more turbulence unnecessarily behind, in the region between back and seatpost

#### Logic-is-your-friend

BANNED
They didn't compare both positions. Obviously because one was for sprinting and one for descending. It's possible that Froome his position is equally aero as Ewan's, but Alaphilippe's position is the best... But obviously both unfit for sprinting.

#### jmdirt

The other moto thread is focused on safety, but some of this could be added to that thread.

I find all this type research interesting. Too often though, they look at a single factor. ie: Ewan's sprint position is aeoro (less drag) but could he produce more watts if he stood up more? Could he be faster? Its somewhat like when they started pushing TT positions: riders punched a nice hole through the air but couldn't pedal or breath as well.

#### Lance Armstrong

Re: AIR RESISTANCE: influence of motorbikes, bike position e

I don't mind motorpacing. It encourages aggressive riding. If you want to be motorpaced, attack, or ride at the front.

#### Logic-is-your-friend

BANNED
Re: AIR RESISTANCE: influence of motorbikes, bike position e

Lance Armstrong said:
I don't mind motorpacing. It encourages aggressive riding. If you want to be motorpaced, attack, or ride at the front.
Depending on which nationality you're from... Camera today, behind De Gendt... but in front of Alaphilippe/Pinot.

#### Orbit501

There was an interesting slo mo of Ewan's back wheel skipping all over the pace during the sprint on yesterday's stage, because he has so much weight over the front of the bike.

I wonder how much that rear wheel skipping negates the obvious aero benefits he gets from his position on the bike?

#### Logic-is-your-friend

BANNED
Re:

Orbit501 said:
There was an interesting slo mo of Ewan's back wheel skipping all over the pace during the sprint on yesterday's stage, because he has so much weight over the front of the bike.

I wonder how much that rear wheel skipping negates the obvious aero benefits he gets from his position on the bike?
It was due to bumps in the road (in combination with his weight distribution). It won't be a problem in many cases.

#### del1962

Wow you posted this today and then the allegations happen, its almost as if you knew

#### More Strides than Rides

2.6 seconds per kilometer (at 54 km/hr) is a big number, but rarely is there a perfect draft for all 60ish seconds of a kilometer for a rider to gain 2.6 seconds. A 6 second draft is egregious, and we are left with 0.26 second advantage?

Important information. Commissars should know, moto drivers should know. But impactful? Only under extreme negligent or dissonant moto drivers. Not zero, but not a cause for total reform like removing motor bikes for drones or anything.

#### Zinoviev Letter

Re:

More Strides than Rides said:
2.6 seconds per kilometer (at 54 km/hr) is a big number, but rarely is there a perfect draft for all 60ish seconds of a kilometer for a rider to gain 2.6 seconds. A 6 second draft is egregious, and we are left with 0.26 second advantage?

Important information. Commissars should know, moto drivers should know. But impactful? Only under extreme negligent or dissonant moto drivers. Not zero, but not a cause for total reform like removing motor bikes for drones or anything.
2.6 seconds per kilometer is at 30 meters distance from the motorbike. Nobody complains about drafting when there’s a moto a full 30 meters away (although the numbers suggest they should). 30 meters is where bikes sit for head on shots on great big Spanish motorways. If there’s a continuous head on shot on roads like these, the camera bike is a lot closer.

#### Pantani_lives

Great research. It makes you wonder how much influence motors have had on famous races in the past. Maybe they should use drones to film the race?

#### del1962

Re:

Pantani_lives said:
Great research. It makes you wonder how much influence motors have had on famous races in the past. Maybe they should use drones to film the race?
Is using drones really feasible?

#### Logic-is-your-friend

BANNED
Re:

More Strides than Rides said:
2.6 seconds per kilometer (at 54 km/hr) is a big number, but rarely is there a perfect draft for all 60ish seconds of a kilometer for a rider to gain 2.6 seconds. A 6 second draft is egregious, and we are left with 0.26 second advantage?

Important information. Commissars should know, moto drivers should know. But impactful? Only under extreme negligent or dissonant moto drivers. Not zero, but not a cause for total reform like removing motor bikes for drones or anything.
I'm a big believer that it's inevitable that polluting helicopters and race rigging motorcycles should and eventually will be replaced by drones. Maybe not next year, maybe in 5 or 10, maybe 20 years. But it should and will happen. Maybe technology needs to improve a bit still, and it won't be perfect from the start, but it's inevitable.

I also don't know what math you are applying. There is 12% less air resistance riding behind a bike that is riding as far as 30 meters (!) in front of a rider. Look at the image in the opening post, of Alaphilippe's "winning ride" of last week. He's riding no more than 10 meters behind TWO motorcycles, on a straight road. According to this study, that means 23% less resistance! That's immense. You are deluded if you think that didn't win him multiple seconds per km as well as thinking that this doesn't happen often or decides races.

#### Sestriere

I've been thinking about drones for cycling coverage before and in more detail again today after the topic came up again. Been looking at what's on the market currently for professional use. There are some interesting problems to solve for this to become feasible which I'd love to investigate in more detail.
Linking up with the topic of the thread, it would also be interesting to see the effects of a drone flying closely in front of a rider.

#### Logic-is-your-friend

BANNED
Re:

Sestriere said:
I've been thinking about drones for cycling coverage before and in more detail again today after the topic came up again. Been looking at what's on the market currently for professional use. There are some interesting problems to solve for this to become feasible which I'd love to investigate in more detail.
Linking up with the topic of the thread, it would also be interesting to see the effects of a drone flying closely in front of a rider.
You wouldn't need to fly closely in front of a rider. First of all, you don't even have to fly "over" the road, you can fly next to it. Camera's will be much more stable than from a bike. Less vibrations and shakes, meaning you can fly farther ahead and zoom closer (which is a problem on a bike due to vibrations). For personal use, check DJI. They have really great quality drones under 1000\$ with great image quality.

del1962 said:
Pantani_lives said:
Great research. It makes you wonder how much influence motors have had on famous races in the past. Maybe they should use drones to film the race?
Is using drones really feasible?
Camera drones are (just like a motorcycle) operated by two people. One pilotting the drone, one doing the camera. The main hurdles currently are the range of control and battery life. It's probably too soon yet, but technology progresses fast, especially once it hits mass production. Even 1000 dollar drones are used for professional TV reports. Image quality is actually very good, it will be more stable than shot from a motorbike. But i don't know the cost and specs of high-end drones. I know DJI has a pro camera drone (for television) for "only" +/-5000 dollars. Very stable and fast. But maybe there are even better drones for 5x that price, that have longer range and better battery. In theory, you could consider having a bus to operate the drones, riding 100 meters in front of the peloton, and swap batteries every 30 minutes, while staying close to the drone (for range). But i think it's still too soon. I guess they'll start on closed loop competitions first, and maybe start experimenting in 5 to 10 years. I 'm pretty sure they're being used on some CX races already since a few years.

But this is just my personal belief/opinion. I'm neither a drone expert, nor an aerodynamics expert.

#### Sestriere

Re: Re:

Logic-is-your-friend said:
Sestriere said:
I've been thinking about drones for cycling coverage before and in more detail again today after the topic came up again. Been looking at what's on the market currently for professional use. There are some interesting problems to solve for this to become feasible which I'd love to investigate in more detail.
Linking up with the topic of the thread, it would also be interesting to see the effects of a drone flying closely in front of a rider.
You wouldn't need to fly closely in front of a rider. First of all, you don't even have to fly "over" the road, you can fly next to it. Camera's will be much more stable than from a bike. Less vibrations and shakes, meaning you can fly farther ahead and zoom closer (which is a problem on a bike due to vibrations). For personal use, check DJI. They have really great quality drones under 1000\$ with great image quality.
Maybe you wouldn't need to fly very closely in front of the riders due to said benefits. However, drones would also offer new opportunities for great action shots due to possible proximity to riders. If you wanted to use this potential it would be interesting to learn something about aerodynamics in that regard as well.
Also, I believe, in some cases you would actually have to fly oer the road, for example when the road is completely/largely covered and sheltered by trees or buildings ot other topographic features.

Yep, I ended up on DJI's website as well.

#### Logic-is-your-friend

BANNED
Sure, but you can adapt to circumstances. You can't jump onto the pavement or ride on rooftops with a motorcycle. But flying a drone close to a rider (in line of sight) is probably not preferable for safety reasons.

PS: don't forget that in many countries there are strict regulations now for drones. Depending on your country, you might need a special license. Just in case you are considering buying one.

#### Alex Simmons/RST

Re:

ClassicomanoLuigi said:
aero matters a lot because power requirement to overcome air resistance increases exponentially with speed.
The relationship between speed and power output is a cubic equation (a mathematical power function), and is *not* an exponential equation. Exponential implies something vastly different to a power law.

#### Alex Simmons/RST

Re: Re:

ClassicomanoLuigi said:
The pressure wave ahead of a moto and why one would not want to be right next to them, I think that is well-known... but the drafting from so far back they prove something new
I ran an analysis of the impact of drafting 12m behind another cyclist. Depending on conditions it provided the following rider with a 20-30W reduction is power required at 40km/h. We can expect that a moto, being a larger vehicle would provide at least this level of benefit at a similar distance:
https://wattmatters.blog/home/2013/02/pour-me-draft.html

At a greater distance I am also not surprised. I've also seen in the data the very small benefit a rider gets on a velodrome from a rider circulating on the other side of the track, i.e. at 125m ahead.

I also did an impromptu experiment and analysis of the impact of having a rider immediately behind, to the side and in front of another rider.

In this case we see the well known results of a significant impact when you are immediately behind a rider, and the small but not insignificant benefit of someone immediately behind you, but also of the negative impact when someone rides next to you:
https://wattmatters.blog/home/2015/08/when-your-ride-buddy-becomes-real-drag.html

#### Alex Simmons/RST

Re: Re:

Logic-is-your-friend said:
I also don't know what math you are applying. There is 12% less air resistance riding behind a bike that is riding as far as 30 meters (!) in front of a rider. Look at the image in the opening post, of Alaphilippe's "winning ride" of last week. He's riding no more than 10 meters behind TWO motorcycles, on a straight road. According to this study, that means 23% less resistance! That's immense. You are deluded if you think that didn't win him multiple seconds per km as well as thinking that this doesn't happen often or decides races.
Aero benefits are not linearly additive in this way.

#### Logic-is-your-friend

BANNED
Re: Re:

Alex Simmons/RST said:
Logic-is-your-friend said:
I also don't know what math you are applying. There is 12% less air resistance riding behind a bike that is riding as far as 30 meters (!) in front of a rider. Look at the image in the opening post, of Alaphilippe's "winning ride" of last week. He's riding no more than 10 meters behind TWO motorcycles, on a straight road. According to this study, that means 23% less resistance! That's immense. You are deluded if you think that didn't win him multiple seconds per km as well as thinking that this doesn't happen often or decides races.
Aero benefits are not linearly additive in this way.
I don't know what you're talking about. Those are numbers from his research, be it in "optimal" conditions behind ONE motorcycle.

#### ClassicomanoLuigi

Re: Re:

Alex Simmons/RST said:
The relationship between speed and power output is a cubic equation (a mathematical power function), and is *not* an exponential equation. Exponential implies something vastly different to a power law.
A cubic equation is an exponential equation, the former type is a subset of the latter.
A third-order power law is also an exponential equation, so the distinction you are trying to make is between the variable and the base ...
Whereas what I said, covers all of the above.
"*not* an exponential equation"
is just untrue, I see what you are saying, though. Better observation would be: that it is a specific kind of exponential function. We're talking about the same thing, imprecise is perhaps better than incorrect
I ran an analysis of the impact of drafting 12m behind another cyclist. Depending on conditions it provided the following rider with a 20-30W reduction is power required at 40km/h. We can expect that a moto, being a larger vehicle would provide at least this level of benefit at a similar distance:
https://wattmatters.blog/home/2013/02/pour-me-draft.html
The "non-draft triathlon" photo on your blog is hilarious, what a joke that is. The triathletes even have a mini-peloton going in there ...
There is a local velodrome where I learned basically how to ride track, but soon decided I would not be any good at track racing. Also very steep banks at 50 degrees, such I fear crashing more than I enjoy the novelty
In road cycling experience, drafting at 12 meters is not going to reduce my power requirement from 300W to 280W, for example... but under the semi-ideal conditions of track cycling, and an experiment in which both cyclists are cooperating for the purpose, that seems believable. And although surprising for the distance between riders, that's what your data on your blog has proven. And moto-bikes, motorcycles with two riders, or especially a team or service car the effect will be huge compared to a single cyclist, like you said. Commissaires let GT cyclists get away with a lot of stuff. Almost anything in a grey-area of rules as long as the racers don't hold onto the cars

#### Sestriere

Just for the lol's I fitted an exponential function to data points generated by the physical relation P(v)=v^3. You start to get a reasonable result (was too lazy to calculate reduced chi squared values to evaluate properly) with 3 degrees of freedom (fit range was 0 to 15), i.e. P_fit(v)=a*e^(b*v) + c
Visually, it looks reasonable although still not perfect. Parameters are: a=133.8 +- 17.4, b=0.2 +- 0.0, c=-223.9 +- 41.9
Parameter c obviously indicates that something is still fundamentally wrong with this fit as it should be 0. You would probably need to add a second summand to your fit function to get a better result. So yeah, you can model a power function with an exp function if you wanted to. That does however not change the fact that the theoretical relation is that of P(v)~v^3, so every attempt to use an exp function is unphysical and not justifiable.

#### Alex Simmons/RST

Re: Re:

ClassicomanoLuigi said:
Alex Simmons/RST said:
The relationship between speed and power output is a cubic equation (a mathematical power function), and is *not* an exponential equation. Exponential implies something vastly different to a power law.
A cubic equation is an exponential equation, the former type is a subset of the latter.
A third-order power law is also an exponential equation, so the distinction you are trying to make is between the variable and the base ...
Whereas what I said, covers all of the above.
"*not* an exponential equation"
is just untrue, I see what you are saying, though. Better observation would be: that it is a specific kind of exponential function
An exponential function has its variable in the exponent, but a power function has its variable in the base. It is a significant difference.

e.g., where "x" is the variable and "a" is a constant:
exponential function: f(x) = aˣ
power function: f(x) = xᵃ

The physics relationship in play here is a power based relationship, not an exponential relationship. There is no physics involved which invokes an exponential relationship. IOW the variable of velocity would never appear in the exponent.

Where v = velocity, the mathematical relationship between speed and power is of the following form:

Power(v) = av³ + bv² +cv

The cubic part is for air resistance, gravity and rolling resistance are linear parts of the equation. There are very small parts (bearing friction) related to the square of velocity. This is for steady state cycling. For changes in speed then of course the change in kinetic energy is also related to the changes in the square of starting and ending velocities.

Here's the equation from the seminal paper on it:
http://www.aerocoach.com.au/math-model

As a result, solving power from speed(s) is relatively straightforward, provided you have all of the relevant input constants (coefficient of drag, frontal area, coefficient of rolling resistance, gradient, mass, wheel moment of inertia, relative wind velocity etc).

Solving speed from power is (mathematically) much harder.

Red Rick and Liiio