Aero - e / Velomobile

Living in the Netherlands I see a few Velomobiles going around among the bicycles.

Velomobiles are interesting for aerodynamics as they are made for low drag condition, this means that they can reach a higher top speed and require a lower force for similar speeds.

Bicycles can be extremely aerodynamic but you might not be surprised to know that the human body on top of it is not. We can change that to some effect by wearing an aerodynamic helmet, tight clothes and changing the riding position but it will still not give a low drag coefficient.

This is where the Velomobile allows for the cleanest airflow around our body, positioning the body in a reclined position and decreasing the frontal area.

Executive summary

The Velomobile provides 2 big advantages compared to a traditional bicycle:

-        The low drag force returns a low amount of power to maintain an equivalent speed to a regular bike. 

-        With crosswind, the velomobile is less prone to additional drag from the wind and can even move from the wind alone above a specific wind angle. 

First the cyclist

The cyclist is positioned in a comfortable riding position.

The result is a frontal area of 0,535 m2 and a drag coefficient of 0,682 at 0-degree yaw. By comparison a Mercedes G-class, which has one of the highest drag currently on the road, has a Cd of 0,540, so no, the human body is not aerodynamic.

To make this point even more clear, the human is responsible for 80% of the drag while the bike is only responsible for the other 20%.

The velomobile

The intent was to create a vehicle with visuals closer to a car than the traditional velomobile. The surfaces are as close as possible to the driver and follow the shape previously defined for the Narrow car project.

As a result, the drag is reduced by 90% compared to the cyclist.

Top speed

Now that we have the drag data at 0-degree yaw, we can use it to calculate the top speed.

First a few parameters:

-        The road is considered flat.

-        No cross wind.

-        The tire rolling resistance is defined at 0,002 and constant with the speed.

-        The human mass is 75 kg.

-        Driveline loss equal to 5%.

-        The power from the human is defined as a constant average cycling strength with a peak power of 100 W at 80 rpm.

For the same strength, the velomobile top speed is double! The velomobile acceleration is a bit slower from 0 to 15 km/h compared to the cyclist due to its additional mass and speeds up on its way to 25 km/h. It then keeps on accelerating to 51,7 km/h.

We can also look at it as how much power is needed for a top speed of 25 km/h.

Power for 25 km/h

About ¼ of the power is required to maintain the same speed of 25 km/h with the Velomobile.

Crosswind

Anyone who has been cycling in the Netherlands knows, the most difficult part is the wind. There is always wind and no matter which direction you are traveling, the wind seems to always be pushing against your movement.

It is important to check crosswind as it not only makes progress more difficult but it could push you off the road in the case of wind gushes.

Drag results with wind

For the cyclist reference, the force in the traveling direction is increasing slightly with yaw until 60 deg when it increases more sharply.

The Velomobile shows a reverse trend with drag decreasing with yaw and becomes negative around 20 deg yaw.

This means that for the regular bike, under crosswind, more power is required to maintain speed while for the Velomobile less power will be required. It does not mean that you can always travel in crosswind and stop cycling but the wind force can be greatly reduced. We will look at the real effect a bit further.

Crosswind also has an effect on the side force and lift.

Side force results

It is not surprising that the side force is higher for the Velomobile with its larger side surface. It is also increasing with yaw compared to the cyclist.

2 important characteristics should be checked:

1.       The force distribution should be centred around the centre of gravity (COG) to avoid too much driver steering correction.

2.      The force should not overcome the tire friction and push the bicycle off the road.

For the first point, we can look at the centre of pressure.

The Velomobile COG is about 40 % front with a 75 kg human. The side force distribution is forward of the COG and gets closer to this point with yaw. This means that crosswind will push the front further sideways and the driver will need to correct slightly to stay on course. With more yaw, this effect becomes smaller.

This is reasonable and additional surface optimization might bring a perfect match reducing driver inputs in crosswind even further.

Parameter 2 will depend on the wind speed and vehicle speed.

Let’s consider the tire side friction force to be 0,6 and the vehicle speed at 25 km/h. The variable is the side wind speed at 90deg to the traveling direction.

-        The yaw increases with the side wind speed which in turn increases the side force.

-        The tire side force depends on the mass and lift. When lift increases, the tire side force decreases.

By looking at the graph of the side force on the velomobile vs the tire friction, the critical condition would be 32 km/h with 90deg crosswind. At this speed, the vehicle could slide across the road.

This critical point can be improved by:

-        Increasing the vehicle speed

-        Increasing the mass (not easy to do on the go)

-        Increasing tire friction (and more importantly to not go on lower grip surfaces such as wet asphalt, gravel, ...)

We saw that crosswind can reduce drag force but how does crosswind influence the power required to keep speed?

We can look at drag force for a specific 20 km/h wind coming from all angles around the velomobile and cyclist.

This graph only shows the impact from aerodynamics (no rolling resistance).

At 25 km/h with 20 km/h wind, the velomobile has the specific particularity to be powered by the wind with crosswind above 45 deg.

The graph also shows that under 30 deg, the drag force is higher than without crosswind.

We then have 4 conditions:

-        Crosswind < 30 deg: more force required than without crosswind for the same speed.

-        Crosswind = 30 deg: equivalent to no wind.

-        30 deg < crosswind < 45 deg: the force required is less than without crosswind.

-        Crosswind > 45 deg: all force is provided by the crosswind to compensate for the drag force.

Some energy is still required to compensate for the tire rolling resistance. The rest of the energy could be recuperated and deployed later with an electric motor.

This second graph shows how much power is required to maintain 25 km/h. This graph includes the power loss from the tire rolling resistance.

All together at about 61 deg yaw, the velomobile is powered by wind alone in these specific conditions.

For the cyclist wind under any angle less then 121 degrees would require more power to move at the same speed. When the wind angle is more it will help you move forward and less power is needed.

For the velomobile this tipping point is at 46 degrees, meaning even wind that comes from a forward direction could reduce the power needed to move at the same speed. And in specific conditions with an angle above 61 degrees it could even mean you do not need power at all and the wind pushes you forward.

In this wind condition, the side force does not exceed the tire side force and both velomobile and cyclist should stay on the road.

Push further

The Velomobile has a relatively flat hood on top of the driver legs, we can use this surface to harvest the power from the sun. This surface is about 0,65 m2 for solar panels and can be combined with a traditional electric bike system.

With the battery fully charged and no help from the solar panel, the range at 25 km/h is 399 km. Compared to the cyclist with an electric motor and battery range of 105 km.

Due to the low drag and rolling resistance, road inclination has a large impact on range:

On flat roads the velomobile has nearly 4x the range of a regular electric bicycle. Once we have some slope the range drops faster on the Velomobile as the rest of the vehicle is very efficient and the power required to go up a slope becomes the largest component.

Interestingly, if the motor could turn faster and reach a speed of 50km/h for the same power and torque, then the range would not be too different from the speed of 25km/h.  

Now, let’s turn on the solar panels on the Velomobile.

The 0,65 m2 solar panels adds 6,7 kg to the total mass.

By adding 0,65 m2 of solar panels with an average annual solar yield in the Netherlands, we can get enough energy per day to charge the battery.

Alternatively, the battery can still be charged in 5h with a 36V charger.

Using the electric motor, the acceleration time and distance are reduced

Conclusion

Let’s summarize the advantages of a Velomobile compared to a regular bicycle:

-        Higher top speed with the same human power on flat roads.

-        Lower human power required for an equivalent steady speed.

-        Sailing abilities under specific wind conditions.

-        Abilities to add solar panels on the hood to charge the batteries or use direct solar power.

There are also some inconveniences:

-        Larger than a regular bicycle and not as easy to park in the street.

-        Sitting low to the ground with lower visibility for the driver and low visibility for the cars.

-        More weight to push up in case of inclination on the road.

A next project could be to solve these 2 inconveniences and make a more usable vehicle for a wider audience.

Thanks to Alexander Antipov who made the male body and shared it on sketchfab.com.