Aerodynamic impact of roof racks
Aftermarket roof racks is a common accessory attached to vehicles for carrying larger items, such as roof storage pods and bikes. When purchasing roof racks, it’s important to understand these racks impact your car’s fuel economy, opting for a cheaper less aerodynamic rack may not save you money in the long term. Here we look at the estimated total fuel consumption of roof racks using a bottom-up approach.
Our estimates and models incorporate real-world data that has been gathered by the Department of Energy. This data looks at vehicle information to asses fuel consumption for varies categories of vehicles, along with roof rack configurations and usage of roof racks. The results show that nationwide, roof racks contribute to 0.8% of fuel consumption for cars and light vehicles. But it doesn’t stop there. Whilst this may seem insignificant this equates to 130 million Gallons a year. This data is derived from vehicles with roof racks that are not carrying equipment. Whilst this is not ideal, the good news is the aerodynamic efficiency of typical roof racks can be improved to minimise this wastage. The average individual in America spends $3000 a year on fuel. Whilst this saving of $24 a year is insignificant. If your someone who does more driving that most or want to reduce your environmental impact, buying aerodynamic roof racks is a great option.
One manufacturer (Whisbar) measured the aerodynamic drag forces of 12 different rack designs in a wind tunnel. It was found that the most slippery model had roughly 1/10 of the drag of the least aerodynamic model. The cost difference between the least and most efficient was around 15%. We had a look at the numbers to see if it’s feasible to justify spending more on aerodynamic roof racks.
Here are our conclusions below.
The average pair of roof racks costs $266, and the average pair of
Average pair of aerodynamic roof racks costs $306
So is it worth the extra money?
So let’s have a look what owning each type of roof rack will cost over 6 years including the added petrol cost due to drag. (assuming you drive the average amount of kms each year)
Having a look at the numbers over 6 years
In conclusion even the sleekest bike carrier imparts drag. The laws of aerodynamics dictate that interrupting the flow of air over an object even by a small amount causes resistance and that resistance requires energy to overcome… and that energy comes from only one source: your fuel tank. aerodynamic roof racks will save you on average $21 a year (to be conservative) However if you drive more than the average person the savings will be greater. Therefore, opting for the aerodynamic roof rack whist, it may cost you more upfront will be cheaper in the long run.
Furthermore, average lifetime of a roof rack is around 15 years but from the 6 years displayed its evident that paying the little bit extra upfront can save you down the track.
Note – With the growth of electric vehicles these percentages still stay relevant as energy savings. However, dollar savings are not relevant.
These experiments have been carried out before and often numbers and percentages can vary depending on how controlled the experiment is, all conclusions however lead to a savings in fuel as expected when roof racks are removed.
Roof racks are one of the most common vehicle accessories in the US. The configuration of roof racks on a vehicle depends on the need. Additional roof racks decrease aerodynamics, resulting in the vehicle using more fuel to achieve the same desired speed.
The equation for this is takes into account Aerodynamic drag (D), Projected frontal area (A)
Drag coefficient (CD), Vehicle speed (V) and Air density (pp)
Aerodynamic drag equation
Based on the above equation installation of a roof rack can increase aerodynamic drag by increasing both the drag coefficient (CD) and the projected frontal area (A). Previous studies found that, for a car moving at 100 km/h, aerodynamic drag typically accounts for 75 ~80% of the total resistance. The additional resistance comes from the connection between the road and your tires. Therefore, having a roof rack on your car increases fuel consumption and energy use.
Doing our part for the environment
With increasing awareness for the environment, I am always looking for new ways to reduce environmental impact. Whilst some measures are small compared to others, like most, it takes people to catch on and collectively reduce their environmental impact, this can be done by removaling your roof racks when not needed. According to Berkeley Lab this would decrease global fuel consumption by 1%. It’s a start, and something worth considering as it also makes financial sense.
Further investigations
Investigations on Volvo xc90 2012 Volvo model with roof racks
The vehicle was equipped with an unloaded roof rack and driven at 43.5 mph, 49.7 mph and 55.9 mph. Fuel consumption increased 2.6%, 2% and 1.1% compared with a clean roof.
Whilst there increases in fuel consumption appear marginal, long term savings by removing unused roof racks can be substantial. Car manufacturers spend millions of dollars designing their cars to be as aerodynamically slippery as possible, so the interruption of that carefully thought-out air flow seems to be undoing all that hard work. It also generates noise, but that’s not what we’re focusing on here.
Conclusion
Whether you are focused on reducing your carbon footprint and looking for ways to reduce your environmental impact, or want to minimise unnecessary expenditure, looking in the aerodynamic roof racks is one way to do this. Same goes for roof boxes as well. As demand for greater innovation occurs, technology around these items efficiently with improve, therefore the energy usage of accessories on vehicles will be reduced.