Adding a roof box to your vehicle can give you extra space for gear and luggage, but it can also have a significant impact on your car’s aerodynamics. Understanding how aerodynamism works with a roof box attached is essential if you care about fuel efficiency, performance, and safety. This article will break down what you need to know about these changes and how they affect your driving experience.
What is Aerodynamism?
Aerodynamism refers to how air moves around objects, like vehicles, and how these objects interact with that air. In vehicles, reducing air resistance improves fuel efficiency and makes the car run more smoothly. This is why modern cars are often designed to be more aerodynamic, with streamlined shapes that reduce drag. When a car faces more air resistance, the engine has to work harder, consuming more fuel.
A roof box, being an extra object sitting on top of your car, disrupts this streamlined design and can create more air resistance.
Roof Box and Air Resistance
When you install a roof box, it increases the frontal area of your vehicle. This means there’s more surface for the wind to hit, creating more drag. The more drag there is, the harder your car has to work to push through the air. This will have a direct effect on your fuel consumption. According to data from fuel efficiency experts, driving with a roof box can increase your car’s fuel consumption by 10-20%, depending on the size of the box and your driving speed.
Adding roof-mounted light bars increased fuel consumption 7.1–12.7% when the vehicles were driven a constant 55 mph and 7.5% on overall driving miles (e.g., mixed speeds)…. When the roof rack was loaded with a ski box, fuel consumption increased 10%, 11% and 12.3% compared with the clean-roof conditions.
https://www.sciencedirect.com/science/article/abs/pii/S0301421516300714
This increase in drag is also influenced by your speed. The faster you go, the greater the impact on aerodynamics. Driving at higher speeds with a roof box can significantly worsen your car’s efficiency. For example, driving at highway speeds with a roof box can reduce your fuel economy by up to 20%.
When a rooftop cargo box was installed on a Corolla sedan, FC increased 8.8% and 20.8% on city and highway driving cycles, respectively. For a Ford Explorer SUV, the FC penalties were 2.5% and 6.2% on the two driving cycles.
https://www.sciencedirect.com/science/article/abs/pii/S0301421516300714
How Roof Box Design Affects Aerodynamics
Not all roof boxes are created equally. Some models are designed with aerodynamics in mind, featuring sleek shapes that minimize drag as much as possible. Others, however, are more focused on capacity and might be bulkier, further increasing air resistance. When shopping for a roof box, it’s important to consider both its capacity and its shape. Opting for a model that is more aerodynamic can help offset some of the negative effects on your fuel efficiency.
In general, longer, lower-profile roof boxes create less drag than taller, boxier ones. Look for terms like “low-profile” or “slim design” when researching roof boxes to minimize the hit on your car’s aerodynamics.
Real-world Impact on Fuel Efficiency
Numerous studies and user reports have shown that attaching a roof box can result in a noticeable drop in fuel efficiency. For example, research by Consumer Reports found that a roof box can lower fuel economy by as much as 5-9 mpg on the highway. That’s quite a significant difference for long trips, especially with fuel costs rising.
Our tests revealed that the sedan’s fuel economy took a significant hit with the rooftop carrier. The Altima got nearly 48 mpg when traveling at 65 miles per hour with nothing on the roof. When just the roof rack was added, the car’s mileage dropped by almost 11 percent, or 5 mpg. But with the rack and rooftop carrier installed, the overall drop was 19 percent, a loss of 9 mpg.
https://www.consumerreports.org/fuel-economy-efficiency/how-rooftop-carriers-affect-fuel-economy/
Users on automotive forums consistently share their experiences, often reporting a drop in fuel economy when adding a roof box. One user stated that they saw a 15% drop in fuel efficiency after installing a roof box on their SUV for a camping trip. Another user mentioned how their vehicle, which normally got 30 mpg, dropped to 23 mpg on a road trip after they added a roof box.
It’s not just fuel consumption either. Increased drag can also affect your car’s handling, especially in windy conditions. You might find that your car feels less stable with a roof box, particularly at higher speeds or during sharp turns. Ensuring that your roof box is securely attached and that its weight is well-distributed can help mitigate some of these issues.
Solutions for Minimizing Drag and Fuel Consumption
While the impact on aerodynamics is inevitable, there are a few things you can do to minimize the effect of a roof box on your car’s performance:
- Choose an aerodynamic design: As mentioned earlier, some roof boxes are more aerodynamic than others. Research models that specifically mention features like “low drag” or “wind tunnel tested” to ensure you’re getting a more efficient option.
- Remove when not in use: If you don’t need your roof box for a trip, take it off. Many people forget to remove their roof box, keeping it on for weeks or months at a time. By only attaching it when necessary, you’ll avoid unnecessary drag on your daily commute.
- Adjust your driving habits: Slowing down can help offset the extra drag caused by the roof box. Driving at slower speeds, particularly on highways, can reduce the impact on your car’s aerodynamics.
- Pack efficiently: Ensure the roof box is evenly packed to avoid any unbalanced weight that might affect your car’s handling. Moreover, the box shouldn’t be overloaded, as this could further increase drag and reduce fuel efficiency.
The Importance of Load and Weight Distribution
Weight distribution is another key factor when using a roof box. The heavier the load, the more impact it will have on your vehicle’s performance, especially on the aerodynamics. Make sure the weight is evenly distributed inside the roof box to prevent any imbalance that could make the car feel unstable. Also, heavier loads increase the center of gravity, making your vehicle more prone to tipping over in extreme situations like sharp turns.
A good rule of thumb is to keep heavier items inside the car and lighter, bulkier items in the roof box. Many manufacturers also recommend not exceeding a certain speed limit (often around 90-100 km/h) with a loaded roof box to reduce air resistance and improve safety.
Environmental Considerations
Beyond the practical aspects of fuel efficiency and handling, using a roof box also has environmental implications. Higher fuel consumption means a larger carbon footprint. According to the European Environment Agency, every extra liter of fuel burned results in 2.31 kg of CO2 emissions. Over a long trip, the additional emissions caused by the roof box can be significant.
If you’re concerned about your environmental impact, minimizing the use of a roof box or choosing an aerodynamic model is crucial. Reducing your driving speed and opting for more fuel-efficient travel alternatives can also help lessen your carbon footprint when using a roof box.
Final Thoughts: Is a Roof Box Worth It?
Adding a roof box to your car comes with clear pros and cons. On the one hand, it provides extra storage for road trips, camping, or moving heavy luggage. On the other hand, it can significantly affect your car’s aerodynamics, leading to higher fuel consumption, reduced performance, and even less stable handling.
However, by carefully selecting an aerodynamic model, adjusting your driving habits, and removing the box when it’s not needed, you can minimize its impact on your vehicle’s efficiency. Taking these steps will ensure that you enjoy the convenience of extra storage without sacrificing too much in terms of fuel economy or vehicle performance.
For those looking to invest in a roof box, always weigh the trade-offs between extra space and efficiency.
