Attaching an accessory to your car’s exterior can have an effect on its dimensions, shape, weight, and aerodynamics. Among all types of bike racks, roof racks are likely the most noticeable in terms of these changes.
The Influence of Car Roof Racks on Fuel Economy and Performance
Car roof racks have a significant impact on a vehicle’s fuel economy, handling, and overall performance, especially when compared to other bike racks. The higher center of gravity and the increased exposure to wind from the front and sides are the main reasons behind this. Notably, the differences between a loaded and unloaded roof rack are more noticeable than the discrepancy between having no roof rack at all.
However, it’s important to note that while these effects may sound dramatic, not all of them are immediately perceptible to the driver. Some effects are only noticeable in specific driving situations or when refueling at the gas station.
Pro-tip: Rear-mounted racks have a much smaller impact on gas mileage compared to aerodynamic roof racks. For the most fuel-efficient bike racks, check out the best hitch bike racks on Amazon now.
Understanding Aerodynamic Drag
Let’s face it: a roof rack will not enhance your car’s aerodynamics. Adding any external accessory to your car increases its wind resistance, including roof racks.
The frontal area of your car exposed to headwinds enlarges with a roof rack, especially when carrying cargo like bikes. The impact of cargo on the drag coefficient and the frontal area depends on the car’s aerodynamics, the rack itself, and what is loaded onto it.
To put these factors into perspective, let’s examine the formula for calculating drag force:
Drag Force = Drag Coefficient Frontal Area Velocity² * 1/2
By understanding this equation, we can grasp how mounting a roof rack affects drag, regardless of the car’s make and model.
The drag coefficient measures a vehicle’s aerodynamic properties. While some cars are more aerodynamic than others, we cannot influence this aspect significantly. An empty roof rack has a minimal effect on the overall drag coefficient and frontal area, as its footprint is relatively small compared to the entire car.
However, loaded roof racks tell a different story. Cargo, especially large items like bicycles, dramatically increases both the drag coefficient and the frontal area. Essentially, two out of the three variables relevant to air drag are negatively influenced by roof racks.
Although there are aerodynamic roof rack designs available, these models achieve only a 10% reduction in drag compared to high-drag models. Considering the minor impact of the rack itself on frontal area and drag coefficient, the extra cost is likely not worth it.
Lastly, wind resistance increases exponentially with velocity. Therefore, speed has the most significant impact on drag and fuel economy.
The Impact of Roof Racks on Fuel Economy
In short, roof racks have an adverse effect on fuel economy. They worsen a car’s aerodynamics by increasing the frontal area and drag coefficient, forcing the engine to work harder against wind resistance to maintain the same speeds.
To illustrate this, consider the following table, which displays the values of drag force based on velocity changes:
Velocity | Drag Force |
---|---|
100% | Baseline (50) |
50 | 80 |
As the table shows, drag force does not decrease as much with velocity reduction as it increases with velocity gain. For example, a 20 mph increase almost doubles the drag force, while a 20 mph reduction only results in a 66% decrease in drag.
This relationship between velocity and wind resistance explains why velocity has the most significant impact on fuel economy. The faster you drive, the harder the engine works against the increasing resistance. At 100 mph, the aero drag quadruples, making it unattainable for smaller, less powerful cars.
In summary, velocity has the most significant impact on gas mileage, and it is also the easiest factor to control. Now, let’s return to the impact of roof racks.
When observing cars with empty roof racks, you might wonder if the effect on fuel economy justifies removing the rack after use. After some calculations, the answer seems simple.
At speeds below 60 mph and in city traffic, driving with an empty roof rack has virtually no noticeable impact on gas mileage (1% to 2% reduction). However, at higher average speeds, there is typically a fuel efficiency reduction ranging from 5% to 15%.
Therefore, it is true that an empty roof rack increases fuel consumption in certain scenarios. Although the small footprint may not be noticeable while driving, its impact becomes apparent at the gas station.
When it comes to loaded roof racks, driving at lower speeds leads to a 7.5% to 15% decrease in fuel efficiency. Meanwhile, traveling at highway speeds above 60 mph results in an average fuel economy loss of 20% to 30%. Trucks experience a slightly lower reduction, around 12%.
The variation in these average values depends on the frontal area and drag coefficient of the base vehicle. Smaller, more aerodynamic cars experience a relatively larger increase in their drag coefficient, while trucks and vans are less affected due to their higher starting coefficient.
Do Roof Racks Slow You Down?
To answer this question, let’s apply our previous knowledge.
With a given throttle input, a roof rack indeed slows down a vehicle by generating air drag. The maximum achievable speed is also reduced for the same reason. Additionally, wind gusts have a greater impact on speed due to the larger area affected by the roof rack.
The impact on acceleration is also present, although it varies with velocity, as we discussed earlier. While accelerating from a standstill at a traffic light produces minimal drag, accelerating to overtake on a highway tells a different story.
You can utilize the “air brakes” on your roof rack to your advantage by reducing speed and saving wear on your brake pads.
Weight Increase with a Roof Rack
On average, a bike roof rack weighs around 15.5 lb (7 kg), which is negligible compared to a car’s weight. The typical maximum capacity of a roof bike rack is approximately 165 lb (75 kg), still relatively light and equivalent to the weight of one additional adult passenger.
Thus, bike roof racks have a minimal effect on the total weight of the car. Adding one bike constitutes around 1% of the car’s weight. Even if you manage to mount four bikes, it would only account for 4% of the total weight. In such cases, the presence of three additional passengers in the car makes a more substantial difference.
It is worth noting that roof racks come in various types, and while bike racks have the largest footprint, other attachments like roof tents, cargo baskets, and roof boxes add more weight. Make sure to check your vehicle’s manual for the maximum weight it can safely carry, including the roof rack. Overloading the car can negatively impact performance, handling, and fuel economy.
The heavier the weight on a roof rack, the greater the negative impact on handling, acceleration, braking, and fuel economy. However, these effects increase linearly rather than exponentially, as observed in aerodynamic drag. The most noticeable differences can be experienced during deceleration (braking) and sharp turns.
Unless you are driving a drag racer with inferior brakes, cars can decelerate more effectively than they accelerate. Thus, while you may notice a slight struggle to reach higher speeds, the effect is minor compared to the changes you will experience in terms of stopping distance. The braking power needed to stop a car depends proportionally on its weight. Consequently, with a fixed braking power, the stopping distance increases. For example, when four adults, each weighing 165 lb (75 kg), and their 441 lb (200 kg) of luggage are added to a 4,410 lb (2,000 kg) car, the 500 lb (220 kg) increase represents a 25% rise in weight and directly translates to a 25% longer stopping distance.
During braking, the weight shifts forward and applies additional load to the front two tires. The same is true when turning, as the front tires bear most of the lateral forces. Due to these heavier weights, front tires may experience slightly more wear. Therefore, it is crucial to ensure the tire pressure is adequate and increase it if necessary, especially when carrying significant loads on a roof rack for extended trips.
Handling Affected by Heavy Roof Racks
Transporting any weight using a roof rack raises the center of gravity of the vehicle. While this is negligible for lighter cargo like bicycles, it becomes more noticeable with smaller vehicles and heavier items such as tents, roof boxes, or other substantial attachments. Always stay within the maximum load capacity recommended for both your roof rack and car.
Handling is where the extra weight becomes apparent, particularly during cornering and heavy braking. In corners, the car may sway or lean more, while braking distances increase with added weight. Additionally, wind gusts from the side and front can have a more significant effect on driving. Although not inherently dangerous or damaging, these factors can be felt while driving.
Interestingly, heavy loads on a roof rack may result in slightly improved fuel economy. This occurs because the car sits lower to the ground, which allows it to utilize more of the suspension travel due to the added weight. However, these opposing effects do not necessarily cancel each other out. Although sitting deeper into the suspension travel is not inherently beneficial and may impact overall handling, especially on rough roads.
Height Problems with a Roof Rack
This point might seem obvious, but it shouldn’t be underestimated. I have witnessed numerous unnecessary (and expensive!) accidents occurring right in people’s driveways.
Bikes placed on a roof rack can almost double the height of a car. From the driver’s perspective, however, the cargo directly overhead is usually out of sight and out of mind. It’s easy to forget, especially considering that traveling with extra cargo on the roof is not an everyday occurrence.
Remember, loaded roof racks cannot fit into most garages or parking decks. On the other hand, traversing tunnels generally poses no problem.
Roof Racks Can Be Noisy or Whistle
Similar to a solar eclipse, this phenomenon occurs rarely, but when it does, it creates a unique experience. Unfortunately, not a pleasant one in this case.
This phenomenon is known as Aeolian noise and can happen when air moves at specific speeds around an object of a particular shape. In the case of roof racks, it depends on the shape of the rack’s parts exposed to wind, the angle at which air hits the rack, and the speed of the moving air. You can influence all three factors by adjusting your driving style or modifying the rack itself.
Think about what you need to do to whistle using your mouth: you require a particular lip shape, airspeed, and angle. If any of these factors are slightly off, you won’t achieve the desired whistle. Similarly, your roof rack needs all these factors to align perfectly to produce whistling. By altering just one of these factors, you can eliminate the noise.
If you haven’t yet purchased a roof rack, consider choosing one specifically designed to prevent this whistling issue. This is particularly important for cars equipped with a sunroof, as occupants are directly exposed to any noise coming from the roof rack.
For specific steps to solve this issue, refer to our separate article on how to stop roof rack noise.
In conclusion, roof racks have a noticeable impact on various aspects of your car’s performance and fuel economy. Carefully consider these factors when using a roof rack and adjust your driving style accordingly. Remember to adhere to the maximum load capacity guidelines for both your vehicle and roof rack. Ensure a safe and enjoyable journey with your roof rack by being aware of its effect on your car and driving behavior.
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