When we add any accessory to a car we affect its performance. An accessory on the inside, especially if it is lightweight, will hardly make any difference. But anything added to the outside of the vehicle will have some effect on the vehicle’s aerodynamics, and probably the balance and centre of gravity. A roof rack will be the most extreme example of this.
Being high on a car’s roof, a roof rack will shift the vehicle’s centre of gravity, and therefore the handling of the vehicle. This applies even when there is nothing on the rack. When we add a load to the roof rack we further alter the centre of gravity; the greater the load the less stable the vehicle is. We can reduce the effect by having a lighter roof rack and keeping the load to a minimum, but we can never be free of this problem.
Flat Roof Rack – Aerodynamics
We can minimise the drag of a roof rack by making it more aerodynamic. This is simple matter in principle – we make the roof rack design streamline. In practice, this means a different custom design for each car roof rack.
Older style 4WDs from the 1950s to the 1970s had poor aerodynamics, often because they were not designed to travel at high speeds or expected to be fuel efficient. As such, the roof racks on these vehicles never tried to be aerodynamic. Newer model 4WDs have improved aerodynamics, though they fall far short of sports cars and even general family cars. So any roof rack for these newer 4WDs will at least try to be streamline, and this helps keep the fuel efficacy at a reasonable level.
A roof rack with a low profile, that is one that is only slightly higher than the roof of the car, offers a couple of advantages. One advantage is that the clearance of the cars is hardly affected, so we will not be in danger of hitting low ceilings in car parks. Another advantage is that there will be lower wind resistance.
A low-profile roof rack combined with a wind deflector should give the optimal aerodynamic results for a vehicle. The only issue, once again, is that this has to be custom-designed for each vehicle. With is because a low-profile roof rack leaves very little margin for error; the rack has to fit exactly onto the vehicle roof, with no space for airflow underneath.
Fuel Efficiency of Flat Roof Racks
When a roof rack isn’t carrying a load there will still be some effect on a car’s fuel efficiency. At low speeds, under the suburban speed limit, this is only about a 1-2% increase in fuel consumption. At higher speeds, above 100km on a freeway, the fuel consumption will increase by 5-15%. This is on top of the already higher fuel consumption we experience when driving at a higher speed.
