A plastic roof rack for a car is called airbox or just autobox. This is a sealed, streamlined container that is attached to longitudinal arches (rails) or cross members and is designed for the safe transportation of large items, freeing up space in the cabin. Unlike open metal baskets, this design protects the cargo from precipitation, road dust and oncoming air flow, which significantly reduces the noise level when driving.
The main purpose of using this element is to increase the useful volume of the vehicle without losing comfort. Drivers choose it when the standard trunk is not enough for skis, bicycles, camping equipment or large purchases. It is important to understand that the term “plastic trunk” is a common one, while technically it is closed external luggage system, certified for safety and aerodynamics.
Technical features and design of the airbox
Modern autobox is a complex engineering structure made of impact-resistant ABS plastic. The body material must withstand extreme temperature changes, exposure to ultraviolet radiation and mechanical stress encountered when driving at high speeds. Stiffening ribs are often provided inside the body, which prevent the lid from deforming under the pressure of a snow cap or when accidentally hit by a branch.
The key element of the design is the fastening system. It can be universal, involving the use of clamps or paws that grip the crossbars, or specialized, designed for specific car models. Tightness is ensured by a seal around the perimeter of the lid, which is pressed with powerful locks. Some premium models are equipped central locking system, allowing you to open both locks with one turn of the key.
The airfoil is another important characteristic. The shape of the box is designed to minimize air resistance, which directly affects fuel consumption. The flat bottom and tapered nose reduce the whistling and wind noise that often occurs with open designs. However, even the most perfect airbox increases the windage of the car, so speed limits when using it are strictly regulated.
⚠️ Attention: The maximum permitted speed with a loaded box usually does not exceed 130 km/h, and for some models - 110 km/h. Exceeding this limit can lead to the cover being torn off by the oncoming air flow or the fastenings being destroyed.
Differences from metal analogues and open systems
When the question arises about what a plastic roof rack is called, many people confuse it with metal baskets or platforms. The main difference is the degree of protection of the cargo. Metal trunk (basket) is an open structure made of rods, where things are simply placed and secured with straps. In such a system, the cargo is completely exposed to rain, snow, dirt and road reagents.
Plastic autobox Provides complete content isolation. This is critical when transporting sports equipment, electronics, clothing or groceries. In addition, a closed case prevents theft of things, since an attacker would need to open the lock, while the contents of an open basket are visible to everyone and are available at any second of the stop.
From the point of view of aerodynamics and acoustic comfort, the closed box also wins. Metal bars create air turbulence that can generate a constant low-frequency hum. The plastic case, especially with a matte finish (shagreen leather texture), better dissipates sound waves and makes less noise on the highway.
Classification by type of installation and fastening
The choice of model directly depends on what is installed on the roof of your car. If you have stock roof rails or cross bars, you will need a box with a universal mount that covers the bar. If any elements are missing on the roof, you must first install the basic kit (foot and arches), and then mount them on them plastic box.
There are several types of grips:
- 🔹 Hook clamps: the classic version, where the legs tightly grip the crossbar and are fixed with screws.
- 🔹 Central mount (T-track): the box is inserted into a special groove on the arch and fixed in one movement, which looks more aesthetically pleasing.
- 🔹 Quick release systems: allow you to remove the box in a couple of minutes without using tools, which is convenient for seasonal use.
It is important to consider the length of the arcs. A box that is too long may block the hatch or prevent the rear doors of a hatchback or liftback from opening. Before purchasing, be sure to measure the distance between the centers of the fasteners and check compatibility with the roof geometry.
When installing the box on a car with a panoramic roof, make sure that the mounting points do not press on the glass, but rest on reinforced areas of the body.
Volume and dimensions: how to choose the right size
Volume autobox measured in liters and varies from 300 to 900 liters. For compact city cars (B-class hatchbacks), models with a volume of 300–400 liters are optimal. They allow you to place several bags or a baby stroller without significantly changing the dimensions of the car. For crossovers and station wagons, medium options with 400–500 liters are suitable.
Owners of large SUVs and minivans planning long trips with the whole family should pay attention to models with a capacity of 600+ liters. Such giants can accommodate skis up to 220 cm long, several bicycles (disassembled), inflatable boats and large travel packaging.
| Car class | Recommended volume (l) | Ski length (cm) | Typical Application |
|---|---|---|---|
| Small class (A, B) | 300 – 380 | up to 180 | Bags, stroller, small sports equipment |
| Golf class (C, D) | 400 – 480 | up to 195 | Family holiday, 2-3 bikes |
| Crossovers (SUV) | 450 – 550 | up to 210 | Ski equipment, camping |
| SUVs/Minivans | 600 – 900+ | up to 225 | Long expeditions, oversized cargo |
When choosing a size, also consider your own physical shape. Loading a 60-liter backpack onto a 1.8-meter-high roof is difficult even for an adult man. If you plan to use the rack alone often, you may want to choose a smaller or smaller model to make loading easier.
☑️ Checklist before purchasing
Materials of manufacture and environmental resistance
The main material for the production of high-quality boxes is ABS plastic (acrylonitrile butadiene styrene). It is a polymer with high impact strength and chemical resistance. It does not rust, does not rot and does not require painting. The surface can be glossy or matte. Glossy models look impressive, but scratch faster and require polishing. Matte (textured) are more practical; small abrasions from branches or stones are less visible on them.
Cheap analogues can be made from ordinary polyethylene or low-quality plastic, which becomes brittle in the cold. When hit in the cold like this autobox may crack. Therefore, when purchasing, be sure to check the quality certificates and operating temperature range specified by the manufacturer (usually from -40°C to +80°C).