Strength of hardtop construction and optional reinforcements

By selecting this optional equipment, you have to be aware of what you can and cannot mount on the hardtop’s roof.

Let's start with the construction of a car such as pickup. The cab of this type of vehicle and the truck bed are elements which are mounted independently of each other onto the chassis frame. Therefore, the passenger compartment and the truck bed vibrate and move in relation to each other when driving. So they must be located at a safe distance so that there is no contact between them. When looking at a pickup perpendicularly from the side, you can notice that there is a break (gap) between the cab and the truck bed. Manufacturers carefully conceal this by using various plastic covers, seals or simply by embossing both the cab and the side elements of the truck bed.

When knowing the basic construction of the car, we can find out about how the roof rails are most often used, i.e. mounting elements to the roof rails fixed on the passenger cab and the hardtop. The elements which are most often mounted include roof boxes, roof platforms for transporting pipes or ladders, ski racks, etc. The rigid connection of two independently operating elements (i.e. the car and the hardtop) introduces stresses into the hardtop’s construction. The consequence of the connection is serious damage to the hardtop components – from damages to the paint coating and laminate structure, through roof collapses, torn out rail attachments to leaks and cracks in the rear doors. In the steel/laminate confrontation, it is the latter that suffers the most damages. It is worth bearing this in mind..

Joining the roofs of the hardtop and the car is possible, but the possibility for a free movement between the components should be left, e.g. by applying slides and making the installation on rubber bumping blocks to cushion the movements of the joined elements. Another important issue which is directly related to the use of roof rails is the load-carrying ability of the hardtop. We receive a lot of questions about reinforcement of the roof to increase its load-carrying ability up to 100kg. At this point, I would like to systematise the issue of the load-carrying ability of the roofs manufactured by our company.

The load-carrying ability of a roof in standard version is 40kg – this is a standard for all types of hardtops. It is possible to make a reinforced hardtop. In this case, after the application of special structure reinforcements, the strength of the hardtop roof is increased to 50kg. However, customers sometimes want to carry items weighing 100kg. To ensure safety, reinforcements in the form of an internal cage dedicated to a specific hardtop and car are used. The cage is mounted directly to the truck bed and supports the hardtop’s structure at sensitive points.

We must also remember that manufacturers of off-road vehicles design the passenger cab roofs for a load of 75kg. This illustrates the strength of the car’s roof which is made of steel, has reinforcements and must provide safety for passengers in case of an accident.

Another aspect that restricts mounting heavy objects on the pickup roof is the elevation of the vehicle’s centre of gravity, and thus the reduction in its driving stability, e.g. when driving at a higher speed around a bend. In extreme cases, this may result in the vehicle rolling over. Please note that pickups have an elevated suspension and the centre of gravity is higher than in passenger cars. When mounting heavy objects on the roof, we significantly reduce the driving stability of the vehicle.

The last important issue related to mounting heavy items on the truck bed is load-carrying ability of the truck bed sidewalls. This information is not readily available and hardly any car dealer is able to provide us with this parameter. According to our experience, the safe loading of a truck bed sidewalls is approx. 150kg. Assuming that the weight of the hardtop is 45-95kg depending on its size and equipment, not many kilos are left for an additional load to be mounted on the roof rails. Overloading the truck bed sidewalls leads to irreversible damages, and the effects are visible both on the external and internal truck bed plating. These may be bends, breaks and ruptures in the surfaces of the sheets. Damages of this type are particularly dangerous as they destroy the vehicle. In this case, the use of a hardtop is not possible.

Therefore, remember to strictly follow the instructions of the manufacturer when using additional accessories. This ensures first of all safety and provides collision-free construction in relation to the vehicle.