Question: “This year I want to go on vacation with a roof box and bike rack. My neighbor thinks I could overload my car with it. What do I have to pay attention to? “
Answer from Dipl.-Ing. Stefan Ehl, test engineer at the expert organization KÜS: “Anyone who wants to go on vacation with their own car or with children and their children has quickly exceeded the legal load limits. Traveling with the entire family and a bicycle fleet often becomes a balancing act that requires precise and, above all, critical analysis of all loads. Several weighting factors must be considered individually and in their entirety.
Keep an eye on the roof load
If the luggage space in the vehicle is insufficient, a practical box can be strapped onto the roof as a supplement. Often a few simple steps are enough to increase the storage space volume by a few hundred liters. Basically, this solution is more recommended for the transport of light objects, because the limiting factor is the roof load, which for common vehicle types is usually between 50 and 100 kilograms.
If you do not know the roof load, you can read this value in the operating instructions. Important: The weight of the roof box itself and the carrier must be deducted from the roof load. Usually both together weigh between 20 and 30 kilograms. Only after subtracting this value do you know how heavy the roof luggage can be. So you don’t have much leeway. In addition, high loads on the roof have an impact on the vehicle’s center of gravity and thus on its driving behavior. The cargo in the box should therefore be as light as possible and also be evenly distributed and secured with lashing straps.
Check drawbar loads
Even with the rear rack, which is mostly used to transport bicycles and mounted on a trailer coupling, the scope for loading is manageable. The limiting factor in this case is the vertical load, which for a car is usually between 70 and 90 kilograms. Here, too, when calculating the maximum load due to luggage, the weight of the wearer must be included in the calculation. If the carrier weighs 20 kilograms and two pedelecs together 50 kilograms, another children’s bike could already exceed the limit. The trailer load applicable for the respective vehicle is also listed in the registration certificate Part I (vehicle registration) in field 0.2.
If you know the loads from the rear rack, the bicycles and the roof box and its contents, you can calculate fairly precisely how much the passengers and luggage in the trunk are allowed to weigh. To do this, you only have to calculate the payload, which results from the difference between the empty weight and the permissible total weight. The curb weight is the weight of a ready-to-drive car with a 90 percent full tank and a driver weighing 75 kilograms. The corresponding values can be found in the registration certificate Part I under G and F. If, for example, the numbers 1,500 and 2,000 are shown here, luggage and passengers should not weigh more than 500 kilograms.
The permissible total weight must not be exceeded
If the four passengers weigh 300 kilograms and add 150 kilograms of the roof box and rear rack, the scope for additional luggage is already tight. But you not only have to pay attention to the gross vehicle weight, but also to the maximum load for the front and rear axles. These values are in the registration certificate part under 8.1 and 8.2. The problem here: the loads can be distributed unevenly.
A rear rack with bicycles, for example, puts a lot more strain on the rear than the front axle due to a leverage effect. This effect of one-sided overloading of the rear axle can be observed more frequently when the rear wheels of heavily loaded vehicles dip deep into the wheel arches and the front wheels are strongly rebounded.
If in doubt, off to the vehicle scale
If the calculations or your gut feeling indicate overloading, it is advisable to drive to a public vehicle scale before going on holiday. If the vacation car is overloaded, this has a negative impact on driving behavior. The vehicle becomes significantly more unstable, has longer braking distances and is difficult to control during steering and evasive maneuvers.
Due to the high risk potential, the legislature provides for correspondingly sensitive penalties in the event of overload. The fine in Germany is currently 235 euros, and there is also one point in Flensburg. Fines can be even higher abroad. “(Text: mh / sp-x | Image: Porsche)