Vehicle weight limits
A common question people ask, is what affect the added weight of people inside a TentBox has on a vehicle roof load limit, and how a TentBox can work with this added weight. Trust us, we’ve done the engineering, and we know it works.
Firstly, it is important to remember that a TentBox sits on the cross bars, which are fixed to the steel frame of your vehicle. The TentBox does not at any point sit on the actual roof of your car.
Secondly, the maximum roof load limit shown in your vehicle/roof bar manual refers to dynamic loads, as opposed to static loads.
Dynamic vs. Static Loads
A dynamic load refers to the maximum weight allowed whilst your vehicle is moving, and the static load is when your vehicle is stationary. A vehicle can actually take much larger static loads than it can dynamic loads.
The reason for the difference is that when moving, your vehicle is turning, accelerating and braking, which all exert lateral forces on the top of your vehicle, and can make it unstable, when driving.
However, when your vehicle is stationary, only a downward force is being exerted directly into the steel frame. Vehicle frames are designed to withstand a force large enough to protect you in a crash, and to take the force of driving over bumps in the road, and changes in road gradient, when you hit them at speed.
In other words, when your vehicle is stationary, the steel frame can easily take the weight of a TentBox, as well as two or even three adults. In fact, it most vehicles can take upwards of 500kgs.
We’ve done the maths below, if you want to delve that deep!
What TentBox can your vehicle take?
In light of the above, you need to ensure that your roof load limit is higher than the weight of the TentBox model you’d like to go for.
The respective weights of our TentBox models:
If you are struggling to find your vehicle roof load limit, a member of our support team will be happy to help. Just get in touch.
Warning: The this section contains maths
The maths below demonstrates how the steel frame of an average vehicle will regularly have to withstand a force of around 700kgs just from driving over an uneven road surface. In other words, the steel frame is incredibly strong.
Let’s take a manufacturers roof load limit of 70kg. Now let’s assume the roof is loaded with 70kg of added weight, and that we’re driving at 80km/h over corrugations in the road surface that are 1m apart.
Now let’s assume the corrugations are 4cm from trough to peak and the tyres and suspension absorb three quarters of that and that the roof is moving by a distance of 1cm.
At 80km/h the frequency in cycles per second (Hz) would be:
frequency = 80 * 1000 / 3600 / 1 = 22.22Hz
Assuming a sinusoidal profile, 1cm peak to peak displacement corresponds to an amplitude of 0.5cm and the displacement as a function of time would be:
displacement = 0.005 sin (2π x 22.22t) where t = time in seconds and displacement is in meters.
Taking the first derivative, we get velocity:
velocity = 0.005 x 2π x 22.22 cos (2π x 22.22t)
Taking the derivative again, we get acceleration:
Acceleration = 0.005 x 2π x 2π x 22.22 x 22.22 sin (2π x 22.22t) Peak acceleration occurs when sin (2π x 22.22t) = 1. So peak acceleration is:
Peak acceleration = 0.005 x 2π x 2π x 22.22 x 22.22 = 97m/s/s
To calculate peak force, use the formula Force = mass x acceleration, using a mass of 70kg which is the maximum roof load of my Hilux:
Peak force = 70 x 97 = 6790N
Let’s compare to the force applied by the same weight when the car is not moving. The force applied by the static weight is 70kg times acceleration due to gravity (9.8m/s/s):
Static force = 70 x 9.8 = 686N
So the peak force is nearly 10 times greater than the static force. Converting the peak force into an equivalent static weight:
Equivalent static weight of peak force = 6790 / 9.8 = 692kg.
Note when the vehicle is moving up in its bump cycle the peak force would approximately equal 692kg + 70kg (total 762kg). The roof is not only speeding up the mass, but it is working against gravity. The peak force when moving down will be much less, since the roof is working with gravity and the rate it can fall is somewhat limited by acceleration due to gravity, although the recoiling suspension and associated unsprung mass will pull the vehicle down faster than gravity. The result is a sine wave with a skew towards shorter, steeper up cycles and longer, shallower down cycles.
So under the assumptions we’ve made, with 70kg of load, the roof would be experiencing a peak dynamic load of around 700kg when driving over corrugations. The assumptions may not be perfect but it does give you an idea of how dynamic load escalates significantly compared to static load and what sort of stress your roof is under when driving over a rough surface.
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