Not at all trivial problem! A vehicle is not rigid. More or less elastic wheels are the the first defence line against hits.
The vehicle does not fall down vertically, it flies horizontally at the same time. This makes difficult to calculate how the wheels meet the next obstacle
Different wheels receive different hits. The vehicle does not fall and rise, it also rotates. It's in 3D.
You need a realistic model of the road
Rolling wheel (assuming it does not fly nor modify the road) meet the road profile. Solving the route of the wheel axis is a numerical equation solving problem. In practice the road can be smooth and the vehicle sometimes flies.
As a coarse approximation assume at first some road profile. Assume the wheel rises as high as is needed to prevent the wheel to sink into the road (=solve the route of the axis at first). If your wheel moves horizontally (= to X) 15km/h, present Y as function of the time. The second derivative is the acceleration input to your wheel.
Wheel elasticity, vehicle mass and frictional losses make a low pass filter. They make the effective hit input smoother. The second derivative should be calculated from the filtered signal.
Calculations like this are coarse. Make some measurements or use proper physics simulation software. Building the simulation model is a major task and the needed software can cost $10k. Making acceleration measurements give to you realistic info much easier.