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For a given vehicle, I implemented a suspension system on four wheels. The system is based on Hooke's Law.

The Problem: The vehicle should not be able to touch the ground. When driving in a spherical container (inside), the suspension gets compressed up to 100%, making the vehicle chassis touch the underground, which leads to unwanted collisions that throw the vehicle around.

Despite that may being a realistical behaviour, our game aims for an arcade-feeling, so I am looking for a formula to implement a maximum compression, so that the vehicle chassis can't come closer to the underground than X percent of the suspension size at any given moment, without actually simulating a physical contact between the two rigid bodys. Thus, I need to apply a fake force to the suspensions.

My current approach: If the vehicle chassis would in fact touch the suspension base (Sorry, I don't know the proper word to describe this. I mean, when the suspension is at maximum compression), a force equal in magnitude and opposite in direction relative to the force pushing onto the suspension would be applied to the vehicle chassis, forcing it to stop moving downwards.

Therefore, I receive my vehicles world velocity V. To get the downwards-velocity, I get the DotProduct of the velocity and the BodyUpVector.

float DownForceMagnitude = DotProduct(VelocityAtSuspension, BodyUpVector);
FVector DownForce = DownForceMagnitude * BodyUpVector;
FVector CounterForce = -DownForce * WeightOnSuspension;

Okay, this pseudo-code works somewhat fine on even underground, when the vehicle lands on a plane after a jump. Driving on a increasing slope however (like driving on the inside-walls of a sphere), makes the suspension reach maximum compression anyway, so apparently my approach is not correct.

I am now wondering what the cause is. My weight calculation only is simulated by VehicleWeight / 4, since the Unreal Engine 4 has no functionality to receive weight at a given location. I am no physics-pro, so forgive me if this is easy to calculate. Could that be the issue?

I do not need a physically 100% plausible solution, I just need a solution that works, and sufficiently stops the downwards motion of my vehicle chassis.

Any help is appreciated. Greetings,

Sebastian Schon
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  • It is very difficult to get such code working without a good grasp of basic physics. Does your approach work on an inclined plane? – Beta May 16 '16 at 16:44
  • Visually it seems yes, but I did not perform 100% assuring debugging for that yet. – Sebastian Schon May 17 '16 at 19:38

1 Answers1

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I had this problem with a futuristic magnetic hovercraft.

I solved it by reducing the force by ln depending on suspensions extension levels like so:

y = ln(ln(x+e))

where:

x = Suspension extension lvl in % (-> 0 being fully compressed)
y = the factor that you multiply the force with
e = eulers number

Here a graphic to help what it will be like:

https://ggbm.at/gmGEsAzE

ln is a very slow growing function thats why it works so great for this.

You probably want to clamp the values (maybe between 0 and 100 idk exactly how your code behaves and how u want this "break" to behave)

Tailor the function to your needs, I just wanted to suggest u use the ln like I did to solve this Problem.

I added e to x first to make it go through 0,0 if u want to make it stop earlier just subtract from x before using ln.

Also notice depending on when/how you calculate / update your suspension this (and any function applied to the force based on the extension of suspension levels) may not work under some circumstances or at all.

t0b4cc0
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