Vehicle's Weird Behavior When Players Are Standing On It

Howdy, I encountered this weird behavior when I was testing my game with multiple players.

  • My car system fully utilizes Roblox’s constraint system, so no use of bodyforces. It only uses spring and cylindrical constraints for the attachments between the wheels and the car body.
  • The car works normally without players on top of it.
  • The driver of the car owns full networkownership of the vehicle.
  • Here you can see what happens when the person standing on the car gets off:
    it all goes back to normal physics behavior.
  • I have a feeling that it may be due to the stiffness of the springs, or the weight of the car, but even if that was the cause this shouldn’t happen under any conditions as its not normal physical behavior.
  • The behavior becomes worse with more player, server latency, or lower ping.
  • Here are more examples that include more players on the vehicle, the movement becomes crazier:
    Keep in mind that I am not placing any throttle input the the vehicle, it is doing it by itself.

I believe this happens due to this update:

You can reproduce this in my game, you will need another player/s to accompany you!

If I missed out on some key details, please notify me and i’ll will get back to you.

Other than that I have no idea what might be the cause of this physics bug, any suggestions of fixing this would be appreciated!


What are the typical Part’s Density Properties of the items in your car?
What happens if you print the VehicleSeat Steer and Throttle outputs to see if they are being affected by the passenger. They shouldn’t be, but might be worth a try.

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I’m confused, are you saying the car starts driving itself when someone hops on?

It is as if the players produced a weight to the car, the more players on top, the weirder it goes?

The players do add weight to the car, but that wouldn’t cause it to move on its own unless the script controlling the car has some feedback from the car’s current position/velocity.

The car moves by itself, its crazy I know, even if you place no throttle/steering to the vehicle for some reason the car moves back and forth

I actually do print out the throttle and steer:

If you play the game you can see this at play!e81ca3709e3e80e7e90b1b820fce9064

I didn’t show it on the original gifs because my gif recording program only allows for a certain size of the gif

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Again, I don’t know how your car is programmed. What exactly do you do to get the car to move or stop. Sounds like there is a script that changes the behavior of the constraints to make the car drive, right?

It’s normal properties, Is not being affected?

I think it has to do with differences in weight. Does it do the same thing when placing a heavy part on car?

It is basically this:

if ThrottleFloat > 0 then	
    Wheel.HingeConstraint.AngularVelocity = 100
    Wheel.HingeConstraint.MotorMaxAngularVelocity= 25
    Wheel.HingeConstraint.MotorMaxTorque= 10000

elseif ThrottleFloat == 0 then	
   Wheel.HingeConstraint.AngularVelocity = 0
    Wheel.HingeConstraint.MotorMaxAngularVelocity= 25
    Wheel.HingeConstraint.MotorMaxTorque= 10000

elseif ThrottleFloat< 0 then	
   Wheel.HingeConstraint.AngularVelocity = -50
    Wheel.HingeConstraint.MotorMaxAngularVelocity= 25
    Wheel.HingeConstraint.MotorMaxTorque= 10000

I don’t use bodyforces, or any of these image

Works as it should, if a heavy part is on top of the car it’s suspension will go down with it. It doesn’t make the car move side to side like how players would

Here I even placed NPC’s ontop of the vehicle and it doesn’t cause the behavior, it definitely is caused by players.

What happens if you make the part smaller, and increase it’s weight and friction?

Ah okay nevermind then.

Edit: I had some issues with players too. Once players would stand on a certain platform I created, it was supposed to move (I added springs) but it just glitched out. Normal parts would work just fine… No scripts were involved.

Does it react different with stiffer suspension?

So car move weird when humanoid is on top, it should be a Roblox error maybe?

I am not 100% sure on that, but it could be the reason!

Yes, but, don’t you have a script in the vehicle that changes the humanoid in a certain way?
I don’t know.

I have a part in my script that makes the players hats invisible when they get on the car, but apart from that there is nothing else that changes the humanoid.

try apart of that script, maybe it fix

When a character is standing/moving around on your vehicle, they apply an equal and opposite force on the vehicle as well. This will have a similar effect as having a “BodyForce” applied down on the points your character are standing. If your car is light enough its possible for the characters to push on the car a bit.

How heavy are you cars?