Raycast suspension chassis behaving inconsistently on different frame rates

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#1

hello!

my chassis utilizes raycast suspension, and everything works fine however it behaves differently and inconsistently on different frame rates; below 30 is practically unusable and upwards of 60 it drives too fast.

here is an example:

ive checked out other forum posts that face the same issue, however they were of little help

here is my code

-- in a while true loop, 
-- delta = game["Run Service"].Heartbeat:Wait()

for wheel_name, original_position in pairs(chassis_configurations.wheel_positions) do
	local chassis_cframe = chassis_primary.CFrame
	local ray_origin = chassis_cframe:ToWorldSpace(CFrame.new(original_position))
	local ray_direction = -chassis_cframe.UpVector * (chassis_configurations.suspension_max_length + chassis_configurations.wheel_radius)
	local ray_params = RaycastParams.new()
	ray_params.FilterDescendantsInstances = {chassis}
	local raycast = workspace:Raycast(ray_origin.Position, ray_direction, ray_params)

	if raycast then
		local raycast_distance = (ray_origin.Position - raycast.Position).Magnitude
		local spring_length = math.clamp(raycast_distance - chassis_configurations.wheel_radius, 0, chassis_configurations.suspension_max_length)
		local stiffness_force = chassis_configurations.stiffness * (chassis_configurations.suspension_max_length - spring_length)
		local damper_force = chassis_configurations.damper * ((spring_length_memory[wheel_name] - spring_length) / delta)
		local suspension_force_vector3 = chassis_cframe.UpVector * (stiffness_force + damper_force)
						
		local rotations_only_wheel_direction_cframe = CFrame.lookAt(Vector3.zero, chassis_cframe.LookVector, chassis_cframe.UpVector)
						
		if string.sub(wheel_name, 1, 1) == 'f' then
			rotations_only_wheel_direction_cframe = rotations_only_wheel_direction_cframe * CFrame.Angles(
				0, 
				-math.rad(chassis_smooth_steer * chassis_configurations.steer_angle), 
				0
			)
		end
						
		local local_velocity = rotations_only_wheel_direction_cframe:ToObjectSpace(CFrame.new(chassis_primary:GetVelocityAtPosition(raycast.Position)))

		local x_force = rotations_only_wheel_direction_cframe.RightVector * -local_velocity.X * chassis_configurations.wheel_friction
		local z_force = rotations_only_wheel_direction_cframe.LookVector * seat.ThrottleFloat * chassis_configurations.torque * (
			math.sign(-local_velocity.Z) == seat.Throttle and (
			1 - math.min(1, math.abs(local_velocity.z)/chassis_configurations.max_speed - chassis_module.chassis_cons)
			) or 1
		)
						
		spring_length_memory[wheel_name] = spring_length
						
		chassis_primary:ApplyImpulseAtPosition(suspension_force_vector3 + x_force + z_force, ray_origin.Position)
	else
		spring_length_memory[wheel_name] = chassis_configurations.suspension_max_length
	end
end

id have to assume this is an issue associated with how i used delta but i am clueless nonetheless

#2

One possible solution to improve the consistency of your raycast suspension is to use a fixed timestep instead of relying on the frame rate. Instead of using “delta = game[“Run Service”].Heartbeat:Wait()”, you can set a fixed timestep value that is independent of the actual frame rate and use it to calculate the simulation updates.

Here’s an example implementation using a fixed timestep of 1/60 seconds:

local fixed_timestep = 1/60
local accumulator = 0

while true do
    -- Get the elapsed time since last frame
    local delta = game["Run Service"].Heartbeat:Wait()
    
    -- Add the elapsed time to the accumulator
    accumulator = accumulator + delta
    
    -- Perform the simulation updates in fixed timestep increments
    while accumulator >= fixed_timestep do
        for wheel_name, original_position in pairs(chassis_configurations.wheel_positions) do
            -- Perform the raycast suspension calculations here
            -- using the fixed_timestep value instead of delta
        end
        
        -- Subtract the fixed timestep from the accumulator
        accumulator = accumulator - fixed_timestep
    end
end

By using a fixed timestep, the simulation updates will be consistent regardless of the actual frame rate, and the behavior of your raycast suspension should be more predictable and stable.

#3

it certainly improved it especially around the 20-30 fps range.

a bigger issue however is the car starts to fling around uncontrollably on really high frame rates (e.g 300 courtesy of fps unlocker).

1 Like
#4

additionally, 20 is where it really starts to spaz out

#5

Another possible solution or mitigation is to use a Physics sub stepping algorithm and divide the physics calculation for a large time step (or frame) into multiple sub frames which reduces instabilities: