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The BackParamB property is relevant when a part’s /BasePart/BackSurface is set to Motor or SteppingMotor and /BasePart/BackSurfaceInput is set to Constant or Sin. For Constant, it determines the constant rotational velocity of the motor. For Sin, it determines the frequency of the motor’s rotational velocity, using the following formula:

MotorVelocity = ParamB * math.sin(workspace.DistributedGameTime * ParamB)

In no other cases is this property used.

Code Samples

Motor Control

This code sample demonstrates how surface properties can be set using only a NormalId (Top, Front, etc). It switches a motor’s -SurfaceInput from NoInput, Constant and Sin to show how each work using -ParamA and -ParamB properties.

-- Paste this into a Script inside a part with a Motor SurfaceType
local partMotor = script.Parent
-- Place a brick called "MovingPart" so it is touching the Motor surface
local partMoving = partMotor.Parent.MovingPart

-- For this example, we use TopSurface, TopSurfaceInput, TopParamA and TopParamB
-- However, this will work for all faces (NormalId): Top, Bottom, Left, Right, Front and Back

-- A function to quickly set all surface properties at once 
local function setFaceSurfaceInputParams(normalId, surfaceType, inputType, paramA, paramB)
	local surfaceName = normalId.Name -- e.g. "Top", "Bottom", etc

	-- Syntax Note: in Lua, part.Something is the same as part["Something"]
	-- The difference is that the latter allows us to use a string ("Something"), while
	-- the former requires use of an identifier (.Something). Below, we build of each the surface
	-- properties below by concatenating the surface name with the property postfix.
	-- Set "___Surface", eg "TopSurface"
	partMotor[surfaceName .. "Surface"] = surfaceType
	-- Set "___SurfaceInput", eg "TopSurfaceInput"
	partMotor[surfaceName .. "SurfaceInput"] = inputType
	-- Set "___ParamA", eg "TopParamA"
	partMotor[surfaceName .. "ParamA"] = paramA
	-- Set "___ParamB", eg "TopParamB"
	partMotor[surfaceName .. "ParamB"] = paramB

local normalId = Enum.NormalId.Top

while true do
	-- Set to NoInput, where the motor will not operate at all
	setFaceSurfaceInputParams(normalId, Enum.SurfaceType.Motor, Enum.InputType.NoInput, 0, 0)
	-- Set to Constant, where motor rotational velocity = paramB
	setFaceSurfaceInputParams(normalId, Enum.SurfaceType.Motor, Enum.InputType.Constant, 0, .25)
	-- Set to Sin, where motor rotational velocity = paramA * math.sin(time * paramB)
	-- Since we're using pi (~3.14), the frequency of rotation is 1 second (per definition of sine function)
	setFaceSurfaceInputParams(normalId, Enum.SurfaceType.Motor, Enum.InputType.Sin, .25, math.pi)