How to calculate internal Y offset when character is sitting down?

[Entelogical]

When a player sits in a seat, the character gets properly positioned. I am trying to find the formula to recreate this

At the moment. the C0 offset is the seat.Size.Y/2, what I’m more interested in is calculating the C1 offset

[Spooce]

@XAXA found the precise solution!

local forceSit = function(character: Model, humanoidRootPart: BasePart, humanoid: Humanoid, part: BasePart)
	local joints = humanoidRootPart:GetJoints()
	for _, joint in joints do
		if joint.Name == "SeatWeld" then joint:Destroy() end
	end

	local leftUpperLeg = character:FindFirstChild("LeftUpperLeg") :: BasePart?
	local rootJoint = character:FindFirstChild("Root", true) :: Motor6D?
	local leftHip = character:FindFirstChild("LeftHip", true) :: Motor6D?

	local seatThickness = part.Size.Y

	local hrpToTopOfSeat = 0
	if leftUpperLeg ~= nil and leftUpperLeg:IsA("BasePart") and leftHip ~= nil and leftHip:IsA("Motor6D") and rootJoint ~= nil and rootJoint:IsA("Motor6D") then
		hrpToTopOfSeat = rootJoint.C0.Position.Y - rootJoint.C1.Position.Y + leftHip.C0.Position.Y + leftHip.C1.Position.Z - leftUpperLeg.Size.Z / 2
	end

	local weld = Instance.new("Weld")
	weld.Part0 = part
	weld.Part1 = humanoidRootPart
	weld.C0 = CFrame.new(0, seatThickness / 2, 0) * CFrame.fromOrientation(-math.pi / 2, 0, 0)
	weld.C1 = CFrame.new(0, hrpToTopOfSeat, 0) * CFrame.fromOrientation(-math.pi / 2, 0, 0)
	weld.Name = "SeatWeld"
	weld.Parent = part

	return weld
end

Original post (approximate solution):

After some experimentation, I came up with this expression for R15:

(HumanoidRootPart.Size.Y + LeftUpperLeg.Size.Z) / -2

Interestingly (debatable), it does not resort to the right leg if the left leg has been severed. If it is missing, the leg term is simply zero.

As for R6:

(HumanoidRootPart.Size.Y + 1) / -2

[happya_x (Happya)]

you could do character:GetExtentsSize().Y

[pilotpug123 (Pilot)]

You could do it like this (at least this is what I think you’re trying to do (custom seat attempted).

Also, apologies in advance for the terrible scripting practices–if I’m being honest I just didn’t care enough to make it look pretty.

SCRIPT MADE IN R6 ENVIRONMENT! MODIFY FOR R15 IF NEEDED.

By the way, it doesn’t work entirely like a ROBLOX seat because you can immediately exit it by holding space. I didn’t feel a need to actually correct this feature.

Setup:

local recent = nil

script.Parent.Touched:Connect(function(p)
	if script.Parent.Weld.Part1 then return end

	local c = p:FindFirstAncestorOfClass("Model")

	if c == recent then return end

	c.Humanoid.Sit = true

	recent = c

	script.Parent.Weld.Part1 = c.HumanoidRootPart
	script.Parent.Weld.C1 = CFrame.new(0, -c["Left Leg"].Size.Z - c.Torso.Size.Y / 2, 0)
	
	local f

	f = c:FindFirstChildOfClass("Humanoid"):GetPropertyChangedSignal("Sit"):Connect(function()
		if c.Humanoid.Sit then return end

		script.Parent.Weld.Part1 = nil

		task.wait(1)

		recent = recent ~= c and recent
		
		f:Disconnect()
	end)
end)

[Entelogical]

I’m going to mark this as the solution

Edit: After further testing, this does not seem to be a 1:1 match but it is close. It would be nice if Roblox would document these things!