[BUG] Gearboxes will output 0 torque most of the time when powering light props

[ProfessorBoxtrot]

Short Description

It seems that when a gearbox is powering a light prop (<200-ish?), it will not apply any torque unless the prop dips below a certain RPM. The source of the issue is on entities/gearbox/init.lua on line 800:

for Wheel, Link in pairs(self.Wheels) do
	local RPM = CalcWheel(self, Link, Wheel, SelfWorld)

	Link.ReqTq = 0

	if self.GearRatio ~= 0 then
		local Clutch = Link.Side == 0 and self.LClutch or self.RClutch
		local OnRPM = ((InputRPM > 0 and RPM < InputRPM) or (InputRPM < 0 and RPM > InputRPM))
		
		if Clutch > 0 and OnRPM then

			local Multiplier = 1
			if self.DoubleDiff and self.SteerRate ~= 0 then
				local Rate = self.SteerRate * 2

				-- this actually controls the RPM of the wheels, so the steering rate is correct
				if Link.Side == 0 then
					Multiplier = math.min(0, Rate) + 1
				else
					Multiplier = -math.max(0, Rate) + 1
				end
			end

			Link.ReqTq = (InputRPM * Multiplier - RPM) * InputInertia * Clutch

			self.TotalReqTq = self.TotalReqTq + math.abs(Link.ReqTq)
		end
	end
end

OnRPM returns false and Link.ReqTq is set to 0, applying no torque to its attached wheels.

Branch and Version

Master, up to date as of posting this.

How to Reproduce

Link a light prop up to an engine, and it will start to spin erratically.

Screenshots/Additional Information

I was able to replicate this bug with 3 vehicles:

I can provide the dupe files if needed. None of them switch gears or use brake/clutch/steer, the only gearboxes in use are differentials and double differentials, but the issue seems to happen for any gearbox.

Video of the tank:
https://youtu.be/NG4omZTKJPo

Video of the quadcopter: (much more dramatic)
https://youtu.be/jy6cHfwHfRU

[CheezusChrust]

Related to/same issue as #134, it's been talked about and most likely would need an overhaul of the whole mobility system to fix this easily. Hopefully it's looked into soon.

[ProfessorBoxtrot]

Ah, somehow I missed that when looking over issues. Hopefully this should still provide some more insight on the problem.