As I wrote in the title, the setElementSpeed() function doesn't appear to change anything ecxept the getElementSpeed() result, here's the function:
 
function brrr ( source )
  if getElementModel ( source ) == 96 then
    outputChatBox(getElementSpeed(source, 1)) --returns 30
    setElementSpeed(source, 1, 100)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if the speed has changed)
    wait(1)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if it stays constant after 1 sec)
    wait(2)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if it stays constant after 3 secs)
  end
end
	here's the ingame chat:
 
	
 
	 
 
	 
 
	 
 
	 
 
	 
 
	here's the whole code:
 
function wait(seconds)
    local start = os.time()
    repeat until os.time() > start + seconds
end
function getElementSpeed(theElement, unit)
    -- Check arguments for errors
    assert(isElement(theElement), "Bad argument 1 @ getElementSpeed (element expected, got " .. type(theElement) .. ")")
    local elementType = getElementType(theElement)
    assert(elementType == "player" or elementType == "ped" or elementType == "object" or elementType == "vehicle" or elementType == "projectile", "Invalid element type @ getElementSpeed (player/ped/object/vehicle/projectile expected, got " .. elementType .. ")")
    assert((unit == nil or type(unit) == "string" or type(unit) == "number") and (unit == nil or (tonumber(unit) and (tonumber(unit) == 0 or tonumber(unit) == 1 or tonumber(unit) == 2)) or unit == "m/s" or unit == "km/h" or unit == "mph"), "Bad argument 2 @ getElementSpeed (invalid speed unit)")
    -- Default to m/s if no unit specified and 'ignore' argument type if the string contains a number
    unit = unit == nil and 0 or ((not tonumber(unit)) and unit or tonumber(unit))
    -- Setup our multiplier to convert the velocity to the specified unit
    local mult = (unit == 0 or unit == "m/s") and 50 or ((unit == 1 or unit == "km/h") and 180 or 111.84681456)
    -- Return the speed by calculating the length of the velocity vector, after converting the velocity to the specified unit
    return (Vector3(getElementVelocity(theElement)) * mult).length
end
function setElementSpeed(element, unit, speed)
    local unit    = unit
    local speed   = speed
	local acSpeed = getElementSpeed(element, unit)
	if acSpeed and acSpeed~=0 then 
		local diff = speed/acSpeed
		if diff ~= diff then return false end
        	local x, y, z = getElementVelocity(element)
		return setElementVelocity(element, x*diff, y*diff, z*diff)
	end
	return false
end
function brrr ( source )
  if getElementModel ( source ) == 96 then
    outputChatBox(getElementSpeed(source, 1)) --returns 30
    setElementSpeed(source, 1, 100)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if the speed has changed)
    wait(1)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if it stays constant after 1 sec)
    wait(2)
    outputChatBox(getElementSpeed(source, 1)) --returns 100 (to check if it stays constant after 3 secs)
  end
end
function bindz (source)
  bindKey ( source, "Z", "down", brrr )
end
function unbindz (source)
  unbindKey ( source, "Z", "down", brrr )
end
function binds (source)
  bindKey ( source, "space", "down", brrr )
end
function unbinds (source)
  unbindKey ( source, "space", "down", brrr )
end
function checkkey ()
  bindKey ( source, "space", "down", bindz )
  bindKey ( source, "space", "up", unbindz )
  bindKey ( source, "z", "down", binds )
  bindKey ( source, "z", "up", unbinds )
end
addEventHandler ("onPlayerSpawn", root, checkkey)