▁in ▁gy ro scop ic ▁tor ques ▁about ▁the ▁roll ▁axis , ▁so ▁that ▁the ▁mechanism ▁has ▁the ▁potential ▁to ▁right ▁the ▁vehicle ▁when ▁t ilt ed ▁from ▁the ▁vertical . ▁The ▁wheel ▁shows ▁a ▁t endency ▁to ▁align ▁its ▁spin ▁axis ▁with ▁the ▁axis ▁of ▁rotation ▁( the ▁g im bal ▁axis ), ▁and ▁it ▁is ▁this ▁action ▁which ▁rot ates ▁the ▁entire ▁vehicle ▁about ▁its ▁roll ▁axis . ▁ ▁Ide ally , ▁the ▁mechanism ▁applying ▁control ▁tor ques ▁to ▁the ▁g im bal ▁ought ▁to ▁be ▁pass ive ▁( an ▁arrangement ▁of ▁spr ings , ▁d amp ers ▁and ▁le vers ), ▁but ▁the ▁fundamental ▁nature ▁of ▁the ▁problem ▁indicates ▁that ▁this ▁would ▁be ▁impossible . ▁The ▁equilibrium ▁position ▁is ▁with ▁the ▁vehicle ▁u pr ight , ▁so ▁that ▁any ▁disturb ance ▁from ▁this ▁position ▁reduces ▁the ▁height ▁of ▁the ▁centre ▁of ▁gravity , ▁lower ing ▁the ▁potential ▁energy ▁of ▁the ▁system . ▁Wh atever ▁returns ▁the ▁vehicle ▁to ▁equilibrium ▁must ▁be ▁capable ▁of ▁rest oring ▁this ▁potential ▁energy , ▁and ▁hence ▁cannot ▁consist ▁of ▁pass ive ▁elements ▁alone . ▁The ▁system ▁must ▁contain ▁an ▁active ▁ser vo ▁of ▁some ▁kind . ▁ ▁Side ▁loads ▁▁ ▁If ▁constant ▁side ▁forces ▁were ▁resist ed ▁by ▁gy ro scop ic ▁action ▁alone , ▁the ▁g im bal ▁would ▁rotate ▁quickly ▁on ▁to ▁the ▁stops , ▁and ▁the ▁vehicle ▁would ▁to pp le . ▁In ▁fact , ▁the ▁mechanism ▁causes ▁the ▁vehicle ▁to ▁lean ▁into ▁the ▁disturb ance , ▁resist ing ▁it ▁with ▁a