▁rear ▁deck . ▁The ▁whole ▁system ▁we igh s ▁about ▁. ▁ ▁An ▁electron ically ▁controlled ▁rear ▁m uff ler ▁is ▁used ▁to ▁keep ▁the ▁B 4 ▁quiet ▁at ▁low ▁road ▁spe eds . ▁The ▁pipe ▁that ▁leads ▁into ▁the ▁rear ▁m uff ler ▁is ▁divided ▁in ▁two , ▁with ▁one ▁pipe ▁equ ipped ▁with ▁a ▁fla pper ▁val ve . ▁The ▁EC U ▁– ▁working ▁with ▁an ▁actu ator ▁and ▁cable ▁– ▁opens ▁this ▁val ve ▁when ▁trav elling ▁at ▁medium - to - high ▁road ▁spe eds . ▁This ▁increases ▁exhaust ▁gas ▁flow . ▁ ▁Body ▁ ▁The ▁bon net ▁of ▁the ▁Liber ty ▁B 4 ▁is ▁made ▁from ▁ 1 mm ▁thick ▁al umin ium . ▁Once ▁fitted ▁with ▁its ▁inter co ol er ▁sc oop , ▁the ▁bon net ▁assembly ▁we igh s ▁ 9 . 5 ▁kil og rams ▁– ▁some ▁ 8 ▁kil og rams ▁l ighter ▁than ▁the ▁steel ▁bon net ▁fitted ▁to ▁other ▁Liber ty ▁models . ▁ ▁Body ▁strength ▁has ▁also ▁been ▁improved ▁over ▁the ▁first ▁ 1 9 9 9 ▁current - shape ▁Liber ty . ▁T ors ional ▁rig id ity ▁has ▁increased ▁from ▁the ▁ 1 9 9 9 ▁model ' s ▁ 2 . 5 2 ▁x ▁ 1 0 6 ▁N m 2 / rad ▁to ▁ 3 . 5 ▁x ▁ 1 0 6 ▁N m 2 / rad . ▁F lex ural ▁rig id ity ▁is ▁increased ▁from ▁ 4 . 8 9 ▁x ▁ 1 0