ARROW 51509AO Aprilia RS4 125 (2011+) Aluminum Slip-on Exhaust "Thunder"Slip on exhaust "Thunder" for Aprilia RS4 125 by ARROW. More and more new bikes are designed with very small silencers. moreover, these small silencers are also fitted in strange positions: these facts made necessary to use smaller and smaller exhaust systems also in the after market production. For these reasons Arrow decided to introduce in the Street Bikes silencers range a kind of silencer designed in the first time for the Off Road program, again