CSR Vibration on overrun

[David.Ashley.Poole]

Hello L7s,

I have a CSR 260 and there is a nasty vibration/mechanical sound on overrun in sixth between 70-90 mph.  It does not occur in fourth or fifth at the same speed/propshaft rpm.  I recently had the gearbox rebuilt by Road and Race and Phil tells me that is caused by propshaft resonance and is a common feature in CSRs.  So I was wondering if anyone else had experience of it and if they had any solutions.  I have contacted carbon propshaft manufacturers and plan to get one made if no other solution presents itself, with the hope that the lighter propshaft weight will push any resonance higher in frequency, and that the increased damping in carbon fibre will reduce the magnitude.  And suggestions welcome, including “buy a new gearbox”...