Cobra Vibration Problem

[Jac Mac]

Originally Posted by Cobra #3170

I believe you are missing something, the joints are normally set up so that both of them have the inboard ends parallel ( these are splined Hardy Spicer half shafts that can be indexed any where radially on the splines. I had one shaft assembled so that the out board side of the half shaft joint was lined up with the inboard side on the other end rather the inboard side being lined up with inboard end of the half shaft. Although rotational velocity will be the same the effective torsional length is slightly longer side to side. The rear end locks up very well under hard acceleration and thus a harmonic could be generated. I am not saying it will, but could, I am looking for a needle in a haystack here so I am making everything as equal as possible. When I was with Ford we had many vehicle vibrations that were related to drive line harmonics that in some cases (Granada for example had huge weights on the tail shaft of the transmission) had to have major fixes applied because we did not understand the source. I have no test time between now and the Reunion so every possibility has to be addressed. This car has sheared several stub shafts, one half shaft joint and one drive shaft joint so the drive line stresses are quite large even though I have never drag raced it. The engine makes 585 LB/ft at 7250 rpm. The half shafts could theoretically see
over 1000 foot pounds at 130 MPH assuming 18% drive train loss and dividing available torque by two. Who knows what that will do to components rotating at 1600 rpm not to mention tires .

WE run a fairly potent TVR/SBF here in NZ & have had similar issues , you must run the half shafts as per the middle item in attached drawing. You should also strive to mount the diff head assy in the optimum position to minimise the amount of plunge in the sliding spline area of the halfshaft throughout the full range of suspension travel along with the amont of angle difference on inner/outer u-joints throughout that range. Tire sidewall height also has an effect on this, 15" rims with higher sidewalls will 'soften' the effect, 17"+ with low profile tires will make it worse.
Richards experience with the 5000/FA cars is the same, CV joints would cure the problem, but that will require quite a bit of rework.