[Fot] vibration

[Scott Janzen]

I used to have the exact same problem with my GT6, also noted in the same spot at Summit Point.  I’m still not convinced it is not related to RPM - in lower gears, you run through this range much more quickly, and I think it’s the RPM combined with the speed and the longer time you spend in this RPM range.

Anyway, I found that both my driveshaft u-joints were slightly out of phase, as were the u-joints on the rear axle half-shafts.  I don’t know what your half-shafts are made with - cv joints, u-joints?  Mine are Spicer splined sliding half-shafts with inner and outer u-joints.  A good driveshaft shop corrected both of these issues, balanced everything, made sure all the u-joints fit tightly in the yokes, etc.

The other thing I spent some time with was the alignment and mounting of the differential.  The GT6 has far from optimum alignment of the differential relative to the transmission - poke around on this subject but here’s one article http://www.markwilliams.com/driveshafttech.aspx <http://www.markwilliams.com/driveshafttech.aspx>
  
Anyway, the best I could achieve was to get the nose of the differential as high as possible and the rear as low as possible, if I remember correctly.  Also, look at the mounts themselves - are they in good shape, tight?  My diff is mounted with all aluminum spacers front and rear, no rubber, and i had the rear ones made eccentrically so that I could adjust the rear up and down a bit.  

It seemed that a combination of these things worked. No one item was the silver bullet.  Not sure if this is a new problem for the car - if it is, I’d  suspect the diff mounts or poor balancing of one of the rotating components.

Of course, we are assuming this is not suspension or engine vibration related - good to eliminate all of those as well, but also relatively easy to check at speed and you can usually feel it if it’s out at a wheel.

Scott

On Jun 9, 2017, at 7:35 AM, kkjjk--- via Fot <fot at autox.team.net> wrote:

 I have a significant vibration with my GT6 which occurs only at speed, in 4th gear starting about 300-400 before redline (approx 6600 RPM).  In any other gear to redline there is no problem, so it is related only to speed.  I had the driveshaft universal joints replaced and balanced with no change in the vibration.  The tires are brand new and balanced as well, although I did have a bit of shaking of the car, so I will have them re balanced again.  But I need to know what I should look at next to find the solution to the problem.  The diff?  Rear axles? And what items specifically?  I have to back off a bit near the end of the straight at Summit Point due to this problem, which sucks.  A longer straight at another track would be even worse! Any help would be appreciated!  Thanks- Jay De Pol
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