Have a really good tire shop true the tires and get them perfectly round.
Balancing will not cure an out of round tire. Once they are round, balance
them. New tires are not always round. This will help if it is a tire problem.
On Friday, June 9, 2017 10:44 AM, Scott Janzen via Fot <fot at
autox.team.net> wrote:
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??
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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