<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">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.<div class=""><br class=""><div class="">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.</div><div class=""><br class=""></div><div class="">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 <a href="http://www.markwilliams.com/driveshafttech.aspx" class="">http://www.markwilliams.com/driveshafttech.aspx</a></div><div class=""> <br class=""><div><div class="">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. </div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">Scott</div><div class=""><br class=""></div><div class="">On Jun 9, 2017, at 7:35 AM, kkjjk--- via Fot <<a href="mailto:fot@autox.team.net" class="">fot@autox.team.net</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><font size="2" face="arial" class=""><font size="2" class=""> 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<br class="">
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