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[Fot] vibration

Subject: [Fot] vibration
From: billdentin at aol.com (Bill Dentinger)
Date: Fri, 9 Jun 2017 18:58:04 -0400
Less likely in the old days when we paid to have new race tires shaved.

Of course, I was likely to flat spot the fronts in the first session.

If you downloaded the Blackhawk Farms track map from their website, you'll see 
a building marked Tire Service.  It's attached to the building that has the 
garages.  They used to sell tires there, but now it is mostly mounting.  In the 
late 1970s and early 1980s, they did a lot of tire shaving there.  I bought my 
first set of race tires there.  I remember watching them shave them.  The shop 
was so full of tire shavings it was like thick, black carpeting.  By the end of 
the day, he'd have buckets full of those shavings. 

Bill Dentinger



-----Original Message-----
From: Elwyn Rolofson via Fot <fot at autox.team.net>
To: barry rosenberg <britcars at bellsouth.net>
Cc: 'Friends of Triumph' Triumph <fot at autox.team.net>
Sent: Fri, Jun 9, 2017 3:30 pm
Subject: Re: [Fot] vibration



What Barry says.


I had one of four new Yokohama tires on my 6 out of round. Chased that for way 
too long.


Bud

Sent from my Commodore 64 

On Jun 9, 2017, at 12:03 PM, barry rosenberg via Fot <fot at autox.team.net> 
wrote:




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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