<VV> Re: LM rear tire shake --- I'm stumped

[Taruffi57 at aol.com]

Can't seem to locate the cause of a shake which I believe should more  
accurately be described as a bounce.  No vibration in the steering wheel so  it's 
apparent that it's coming from the rear.
 
I'll describe the car:
Rims:  Cragar 7X15 black steel - look like NASCAR rims. 5 stud holes  are 
LARGER than
the studs.
Tires: new 215/50X15 BFG G Force Sport
Stock rear drums
New U-Joints, 75 Duro Lon's bushings, stiffer Stock Car Products 350  lb. 
rear coils to equal the height of stock fronts which were cut 2  coils.  I have 
changed to slightly longer studs.
4 spider POSI, Jeff Rapp lightened flywheel/pressure plate, stock  disc.
Built 140.
 
Car "bounces" from about 60 to 70 mph then lessens with speed but it's  still 
there pretty much all the way up.  My tire shop is a real pro with  the 
latest in computerized balancing and diagnostic equipment and today, we  pulled all 
4 wheels and re-checked run-out, balance & out-of-roundness  and everything 
looked good.
Because the rims have a center hole slightly larger than the center  locating 
boss on the drums, I had a machine shop make me up 2 alum. spacers  which fit 
closely over the 5 wheels studs and have a center boss more closely  fitting 
the rims center hole.  This spacer is 1/4" thick.  I did this  because I 
thought that in mounting the rims onto the studs, rims were not  getting exactly 
"centered".  This $160 addition did not improve the  shake.
 
Everything seems right and balanced but does anyone know if the halfshafts  
have been a problem for balance in the past?
 
Running out of ideas and the car is no fun to drive shaking the way it  is.
 
Joe Dunlap
Cen. Florida