Re: Tunnel Slap!

[jack@domain.elided (Jack Hagerty)]

>From: Mike Grace <mgrace@domain.elided>

>I've got a scary-sounding problem with my '74 GTV. When I hit nasty
>expansion joints in the freeway at 70 mpg (indicated), I frequently get a
>severe WHACK! from the driveline slapping into the side of the tunnel. It
>seems to be the donut joint which hits, but the U-joints, donut,
>trans-mount, and motor mounts are all new, the driveline was recently
>dynamically balanced *on the car*, and there's zero (or minimal) vibration
>felt at speed. The center support is not new, but it appeared good to me
>and my mechanic. What's up? This dynamic interference is difficult to
>pinpoint, but definitely unacceptable.

Man, I"m glad to find SOMEBODY else with this problem!

My '74 GTV has been doing this for over 10 years. At least it was consistant
enough that I could take a trusted mechanic (Dr. Bob at Import Doctors) on
a ride and have him hear for himself. It seems that the problem is not that
rubber peices you mentioned are worn out, but that they're new (?!). This
first started happening on my car after replacing the center support and
aft (gearbox) motor mount. Apparently this raises the tail of the gearbox
high enough that one of the rotating pieces, we could never determine which
but suspected a U-joint knuckle, will contact the tunnel on severe bumps.
The solution we found was to place a single flat washer between the support 
and the body. This drops the center support bearing about .05" which was
enough to prevent the contact at speed. 

We've no explanation why this should happen other than maybe there was a
subtle change in the aft motor mount over the years (maybe unintenional if 
they changed vendors) that messes with the tolerances.  I've never heard of 
this happening with a Spider, so maybe the GTV has a slightly tighter 
driveshaft tunnel.

- - Jack

+---------------------------------------------------------------------------+
| Jack Hagerty                |                                             |
| Robotic Midwives, Ltd.      |    "What luck for leaders that men          |
| Livermore, CA		      |    do not think.  The winner is never       |
| jack@domain.elided   |    asked if he lied." - Adolph Hitler       |
| (925) 455-1143 (voice/fax)  |                                             |
+---------------------------------------------------------------------------+
|        ARA #97, NAR #55105, LUNAR #002 / TRA #3943, Aero-PAC #168         |
+---------------------------------------------------------------------------+

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