sliding blocks, links, and all that

[JHertzman@xxxxxxx]

Following Eric Storhok's question in AD7-393 Richard Welty , Will Owen, Simon
Favre, and Richard Welty again offered comments, criticisms and rebuttals
which leave me seriously addled.

In the CRH the first rear suspension modification (Gr. 3A p.1) is eliminating
the rubber silent-blocks from both ends of the lower radius rods and replacing
them with suitable Heim joints or Unibals, either locally obtained parts or
special Autodelta parts. The reason is (I think) to eliminate mush (note
precise technical term) from the lower radius rod axle positioning without
limiting the vertical movement of the axle in a shallow arc, probably at the
expense of increased NVH.

P.3 & 4  cover springs, not relevant here.

Gr. 3A p.5 covers torque tees and sliding blocks. If not going to the sliding
block, one replaces the steel torque tee with either of two lightweight cast
aluminum units, the first (a pure weight-saver) using stock rubber bushings
and the second with a "unibal" joint in place of stock rubber, matching the
lower radius arms. 

Here I must question Will Owen's "in contrast to the earlier a-bracket, which
wouldn't work at all without lots of rubber in the bushings". The torque tee
of the 105/115 chassis is the same in principal and geometry as the rubber-
bushed upper A-arms of the Giulietta, 1900, 2000 and 2600, and "lots of rubber
in the bushings" is no more necessary here than it is in the lower A-arms of
an IFS, which can have metal-to-metal bearings, either cylindrical or
spherical depending on the other details of the design. Note that the 159 GP
car used lower radius rods at the ends and an upper A-arm in the center with a
DeDion rear axle, almost certainly with metal ball joints rather than rubber
bushings 

Simon Favre's point that "You're comparing two very different animals here.
The watts linkage on transaxle cars was used with a deDion tube (no matter how
you spell it' The GTA and GTAm, all had a classic live rear end" escapes me.
Sliding blocks, Watt's linkages, Panhard rods and any other locating systems
will work equally well on any axle system which establishes a rigid connection
between the two wheels, regardless of where the differential is.

The lower half of Gr. 3A p.5 covers "Sliding Block Conversion - lateral Axle
Location - System "CRB". It says "The purpose of the sliding block conversion
is to lower the rear roll center, lower the rear body, and provide better
lateral stability of the rear axle." (Note in passing that the sliding block
does not eliminate the need for the torque-arm function of the torque tee,
only the lateral location function. The torque tee is replaced by a
crossmember and a torque rod assembly.)

The second and third purposes mentioned for the sliding block conversion-
"lower the rear body, and provide better lateral stability of the rear axle"
do not seem compelling. People lower rear bodies at will without finesse, and
use of ball joints in place of rubber bushings should provide for lateral
stability. My understanding has always been that the sole purpose of the
sliding block was to lower the rear roll center and eliminate (or at least
mitigate) the gyrations that can come with a steeply inclined roll axis.

I also question Will Owen's suggestion that "The center point of the Watts
link on the Alfetta/Milano axle describes a shallow S curve". The geometry of
the Watt's linkage has been well understood since Watt patented it in 1784; it
was a straight-line motion then and it still is. I also don't follow Richard
Welty's statement that "a 'correct' watts linkage in a car would anchor the
center link to the body, and the end links to the axle". If you have a Watt's
link locating two things relative to each other it shouldn't matter which
name, "body" or "axle" you assigned to "thing A" and "thing B". The geometry
is the same.

I can't speak with certainty about the earlier uses of the DeDion with a
triangular radius rod - Ricart's system - on the 512 of 1939, the slightly
later 162 and the 163 of 1941, but Alfa's first certain uses of the
triangulated upper arm and the Watt's link were almost contemporary; the
triangulated upper arm and lower radius rods on the 1900 in 1950 and on the
159 GP car in 1951, and the Watt's link on the 6C 3000 CM in 1952 and on the
Sportiva in 1954.

The sliding block system was, as far as I know, used only on the GTA through
GTAm cars, with the purpose of lowering the roll center and with the
disadvantage that the bronze block slid between two wear plates, lubricated
only by an amalgam of lost oil and road dirt. 

But back to Eric Storhok's question: "I'm considering making a GTAm replica -
- -". If making a "replica", as nearly as one can, rather than a "look-alike",
it would seem to me that replication would be the good-enough reason for a
particular choice which he is looking for. If making a "special", then mix-
and-match ad-lib.

Cordially, 

John H.

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End of alfa-digest V7 #395
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