Join Date: Jan 2005
Location: East of Seattle
A sliding block primer
All this talk about a sliding block may be foreign to some of you. Hell, I didn't even know what they were until a few years ago, and I've been into Alfas for at least a decade. I'm going to try and explain what I know about them, and someone please correct me if I'm wrong:
In a stock Spider (and other roofed models) there is a stout trunnion bar, or "T-bar" that keeps the rear axle from swinging left and right in the turns, and limits fore-aft movement in acceleration and braking. Other cars with a solid rear axle keep it in place with leaf springs, and those with wiggly coil springs in the back used a Panhard bar or a Watts linkage for lateral fixation along with trailing arms to control fore/aft movement. Today, the only vehicles with live rear axles are trucks (and Mustangs until 2015). My kickass F-350 has leaf springs, and my wife's 4Runner uses a Panhard bar.
Perhaps the more important benefit to the sliding block, Panhard bar and Watt's linkage is that it lowers the car's rear roll center. I'm not going to pretend like I totally understand roll centers, but it seems like the T-bar gives the car a higher one, and you want a lower one to help the car stay stable through the turns.
For some reason, Carlo Chiti, chief racing engineer for Alfa/Autodelta, came up with this sliding block solution to locating the Alfa rear end and lowering the roll center on the legendary GTA and GTAm instead of using the aforementioned solutions. There's a picture below I took of an early example on a GTA. Study it, and you'll see how the axle is limited in it's side-to-side motion by that stout aluminum yoke, braced by those aluminum diagonal struts side to side, and that smaller steel bar for fore-aft fixation of the yoke. A trailing arm fixes fore and aft movement of the rear axle, also pictured. What you can't see is that the rear differential has a plate with a peg affixed that carries a bronze block. That block slides in the yoke's slot up and down with the rear differential through its suspension travel.
Bronze is naturally self-lubricating, but tolerances must have been tight against the yoke, because apparently there was a common issue of the block binding in the yoke's slot, thereby locking up the rear suspension. So the yoke had to be lubricated frequently to keep the block sliding. I believe this problem can be fixed with modern materials such as a Delrin sliding block or yoke liner, or something more complicated and craftier that I have in mind.
69 Spider, 2L, street; pursuit of happiness
69 Spider, 1.8L, race; happiness of pursuit