Pics of a Duetto racer buildup - Page 36 - Alfa Romeo Bulletin Board & Forums
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post #526 of 764 (permalink) Old 12-20-2014, 11:05 AM Thread Starter
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Originally Posted by Alfar7 View Post
The carbs need to be syncronised.....
Richard, I have to laugh because you're probably right. After all of my fiddling with the carburetors last year, the only thing I checked after reassembly was that wide-open throttle at the gas pedal translated to wide-open throttle at the carbs. Gotta add carburetor synchronization to my list of projects.

In all of my mucking around with this car, I sometimes surprise myself that I haven't tuned it to a standstill (again).

69 Spider, 2L, street; pursuit of happiness
69 Spider, 1.8L, race; happiness of pursuit
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post #527 of 764 (permalink) Old 12-20-2014, 08:26 PM Thread Starter
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While I was at it, I remade the bracket that goes from the transmission to the exhaust. I used more of that really strong flat sheet of Boeing surplus 7075 aluminum that I used on the passenger seat mounts, and braced it with a piece of 6061 L-channel.
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post #528 of 764 (permalink) Old 12-22-2014, 08:52 PM Thread Starter
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I had mentioned in an earlier post that I had relocated the fuel pump to clear the way for the future sliding block. Previously, it was on the forward outer side of the spare wheel (now a fuel-cell) well. Now, it's in the trunk. Before, I could hear the distant rattle of the pump, but now, it sounds like there's a spastic gorilla wearing tap shoes back there. Anyway, here's a picture of it for completion's sake. Geez, that thing makes a racket!
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post #529 of 764 (permalink) Old 12-22-2014, 09:05 PM
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Geez, that thing makes a racket!
Did you mount it with rubber isolators? Can't see from the pic.
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post #530 of 764 (permalink) Old 12-22-2014, 09:21 PM Thread Starter
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Yes, Snake Man, rubber mounts are on the fuel pump. Still very loud!

With the exhaust and the fuel pump out of the way, there is space now around the rear axle for the sliding block. Why am I set on the sliding block when everyone else runs Panhard bars, or Watts links? It's a clunky and inelegant solution to locating the rear axle and lowering the role center. But it's pure Alfa and that appeals to me. And what's good for the quarter million dollar GTA goose, is good for the fakely vintage racing Duetto gander.

Speaking of GTA's, when I was researching the sliding block, I poked my nose into the rarified atmosphere of the GTA subforum of this website. There, I made the acquaintance of Ken G (not the saxophonist) who has a real GTA and also reproductions of the sliding block parts. So I got a set from him. Thanks again, Ken!

I was planning on cooking up my own sliding block set up, but I couldn't pass up the chance to have something more closely resembling the real deal. I still have to make up a couple of brackets and odds and ends, drilling holes, heat treatment and what not, but here it is.
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post #531 of 764 (permalink) Old 12-22-2014, 10:06 PM
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I look forward to milled titanium copies (for my own Duetto).

PS. Loving your thread.
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post #532 of 764 (permalink) Old 12-23-2014, 01:14 AM
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Watts linkage is superior to the sliding block and requires no maintenance.
Pete

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post #533 of 764 (permalink) Old 12-23-2014, 05:05 PM Thread Starter
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**sigh**

69 Spider, 2L, street; pursuit of happiness
69 Spider, 1.8L, race; happiness of pursuit
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post #534 of 764 (permalink) Old 12-23-2014, 07:13 PM Thread Starter
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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.
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post #535 of 764 (permalink) Old 12-23-2014, 07:19 PM
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Would a roller bearing in place of the block work? Maybe even a "captured" one which would run in a race?
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post #536 of 764 (permalink) Old 12-23-2014, 09:19 PM Thread Starter
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Good thinking, Mr. Maserati. I've given roller bearings a lot of thought. Maybe a block/bearing hybrid. The easy route would be to just machine a block of Delrin/acetal and call it good.

If the car pulls 1G in a corner, then half or more of that at the rear wheels is about 1000-1500 lbs held at bay by the sliding block. I'm sure a hunk of Delrin can handle it, considering that Delrin rollers are taking the weight of a large steel cantilevered platform with an '87 Carrera on top of it. Pic below.

As I mull it over, I got started on the plate that will sprout the pin that carries the block. Though I'm not sure of the final dimensions, nor how I'll attach it to the differential, I figured that the piece will take shape as I rough it out. A trial and error process. The same goes for the pin.
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post #537 of 764 (permalink) Old 12-23-2014, 10:39 PM
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Many racers in the 60's used the sliding block, and had plenty of troubles with it seizing; Adding high pressure grease before every race, and re-greasing during long races was common. Eventually we milled rectangular pockets in the sides of the bronze block, about 5 mm deep, and added fitted delrin inserts. I suspect using a solid delrin block will creep or deform pretty fast, as the inserts themselves didn't last too long.

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post #538 of 764 (permalink) Old 12-24-2014, 08:59 AM
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I'm no fabricator so I am second guessing my original roller bearing suggestion. Would that not also seize or bind under hard cornering?
In my mind my original idea was a larger roller bearing located on the pin or as the pin which would ride in the vertical track pictured, (but with that track having smooth bearing races for the bearing to ride in).
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post #539 of 764 (permalink) Old 12-24-2014, 10:05 AM
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Cool

Whatever----Sliding block rear suspension is V. Cool.

Bob,
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post #540 of 764 (permalink) Old 01-19-2015, 07:22 PM Thread Starter
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The holidays slowed me down, but now I'm back to business.

I've been busy making brackets to mount up a sliding block on the differential. I lined up the bracket mounting angle to match that of the diff filler hole. At some point, Autodelta used that threaded hole to mount the sliding block's pin. That's going to be part of my plan. But I'm not comfortable with the aluminum diff housing alone supporting the forces imparted on the pin. I have visions of the the diff cracking over time. So I fabbed up that steel plate to girdle the pin for additional support.
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