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Creating the lightweight Mopar axle for the Valiant

by Al Bosley

The axle is one of the longest-lead tooling items on a car. Thus, the Valiant axle was planned long in advance, before details of the engine, transmissions, or even the full details of the vehicle were known.

In early 1957, when I was working for Gale Smith (the manager of general chassis design), I had responsibility for the rear axle, drivetrain, and rear suspension. Believe it or not, at that time, the front suspension and rear suspension design were in different departments at Chrysler. Incredible.

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I was instructed to start designing a new rear axle with Evan Jones, the manager of the "axle lab" - an experienced expert who had been responsible for rear axles for many years. Back when we had the project engineer system, he was responsible for axle design and all the calculations, including all the gear work and bearing loads.

The drafting department took his sketches and specifications and put them on paper (drawings for release to manufacturing and purchasing.) From 1955 through '58, graduate engineers were introduced into the what had been called the Drafting departments; the design engineers begin taking over the load, stress, and configuration work, with the labs concentrating on test and development.

Evan and I started on this axle, not knowing what it was for, at the beginning of 1957. We had a lot of discussion about what was required; but we were told very little what car the axle was for, how long the pinion should be, what the vehicle track was going to be, or how much power or what the weight the car would be: only that it was a "light, smaller" car.

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Evan had talked often with the manufacturing guys over at Lynch Road Axle, and they were not happy with the Hotchkiss "banjo" axles used by Chrysler at the time. After some careful analysis we concluded that, for the first time at Chrysler, the seven-and-a-quarter axle - a carrier tube rear axle - was the way to go, because that allowed us easy flexibility on tread, was more rigid, and avoided the manufacturing issues with the banjo axles. ( With "banjo" axles, the differential is in a removable carrier assembly; when the carrier is taken out, the empty housing looks something like a banjo.)

I was always disliked the two-piece drive axle arrangement that Chrysler had traditionally used, with tapered outer end shafts that fit in a separate flange and had a nut and key that held it together. They were always a pain to get apart if you wanted to get to the brakes or change axle shafts. I proposed a flanged shaft like many other brands used.

"Hot upset" is a process whereby the end of a bar is heated and forged back on itself to form the larger flange. Videos: the old way and the modern way (shown in slow motion).

There was some discussion because the Chrysler-style axle shafts, since 1924, were always straight with a taper and a bolt-on flange. The people at the axle plant said, "Well, you know, they'll be a little more expensive, but they're not that bad. We can hot upset those."

Then Evan and I worked together over a drawing board, making some sketches of the Valiant seven-and-a-quarter axle. We had a designer who had been doing our design work for many years (really, just changing existing axles), and we put him to work making a detailed, specific layout and putting the pieces together in that axle.

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We started on that axle not really knowing much about it. As time goes on, when the engine and car specs were firmed up, Evan and the axle design team (after I has moved to the Midland Avenue Valiant design group) re-studied it. By that time, they knew the target vehicle weight and approximate power they would get from the engine, although it had never been run.

The axle lasted a long time, and was changed several times. The drawings were released around mid-1958 to Purchasing, Manufacturing, and Lynch Road Axle; and they started designing the tooling for it, ordering the gear-cutting machines, getting the casting patterns done, and taking care of everything else that was needed.

"I wish we had..."

There were two things in that axle that I wish we'd done better.

First, Evan and I never thought that someone would want to run 300 horsepower through that axle. They didn't, but they wanted to, and the differential gears and the pin were too small. The Sure Grip versions held up, or so I'm told.

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The other thing I wish we had done differently was something we did to save weight. In an eight-and-three-quarters axle, the main drive gear pumps oil up to the front bearing along a passageway on the top of the axle casting and there's a little void along the bottom of the rear bearing so the oil can get back. We left that out to save weight on the seven-and-a-quarter, because we thought the oil would come out through the rear bearing. It turns out that both bearings didn't get enough lubrication, which made the axle run a little hot. Perhaps if we had realized that someday we would want to put a 340 Six-Pack engine in front of it, we might have thought about it a little longer

Most of those things got changed, later. I think the weight and cost savings for 3 or 4 years was a good decision.

The small five-bolt circle

In the Valiant task force over at Midland, we were asked how big should the axle flange be. We made a choice to use a smaller bolt circle both to make the axle shaft upset easer and to save weight.

The Valiant has a smaller bolt circle than the Chevy and Ford, but with five wheel bolts; the Falcon and the Chevrolet had four wheel bolts on a larger bolt circle. The wheel makers told us that the wheels would be okay; thus, we saved some diameter on the flanged axle shaft.

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We also went all right-hand thread, which caused a storm in Engineering, because since 1924, we had left-hand threads on the left side of the car and right-hand threads on the right side of the car - which always seemed to me inviting broken bolts or studs when trying to get a wheel off.

I did that without really asking anybody, and got in deep doo-doo afterwards.

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