Steering Geometry
When Retrofitting Checker Disk Brakes
to Pre-1968 Checkers
Lane Darnton

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When considering a disk brake retrofit to a pre-disk (1970-earlier) Checker, one question that comes up is “Which Checker disk brake steering knuckles will fit my pre-disk Checker?”. Another is “Is there a difference between ‘rear steer’ and ‘front steer’ Checker hardware in terms of what will work on my pre-disk Checker?”

The answer to the first question is “any 1968 – 1982”. This means ‘68-‘70 (drum brake) Checkers already have the correct knuckles and steering arms for a disk retrofit. Note that pre-‘68 cars need a ’68-later lower A-arm end fitting. This is because Checker flipped the lower ball joint over in ‘68, with the ball joint body pressed into the steering arm (not the A-arm end fitting as on earlier cars), so the ball stud pointed DOWN into the new A-arm end fitting. By the way, one benefit of this change is that coil spring replacement is rendered trivial, an advantage for fleet cars such as taxis – and for you!

The answer to the second question is a resounding “yes!” The 1978-1982 “front steer” steering arms will not work on your pre-‘68 Checker. They do not have the correct angle to the centerline of the car to induce the right amount of dynamic toe-in when you turn the wheels.

To understand this, let’s review the basics. When you turn your car, the following things are true:
1)    each wheel follows the arc of a circle,
2)    these circles have a common center, and
3)    each circle has a different radius. For the rear wheels this is not a problem – they just follow along like a trailer – but each front wheel must be correctly steered along its separate circle, or else the front tires fight each other and scrub themselves to death. Here’s a sketch:

Note that the outside front wheel follows a larger circle than the inside front, so it needs less steering angle. This dynamic toe-out depends on the wheelbase and track width of the car (to the center of the tread contact patch). For Checkers these values are 120” and about 65” respectively for models A11, A12, & A12W. (A11E and A12E wheelbases are 129”.) Using these values, one can calculate the required dynamic toe-out as a function of the inside wheel angle:


Look at the “Neutral” curve. Simple, huh? If only. There are two problems: 1) how do you get the front wheels to do that? and 2) this curve only applies
at extremely low speeds, when there are no cornering forces.

Let’s address the 2nd problem first. Due to lateral weight transfer and body lean in corners, the outside and inside wheels do not share the cornering load equally. The degree of inequality depends on many factors – the height of the center of gravity, the stiffness of the anti-sway bar, the car’s speed, spring stiffness, etc. –  but the result is the outside tire takes more cornering load, so it needs to operate at a greater slip angle than the inside tire. It therefore needs to be turned IN slightly more than the inside tire (relative to the circle it is forced to follow), and so its dynamic toe-OUT needs to be LESS than indicated by the neutral curve. Got that?

How much less? It varies from car to car, but let’s say *roughly* half (see “1/2 neutral” curve above). Why half? Because the outside front tire generates *roughly* twice the cornering load of the inside front tire for a passenger car driven “normally”. The slower the car is cornered, the higher this value should be (closer to “neutral”). The faster you corner, and especially if you drift, the lower this value should be (closer to zero dynamic toe-out). On race cars, the required dynamic toe-out reaches zero as the inside front wheel lifts off the ground, which is fairly common.

Now back to the first problem: how to get the front wheels to do that. The answer: by bending the steering arms IN toward the car centerline (for rear steer cars). Why does this work? Well, the details involve trigonometry, but the upshot is, it makes the inside wheel steer more quickly than the outside wheel at all turn angles. Thus, the toe-out increases steadily (quadratically) with turn angle, as the graph shows is required. The more the inward slant of the steering arms, the more dynamic toe-out. Checkers “want” about 1.5 degrees of toe-out at a
n inside wheel steer angle of 20 degrees, as shown by the gray dashed "1/2 neutral" curve.

So, how much steering arm angle produces that much toe-out? Let’s add to the graph:

The two added curves are labeled: ’68-’77 (rear-steer) steering arm (on a rear-steer car), and ’78-’82 (front-steer) steering arm (again on a rear-steer car, but facing aft). The inward slant of these steering arms is 8.14 and 2.39 degrees respectively, whereas the inward slant that would produce “neutral” and “1/2 neutral” toe-out are 10.44 and 20.88 degrees respectively. The ‘68-’77 arm is about 1/3rd neutral, which tells me that Checker believed their taxis were cornered hard enough to shift 3/4ths of the front cornering load to the outside front wheel. The ’78-‘82 arm, if retrofitted to a ’77-earlier rear steer Checker, gives another factor of 3 less toe-out than that. (So why do they work on ’78-’82 cars? That’s another story.)  Use the wrong arm and you’ll get 3x faster tire wear, very uncool tire squeal in slow, tight corners, particularly in parking lots, and worst of all, reduced cornering force because the tires spend part of their available force fighting each other. My car did all these things.

So. Imagine you are in Joe Pollard’s Mojave, CA junkyard with a pile of Checker steering arms in front of you. Which ones do you want? I wondered this too, since the ones I had were clearly wrong, so I drove out there and spent four windy hours underneath junk Checkers with a digital camera and a tape measure, and took home a boxful of arms courtesy of Joe. Check out these two photos.


The differences in steering stop castings and inward slants are the clues. Both arms have a steering arm length of 7.5” (ball joint center to tie rod hole center), but the ’68-’77 arm has a tie rod inboard offset of 2.25” from the arm mounting surface, while the ’78-’82 offset is only 1.5”. Since the center of the ball joint itself is offset inboard from the mounting surface by 1.19 inches, the resulting inward offsets from the ball joint are 1.06" and 0.31", so the inward slant angles (ball joint to tie rod end) are arctan(1.06/7.5) = 8.14 degrees and arctan(0.31/7.5) = 2.39 degrees respectively, as stated above.

Here’s an interesting side note. In early-to-mid 20th century automobile design, a quantity known as the Ackerman Angle was used as a guide to what the steering arm inward slant ought to be. This was the angle the arms would have if you extended the line formed by the steering axis (i.e. ball joint or kingpin center) and the steering arm end (tie rod end mount hole), and made it pass through the center of the rear axle. For a Checker, this angle is about 14.6 degrees and would produce 70% of neutral dynamic toe-in. From this we can deduce that those early cars
, most of which had no anti-sway bar, were designed for sedate cornering, and squealed their tires if you misbehaved, just like in the movies – and the ’51 Buick Roadmaster I drove in my teens.

Now you know why.