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Technobabble: May 2002

That's My Point

Dave Coleman
May 1, 2003
0212scc_techno01_z Photo 1/1   |   Technobabble: May 2002

After two and a half days of working on the $3,000 Silvia project, we're finally ready to go. Tired, cold, and bleeding from every knuckle, I crank the new SR20DET to life for the first time and prepare to limp 400 miles home. The engine looks perfect and runs well, but the tired, old chassis has seen more than 250,000 miles. The shot bushings, suspect alignment and the steering rack's torn boot and leaky fitting should've--but didn't--stop us from taking the long way home.

Down the road, I steer into the first turn of the narrow, twisty two-lane and am amazed. I turn to my passenger/victim and exclaim, "Wow! This car has a great Dave Point." The Dave Point, as regular readers of this technodrivel already know, is "the point where the steering axis crosses the ground plane."

The steering axis, of course, is the imaginary line in space each front wheel steers around. With a double-wishbone suspension, it's the line connecting the upper and lower ball joints, for example, or on a MacPherson strut car, it's the line connecting the top of the strut and the ball joint. Follow this line all the way down to where it hits the ground and you have one of the most important points in the fascinating world of steering geometry. Still, until September 1999, this point had no name.

One of engineering's greatest weaknesses is its dry, unimaginative language, so as a service to the engineering community, I took the point where the steering axis crosses the ground plane under my wing and relabeled it the Dave Point. End of history lesson.

My new Silvia's steering, despite its age and questionable condition, has incredible straight-line stability, great feedback and a very natural buildup of steering torque. You can thank my point for all of that.

The Dave Point's position relative to the center of the contact patch is one of the most important factors determining steering feel, torque steer, and the car's tendency to wander around on uneven pavement. Looking at just the contact patch, you can imagine the Dave Point as a pivot, and the center of the contact patch as the point where all road forces push on the tire.

If the Dave Point is significantly inboard or outboard of the contact patch's center (a measure known as scrub radius) acceleration and braking forces, impacts from bumps and uneven pavement all try to turn the wheels. If, on the other hand, the Dave Point is directly in front of the center of the contact patch (a measure known as caster trail), everything but acceleration simply pushes the wheels straight. The front wheels of a shopping cart, for example, have a lot of caster trail, and are so stable they can be trusted to point in the right direction on their own.

Adding caster (tilting the top of the steering axis back) tends to move the Dave Point forward. Simply moving the entire steering axis forward also moves the Dave Point in the right direction, but that's much harder to change if you didn't design the car. The 240SX has 6 to 7 degrees of caster, which is a lot compared with about 5 degrees for a Miata, and about 2 or 3 degrees for a Sentra. This caster also helps improve cornering grip. As you turn the wheel, some of that caster becomes camber. The effect is relatively minor, because you don't actually turn the wheel much in a normal corner, but you only have to look at the severe tilt of the front wheel in our cover shot to see the effect.

Straight-line stability is nice, but it's really the kind of thing your grandmother would look for in a car. The Dave Point is all about fun. This is why the Dave Point likes drifting. Drive in a straight line and all the drag from the tire rolling along the road pushes the wheel straight. Well, the same thing happens when you're sliding. Pitch the car sideways and suddenly the road is pushing sideways on the contact patch until it falls again behind the Dave Point. Throw a 240SX into a drift and you barely have to hold the steering wheel. The Dave Point and the contact patch do most of the work; all you have to do is make little corrections and stay on top of the throttle.

I once rode along with Rhys Millen as he drifted a Porsche 996 around Buttonwillow Raceway Park. He actually let the Dave Point do all the work. He would simply pitch the car into a slide, completely let go of the wheel, and then catch it when it was pointing where he wanted.

Next time you do a perfect drift in a Miata, 240SX, or perhaps even a 996, just remember whose point is there to help you.

By Dave Coleman
94 Articles

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