Steering Theory 101

Steering basics

When I started to measure the toe-in on the original suspension to get a baseline, I found that the front suspension design is very poor by today's standards. There was horrendous amounts of bumpsteer through the suspension travel. I expected to see some bumpsteer at the far ends of the suspension travel but not with just a small amount of movement. I found out that Steve Smith from Steve Smith Autosports has many excellent resources and books available from this site on chassis and racing suspensions.

Bumpsteer is often quoted but few people explain what exactly it is. So here goes: there are 3 planes of rotation in an independant front suspension: upper control arm, lower control arm and tie rod. All have to rotate on the same plane and arc. The upper and lower control arm are fixed and cannot be adjusted. The tie rod length can be adjusted and usually is for toe-in settings.

If the tie rod length is longer or shorter than the control arm's arc, then when a wheel hits a bump, the tie rod will either pull the wheel in or push the wheel out. Hitting a bump causes a change in the steering hence the term bumpsteer.

King Pin Inclination Angle and Scrub Radius

Important! Read the section called "The truth about scrub radius!" that follows.

Most people have heard about caster, camber and toe-in. Few have heard about scrub radius and kingpin inclination angle. These two are directly related and determine how much effort is required to steer the car and how responsive the car is. The kingpin inclination angle also determines the natural tendency of the car to straighten the front wheels after turning.

Kingpin inclination angle (also called steering inclination angle) is the angle drawn from the center of the upper ball joint axis through the lower ball joint axis. By offsetting the ball joints, the effect is to produce an vertical arc as the wheel turns. The top of the arc is when the wheel is pointing straight to the front. As the wheel turns, the arc causes the body to lift up. All in all, the weight of the front end tries to keep the axle at the top of the arc which is the position when the wheel is pointing straight forwards.

Proper kingpin inclination angle will have the line drawn between the two ball joint axis meet the centerline of the front tire where the tire contacts the pavement or slightly to the outer side. The distance from the tire center to the kingpin inclination angle contact point on the road is called the scrub radius.

The two major factors that affect the scrub radius are wheel height and wheel offset. Taller wheels than stock will push the scrub radius farther out from the center line which is acceptable to a point. Shorter wheels will pull the scrub radius in towards the frame. This is not desirable as it makes the steering heavier (more effort to steer) and can cause unwanted bumpsteer while braking.

Wheel offset changes the scrub radius by repositioning the tire centerline with respect to the kingpin inclination angle. An offset that moves the tire centerline out causes the same problems as shorter tires. Combine the two: shorter tires and wheels that offset so that the tire centerline is moved outwards and it spells big trouble. The type of trouble is fast wearing tires, suspension components, steering components and axle bearings not to mention bumpsteer while braking and heavy steering.

I found that the combination of the original spindle and ECI disk brake kit moved the tire centerline outwards quite a bit. I picked up a pair of a front wheel drive 1981 Cadillac Seville's rims for the front end which have a whopping 2.5" negative offset. This reduces the kingpin angle dramatically and allows a greater selection of tires to be used. Supposedly, S10 4x4 pickups with steel rims have 2" offset - I couldn't find any to confirm this.. I will use 1977 Grand Prix steel rims for the rear, they have 0.25" negative offset which positions the wide rear tires evenly in the wheel well.

The truth about scrub radius!

After literally months of working with different spindles, rim widths, wheel offsets and calculating the scrub radius, I settled on 73 Nova spindles. As a matter of course, I calculated what the scrub radius would be on a 73 Nova. Guess what? It was way off from all the ideal values. It was about 2" positive where everything I read indicated that it should be about 1" negative! What gives?

I went and had coffee with the steering and suspension expert at the local technical institute who laid out the story. Scrub radius is very important for front wheel drive cars to minimize bumpsteer caused by the front wheel drive. It was also very important when the brake system was split diagonally on the first 2 reservoir master cylinders. In case of a brake failure, diagonal pairs of wheels (front left/rear right for example) would be paired up. If the scrub radius was positive, then the vehicle would pull hard to the outside. With negative scrub radius, the vehicle would hopefully slow down somewhat in a straight line.

So what that means is that scrub radius in rear wheel drive cars with front/back split reservoirs is not critical. Those of you that have driven 67-69 Camaros and 68-74 Novas would know that these vehicles drive perfectly fine with their 2" positive scrub radius. Did I ever waste months of work to find this out!

Ackerman angle

Ackerman angle is the difference in steering angle of the 2 front wheels when your vehicle goes through a turn. The tire closer to the corner has a different turning radius than the outside wheel. The Ackerman angle takes care of this difference. Where is it located? - the steering arms. The angle that the steering arms move away from the rim is the Ackerman angle. The old rule of thumb is that the left and right steering arms should point to the center of the rear axle. Newer calculations indicate that the point of convergence should be at a distance of 60% from the center of the rear axle.

How important is the Ackerman angle? Not much really. It is only important when you are driving very slow and turning. For example when you are parallel parking or something like that. During normal highway speeds, the front wheels barely turn when executing a corner - not enough turn for the Ackerman angle to influence.

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Copyright Jan 16, 2011 Eugene Blanchard