Wagon and Pedal Car Steering Geometry

Correct steering geometry makes your family car or a toy corner without excessive tire scrub during tight turns. The wheel on the inside of the turn is best angled more sharply than the outside wheel. The mechanical solution has been known for hundreds of years and is known as the "Ackerman" arrangement beyond 1818. It's easy to see and design without the math backing it up. Lines through the vertical pivots, the kingpins, and the tie rod end pivot points (on each side) need to cross at the center of the rear axle. This makes for very low rolling direction error even at hard over angles. When steered off center, lines extended from all axles converge together at the turn's center of radius on the inside of the turn- straight out from the back axle. A well designed tie rod is longer than the kingpin span when placed ahead of the axle and shorter if mounted behind. The low geometric error allows for very hard over steering limits and highly maneuverable wagons- such as the VR280G wagon fleet aboard the Queen Mary hotel ship or the Berlin wagons at a Texas children's hospital. Kingpin and tie rod steered vehicles that we sell with the most accurate geometry are: Kettcars, Valley Road wagons, Speedway wagons, Berlin model F600- only, Caterpillar Dumper, and Scale Models wide-front pedal tractors.

Six and eight wheel wagons have only slightly less precision from use of a shared design. A center pivoting solid front axle or "fifth wheel" steering on Radio Flyer wagons, and narrow front Scale Models pedal tractors are inherently correct, but they may include turn angle limits to prevent instability from wheel tuck-under. Hochstetler wagons with a design shared with four-wheel steer versions cannot easily adapt to the Ackerman system since both ends are linked. All Hochstetler wagons and smaller Berlin wagons have less accurate (at hard angles) parallelogram kingpin and tie rod arrangement.

A further steering geometry refinement is the use of kingpin inclination to induce a camber (side tilt) change with steering input, with the intent to cause benign front end breakaway at the cornering limit. The rear axle of Kettler Navigator tricycles also have inclined kingpins, more below. A secondary benefit of this design is that a tighter turning radius can be allowed within safe handling limits. The two kingpins (vertical steering pivots) are tilted off vertical so that their tops are closer to each other than the bottoms. The kingpin to axle angle is such that the wheel is vertical when steered straight ahead. When steered to 45º off center, the outside front wheel adopts a positive camber of half of the kingpin tilt- biasing the loading toward the outside sidewall by that number of degrees. The primary result of the camber change is to diminish the grip at that tire relative to the others. Thus the front end with the stressed outside and lightly loaded negative cambered inside tires will break away before the back end during hard cornering- making for an understeer condition.

Understeer is also called "push", "plowing" or "tight" handling. The famous book by Ralph Nader "Unsafe at Any Speed" about dangerous oversteering characteristics of rear engine cars in the 1960s has caused automobile makers to be quite sensitive to oversteer or "loose" handling cars in the hands of the driving public. Highly adjustable race cars are generally finely tuned close to "neutral" between loose and tight except for intentional loose handling on dirt or gravel racing surfaces.

There are ways to tune in the relative cornering grip between the front and rear by means of "roll stiffness distribution" in the frame, suspension and tire selection and pressure, among others. One way to measure the handling balance is to take the surface temperature of the tires to get a view of the immediate past cornering stress. Other ways are to highlight the short term wear pattern with paint dots or to look at the markings left on the pavement. See below for a report on cornering force measurement of a pedal car for evidence of cornering balance in a car without kingpin inclination as evidenced by marking left on the pavement.

The Kettrike Navigator and Air Navigator tricycles have lockable caster steered rear axles designed to work intuitively for the person controlling the pushbar. Both or either end steers or locks straight ahead for use in one of several different modes. The unique feature of the Navigators is the moderately complex geometry of the steerable rear axle. Applying force on the pushbar toward the left from far behind the center of the tricycle steers the trike right from rear wheels steered forward side of each tire to the left. The kingpins (steering pivots) are inclined toward each other at their tops with spindle links made to keep the tires vertical when steered straight ahead. Kingpin inclination makes for stable self-centering at low speeds since steering in either direction raises the ride height in opposition of gravity. The kingpin inclination also is what makes the axle act like casters. When the bottom of both tires experience a sideways force in a given direction it will cause a matching tilt to the outside along with a steering action in that direction. The rear steering axle thus yields to the same direction as it experiences a cornering force. Without a pushbar in play and a rider on the pedals, the unlocked rear wheels will stay close to the center position at low speeds but will begin to assist slightly in the turning direction with increasing speed. At speeds beyond what could be pedaled this could be undesirable.

The moral: Remind children to limit speeds to the tire's "A" speed rating of 20 mph or lock the rear steering out. As a plus, the simulation of oversteer does seem to take the base out from under the possibility of a rollover. If you could go fast enough this could also be useful training for someone who will one day drive a car on rain or snow. Counter steering at the handlebars must be learned fast- otherwise the tail will hang out more than a good-ole boy outrunnin' the revenuers. In summary, the rear steering is a great feature that can be disabled for use like a regular tricycle.

Measuring Kettcar Cornering "G" Force

This question had to be answered. How many Gs can a Kettcar pull in cornering on fresh asphalt?  Just for comparison, a passenger car will corner at from 0.60 G on up to around 1.00 G (Ex Corvette C5), 0.80 G is typical for modern cars on low-profile radials. Race cars can far exceed that up to 5 Gs with the use of lots of areodynamic downforce, high banking and sticky tires. A fighter jet can corner through the air up to CLASSIFIED Gs, more than the mere passenger can take. As early as the 1920's experimental aircraft have exceeded 10 Gs of cornering force in air.

First the methodology. Radius of the skidpad is 15 feet, marked with a line- also works well as the 3-point line at the basketball goal. Time the moving car through part of an arc to get the speed in feet per second. Calculate cornering in Gs with the formula: V squared over Radius divided by G (32.2 feet per second squared). Results: The smaller Kettcars (Grand Prix and XR-12, Sportster) could not be ridden fast enough to reach the cornering limit on a 15 foot radius curve, thus all further tests were done with a Kettcar Safari (Same chassis as a Formel 1). There was a running Bonneville start and then a timed portion of the arc to get the speed in feet per second. The 10 year old who weighs about 70 lbs. reached 0.52 G with massive understeer. Ralph Nader would have been proud of the way the front end broke loose first and just plowed. But with the author on board, at 69" and 160 lbs., the car would roll and eject the rider every time at around 0.55 G, without breaking traction. Being top heavy really reduces the ability to cut seriously hard turns. The moral is, the size and age limits are evidently for safety and not because the car can't take the abuse.

--A member of the Society of Automotive Engineers, the author, Dave McCusker, Jr., has been a design consultant for two products. He checked his life insurance before exploring the performance envelope at the cornering limits of toys. His mama also reported that his first word was "car".