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Derby Tech - September, 1985

by Ollie Brower

Each derby racer has only so much potential energy. When some of this energy is used due to a poor steering system, the racer is slowed down unnecessarily. I want to talk about some physical effects of steering and how to avoid energy losses.

When the front axle of a racer is moved from perpendicular to the racers momentum direction, energy losses will occur because of forces acting in a direction other than that of the racer (Figure 1). This energy loss occurs whether the driver, or the uneven surface of the track changes the racer's direction. These energy losses are caused by the horizontal reaction of the ground acting on the wheel. This reaction can arise because the racer is undergoing lateral acceleration in a turn. It can also arise when the steering is deflected sideways by track surface conditions. The turned or deflected wheels will begin to carry the racer sideways. This lateral acceleration creates a force and reaction between the wheel and the ground, acting in a skidding motion which results in energy losses.

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The gyroscopic reaction of the wheels to the turning and tilting caused by the track surface add a small loss of energy. This loss is larger for the front wheels, as the effect the front wheels have is about half that of the rear wheels, due to the larger distance travel- ed. All in all the gyroscopic energy loss is very small

The tilting of the wheels due to surface irregularities, the bouncing and give of the axles, and improper spindle alignment also cause energy losses. The wheel has a natural tendency to roll in a circle toward this tilt. This tendency causes tire and ground contact drag to increase, which results in even more energy loss.

When the axle is turned while the racer has forward motion, the wheel on the outside of the turn moves on the spindle toward the racer's body, and the wheel on the inside of the turn moves toward the outside of the spindle. The more space the wheel has on the spindle (Figure 2), to move laterally the less chance the wheel will slap up against the square stock of the axle, or the wheel pin, causing bearing side thrust. But on the other hand, you will have much more steering control of the racer if this space is smaller. This area can also be made more streamlined while making it smaller. This also reduces the back and forth movement of the racers body (through normal vibration going down the hill) , changing the racer's direction. This space can be reduced with the use of a rubber "0" ring, which also helps streamline the transition area from the square stock of the axle to the round spindle.

(Online editors note: unfortunately rubber 'O' rings are currently illegal - with Z-glas wheels it is often necessary to place a washer between the square stock and the wheel to keep the wheel from rubbing against axle fairings)

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By off setting the kingpin hole in the front axle (Figure 3), the steering geometry will become more stable. This is due to the trailing axle caster affect. The axle will be trying naturally to stay in the straight ahead position, resulting in the contestant driving a straighter course. This hole should be between 1/16 and 1/8 of an inch in front of the center line of the axle.

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The rocking of the racer's body also has a negative effect on steering stability. This body rock causes the steering cables to loosen on one side and tighten on the other side causing steering errors, and a loss of energy, which slows the racer down. This can be reduced by reducing the amount of body sway from side to side, and by aligning the steering cable positions.

When mounting your axles, do so with a method limiting side to side movement of the body. This can be achieved by blocking the rear axle with some rubber component (one example is shown in Figure 4). Both axles do not need to be blocked. One axle should be able to allow the wheels to roll over track surface irregularities with little or no resistance., Be certain that the car's body is not resting on top of the axle. Have at least 1/8 of an inch clearance on top of the axle. This opening may be covered or filled in with rubber.

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This is most easily accomplished by using horizontal kingpins. These horizontal kingpins also allow for a maximum amount of axle flex. This is because the axle flex resisting area is held to a minimum. Of the two axles, the one that should be allowed to rock up and down freely is the front axle. The reason being, is that the front wheels travel farther than the rear wheels getting to the finish line. This is because of steering maneuvers, which allow the rear wheels to track at a shorter distance. The axle that is absorbing and using more energy, for the same distance traveled (ramp to finish line) , should be the axle that travels the shorter distance (rear), thereby loosing less total energy. The front axle has much less vertical resistance than the rear axle, which means that the front axle will use much less energy going over a pit or bump on the track surface.


The steering cable positions should be exactly centered, and exactly the same distances from the kingpin (Figure 5). Cables exiting the body should be on a horizontal plane and centered with the axle.  The steering cable stresses should be as evenly distributed as possible. Any rocking of the body when the cables are exiting the body from slightly different spots, or on angles not on the same plane as axle movement, will cause uneven pressures on the steering cables. These uneven pressures will cause the axle to turn slightly with no movement of the steering wheel. This turning along with the turning that the driver must do to correct the racer's direction causes a large loss of energy. These uneven pressures also cause the front axle to bend backward (when the pressures are tighter) and forward (when the pressures are loose) , which causes the front wheels to run out of alignment, meaning even more energy loss.

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Center of axle (a), cable exit hole (b), and center of pulley (c;, all are the same distance from ground on a horizontal plane.

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The ideal situation would be to have the steering pulley (one), in the center of the racer (kingpin to kingpin) . It should be the minimum distance back from the front kingpin, which is 24 inches. The cable exit holes should be elongated about 3/8 of an inch up and down. when the racer rocks from side to side, and back and forth while rolling over uneven track surfaces, the body flexes, but the cables will not hit the body and move the steering cables, causing steering errors. These uneven pressures are reduced to almost nothing, because the pulley is in the center of the car on the same plane as the horizontal kingpin. when the body rocks the pulley also rocks the same amount. This 3/8" elongated hole can be filled with rubber having a slit, allowing the cable to move up and down, and still maintain the proper aerodynamics and tension. This rubber also reduces cable vibration, which will reduce steering cable air drag.

Place the cable attachment points exactly the maximum distance allowed from the axle square stock (2" Sr. or 2 1/8" Jr.). This will keep the frontal area of the cable as small as possible, and the head-on profile will become slightly more streamlined.

It is possible to reduce the amount of cable exposed by cross angling the cables to the opposite pulleys (if two are used). This also shortens the length of the cable submitted to vibration, as it enters the body much sooner. The large problem with this is twofold; (1) The head-on profile of the cable becomes more round as opposed to being oblong, and the coefficient of drag goes up noticeably. (2) The steering becomes more difficult because of much more uneven steering cable pressures. This alone (#2), is reason enough not to do it.

Never loosen or tighten your cables during a race without checking the alignment of your axles, as these different pressures on the cables will drastically change spindle direction.


Offset the kingpin hole in front of center of axle for more stable steering.  Use horizontal kingpins for better axle flex characteristics. Center and perfectly align steering cable positions for equal steering pressures which keep axle spindles straighter. Use one steering pulley, and position it on the racer's center line 24 inches back from the center of the front axle. Elongate the cable exit holes about 3/8 of an inch to prevent uneven cable tensions. Never loosen or tighten steering cables during a race without rechecking axle spindle alignment.

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