By Pro-race and Proxy II entrant Pete Sardella

There are a few basic principles that we must acknowledge before building a frame: 1) The 3 Point Principle; 2) The Weight of a Frame; 3) The Frequency and Cross-flex of a Frame.

The 3 Point (Triangle) Principle Frames work on a three-point principal. The center of the guide pivot and the rear tires make up the "triangle." The smaller the angle created at the guide pivot the deeper your car will go into a turn; the angle at the guide pivot is inversely related to the distance between the guide pivot and the rear axle -- the angle gets smaller as the distance increases. The "increased" distance enables your car, aside from going deeper into a turn, to be less responsive to your trigger, which is a welcomed trait on a small tight track. The "triangle" principle works best when the front wheels are able to "float" on the track surface. The "float" effect can be attained by incorporating an iso-fulcrum (hinged front axle tube), or an oversized axle hole in the front wheels, or an oversized axle tube.

The Weight of a Frame

The next issue in building a frame is its weight. Here we must look at the variables that determine the weight of a car. The variables are weight of body, weight and strength of motor, track size and tightness of turns, track surface texture and finally the material being used to make the frame.

  1. Body weight: The heavier the body the heavier the frame. The frame must counter the tilt factor of the body; as you increase the body weight you will need to also increase the frame weight.
  2. Motor weight and strength: The heavier the motor the heavier the frame. Heavier motors will need more weight up front in order to maintain the proper front to back balance. Keeping the weight of the motor the same (same setup) but installing a more powerful armature will necessitate extra weight to be added to the frame.
  3. Track size and turn tightness: The larger the track the lighter the car. The increased running surface of a large track allows the body to generate and transfer more downforce to the frame; therefore your frame can be lighter. With the limited downforce attainable on a short tight track a heavier frame is needed. Smooth track surfaces offer more "bite" therefore a lighter frame can be used; bumpy track surfaces require a heavier frame to obtain better traction.
  4. Materials: The stronger the frame material the lighter the frame. Brass frames are heavier than steel frames since brass is a softer yet heavier material than steel. A thicker brass plate is needed in order to attain the strength of steel. The "new" frame material - 6160 aluminum - used in wing car frames is lighter and as strong as steel. It has become the modern lightweight frame material.

The Frequency and Cross-flex of a Frame:

A slot car frame is like a rolling tuning fork. The vibrations induced by the motor (starting, topping out and braking), out of round tires, bad bearings, and bumpy track surfaces, etc. can send the frame into chaos causing you to de-slot. To control these unwanted vibrations a frame must be built very stiff in the front to back direction; adding a reinforcing rail atop the center rail or pan generally does the trick. A frame should not flex when downward pressure is applied between its guide flag and rear end! The stiffness raises the frequency point of a frame; the terminal velocity of a frame increases as the stiffness increases. Note: this does not apply to one-piece wing frames.

Cross-flex is the side-to-side flex of the rear end while keeping the front end stationary. The tightness of the turns on a track and the power of the motor determine the amount of cross-flex. Too much cross-flex will cause your car not to straighten out fast enough exiting a turn in order to accelerate up the straightaway. Too little cross-flex will cause your car to chatter and de-slot through the turns. It is best to start out with too much cross-flex and gradually stiffen the main rails by soldering piano wire on top of them. Start from the front of the rail and work your way back with longer pieces until the desired amount of flex is attained. This method is preferred to adding lead.

Pete's Proxy II entry (lovely isn't it!)

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