A few tips on chassis design by DavidLawson

Southend Slot Racing Club members employ all their ingenuity when building cars for the club modified saloon class. The general trend until now has been to use the Ninco Mercedes C Class or McLaren GTR chassis trimmed to fit the regulation Mercedes or Calibra Scalextric bodies, fitted with a variety of motors such as SCX, Ninco NC2, OzRace, ProSlot V12, Red Bull and Scalextric Mabuchi.

A variety of wheels are used, generally from the Ninco Sports GT range with both standard tyres and softer compound treaded and slick tyres and other makes such as the excellent ProSlot grooved slicks. To reduce the risk of tramping it is a common practice to glue and true the chosen tyres.

There are a variety of opinions and approaches regarding achieving a handling balance. One view is to experiment with ballast and to move small amounts of weight around the chassis in testing until the car performs to the drivers liking with good balance, handling and good anti de-slot characteristics. Another approach is to keep the chassis standard and try a variety of motors that give a suitable top end speed combined with driveability that doesn’t unsettle the chassis mid corner.

Some club members have added brass plates to large areas of the frame or tried the Excalibur chassis and one member has a scratchbuilt piano wire and brass chassis in development although this has not been raced to date.

All these permutations of the various components produce an interesting array of slot cars and with a championship being introduced for next season and the natural desire to try anything that might help to give the extra ‘edge’ it is possible that more scratchbuilding and experimentation will occur to produce the Holy Grail, that superior winning car. To provide more fuel for thought, the following ideas on chassis design may help everyone to produce that perfect car.


The 3 Point (Triangle) Principle

Chassis handling characteristics are influenced by a 3 point principle. The centre of the guide pivot and the track at the rear axle make up the ‘triangle’. The smaller the angle created at the guide pivot the deeper the car will go into a corner, the angle at the guide pivot is inversely proportional to the distance between the guide pivot and the rear axle – the angle gets smaller as the distance increases. The longer wheelbase enables your car, aside from going deeper into a corner, to be less nervous to your throttle input, which is a welcome characteristic on the tighter corners.

The ’triangle’ principle works best when the front wheels are able to float on the track surface. The float effect can be obtained by incorporating an iso-fulcrum (hinged front axle tube), check with Derek White regarding eligibility before trying this, or alternatively an elongated front axle hole should produce a similar result.


The Weight Of A Chassis

Let’s consider the factors that determine the optimum weight of a chassis, the variables are weight of the body, weight and power of the motor, track length and radii of the corners, track surface texture and finally the material being used to construct the chassis as highlighted in this box.

Key Factors

Body weight: The heavier the body the heavier the chassis needs to be. The chassis must counter the tilting force of the body, as you increase the body weight you will need to also increase the chassis weight.

Motor weight and power: The heavier the motor, the heavier the chassis. Heavier motors will need more weight at the front in order to maintain the ideal front to back chassis balance. Should you change a motor in an existing chassis for a more powerful motor of the same weight you may need to add weight to restore the chassis balance.


Track length and corner tightness: The larger the track, the lighter the car. The increased length and higher speed of a large track allows the body to generate more downforce to the chassis, which can therefore be lighter. Uneven track surfaces require a heavier chassis to obtain better traction and smooth track surfaces offer more grip therefore a lighter chassis can be used.

Materials: The stronger the chassis material the lighter it can be. Brass chassis are heavier than steel ones since brass is softer and therefore a thicker gauge is required in order to achieve the same strength as steel.


The Frequency And Torsional Rigidity Of A Chassis

A slot car chassis is like a tuning fork. The vibrations induced by the motor (accelerating, top speed and braking), out of true tyres, poor bearings and uneven track surfaces will vibrate the chassis possibly causing de-slots. To minimise these unwanted vibrations a chassis must be built very stiff longitudinally, adding a reinforcing rail along the centre line can assist. A chassis should not flex when downward pressure is applied between the guide and the rear axle.

Torsional rigidity is the side-to-side flex of the rear end while keeping the front end stationary. The tightness of the corners on a track and the power of the motor determine the amount of flex. Too much of this flex will cause your car not to straighten out fast enough when exiting a corner and delaying full throttle opening onto the straight. Too much torsional rigidity will cause your car to

chatter and de-slot through the corners. It is best to start out with too much flexing and gradually stiffen the chassis by soldering piano wire on top of the main rails of a scratchbuilt or strengthening a Ninco. Start from the front of the chassis and work your way back with longer pieces until the desired amount of flex is attained. This method is preferable to adding ballast weight.

Perhaps when you start planning your next build or modification these notes may assist you in finding that extra fraction of a second that could make the difference between winning and losing.
  David Lawson