da 

 SLOT CAR CHASSIS

Why are the cars built like that?

Some slot cars look complicated, others look simple.  Why are they like that and what are the important features?

What are they trying to achieve?

The  main things that racers have tried to improve over the years are speed, drivability and reliability. 

By speed I mean faster lap times which includes cornering speed and braking as well as straight line speed. In fact the top speed of cars isn't that important on most circuits - cornering speed is much more important. More powerful motors are an obvious route to more straight line speed, but this is a compromise in that lighter motors help cornering speed, and less powerful motors make a car easier to drive in corners.  

For maximum performance driver needs to be able to consistently push the car to the limit of adhesion. Even the very best drivers cannot drive a series of laps that are perfectly identical, and even if they could the track conditions are not perfectly consistent.  There is a limit to human reaction times. (Research on racing drivers shows that their reaction times are not much faster than the average member of the public. What racing drivers are good at  is detecting the early signs of what the car will do next and providing a correction in good time.)  

It turns out that a slot car travels quite a few car lengths during the human reaction time, so the behavior of the slot car has to be progressive enough for the driver to detect what it's doing and apply the necessary correction without it all happening too fast for human reaction time.  The slot car driver can see the tail of the slot car sliding out on corners - this shows the limit of adhesion has been reached and if the car can be developed to react slowly enough the driver can react with the appropriate throttle movement.  Something the size of a slot car will naturally spin too quickly for anybody to react, so a major part of car development is making the car react slowly enough for normal reaction times.  

By contrast, if a car tips out of the slot when it reaches its limit, then it all happens far to quickly for anybodies reactions to prevent it, and the best you can do is try it a bit slower next lap.  If a car slides on the limit, it is possible to see it is someway short of the limit and push a bit harder next lap. If you do push a little too hard next lap you'll waste a small fraction of a second going too far sideways. If a car tips on the limit, it looks exactly the same if  it is someway short of the limit or right on the limit.  If you do push a tipping car little too hard next lap you'll deslot, and waste a few seconds being marshaled.  Even if a tipping car had the same ultimate cornering speed as a car that slid on the limit, most of the laps with the tipping car would be slower, and it would fall off more frequently.

Reliability?  I'll use a quote attributed to various motor racing luminaries "to finish first, first you must finish".  Higher speed puts more strain on many components, so something that was reliable at low speed may fail at higher speeds. Gears that are adequate for low power motors will last no time on powerful ones, conversely gears that are needed for high performance cars may provide little or no advantage on low powered ones.

What do you do to achieve all that?

There are a few features that can be explained in scientific terms, but most parts of the car have been developed by trying different things and adopting the features that work.

A low center of gravity has always been recognized as important. This is why the main weight of the chassis is mounted as low as possible. It is also one reason why racers seek lower and lighter motors. The highest part of the car is the body, so particular attention is given to light bodywork. Most people find it intuitively obvious that a low center of gravity is an advantage, it is easy to see the car can corner faster before it tips over if it's as low as possible. Less obvious is the fact that tyres provide more grip if evenly loaded, rather than nearly all the load being taken by the outside wheel.  

The motors are mounted parallel (or more often at a shallow angle) to the back axle like the car shown below. 

This Mack  chassis was raced by Graeme Stephenson  to win the 2009  National 1/32 Saloon Championship.

Originally cars had the motor at right angles to the back axle -  "in line" as it is known. The rules insist that formula one cars are built "in line" to this day (see photo at the top of the page). Spur gearing is more efficient than crown gears needed for "in line" motors.  Spurs are more tolerant of sideways movement in the back axle, and allow the motor to be mounted at any height relative to the back axle. Armatures rotating parallel to the back axle, but in the reverse direction (which they always will be with a single spur and pinion gear) produce gyroscopic forces which help the car corner better: there is also an advantage in torque reaction. 

The chassis are designed to flex and the various parts move in carefully controlled ways. As well as flexing the chassis have a damping effect built in. 

A controlled amount of body movement relative to the chassis is important.  This idea has been known since the early days of slot racing. In those early days, many cars had hard bodies attached to the chassis with screws.   It is reputed that the significance of having parts of the chassis moving was discovered when somebody forgot to tighten the screws up and found that the cars went better with the screws a bit loose allowing the body to float. Over the years people have tried various different arrangements, and found what works best. I've never found an entirely satisfactory explanation for why this works - but it certainly does work. Even simple production chassis are made from two separate pieces, allowing a simple form of floating body.

In recent years cars have got lighter - competitive cars used to weigh in at  over 130 gms - many of the top cars are now down around 75 gms.  Modern tyres work best with light loading.    Producing satisfactory handling in 1/32 cars always used to be achieved with lots of weight down low (typically 1.6mm brass with lead on top). Rare earth magnets have allowed lower lighter motors to be produced. Light weight motors have enabled the whole car to have a low enough center of gravity with much less metal low down in the chassis (typically 1mm steel with little weight on top). Getting the center of gravity low enough is only half the story, the car has to be controllable within the driver's reaction time. Getting a light car to slide controllably has been all about chassis development, and smooth motor characteristics.

The 1/32 saloon cars are obliged to use full size C can motors. These are the same size as the motors introduced in the early 1970s (although they are about 5 grams lighter).

Copyright ©  2001 C.Frost  (minor revisions 2009)  photographs copyright C.Frost 2009    All rights reserved