Resistance controllers
Resistance controllers are the basic controllers that have been used to control slot car from the earliest days.   

In their simplest form they consist of an off position where nothing is connected, and a variable resistance complete with a low resistance full power position. This allows the driver to stop the car or provide various amounts of power up to full power.  These were used in the early days of slot racing, and some home sets controllers are still built this way. These have just 2 wires connecting them to the track (left hand diagram above).  (Or just the black and white wire in the trigger diagram)

In the improved form, controllers have the off position replaced by a brake terminal which connects the two pick up rails together.  These have 3 wires connecting them to the track (right hand diagram above)..

It should be noted that the resistance is connected to the full power band and is NOT connected to the brake band.   Occasionally people describe these as rheostat controllers, this can lead to a  misunderstanding about the connection to the brake band.

If overheated the most common failure is the wire burning out near the full power band.  This can be repaired by unwinding one turn and soldering the end on, or by replacing one turn with another piece of resistance wire.

The resistance usually consists of a coil of resistance wire wound on a ceramic core.   This is usually NiChrome wire - a nickel chromium alloy that had good resistance properties, is quite hard and will stand very high temperatures.  Unfortunately NiChrome  is not easy to solder. Satisfactory joints can be made by abrading the wire with fine emery paper, tinning immediately with an aggressive flux, then twisting the tinned wires together and flowing some more solder on the tinned wires.

An alternative resistance wire is constantan - a copper nickel alloy with very good resistance properties (in particular its resistance changes very little with temperature). This is softer than NiChrome and much easier to solder.

Before electronic controllers came into general use, adjustable resistance controllers were quite commonly used. Some designs had 2 or more resistors, some separated the contact and the resistors.  These days electronic controllers provide the adjustability drivers want, rather better than could be achieved with resistors.  With the exception of the more advanced resistance controllers sometimes used in HO scale slot racing, resistance controllers are now almost entirely the straight forward single resistor type which have the merits of low cost and simplicity.  

A frequently asked question is what value resistance controller / how many ohms do I need?  (This is a non issue for adjustable electronic controllers -  just twiddle the knob to give you whatever you need.)  

Unfortunately what value resistance controller you need may not have a single answer - you may need more than one.

• Motor power - higher powered motors take more current, so a lower resistance is needed to slow them down.  Set against this higher powered motors make the car go faster so they need more slowing down for corners.  Mostly the first factor is more important so higher powered motors most often need a lower resistance controller.

• Chassis handling - more power can be applied in corners with a better handling chassis. So the motor doesn't need the power reducing as much. So better handling chassis usually need a lower resistance controller.

• Tyre / track grip - cars corner faster with  more grip. So the motor doesn't need slowing down as much. So more grip usually need a lower resistance controller.  That's true whatever is providing the extra grip - for example stickier tyres, magnet traction or aerodynamic downforce.

• Tight corners - cars need to go slower round tighter corners. So the motor need slowing down more. So tracks with tighter corners usually need a higher resistance controller.

• Track power - cars go faster with more track power. So the motor need slowing down more for corners. So tracks with higher power usually need a higher resistance controller.  This is true for tracks with a higher supply voltage. It is usually true of tracks with better wiring because more of the supply voltage actually reaches the car at the far side of the track.  However, the current rating of the power supply doesn't normally make a difference to the controller resistance needed ( a motor that needs 5 amps still takes 5 amps even if the power supply can produce 10, 20, 40 or even more current.)

• Driver choice - some driver's driving style is suited to a lower resistance controllers than others.

So where do you start? I know some people will have different ideas, but as a starting point  HO 60 to 120 ohms   Basic home set 1/32 45 to 75 ohms  Quicker home set type cars 15 to 45 ohms  hard tyre cars with metal chassis 5 -15 ohms,  Sponge tyre cars 2- 5 ohms.