Those were excellent but highly technical explanations.
Please, don't anyone take this to heart, but I am going to have a shot at a simpler man's guide. I am going to leave out all references to volts and amps and also refer as little as possible to ohms - purists may shudder suitably now and then stamp off in high dudgeon!
The motor and the car
Obvious starting point - motors need electrical power.
In theory, you could run a car under the control of just a simple press on/release off switch and this is how the earliest slot cars WERE operated. But, in practice, this is too violent an extreme between full power and no power, so we really need variable power to provide variable speed on different parts of the track.
The variable speed is provided by a controller.
To keep this simple, we will leave out electronic controllers and brake circuits.
Essentially, the controller works by passing the full electrical current from the power supply through a tightly wound coil of high resistance wire called a resistor, which dissipates some of the excess power as heat.
A contact on the end of the trigger or plunger slides along this resistor coil and the amount of useful current that actually reaches the car depends on just where the contact is along the length of the coil. The more of the resistor wire that the current must traverse, the less of it can reach the car on the track.
When at maximum, no power passes through the resistor at all - it all goes straight to the car and, in this situation, it makes NO difference what the resistance of the controller is as no current goes through the resistor. The car will get ALL the power, completely regardless of the resistor rating.
As soon as the trigger is backed off a little from maximum, the current is diverted through just a few coils of the resistor, loses a little of its power and the car slows down
As the trigger or plunger is moved further and further back from the maximum position, the power is diverted through more and more coils of the resistor and so less is available to run the car, so the car slows down more. Eventually, the contact reaches the other end of the resistor where almost all of the power is dissipated as heat through the full length of the resistor coils and almost nothing reaches the car. A fraction past that, and the contact on the trigger/plunger reaches a point where all power is cut off from both car and resistor.
Hopefully, that was fairly easy to follow.
Now, let's look at the slightly trickier bit - the maximum amount of resistance provided by that resistor - the point that was asked to be explained
It is perhaps not entirely obvious that cars need a bit of power just to overcome inertia and to start moving at all. Well, obvious or not, they do!
After accepting that, I think it's probably easy to accept that a HEAVY car will need more power to get moving than a light one would and also that one with heavy magnetic assistance will act similarly, because of the drag of the magnet. The same applies to a car with high friction or low numeric ratio gears. All need more power just to get them moving.
A nice, average, 'normal' car might need, say, 10% of the total power just to get it moving, but a high drag car could easily need 50% of the power to do the same thing and an exceptionally high consumption motor might need as much as 90% of the available power to get off its ass. This translates to 10%, 50% and 90% of the controller travel, just to start the car moving!
Let's take the 50% case.
If it takes 50% of the controller travel to move the car, then that only leaves 50% of the travel to control its speed between a dead crawl and max speed. This is heading back towards that on/off, all or nothing, switch scenario - not very satisfactory. The answer is to use a lower resistance coil for these cars. The principle here is that, if the resistor is changed for one with half the resistance, then the trigger will not need to be pulled as far in order to let the same amount of current through as before. Therefore the user regains much of the 'lost' travel for finer speed control.
In the 90% case, an even lower resistance would be needed to regain a good amount of fine control.
So, these low resistances are good for high consumption cars, but the opposite holds true for low consumption cars. If you use a low resistance controller on a low consumption car, even a small throttle setting will permit too much current for this car and it will hit close to maximum speed on tiny throttle settings. It will be just as undrivable as the high consumption car using a high resistance coil. The difference is only a matter of which end of the controller setting becomes uncontrollable.
Now to mention the ohms word just lightly!
Ohm is a standard unit of electrical resistance.
A high resistance controller suitable for low consumption motors is rated at around 60-70 ohms.
A medium resistance is rated at around 25-45 ohms.
A low resistance for only the very highest powered cars could be anywhere between 5 and 0.5 ohms.
At the risk of being shot at, that third category is far too low for the average home slot car and I would advise that light or no mag cars will generally be happy on 60/70 ohms and the high mag home cars will be happy on 30 ohms, but your mileage may differ a little, as they say.
People may argue about exact figures to their heart's content, but this is just intended as a simple guide to give you a general feel for what it's all about.
I do hope it helps, or I have just wasted a couple of hours of my time!
