[slotloco]

Part 3. Speed and Torque.
-Motor speed and ratio
You may intuitively think that the ideal ratio is related to speed and that when changing your motor, you can make a calculation so as to make the car's performance equivalent to the older motor as a start point.
Eg I'm changing my NSR king22 to a king30 to improve performance
22/30 = 0.73
This might lead you to want to change your ratio from (for example) 3 to 4.1 (so that the rear wheels will top out at the same speed) but this would be a mistake.
Gearing is less related to motor RPM than is is to torque.
So if changing a motor that's optimally geared, to a faster one of the same G/CM rating, the existing ratio should provide a good start point.


B. Non fixed pitch.
A dirty little fix employed by slot car manufacturers to maintain a fixed motor to axle distance.
Ideally, all of the tooth wheels that are running together would be of identical pitch (represented as a TPi "teeth per inch " number)
However, Companies like NSR and Slot.IT play fast and loose with gear pitch, especially on angle winder and sidewinder gears so that they can avoid having to design a system where the motor would have to be on a sliding mount to account for the different gear diameters that such a fixed pitch design would require.
Instead, the manufacturers stretch and squeeze extra teeth or fewer teeth onto the same diameter gear.
Practically, such designs work but if the mismatch is pushed too far, the gears cease to work efficiently.

AlanW

Very useful broad view Alan. I am sure you know a lot more about gears than me, but there are a couple of points in your post that had me thinking... close but no cigar.

First one is about the gearing. The theory about speed and torque is correct, but in practical terms, since we all here, probably run on smallish tracks, is that if you threw that King 30 into replace a King 22; but didn't change the ratios, you would gain little extra straight line speed before having to brake again, whereas it you geared it a little shorter, so it got closer to winding out to potential on small said track, it would be accelerating faster, and would have more brakes, so you would surely look for the gear ratio (almost certainly shorter) that got you to the highest peak velocity the quickest, simultaneously with the latest possible braking point, before beginning to brake.
I know as I upgrade motors, I look to run shorter/lower ratios to get the best lap times, even if the new motor has as much torque as the old one.

The dirty fix, I like that term. But I think you are confusing the outside diameter of the gear with the pitch circle diameter.
I believe the basis of this is changing the pitch circle diameter, to keep the effective pitch of the pinion as a differing circle diameter as close to the 0.5 modulus mid point as possible.
If for instance I swapped a Slot.it 6.5mm 10 tooth sidewinder pinion for a 12 tooth, the contact point of the 12 tooth, would be further out from the centre of the motor shaft, and the pitch wouldn't change by x 12 ÷10. 

Have a look at the different tooth shapes here, and how you might expect the two pinions to contact at a different pitch circle diameter point with a straight toothed spur gear

 

  

So the "pitch" or modulus value doesn't change as much as we would assume it does.



Does that all, those both make sense?