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Gears pitch & mesh: A look into Slot.it variable pitch gears
Our club races a Group-C class based on Slot.it models. The cars are required to use standard inline, non-offset motor pods; crown gears and pinions are free choice within the Slot.it catalog.
The series is very competitive and we are always looking for a way to get better lap times. After replacing the tires with optimal alternatives to our track, I started experimenting with different gear/pinion configurations; that’s when I ran into problems.
There are several ways to gauge the limit of a slot car. In my case (actually in our track case), my car needs to be able to go through turn 4 and 5 at full speed (see image).
If my car deslots at full speed on those turns, I know I need to increase the gear ratio.
At one point in time I was using a 10X28 gear configuration (2.8 ratio), the car was not desloting on turns 4/5 and the lap time was around 4.2 seconds. Switching to 10X27 (2.7 ratio), the car desloted on the turns mentioned. At this point I knew that a 2.7 gear ratio was too much but a 2.8 worked. Still wanting to extract more from the car I looked into available ratios between 2.7~2.8; the only available option was a 11X30 configuration that would give me 2.73 gear ratio.
After installing the new pinion/crown (11X30), the car was extremely loud and the best lap times I was getting were around 4.5 seconds. How could I get 4.2 seconds lap times with a 2.8 ratio and suddenly go to 4.5 seconds with a 2.73 ratio?
BTW: Gear ratio is the amount of turns the motor needs to generate in order for one turn at the wheel. By decreasing gear ratio you increase RPM at the wheel. Gear Pitch:
Gear pitch (more precisely Diametral Pitch) is the number of teeth of a gear per inch of its diameter. On a 48 pitch gear you will find 48 teeth per diametral inch; on a 64 pitch gear you will find 64, and so on.
With that in mind, when you have fixed pitch gears, their diameter will change depending on the amount of teeth. A 48 pitch 30 teeth gear is larger in diameter than a 48 pitch 28 teeth gear.
Gear Mesh:
To put it simply, a perfect gear mesh is obtained when both gears used have the same diametral pitch.
Why is gear mesh important? Well, simple physics; the better your gear mesh is, the less friction it will generate.
Slot.it Inline Gears:
It didn’t take long to understand the problem with my 11X30 gear configuration. If you look at a Slot.it crown gear, you will notice that there is a channel designed to accommodate the tip of the motor shaft, thus stopping the free lateral movement of the rear axle setup (the crown channel acts as a stop collar). See picture:
With this design, Slot.it has limited the diameter of their inline pinions to 5.5 mm. This was done in order for all their pinions to fit within the space of that channel.
Well, as we already saw, fixed pitch gears change in diameter according to the their tooth count. Since Slot.it pinions have different tooth count but the same diameter, we can conclude that they must fluctuate in pitch.
Slot.it does not publish the pitch of their gears. After some research I came across this formula: Diametral Pitch (P) = (Number of teeth +2) / Outside Diameter.
Using the 5.5mm diameter for the pinions, I was able to determine their pitch:
Teeth Pitch 8 46.18 9 50.80 10 55.42 11 60.04 After finding the pinions pitch I looked into finding the same information on the crown gears. The crown gears all change in diameter. After some measurements and calculations, all the Slot.it crown gears have a fixed pitch set at 48.
With that information I was able to understand why the 11X30 gear combo was not performing. The different in pitch from pinion to gear was 25%. If we use a perfect mesh as basis, where the pitch is equal on both gears, we could say that the mesh from that gear combo is generating 25% more friction.
By looking at the pinion pitch table, the “best” match for Slot.it’s fixed pitch gears will be their 8T pinion., with only 3.8% difference in pitch. Second best is the 9T pinion with 5.8% difference in pitch. The 10T pinion has 15.5% in difference.
With the variances in pitch, I have discarded the use of Slot.it 10T and 11T pinions with their crown gears.
One alternative to using a 10/11 teeth pinion would be to chop the gear channel off and use an fixed 48 pitch pinion. In our club series case, this is illegal and I will have to get the most of a 8 or 9 teeth pinion.
Slot.it Sidewinder & Anglewinder Gears:
With sidewinder and anglewinder setups, the variable pitch extends to pinions and gears. On both cases there is no adjustment for the distance between motor shaft and axle. This means that both pinions and gears need to have an overall width that does not change.
Take a look at the complete Slot.it Gears Pitch Table and you will be able to identify the best gear combo to run with your setup.
Variable pitch gears are common, Slot.it is not the only manufacturer to design the gears this way. The Slot.it Gears Pitch Table was created simply because I needed the information for their products.
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Ary,
Thank you for sharing this valuable information. I never considered there would be gear compatibility issues with Slot.It gears. It does make perfect sense that you cannot maintain a pinion pitch by varying tooth count in a same diameter gear. If you machine the crown hub to eliminate the ridge, it would be possible to run a 10 tooth 6.5mm pinion with any Slot.It crown gear, but since gear modification is not legal, we couldn’t race with that combo.
Jim J.
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