Originally Posted by xniperliams
Thanks for the info. But doesn't more teeth making contact means more friction too?
Also, how do you determine what gear ratio for certain motor? Do you use the turn measurement and calculate it in some way? If you could give an example, I would really appreciate it.
Thank you again for the enlightenment.
In theory, maybe (but a very slim maybe).
Consider this. You are trying to push your (real) car. Use both hands, then use just one finger. Which has more friction? You put the same amount of force but it is distributed differently when you have a larger contact area. Friction depends both on force and contact area. When you have a larger contact area, you have less force on the same unit of area, so it is not clear if in the end there is more friction or not. What is clear though is that when you have one tooth (let's say for the sake of argument) in mesh, you have to put all your force on that tooth which means you have to have a very tough pinion/spur whereas when you have a number of teeth engaged you distribute the force amongst them.
There are other ways to minimise friction, for instance using special materials with intrinsic low friction coefficients like delrin, teflon, duraluminium, special coatings, change the tooth prifile, etc. each with its own application.
Besides all that, if you don't get the gear mesh play (backlash for americans) properly adjusted you will cause more friction than you are trying to avoid by choosing one size spur over another by entire orders of magnitude!
One more word of caution. Kinetic momentum is one very significant difference between spurs of different sizes. Think of them as flywheels. A large one will spool up slower and slow down slower. A small one it's the other way around. Kinetic momentum differences are large because they depend on mass distribution and are proportional to the square of the radius, so the value goes up quickly. There are ways to go around this, by using spurs drilled so most of the mass is concentrated at a smaller radius or spurs that have narrower edges and thicker cores, etc.
To choose the right gear
ratio (not diameter
ratio!) spur/pinion is a function of motor, internal ratio of your car and track where the car is used. Check with the guys you are racing against (ask for either FDR or rollout) and that should get you in the ballpark. Keep an eye on motor temperature to tell if you're too high geared and speed to tell if you're too low. There are all sorts of rollout/FDR calculators on and off line you can use then to explore the possibilities your car offers before diving in and buying pinions and spurs ad nauseam. Or you can make your own excel spreadsheet, it is that easy.
I like to use 64 pitch gearing so I start by measuring the diameter of the spur that came with the car (if it is not 64 already) and find a 64 pitch spur close to that. This ensures I am not going to have trouble with the adjustment available on the motor mount. Only after that I choose the pinion as explained above (car internal ratio, motor, track)