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Old 05-19-2009, 09:31 PM   #1
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Default Spur Gear size

Hi experts! I got a question regarding spur gear. What is the benefit of using a bigger one? In the end we have to use a bigger pinion to maintain the gear ratio anyway. So wouldn't that make no changes in the outcome? Please enlighten me, thank you.
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Old 05-19-2009, 09:51 PM   #2
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It's more about the motor placement within the ratio you need to run. My personal preference is to keep the motor as far forward as possible running off-road (T4 and B4). I can't speak for on-road cars.
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Old 05-19-2009, 09:53 PM   #3
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Hi experts! I got a question regarding spur gear. What is the benefit of using a bigger one? In the end we have to use a bigger pinion to maintain the gear ratio anyway. So wouldn't that make no changes in the outcome? Please enlighten me, thank you.
Using a bigger spur generally gives you smoother engagement, since there are more teeth making contact. This also tends to be quieter as well. In theory, also less wear.

However, for high turn brushless motors (17.5, for example), you might not be able to get to the correct gearing when using a larger spur, since you would need a REALLY big pinion. It might not be able to fit in your chassis.
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Old 05-19-2009, 10:09 PM   #4
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Big pinion, small spur more top end(more heat)..

Big spur, small pinion more toque. Less top end. (Less heat) less wear, quicker take off.

You'll wanna gear your car to the amount of turns of your motor and the size of the track.

Simple long straight away tracks you'll want low gear ratio.

Complex tight tracks you'll want more torque. You'll want a higher gear ratio.
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Old 05-19-2009, 10:10 PM   #5
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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.
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Old 05-19-2009, 10:17 PM   #6
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The smaller the spur and pinion the less friction between the two...........

Another characteristic is motor placement.

Also torque effect of larger spur with the surface speed of the pinion at lower RPM. (Some say it's myth)
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Old 05-19-2009, 11:29 PM   #7
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Originally Posted by xniperliams View Post
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)
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Old 05-19-2009, 11:45 PM   #8
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Your explanation about area of contact and force really make sense a lot I think. I missed that part out when I was analyzing this matter. A real enlightenment. Thank you big time for that.

And yeah the simple measurement I have been using is temperature. I would run the car for 5-10 minutes and check the temperature of the motor and ESC. This may not be the most accurate but I would have to learn more before I go into it more in depth. Cheers.
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Old 05-19-2009, 11:59 PM   #9
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That's good enough.
Some people go to the trouble of buying infrared temperature gauges and all that paraphernalia but I think if you just use your finger touch is close enough. Even when you start by using a temperature gauge, in the end you will build experience and be able to tell by touching if you're too hot. If you can't touch your motor after a few laps (you don't need five minutes, just five laps) for at least three seconds or some such, you're too hot. After a while you'll get your finger "calibrated"! Then again things start smelling funny when they get hot, hehehehe!

Speed will tell you when you're too slow (low geared).
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