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  1. #1
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    Seems like this will be classified as a dumb question

    Hey all I own a traxxas 1/10 Ford fiesta rally, I'm running the venellion combo with a center diff and a few other upgrades. I got into this hobby in December so please keep that in mind, my question is this. If I am wanting to do more gymkhana style drifting and driving, on road and off, do I need a bigger (more teeth) spur and smaller (less teeth) pinion? The reason I ask is for everything I've read I'm still extremely fuzzy on the spur, pinion thing. Also can anyone translate pitch in regards to what it does for the car? I've read several different comments that say it doesn't matter about Spur and pinon as long as you have the correct pitch?

  2. #2
    RC Qualifier Sp2deSummit's Avatar
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    Here's a thread about pitch: https://forums.traxxas.com/showthrea...ch-vs-32-pitch

    If you have a bigger spur, your top end speed will go down, but you will get better acceleration/torque and less motor heat. A smaller spur will give you more top speed, but less acceleration and torque, and more motor heat.

    The same thing goes for the pinion, just the other way around. Smaller pinion = less speed but more torque/acceleration and less heat. Bigger pinion = more top speed but less torque/acceleration and more motor heat.
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  3. #3
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    Pitch refers to the size of each tooth on the gear. For your gears to fit together properly, they have to be the same pitch.

    In the non-metric system, pitch refers to the number of teeth it takes to get around a 1 inch diameter circle. So a 32 tooth, 32 pitch gear is one inch in diameter, and a 44 tooth, 44 pitch gear is one inch in diameter. On the 44 pitch gear, each of those 44 teeth is smaller.

    Pick a gear pitch and stick to it. I believe that your Rally came stock with 32p gears, so when you select a pinion or spur, just make sure that it is 32p and its teeth will be compatible with your stock gears.

    What really matters about pinions and spurs is the ratio of the two tooth counts. The motor turns the pinion, the pinion turns the spur, and the spur (through the slipper and differentials) turns the wheels.

    Your pinion, the one on your motor - turns your spur, the one in your transmission.

    The pinion is fixed to the output shaft of the motor, so for one revolution of the motor, the pinion turns one time, which means it turns x teeth, where x is the number of teeth on the pinion. Each time the pinion turns by one tooth, it turns the spur in the opposite direction by one tooth.

    So a larger tooth count on the pinion will cause the spur to travel farther around its circumference (a larger portion of a single revolution of the spur) each time the motor turns. This means a larger pinion will cause the truck to be faster (each time the motor turns, the larger pinion results in the spur traveling around by more teeth). But turning the spur faster requires more power (volts x amps), and since the voltage of the battery is constant, that means more current (amps). That results in more heat in the motor and ESC.

    Big pinion (gear on the motor) = more top speed, more heat.
    Small pinion (gear on the motor) = less top speed, less heat.

    Now think about the spur. Holding the pinion tooth count constant, each time the motor shaft rotates the pinion turns the spur by x teeth. Let say the spur has y teeth, and to keep it simple, let's say that y=4x. So our hypothetical gearing might be 15/60. Each time the motor rotates, that 15 tooth pinion turns the spur by 15 teeth, which in this example is a quarter turn. Each time the motor rotates 4 turns, it turns the spur by 4x15 teeth, or 60 teeth, which is one revolution of the spur. This ratio is called the gear ratio, and it is what we use to manage top speed and heat.

    If we go up on the spur size from 60 to 65, when the motor turns 4 times, it still turns the pinion by 4x15 teeth, or 60 total teeth. So now our spur has not quite made it all the way around (it has 5 teeth left to go) in 4 motor revolutions. The motor has done less work (power). So it is drawing less current and will run cooler. However, since our motor is rated to do z revolutions per volt applied, we have less top speed, because each revolution of the motor translates to less rotation of the wheels.

    If we go down on the spur size from 60 to 55, when the motor turns 4 times, it still turns the pinion by 4x15 teeth, or 60 total teeth. So now our spur has gone around more than once - 5 teeth more than once. The motor has done more work, so it is drawing more current and will run hotter. Also, since our motor turns at z revolutions per volt applied, our truck has a higher top speed, because each revolution of the motor is turning the wheels more.

    You can use a larger pinion with the spur you have, and it will make the truck run faster and hotter. The only adjustment you will need to make is to set the gear mesh, and Traxxas support has a good video on that.

    You can use a smaller spur with the pinion you have, and it will make the truck run faster and hotter. The only adjustment you will need to make is to set the gear mesh, and Traxxas support has a good video on that.

    Within reason, you can choose your pinion and your spur tooth counts independently, being mindful only of how you are changing the ratio. When you change either or both of them, you will need to reset the gear mesh, but that is not hard.

    Remember, always make sure the spur and pinion you choose are the same gear pitch. The Slash 4x4 (and I assume your Rally would be the same but I do not know) comes with 32 pitch gears, which is the same thing as metric "0.8 mod."

  4. #4
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    A common source of confusion is that when people say "higher gearing" it is unclear what they mean.

    The pinion is the smaller of any two gears that mesh. In our application, a reasonable sized electric motor does not have the torque to push our vehicles without gear reduction, so the pinion is on the motor output shaft.

    An engineer would say your gear ratio is equal to pinion / spur. (Well, actually, an engineer would say those are both spurs, since a spur is a gear designed to transmit motion between parallel shafts, and that is what we are doing here, and one of the spurs is the pinion, since it is smaller, but that's not important right now.) So in my post above with a 15 tooth pinion (the name RCers use for the gear on the motor) and a 60 tooth spur (the name RCers use for the gear that meshes with the pinion), an engineer would say that you have a gear ratio of 15/60 = 0.25. Going to a larger, 20 tooth pinion gear, which makes the truck faster (because now the truck goes farther each time the motor turns), would result in a gear ratio of 20/60=0.33. Gear ratio up = faster truck.

    Unfortunately, it is not at all uncommon for people in RC to look at a 15 tooth pinion and a 60 tooth spur and say that they have a gear ratio of 60/15=4.0 (my pinion has to turn 4.0x for each time my spur turns). When they want to go faster, they put in a 20 tooth pinion (so their truck goes further for each motor revolution) and get a gear ratio of 60/20=3.0 (my pinion has to turn 3.0 times for my spur to turn once). So what these folks call their gear ratio went down, even though the top speed of the truck went up.

    Most of those same people would call the 20/60 setup "higher gearing" or say their truck is "geared higher" or "geared taller" despite the fact that their gear ratio (as they calculate it) went down compared to 15/60.

    If someone tosses out a decimal ratio less than 1.0, you know they are talking about pinion/spur. If someone tosses out a ratio greater than 1.0, you know they are talking about spur / pinion.

    If someone says you need taller or higher gearing, they mean you need a bigger pinion or smaller spur (or both). If someone says you need shorter or lower gearing, they mean you need a smaller pinion or bigger spur (or both).

    If someone says you need to go up or down on your gear ratio, unfortunately, you will have to ask them more questions, because different people mean different things when they use the term "gear ratio."

  5. #5
    RC Qualifier Sp2deSummit's Avatar
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    Wow! That makes my answer seem small...

    I realized that I never answered your question about changing your pinion/spur to suit your driving style. It depends, do you want to go faster or slower? If you want to have the tires break out more often try a smaller pinion/bigger spur at first, so there is more torque to overcome friction. Or if you want to go faster and accelerate slower (less tire break outs), get a bigger pinion/smaller spur. I hope this answers your question!
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  6. #6
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    Thank you so much for the responses, I think I finally have a good grasp now. I do have a follow up question, I am running 32 pitch 9 tooth pinion and 54 tooth spur, but I still seem to be getting a lot of motor heat, did I happen to go to far to the small extreme? if it matters I am also using a center diff with 20k weight Traxxas diff fluid and I did put 1 mil diff fluid in my rear diff.
    Last edited by xXxRomeo; 05-06-2019 at 02:19 PM.

  7. #7
    RC Qualifier Sp2deSummit's Avatar
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    You might've gotten too small a pinion, do you have another one you can try?
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