Gears: Pinions and Spurs

This article is going to mainly pertain to ground vehicles, nitro or electric. We’re going to go through the gearing of these drive trains and figure out what all the numbers mean. You’ll learn the difference between a pinion and a spur gear, the important characteristics of gears you’ll need to know, and you’ll learn how to choose the right gears to get more speed or more torque out of your car or truck. Most of the concepts are easy to get on this subject, especially if you’ve ever ridden a 10-speed bike before. The same gear principles apply to your R/C vehicle.

It doesn’t matter if you’ve got a nitro vehicle or an electric one, the principles of gearing apply to both in the same way, the gears are just different sizes, that’s all. The biggest difference between the two is that sometimes nitro vehicles have two speed transmissions. In either case, however, the gear that’s mounted on the end of the motor or engine is called the pinion gear. It’s the smaller one that drives the bigger gear called the spur gear. Most cars or trucks are sold with gear sets that will give you average speed and average amounts of torque. Options are usually available up or down with both the pinion and the spur gear to give you a number of variations of gearing.

Two of the characteristics of every gear that you need to be aware of if you are going to make changes to your vehicle are the number of teeth, and the pitch of the teeth on the gear. The number of teeth is easy to figure out – you simply count them. Sometimes this number is stamped on the side of the gear: for example 52T for fifty-two teeth. The more teeth a gear has, the bigger the gear. The pitch of a gear in layman’s terms refers to the size and distance between any two adjacent teeth of the gear. This is important to know before you go changing one of the gears, because one will have to mesh properly with the other. Be aware that you cannot mix gears of different pitch sizes. If you do, you will end up stripping the teeth from one or both of the gears and your vehicle will immediately go nowhere. Standard pitch sizes in R/C are 32P, 48P and 64P.

Before you go and start playing with gears to find more speed, you should know what gear ratio is and how to calculate it. It’s really a simple concept actually. Gear ratio is the ratio of teeth between the pinion and the spur gears. Say for instance you have a 24T pinion and a 72T spur. Your gear ratio would then be 24:72 which is reduced to 1:3. That means that your pinion gear is going to rotate three times for every revolution of the spur gear. In other words, you will get one third the rotations out of the spur gear as you will the pinion. So if your motor is running at 24,000 RPM, your spur gear will move the transmission at 8,000 RPM. This process of reducing speed through gears is known as gear reduction. This is necessary because motors and engines operate at speeds that are too high to work efficiently in any drive train. There simply is too much speed and not enough torque.

OK, now you’re ready to get some more speed out of your car or truck, but how do you change the gears to do it? Let’s go back to our example. We want to change the spur gear to get more top end speed, so the spur gear will have to rotate faster. A larger gear with more teeth will make the gear ratio larger, causing the spur to move slower, so that won’t work. We have to go the other way. You want a smaller spur gear with less teeth for more speed. Take a spur gear with 70T to our 24T pinion and you get a gear ratio of 24:70 which reduces to 1:2.92. If the same motor runs at 24,000 RPM you will get the spur moving now at 8219 RPM. Amazing, with only two less teeth, we got an additional 219 RPM, or 3.65 revolutions per second. That’s cool, but can’t we do better? Sure we can. Let’s now change the pinion gear. To get more speed, we do the opposite of what we did with the spur gear. You want a bigger pinion with more teeth to get that gear ratio where we want it. Let’s go with 26T pinion and the 70T spur. Now our gear ratio is 26:70 which is reduced to 1:2.692. At 24,000 RPM on the motor, you get 8915 RPM on the spur gear. Just by changing each gear by two teeth, we get an additional 915 RPM or 15.25 revolutions per second.

Remember that it doesn’t matter whether you’re running electric or nitro powered vehicles, these calculations are the same for both. If you ever get stuck in figuring out if you need a larger or smaller gear size, just think of riding a 10-speed bike. (In this case though, your “pinion” is actually on the rear wheel, not the pedals.) You shift gears to get more speed and when you do, the two gears approach the same size. When the two gears are the most opposite, large and small, you get slower speeds but more torque, meaning you can take off easier from a stand still. This same principle applies to any gear system, including R/C. So now you’re ready to leave the competition in the dust, right? Installing gears on cars or trucks is not always easy to do, but you will notice a difference in performance as we have mentioned above. Next time, we’ll look at the differences between the radio control systems available on the market. Until then, go out, have fun and please support your local hobby shop.

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