The RC Car Gear Ratio Explained and Calculated

For every mating pair of gears in an RC car, there exists a gear ratio. The gear ratio is simply the number of teeth of one gear divided by the number of teeth of a mating gear. Gear ratios are important as they allow us to better control speed and torque. We are able to sacrifice speed in order to benefit from increased torque.

This relationship allows us to achieve optimal output RPM by adjusting the gear ratio. Another point worth noting, it is much easier to extract power from a high RPM brushless motor vs a slower turning motor.  It is all made possible by utilizing the gear ratio.

How Gear Ratio is Calculated

The gear ratio is calculated using the number of teeth on a gear. The other option instead of using the number of teeth can be using the overall diameter of the gear. Either way will work, for simplicity and consistency, we will use the number of teeth as our preferred method. Watch the video for a more visual demonstration.

Determine the number of teeth on the two meshing gears that you are calculating the ratio between. Use the number of teeth on the spur gear  and divide by the number of teeth on the pinion gear.  The resulting value will more than likely be greater than 1.0. A good way to always remember this relationship is dividing the Output gear by the Input gear.

Multiple Gear Sets and Ratios

Gear ratios can be confusing when there are multiple sets being used. In the example below, the output shaft of gear set number one is connected to the input shaft of gear set number two. The gear ratio is determined in each set first. The above method is used to calculate the ratio in each set. Next, the calculated gear ratios for each set are then multiplied together. The resulting value is the gear ratio for the entire system.

2 Meshing Sets of Gears - Output of Set one Connected to Input of Set 2
2 Meshing Sets of Gears – Output of Set one Connected to Input of Set 2

You may also use the RCI gear ratio calculator.

Determine Torque Multiplication from a gear Ratio

Torque multiplication always occurs when there are two meshing gears of different number of teeth. The torque multiplication can be calculated directly based off of the gear ratio. As an example, we will use a gear ratio of 10:1. For every 10 turns of the input shaft, the output shaft turns once. We can use this exact number and multiply it by the amount of torque the motor produces. For example, if we say the brushless motor can deliver 0.05 ft-lbs of torque at the input shaft, we would expect 10 times this at the output shaft. The resulting value would be 0.50 ft-lbs of torque at the output shaft.

Putting it together – Application of Gear Ratio Sets in RC

In many RC vehicles, multiple sets of gears are used. Quite commonly, one set will almost always be found directly on the motor itself. The pinion gear on the motor is the first gear that is in a set. The next set of gears in a transmission is typically the differential on a shaft driven drive train. The output shaft going to either the front or rear differential contains a pinion gear. This pinion gear mates with the differentials ring gear, that ultimately drives the differential. In order to calculate the gear ratio, the Motor pinion gear mated to the spur gear must have the ratio calculate first. Next, calculate the gear ratio of the differential setup. Lastly, take the 2 resulting gear ratios and multiply them together.

The total value that you have as a result is what would be considered as your final drive ratio. For every one turn of the output shaft leading to a tire, the final drive ratio represents how many times the pinion gear on the motor shaft must turn. You could then use this value to determine the total amount of output torque produced as long as you know the total input torque.

Why use a Cap Pack on an RC Car? (Capacitor Pack)

A look at a cap pack.

A Capacitor pack is a fancy name for a bank of capacitors. That is a bunch of capacitors that are electrically wired together to form what we would know as a pack. Each capacitor is wired in parallel to each other. The idea is to place the cap pack as close as possible to the ESC on the battery to ESC wires. The purpose is to smooth out any voltage dips that occur during operation.

What a Cap Pack will NOT help / Capacitor Boost

You may have heard that a Capacitor bank helps with the performance of a Radio Controlled vehicle. This is one area that we will need to set straight right away. Capacitor Boost, ya, not really a thing. Cap packs are not installed to increase the top speed of our RC car. They will also not help to increase the acceleration we can achieve out of our car. Essentially, cap packs do not store nearly enough energy in order to discharge this to the motor increasing power potential. In fact they would not store enough energy to operate the Radio Controlled vehicle for even a split second. For this reason we should not expect any improvements to RC car performance.

Purpose of a Cap Pack (Capacitor Pack)

When an ESC is powering a brushless motor, the ESC must turn on and off power to each of the motors winding’s. The main purpose of a capacitor is to fill the void in voltage as the ESC is switching the motor on and off. An example of where the voltage void comes from would be when the battery is under load. As the battery is under load, the voltage tends to drop creating a voltage drop that could be read across the ESC. As the ESC turns a winding in the motor off, the battery unloads and causes a potential dip and spike in voltage. It is possible during this spike that the voltage can be higher than the source voltage. The capacitor bank is able to fill in the void in order to maintain a more constant or steady voltage across the ESC power input. Now let’s look at which RC car can most benefit from more capacitors!

Does my RC Car need a Capacitor Pack

RC Cars that pull a lot of power and are pushed very hard for short duration’s of time tend to be the offenders that are hard on ESC’s. It doesn’t matter if your ESC is rated for 200A and you are only using 100A of that current handling capacity. What is critical is as we noted above, the cyclic voltage drop that occurs across the input side of an ESC. Using a Cap pack in these high demand applications can only help your ESC by reducing the variance in voltage.

Examples of a few RC Car applications that would benefit the use of a cap pack include:

  • RC Drag Car
  • RC Speed Car (Car specifically designed and raced for achieving highest maximum speed)
  • A vehicle that has been geared for a high rate of speed similar to above

What Cap Bank do I Require? (Capacitor Pack)

There is no magic solution as to the exact specifications you require in your application. What we do know, is that a Cap pack, will not harm your RC vehicle in anyway other than to simply occupy space physically. Therefore, it is recommended to choose a Cap pack that has a minimum of 800μF.

When running your RC Car, keep an eye on the temperature that the capacitors are reaching. Higher temperatures indicate that the capacitors are being taxed more significantly. The cooler the temperatures are, the better off your components will be. The result would be prolonged lifespan of your ESC.

To look at the installation details of a Cap pack, visit the Cap Pack installation part of the extending ESC wires page.

Where to use Digital vs Analog Servos in RC Vehicles

In the previous article, we covered the differences between the Analog and Digital servo. In this article we are taking that a step further and we are going to look at specific applications.

Applications Best Suited for Digital Servos

Digital servos are best used in an application that requires best performance. Two factors must also be kept in mind. Digital servos will not be the cheapest option and will also be power hungry. Make certain that your budget can allow for a digital servo and that you have enough power from an ESC or battery to supply the digital servo.

Primary reasons you would select a digital servo over an analog servo are for maximum holding torque, smooth torque delivery, and fast servo response time. Digital servos are best used for primary control functions that allow an RC vehicle to change direction. Here is a list of applications that would benefit from the advantages a digital servo has to offer.

  • RC Car Steering Servo
  • RC Airplane Flight Control Surfaces
  • RC Helicopter Swashplate Controls
  • RC Boat Steering Servo
  • RC Boat/Car Throttle Servo (Brake function on the Car as well)

Applications Best Suited for Analog Servos

Analog servos are a cost effective option where performance is not required. Below is a list of applications where Analog servos would work well.

  • RC Car Transmission Gear Selection Servo
  • Differential Case Locked vs Open Selection Servo
  • RC Airplane Gear Door servo

You are probably wondering at this point about all primary functions for RC applications being best suited for a Digital servo. When considering performance this is absolutely true, however, it does not mean that Analog servos don’t work. In fact there are many applications that use Analog servos on primary controls.

Analog servos can be used for primary controls for smaller scale radio controlled vehicles. It is not recommended to use analog servos on larger scale vehicles simply from a safety stand point.

The Difference between Analog and Digital RC Servos

If you’ve had to ever purchase a servo or replacement servo for your application, you may have stumbled in to these terms. What is the difference between an analog servo and a digital servo?

Analog vs Digital Servos
Analog vs Digital Servos

If I were to grab a bunch of digital and analog servos, ripped the labels off and asked for you to hand me only the digital servo’s, it would be very challenging. The reason is, there really isn’t a difference between digital and analog servos on the outside. In fact, there really isn’t much of a difference on the inside either. Both types of servos contain the same gear train, same 3 wire lead to the Rx, motor, case, and even potentiometer to determine the servos position!

Where the real difference lies is within the circuit board found in the servo. In a servo the signal sent from the receiver to the servo gets process by the circuit board. From here the signal is converted in to power pulses that are sent to the motor inside the servo.

The Analog RC Servo

The circuit board in an analog servo receives the signal from the receiver and then outputs a power signal to the servo motor. The frequency at which the output occurs is 50hz. This means in one second an analog servo would only apply an output every 20 milliseconds. It may seem like a very short period of time, however, 20 milliseconds (0.02 seconds) is a long time in any control system.

In order to create movement of the servo arm, the controller would apply full voltage to the servo motor. If there is little servo arm motion required, the circuit would turn the motor on only for a split second within this 20 millisecond period. However, if the servo arm would have to rotate a significant amount based on control input from your transmitter, the servo would apply a longer duration on time to accomplish this. The on time during each cycle is what determines the amount of power that the servo may actually output. Below is a list of advantages and disadvantages for an analog servo.

Analog Servo AdvantagesAnalog Servo Disadvantages
InexpensiveSlow Response
Low Power ConsumptionLarge Deadband Zone
Low Frequency Audible NoiseWeak Holding Torque
Poor Resolution

The Digital RC Servo

A digital servo receives a signal from the receiver onboard the RC vehicle and translates this in to a pulse sent to the servo motor. The rate at which pulses of power are sent to the servo motor are much higher. You can expect that a digital servo would send a pulse to the servo motor at a rate of 500hz. This is equivalent to every 0.002 seconds. The delay in time before the next pulse of power is much better than an analog servo. It is this very reason that provides a digital servo with many advantages. Below is a list of advantages and disadvantages for a digital servo.

Digital Servo Advantages Digital Servo Disadvantages
Fast ResponseExpensive
Excellent Holding PowerHigher Power Consumption
Minimized Deadband ZoneHigh Frequency Audible Noise
Excellent Resolution

Why do Digital Servos Whine?

Is it ok if my digital servo makes noise when there is no control input or external forces acting on the servo? The quick answer to this question is yes. It is quite normal for a digital servo to sing to you. The reason for this is because of the high frequency pulses that get sent to the motor within the servo. You are in fact hearing these high frequency pulses that the motor is receiving.

In the next article, we will look at the application of digital vs analog servos. We will also make some interesting comparisons of specifications.

5 Reasons why the Spektrum iX12 is a Great Radio

iX12

I’ve owned the Spektrum iX12 now for well over a year. After tens of hours of ownership and the radio being bound to several models, there are many features and advantages of this radio that stands out. Just for the record, I bought this radio with my own money, and the thoughts shared here are of my own opinion.

#1 – First Air Radio from Spektrum to Include a Tablet

I’ve been waiting for this for quite some time. Many manufactures of a variety of products are beginning to utilize the Android operating system.  Not that I want to go on the internet or play mobile games on my transmitter. There’s more than enough devices that allow for that. What I was looking for is primarily an option to be able to do more with my radio. Options that were important to me at the time of purchase were programmable voice read outs and alerts, customization, more 3 position switches than I previously had, reliability and a decent amount of channels. This is exactly part of what the IX12 has to offer.

#2 – Highly Customizable Menus and Options

As mentioned above, one prime reason that I wanted to purchase the iX12 was due to the menu’s that are built in to the app installed on the tablet in the radio. All toggle switches, buttons and sliders built in to the iX12 are completely programmable. Any voice read outs that you need to be tied in with functionality on the radio such as RX voltage or model GPS speed can be pulled using macros. This allows you to customize the voice read outs / alerts to suit your requirements. 

You can have a pre-flight check list that the radio will force you to go through based on your own setup. The check list functionality is fully customizable. You may also setup the flight check list to be certain switches are in the correct position.

#3 – Spektrum IX12 has the same great feel as the DX18

Although the DX18 and the iX12 appear as different radios, they contain many of the same lines. The iX12 feels very comfortable in your hands. Even with the included Lithium battery pack and tablet, the radio does not feel heavy. Button and switch positioning on the iX12 feels natural, and buttons have great response. I have not owned a DX18 but I do know when you have both radios in your hand, they both feel great. 

#4 – Spektrum IX12 is built on a Reliable Platform

The very first question I had before purchasing the radio was, what happens if the tablet quits while flying? This would be a very common question coming from anyone who has used this technology before. Tablets are running quite a bit of software in order to function as they do. If you’ve owned a tablet, you know that they do crash from time to time. The tablet crashing should not allow your model to crash.

Obviously Spektrum has thought of this, and makes certain that you can operate your model without needing the tablet. You may lose your custom voice message when a switch is activating but you won’t lose your model aircraft.

Interference with the Spektrum iX12

Nope, none here. I am easily over 200 flights just on one airplane alone.  Not once have I experienced a glitch or interference where I could not correctly control the model.  This is true for the 10 years or so that I have owned Spektrum radios and not just on the iX12.

I do have one crash on this radio. I was fighting the aircraft for about a minute until my thought process ran out of CPU power and the plane went in. After walking up to the model, it was apparent that a control surface was stuck. After further investigation it has been determined that a Y servo extention was not providing a reliable connection between the rx and the 2 elevator surfaces. Not iX12 related.

#5 – Easy intuitive menus on the Tablet and App

This one is easy to explain. You have to keep in mind that there are so many more features on this radio when compared with any other Spektrum radio for air models.  Even with all the additional stuff crammed in to the iX12, the menus feel very intuitive. What probably helps me is I’ve owned an Android based operating system on my cell phone for many years prior to owning the iX12. If you can work your way around a tablet or cell phone, working the iX12 will be a breeze.

Coming from the typical Spektrum radios of the past with a roller button and no android experience, will be a change of pace for sure. Once you are inside the AirWare software app installed on the device, learning where to access all the key information and model data will be learnt fairly quickly.

You no longer have to use a scroll wheel to scroll through a bunch of options you don’t need just to get to the option that you are looking for. Simply open up a menu and select the option that you are looking for. Navigating through the menu options is accomplished by touch screen functionality only.