RC (Radio Controlled) cars have come a long way since their inception and are now more sophisticated and technologically advanced than ever before. Many enthusiasts are always looking to fine-tune their vehicles for maximum performance on the track. This is to of course remain competitive against others. One of the key components that can greatly affect the performance of an RC car is the droop screw.

In this post, we will explore the function of RC car droop screws, how they work, and why they are important in fine-tuning the suspension setup of an RC car. We will also discuss how to adjust the droop screws to optimize the performance of your RC car on the track.

What are RC Car Droop Screws?

Droop screws are simply fasteners that are installed on the chassis plate of the RC Car and contact the suspension arms. They control the amount of suspension compression when the car is under its own weight. They really are essentially limit screws that restrict the amount of downward travel of the suspension arms.

How do Droop Screws Work?

Droop screws work by simply adjusting them to suit the desired performance required. The fasteners are generally situated on the metal chassis of the car. the body of the screw then comes out the other side of the chassis plate to make contact with the control arms of the RC vehicle. When the screws contact the arms, this is where the limitation in travel is established. One may drive a droop screw further in to limit the amount of downward travel. Or, on the other hand, turning the screw out to provide maximum downward travel.

Benefits of Droop Screws

Restricting Travel

Restricting suspension travel would be more common for RC on road vehicles. This would allow the RC Car to have a stiffer suspension setup when the car is in its natural resting position. Driving the droop screw further in would then further lower the car and stiffen up that initial suspension travel from the natural resting position. Stiffer suspension decreases body roll while cornering, accelerating or braking.

Pros: Stiffer Suspension for on road racing. Lowered appearance or stance.

Cons: minimized grip when the car travels over a bump. Limited Suspension Travel

Un-Restricting Travel

Adjusting the droop screw out so that you get maximum travel, decreases the amount of pre-load on the springs. In this configuration, the natural resting spot in the suspension is not obtained until the RC Car is completely under its own weight. This way the cars weight acts as a force that settles in and matches the force created by the springs in the suspension. As a result, If the car is lifted up slightly, the tires will remain in contact with the ground. This setting is perfect for off-road vehicles and is absolutely critical for rock crawlers where the tire and suspension requires maximum travel.

Pros: Maximum grip when a car travels over a small or large bump in the surface. Maximum Suspension Travel.

Cons: Can not easily lower the car using Droop Screws. Can not pre-load the suspension increasing the amount of spring force when the car is in its natural resting position.

How to Adjust RC Car Droop Screws

To adjust the droop screw, you will need a droop gauge and a droop screw driver. The droop gauge can really be anything that allows you to understand the amount of droop your car has. Consider a ruler for simplicity. There are fancy gadgets and blocks out there, but you really don’t need these. More detail on how a ruler will be used below. The droop screw driver is generally a standard screw driver that is used to turn the screw in or out. This is to increase or decrease the suspension limit.

To adjust the droop screw, follow these steps:

1. Measure the Fully Extended Height: Start by measuring the distance between the bottom of the chassis and the ground. This measurement is taken with the car on a level surface and without any weight on the suspension. To do this, the cars weight must be fully supported by something other then its own suspension. Example: use blocks under the chassis plate, positioned at a height to allow the tires to barely touch the ground.
2. Measure the droop: Next, grab your trusty measuring device and measure the amount of compression the suspension undergoes when the RC car is now under its own weight. Measure the same specification as in step 1, but the only difference is blocks are not supporting the car, the car is supported under its own weight. Pro Tip – With the car under its own weight push on the car a few times compressing the suspension and allowing the suspension to lift the car back up in to its normal or natural resting position.
3. Adjust the droop screw: Based on the measurements taken in steps 1 and 2, adjust the droop screw as needed to achieve the desired amount of droop. Turning the screw in will decrease droop, while turning the screw out will increase the amount of droop.
4. Re-measure and Confirm: After adjusting the droop screw, re-measure the ride height of the car vs droop. If the desired amount of droop or the desired ride height has been achieved, you are good to go. If not, continue to adjust the droop screws and re-measure.

It is recommended to make small adjustments at a time. Re-test the car after each adjustment to ensure that you are making progress to your desired result.

I don’t know what it is but there’s something in me that will just not allow anything to remain in stock form. This includes the LEGO Passenger Train. The LEGO Train is a great toy for younger kids really exposing them to many different useful life skills. They learn to read instructions and convert this to actions. Toys that provide this type of learning, is always something that is perfect in my books. What does this mean? We need to get one. Just blame it on the kids, you know you will love it too!

Why Convert the LEGO Passenger Train to RC?

It’s quite simple actually. Many hobbyists have parts in their bins that aren’t being used. The LEGO Train is a perfect candidate to parts that are otherwise just collecting dust. More specifically, converting the Passenger Train over to RC will unlock so much potential. The LEGO factory controls called “Powered-Up” have limited speeds that the Train can be operated at. The level of precise control is lacking. Upgrading the unit to Hobby Grade RC equipment can provide many benefits in this regard.

What Features will RC Equipment Provide?

As mentioned above, the biggest advantage of RC equipment is the full range of throttle. In other words, we will have the ability to input a throttle command between 0-100% resulting in virtually limitless control. Our train will have the potential to apply brakes. Imagine how useful this can be when you would like to travel down along a negatively sloping section of track. Brakes will allow the train to come to a full stop. In addition, the train will continue to have a reverse function, however, the function will have 0-100% throttle in the reverse direction. This is unlike the stock train that has a limited number of steps.

Most Cool RC Train Feature

The feature that I am most excited about is the ability to program in “momentum.” Rather then allowing the train to accelerate instantaneously, a delay can be setup to mimic the train having a lot of weight. This will help for two reasons. First reason, well, it’s more realistic while accelerating and decelerating. Second reason is without the delay or some control, the train would be fed enough power to do a burnout down a 6 foot section of track before finally hooking up and getting to top speed. To remove this as it could be a costly wear item, simply just program in a throttle delay.

Other Small Features

Other features such as limiting the potential top speed will be easy to do. For those of you that know RC fairly well, this could easily be done with EPA’s of the throttle channel.

I will be adding a timer that will display how long the train has been power for that specific run. Everytime the train is stopped, the timer will also stop. For every minutes there will be an audible call-out. Having a throttle kill switch or an “arming” switch is possible but totally not needed. In fact most of all of this is not needed but that’s not at all the point.

Features that could be added to the LEGO Train Soon

There are a bunch of features that could be added later. This includes telemetry data as an example. Imagine having the ability to take a look at the voltage of the train when it is underway. Better yet, upon certain levels of voltage, the percentage of battery remaining is called out. This can quite easily be setup with the right hardware. Even telemetry that monitors speed, location and direction can be added. Many more options similar in nature are possible.

Don’t forget to watch the video of this RC Lego Train

Hardcore RC car enthusiasts are always looking to push the envelope. A big part of doing this is understanding what is happening within the power system. Without this level of understanding it is very easy to get yourself in trouble and that’s when your power system starts smoking. Something to avoid at all costs!

The largest challenge in RC is how can we understand or better predict the amount of current that our cars need? Therefore, we are going to discuss why it is difficult to predict how much current an RC car motor will draw. Also, why is this more difficult than predicting current draw for RC airplanes.

The three main components in an RC Car

To demystify the mechanics behind the current pulled, let’s introduce power system components and their respective specifications. This will serve as an example for our RC car.

LiPo Battery

First off, the LiPo battery (lithium-polymer battery) has its specifications typically marked on the front of the battery. In our example consider the specification of 77A (2.2Ah x 35C continuous rated). This 77A figure identifies the amount of current that the battery can generate.

Electronic Speed Control

The electronic speed control (ESC) has its rating also marked right on the front of it. In our example, let’s consider a rating of 60 amps. This is the maximum continuous current that it can support.

Brushless Motor

Lastly, the brushless motor has it’s specification accessible from the manufacture’s spec sheet. Consider our example to be able to deliver approximately 75 amps of current.

What do these Specifications Mean?

This is the exact question, what significance do these specs hold in terms of determining how much power this motor can draw?

A clear and concise answer to this: None.   

Despite these specifications, we are clueless as to how much power our brushless motor is going to draw.

Not that it is completely impossible to determine how much power brushless motors can draw. We can check that with a dynamometer for measuring mechanical power generated. We can gauge the amount of power that the motors are generating, the current being drawn and the amount of electrical or mechanical power being consumed or given out with the dyno.

With these values, we can depict the amount of current that the motor drew for a specific load of the motor.

Doesn’t this sound so easy? Nope, not at all.

Load on a Brushless Motor

The load refers to the amount of current drawn and is arguably an important part of the system. The actual current that the motor will pull comes from the Io value. (no-load value) Essentially the current drawn at zero load. Simply put, the Io value is a waste of energy since you won’t get any mechanical torque from the motor.

Interestingly, once the motor is loaded, (using a propeller) the current value starts increasing past the Io value. Which is about 2-3 amps for the motor used here. Then we increase the load further by placing a larger propeller onto our motor and skyrocketing the current up to 30-40 amps. Remember that the bigger load is represented by the bigger propeller used.

The point we are trying to make here is that as you increase the load on your motor, without interrupting anything else, the torque output of the motor must increase. The load is quantified as mechanical torque required by the motor. The translation from mechanical to electrical is torque being equal to current drawn. As torque output of the motor increases, the motor will draw an increasing amount of current.  

The Difficulty in Determining Torque / Current for RC Cars

If life were simple, we would be easily able to predict the torque output of a motor. But, that’s not how it goes. There are so many factors or variables to consider that make up the torque requirement and thus current draw.

How about an online calculator? Although there are online calculators for RC airplanes or EDF jets, there is no such calculator for an RC car.

Why?

Let’s Discuss Influencing Factors

Because there are more factors involved in this type of situation than one can imagine. For instance, consider placing a brushless motor in an RC car and drive it at constant speed. There would be a significant difference in the power required to drive at constant speed vs accelerating up to that speed. Even if the RC enthusiast were to accelerate lightly vs aggressively, there would be a considerable difference in the amount of current being pulled[RT1] .

Similarly, if you opt for a larger tire, the same effect would be noticed. When we alter the RC car, by increasing load, we are asking the motor do more work. Even if we don’t alter the RC car, and drive it on grass, we would notice that the grass would cause the motor to load up. This is due to the friction between the tires and surface, causing the torque to rise.

Another example that can influence torque required is a strong headwind or tailwind. The greater the headwind speeds, the greater the amount of power required to penetrate the wind. The amount of current drawn is very difficult to estimate.

After all of this where do we go from here?

There are a couple means of determining how much current your system will draw. The best way is to place a datalogger within your RC Car and record the amount of current your system pulls. The big down side to this, is of course, it’s not a prediction. You at this point would already have all the components.

To make the best prediction, we recommend doing a bit of research on the internet. There are more than likely many RC enthusiasts out there with the same RC vehicle as you and based on their power system and results, you can gather data and expectations. If you can’t find someone who has the same power system as you, choose the next best thing. That is the most similar power system you can find.

Key Takeaway

First, the specifications of your RC car’s motor won’t tell you anything about how much current it will pull.

Secondly, the load that you place on your motor, determines how much power or current the motor will essentially draw. Thirdly, the load equates to the amount of mechanical torque required by the RC car.

Lastly, it is possible to predict the current drawn by utilizing someone else’s experience. Find another system out there that most closely matches the one you plan to use.

We hope this was an informative guide for you to understand these challenges and how to better overcome them. Be sure to check out the YouTube video here for more detail.