I’m new to this sport and not only am I coming to terms with learning to drive, I’m also all at sea with understanding the underlying principles of Final Drive Ratio (FDR) and Transmission Drive Ratio (TDR), the different sizes of spur and pinion gears, etc. The list goes on and on. I feel I need to be a practising physics professor before I touch the RC car on the track and squeeze the trigger. But I also enjoy learning and understanding things and thought it might be worthwhile adding a section to the website where these things can be shared, discussed and maybe even understood. If you’re interested! If so, here is a noob’s perspective on how our 1/10th scale on-road touring car transmissions work. And I’d be more than happy for anyone with experience and a better understanding than I, chimes in to correct me along the way.

What are you torqueing about?

I set out specifically to understand what impacts changing spur and pinion gears have on a car’s performance. This is the focus of this specific discussion. It won’t go in to car setup such as camber, toe, droop etc (which may be a follow-on topic as I learn about those too!) but will concentrate on the specifics relating to gearing and how to calculate theoretical top speeds based on different ratio settings. Here goes ….

Transmission Drive Ratio (Internal gear ratio)

TDR is the ratio for how many revolutions it will take the centre gear (attached to the spur gear) to turn the differential pulley. I have a Destiny RX10-SR in the “stock” kit which comes with a 37T diff set containing 37 teeth. The Centre gear which connects to the differential pulley has 20 teeth. Effectively, this means the transmission ratio is 37/20 or 1.85.

This will vary between car makes and models. The Destiny RX10-SR for example comes in kits with a 38T and 40T differentials that alter the TDR to 1.9 and 2.0 respectively.

Pinion and Spurs (Driver gear ratio)

The pinion and spur gears collectively make up the driver gear ratio and the second half of the Final Drive Ratio equation listed below. It is calculated by dividing the number of teeth on the Spur gear by the number of teeth on the pinion gear. In the example diagram here, using a spur gear of 96 teeth (96T) and a pinion of 50T, the gear ratio is 1.92.

Final Drive Ratio (FDR)

FDR is calculated by multiplying the internal and driver gear ratios calculated above. They have a direct bearing on acceleration and top speed. In our Bendigo Spec Class competition, the ratio here has been set to 3.5 so all racers need to align a spur and pinion combination that directly aligns to this. It’s not common a ratio here is predetermined in racing classes as racers are usually free to determine a combination that meets their driving style or track conditions for acceleration and top speed.

Our Spec class however is designed for close competitive racing and by enforcing a FDR, it ensures all cars will have similar top speed and acceleration attributes.

Rollout

Rollout is the other attribute that can impact top speed. It is the measurement of how far the car will move forward for every turn of the motor. This is what we need to determine max speed calculations (to follow).

To calculate roll-out, we need to think all the way back to school maths and recall Pi; the calculation of a circle circumference. That value is 3.1415. Next, we need to know the tire diameter. New tires with no wear for my Destiny are 63mm.

Tire diameter * Pi (63 * 3.1415) = 197.9145

Spur / pinion x internal gear ratio (96/50 * 1.85) = 3.552 (or FDR)

Roll-out is the top value divided by the bottom value.

= 197.9145/3.552 = 55.72 mm per motor turn

So let’s now see what happens if the tires wear down 2 mm …

Tire diametre * Pi (61 * 3.1415) = 191.6315

Spur / pinion x internal gear ratio (96/50 * 1.85) = 3.552 (or FDR)

Roll-out = 191.63/3.552 = 53.95 mm per motor turn

This means a worn tire not only impacts traction, but also speed as the tyre diameter reduces and lessons the distance travelled for every turn on the motor. You can counter this by changing your spur/pinion gears but this will have other impacts as discussed below.

Putting it all together

So what do these values even mean? Changing this ratio through pinion / spur combinations impacts the power and top speed. A smaller tooth pinion gear will result in a higher FDR meaning the motor will turn the pinion more revolutions resulting in my torque and faster acceleration. Because the FDR is now higher though, the motor ends up needing to turn more times to turn the wheels due to the higher differential in the spur / pinion combination. So whilst increasing the FDR increases acceleration, it also decreases top speed.

Conversely, if you increase the pinion gear, you are making it closer in size to the spur and decreasing the FDR as a result. Just like riding a bike and trying to start out in a higher gear, it is more difficult to get started so acceleration is impacted. However, because the spur / pinion combination are more closely aligned, the motor does not have to work as hard and can attain a better top speed.

To summarise:

* Increasing tooth count on pinion or Decreasing the tooth count on spur will decrease FDR resulting in a decrease in acceleration but increase in top speed

* Decreasing tooth count on pinion or increasing the tooth count on spur will increase FDR resulting in an increase in acceleration but decrease in top speed

Experienced racers will alter the FDR based on track conditions for greater speed down long straights or for better acceleration for tracks with an emphasis on cornering.

Enough! Tell me how fast it will go!

Ok. So now we have our tyre diameter and all our ratios, we can work out our theoretical top speeds with one additional consideration; the motor. Motors are rated for a certain maximum Voltage which consequently limits the maximum RPM (Rounds Per Minute). With this in mind, here’s how we go about it.

Electric Motors are often rated with a KV number such as 2000 RPM/Volt DC. My Trinity 21T motor (2000KV) functions at 4.2V per cell (2 cells) which equates to 16800 revolutions per minute (4.2 * 2 * 2000 KV)

From this 16800 RPM value we divide it by the FDR (in this case 3.552). This value equates to how many times the wheels will turn in a minute; in this case 4730.

Next, we take the diameter of the tyre (63mm) and divide it by Pi (3.141), further divide it by 1000 (so we get the distance from millimetres to metres) and finally, multiple it by 60 to take the value from minutes to hours. This equates to:

0.63 / 3.141 / 1000 * 60 = 0.012

Finally, we take the 4730 * 0.012 and this is our kilometres per hour! 56kph

Let’s turn things up a notch and keep the same voltage, tyres and ratio’s but swap out the 21.5T motor for a 13.5T.

13.5T 3000 KV motor would be:

4.2 * 2 * 3000 KV = 25,200 RPM

25200 / 3.552 (same FDR) = 7094

7094 * 0.012 = 85kph theoretical maximum.

If all of that does your head in, there are some great gear ratio apps on the Google Play and Apple IOS stores that does all of this for you. I found that until I understood the concepts here, I struggled to even know what the apps were referring to for input values. So manually doing this here helped me in understanding the apps for future and easy calculations.

The Noob Summary

The irony for me is that this is all theoretical and doesn’t actually impact me as someone still learning to steer the damn thing around a track without crashing and braking parts. But what I have taken away from this is that there are so many ways to change things that it is daunting to know what changes where, would be the best place to start. In relation to the gearing considerations above, it can be simplified to lowering the FDR for tracks with long straights where you want speed over acceleration or increasing the FDR for tracks where you want greater acceleration out of corners on winding tracks.

I’ve found the exercise in trying to understand this and documenting it beneficial to me and I hope you take some value out of it as well.

This noob has a lot to learn!]]>

As touched on in my previous post about gears, the Newbie Corner is intended to be an information draw from a newcomer to the hobby to share my experiences (the newbie) with others so that you can hopefully learn from my mistakes! And this second post is about batteries and not-so-coincidentally, my biggest mistake to date.

I relied on others with experience to kit me up with my first car (now about 6 months old) including the motor, ESC and batteries. I didn’t really understand the figures listed on the batteries but had confidence in those who chose for me. What I didn’t understand, and was never really explained to me, was that these batteries need to retain a certain amount of charge in them or they will be severely impacted in future. I’ve already made that mistake and the 2 batteries I have purchased upfront in my initial foray in to the hobby, are now both depleted in their ability to perform optimally. I’m not going to replace them yet as they still function and I’m still learning in any case, so the impact on me is still negligible.

But I did want to understand where I went wrong and understand why letting the batteries fully discharge causes damage to them. Hopefully this post gives you some insights and a better understanding of what all these numbers on battery packs mean!

LiPo

Lithium Polymer. These are the types of batteries used in our RC car racing. But there appear to be a ridiculous number of variants by way of measurements / ratings on these suckers. My initial purchase was 2 x Team Zombie 5500 mAh 75C 7.4v batteries. Let’s start with what all of these numbers actually mean. And I’m going to start with the easiest one first.

Voltage

I say easiest as it is going to be consistent among all batteries we use in the on-road touring class cars. 7.4v. The batteries we use are commonly known as 2S which represents 2 cells in series format, meaning you combine the voltage of each cell together for the total voltage. Each cell, unsurprisingly is nominally 3.7v so the total nominal voltage rating is 2 x 3.7 or 7.4v as you will see on any 2S LiPo battery we use.

The nominal value is essentially the “resting” value that is used to benchmark and compare against different battery types but it isn’t the maximum capacity. Each cell will charge to 4.2v and conversely, the minimum safe state that you should not go under (as I did; refer opening remarks!) is 3.0v/cell. So the nominal measurement of 3.7v/cell is essentially the middle mark between the 4.2 (max) and 3.0 (min safe) values.

So what does that voltage value actually mean?

It is essentially the measurement of how fast your car will go. In the Spec class series, we are using a Hobbywing 3650 21.5T brushless motor. The 3650 motor is rated at 1750kv or, the ability to turn 1750 RPM for every volt powering it. This motor therefore, powered by a 2S battery providing 7.4v input, can provide 7.4 x 1750 (12,950 RPM) or, at maximum capacity 14,350 RPM (8.4 x 1750). All of our on-road racing classes dictate a 2S battery with 7.4v (common stock racing spec) so that the power provided to the cars are all equal.

Capacity

The 5500 mAh reference refers to the capacity of the battery, or, how much power the battery can hold. It’s pretty easy to draw a conclusion that the larger the number, the better the battery but this isn’t really the case in our racing environment. The unit of measure here is milliamp hours (mAh), or, how much drain can be put on the battery to discharge it in one hour.

Our races only go for 6 minutes. A fully charged battery pack at 8.4v will provide sufficient full power for this race duration so a battery pack of size 5500 mAh will provide sufficient power for a race the same as a battery 8200 mAh. The 8200 mAh battery would power a car longer than a 5500 mAh battery, but in the context of a 6 minute race, this is not necessarily a primary consideration. The higher the mAh rating too, the more weight in the battery and this also can be worth noting when considering your car race weight.

The C rating

My Team Zombie 5500 mAh battery has a C rating of 75. So what’s with this? The C rating on a battery refers to the “Capacity” rating or more succinctly, the rate that it can be discharged safely without harming it. My battery is powering a Hobbywing ESC with a continuous power draw of 45A and a burst draw of 260A as well as the Justock motor draw of 35A.

With my 5500 mAh battery, I can determine the following:

75C = 75 x 5.5 (capacity) = 412.5A. Any more draw than this would at best, reduce the lifespan of the battery or at worst, burst in to flames. I’m OK here with my 75C rating as the ESC and motor, at burst capacity totals 305A, well under the 412.5A capability of the battery.

Most batteries these days have 2 distinct C ratings; a continual rating and a burst rating (10 seconds). By understanding the ratings on these and the continual and burst specifications on the devices the battery is powering, you should be able to determine whether the C rating on your battery is sufficient.

Resistance

From what I’ve read in my research here, the C rating is a bit of a real debatable topic in terms of performance benefits. Some swear that a larger C rating is better but the more measured response here is that as part of the manufacturing process, the higher C rating usually (but not always) results in lower resistance and this is indisputably tied to the performance of the battery.

Internal Resistance (IR) isn’t something that you will find as a rating on a battery as it is variable and will alter over time. The only real valid way to measure resistance is through a battery charger as part of the charging process. So what is it?

Resistance has to do with your battery’s health. As a LiPo battery is used, a build up forms on the inside terminals of the battery and the resistance value increases. Effectively, this is a measure of the battery’s efficiency. After many, many uses, the battery will simply wear out and be unable to hold on to any energy you put in during charging – most of it will be lost as heat. If you’ve ever seen a supposed fully charged battery discharge almost instantly, a high IR is probably to blame.

I won’t (can’t really) go all physic mode here and adequately explain the correlation between Ohms Law and how resistance works; there are far more cleaver people (and blogs) that can run through this. The relevant take-out here is that a battery that is displaying a high IR will have an impact on its efficiency. I got a good gauge for how to start looking at your battery health from a website I used in researching a lot of this information at https://rogershobbycenter.com/lipoguide/ and I encourage you to read their blog in full !

To use their guide on battery health though, you can consider the following resistance measurements when read from your charger as a gauge for when to consider replacing your batteries:

* Between 7 and 12 mΩ is reasonable per cell.

* Between 12 to 20 mΩ is where you start to see the signs of ageing on a battery

* Beyond 20mΩ per cell, you’ll want to start thinking about retiring the battery pack

Also note that higher internal resistance equals higher operating temperature. The higher the operating temperature, the greater the risk of the battery catching fire!

Charging

Battery chargers come in many shapes and sizes. I am now using an EV-Peak AR1 which can charge all the way up to 25A compared to the original charger I had that was restricted to 5A.

As for the charging process itself, the charger will accommodate programming based on the mAh rating and number of cells. If you are reading this before you’ve purchased a charger, make sure you get one with capabilities to charge the cells in balanced mode. Balancing is a term used to describe the act of equalising the voltage of each cell in a battery pack. We balance LiPo batteries to ensure each cell discharges the same amount. This helps with the performance of the battery and is also crucial for safety reasons.

Charges are also used to discharge the batteries as well and there is a process for this too. Most chargers run a storage mode and LiPo batteries shouldn’t be stored at full charge nor should they ever be discharged below 3.0v/cell. The optimal storage should be 3.8v/cell.

The Noob Summary

This is by no means a definitive guide on batteries but I hope, if you’ve managed to read this far, that you got a few takeaways from this. I’ve learned a lot in how to interpret batteries and how to charge, discharge (store) and evaluate their health. So hopefully this helps you too!

One final shoutout yet again, is to the website I gleaned the majority of my research from; Rogers Hobby Centre. Thank you for the comprehensive guide. I encourage anyone who wants to read deeper than what I have provided here, to check out their guide here: https://rogershobbycenter.com/lipoguide/

Next time, I’m going to look at the ESC.]]>

So what is required from a race car to score and what scoring formats are applied here? Let’s start with the mechanism for scoring; lap and race timing.

RC cars are fitted with a small transponder which gets powered by the car’s electrics during a race. This is referred to as an “active” transponder where Radio Frequency ID (RFID) of the unique transponder number is emitted to a timing loop as it passes over the platform. The track has an embedded loop across part of the track that reads the RFID data from the transponder as it crosses and feeds this information back to timing software that runs on a standard PC / Laptop.

Transponders usually cost around the $120-$150 mark although some clubs (such as ours) have a stock of these that can be loaned to racers who do not have their own personal ones equiped in their cars.

At race meets, the timing software allows for multiple formats of race meets and competitors are entered in to their appropriate race classes with their personal transponder number input to track their times.

So how are these times used to score?

Different race meets can have different scoring formats depending on the duration of the meet (single day event or multi-day event) but usually follow a similar format of timed practice, qualifying and races (pretty much the same as any common motorsport racing format!).

Session times are usually 5-6 minutes in length and for practice sessions, usually used to get your car configured optimally for the track and conditions. Ideally, during practice sessions, you are aiming to bring your lap times down. It’s not uncommon to see lap times reduced by 2+ seconds when tweaked across practice and qualifying sessions.

Practice sessions (along with Qualifying sessions) can also be where grading is performed. This is common where there are competitions with too many competitors to fit on a track at the same time. A race may be capped at 10 cars but if there are 20 competitions, the top 10 fastest cars may be graded in the first round and the second 10 in the next round.

Qualifying sessions are used to determine grid positions for races. In this format, cars start the qualifying session (same 5-6 min format as actual races) but do not start from a grid but are staggered (spaced) at the start of the session. At completion of the timed duration, racers must complete the lap they are currently on and the results are used to determine best times.

How do you interpret the race times?

At completion of a qualifying session, positions are based on the least amount of time taken to complete the most amount of laps. This will read in the format of laps/time where 16/6:08.983 means there were 16 laps completed in 6 minus 8.983 seconds. This would beat a time of 15/6:03.455 because even though time of 6 mins 3 seconds is 3 seconds better than the other time of 6 mins 8 seconds, this racer only managed 15 laps so was a lap down on the first racer.

At completion of qualifying rounds, racers start ALL races on the starting grid based on the order they finished in qualifying. Some points systems / championships may award points to the Top Qualifier (TQ) as well.

Races are then run on a pre-determined number of races with a cumulative points system based on position finishes to determine an ultimate winner based on most points]]>

Starting out can be daunting but the club is very supportive and helpful in providing advice and assistance in understanding all components and requirements to get you up and running fast. We can introduce you to the sport via our club cars to give you practice and experience before delving in to your own purchase (which we can also assist with).

BORRCCC is a social and family-oriented club fostering values of camaraderie and fun. Come check out one of our race days and talk to some of the racers; it’s a great way to learn!

Essentially, a race car consists of the following elements:

- a chassis (either Ready-to-run or in kit format to build)
- a radio controller
- a motor (varying speeds and capabilities!)
- an electronic speed controller (ESC)
- a servo (steers the front wheels)
- a battery / batteries (LiPo)
- a transponder (records your lap times)
- a range of tools and accessories!

You tend to start off with the basics when getting in to RC cars. Many of our club members run with this kit whilst others go on to accumulate a complex and vast “pit bag” when racing competitively across the state, country or internationally!]]>