Gearing Explained

Let’s take a look at your transmission and see just what kind of tuning options you might have.

Gear Ratio
The primary tuning option relating to your car’s transmission is the ability to change gear ratio by using different spur gears or pinions. Before we talk about the effect of gear ratio changes, let’s spend a moment sorting out the terminology.
Gear ratios are most often quoted in the form “2.0 to 1”. This can be represented in writing as 2.0:1. This means that the motor must rotate 2.0 times for the car’s driven wheels to complete one full revolution.
The internal ratio of the G4’s differentials is 2.526 to 1

The formula for calculating gear ratios looks like this:
(# Teeth on spur divided by # teeth on pinion) multiplied by Internal diff ratio = Final Gear Ratio

We’ve already seen how gear ratios can be represented as numbers (e.g. 7.8:1). The tricky part is in describing ratio changes in general. If you put a bigger pinion on the car, the ratio will change to a small number (say 7.4:1). Whilst the numerical figure has become smaller, the actual gear ratio 7.4:1 is said to be a ‘higher’ ratio than ‘7.8:1’. Similarly, moving to a smaller pinion will produce a ‘lower’ ratio (say 8.2:1). Without wanting to confuse you, changing the spur gear has the opposite
effect. A smaller spur gear will result in a ‘higher’ gear ratio, and a larger spur gear will give you a ‘lower’ gear ratio.

Confused? Stay with me.
Gear ratio changes do a couple of things. Let’s look at both the ‘lower’ and ‘higher’ gear ratios separately to see what we find.
A lower gear ratio will mostly give you more run time and more acceleration. It’s also generally easier on your motor.
A higher gear ratio will generally give you more top speed, and less run time. It’s also tougher on your motor.
Once you get to a certain ratio point (lets call it the ‘optimum ratio’) continuing to
change to a higher ratio will do nothing but damage. It will result in your motor overheating and being damaged, and in extreme cases, your car may actually go slower.

How’s that. Did you understand it all? Have another read, and think about it carefully.

Look at this simple chart, which might help make things clearer.

Pinion = Spur = Gear Ratio = Gearing = TopSpeed = Acceleration = Run Time
Bigger = smaller = higher = up = more = less = less
Smaller = bigger = lower = down = less = more = more

Hope that helps! For help on choosing your actual gear ratio for any given motor or track, consult or check with the local fast guys.

souce AFM

To add to this topic i have copied a post by Devyl to add here.

From a website i found ages ago:

The reason for different sizes of teeth is a trade off between strength and efficiency. In rough terms bigger teeth are stronger, but less efficient.

Identifying different gear iszes depends on if the teeth are Metric or Non Metric, but before we get into that, we need to define some basic terms:

Number of teeth N: This is easy – it’s the number of teeth on a gear! Teeth being the bits that stick out!

Pitch Circle Diameter: PCD - Imaging two gears in mesh, draw two imaginary circles, each centered on the center of one of the gears that just touch each other at the point where the gear teeth touch. If the gears were wheels this is how they would drive each other. The diameter of each of those circles is the Pitch Circle Diameter of each gear.


NON METRIC GEARS
Diametral Pitch DP: - Is the number of teeth N of a gear divided by Pitch Circle Diameter (in Inches) DP = N / PCD

Or PCD = N / DP

HPI gears are either 32, 48 or 64 DP usually. So the Pitch Circle Diameter of a 64 DP gear with 64 teeth would be exactly 64/64 = 1 Inch.

Pitch Circle Diameter is hard to measure since it is measured part of the way down the gear tooth face. Outside Diameter OD is much easier to measure:

Outside Diameter OD: The diameter of a circle through the tips of the teeth. Easy to measure with a vernier!

Luckily on small gears, the part of the tooth that is outside the pitch circle (the addendum if you need to know) is roughly equal to the DP. So to measure the Outside Diameter we just add 2 (one for each side) to the number of teeth and divide by the DP:

OD = (N + 2) / DP

Like this: 18 tooth pinion, 64 DP

The OD would be (18 + 2) / 64 = 0.3125 Inches

also a 120 tooth spur gear, 64 DP

The OD would be (120 + 2) / 64 = 1.906 Inches


Metric Gears
Metric gears (Tamiya and some other manufacturers) are given a different designation, which is called the Module.

The Modules used are 0.4, 0.6 and 0.8, (with 0.4 being tiny teeth and 0.8 being big lunking teeth.)

The module is actually measured in mm (So 0.4 stands for 0.4 mm)

To use the module in place of the DP we can calculate the OD of gears as follows:

OD (in millimeters or mm) = (N + 2) x Module

An 18 tooth pinion, 0.8 Module

The OD would be (18 + 2) x 0.8 = 16 mm

A 50 tooth spur gear , 0.8 Module

The OD would be (50+2) x 0.8 = 65 mm


To convert between Module and DP to compare sizes:

Metric Module = Exact Equivalent DP

0.8 = 31.75

0.6 = 42.333

0.4 = 63.5



As you can see, the 0.4 Module is almost the same as a 64 DP and the 0.8 is almost the same as a 32 DP (they could probably be interchanged without much damage), but the 0.6 is much different from the 48 DP and if mixed together, the harder of the two gears would eat the other one for lunch! It is never a good idea to mix Metric and Non-metric gears, unless you are desperate!



SUMMARY and test!
So if you’re not sure what gear you are looking at, do this:

1. Measure the Outside Diameter OD in Inches (and in mm)

2. Count the number of teeth.

3. Divide the number of teeth + 2 by the OD in inches.

If the gear is not metric, the answer should be almost exactly 64, 48 or 32. Now you know what gear DP it is!

BUT if the answer is something else,

4. Try dividing the OD in millimeters by the number of teeth + 2

The answer should be almost exactly 0.8, 0.6 or 0.4, if the gear is metric. Now you know the Module of your Metric Gear.

How to choose gearing.
When it comes to the perfect setup for winning races, a key ingredient is the gearing of your car or truck. All that monster powerin a pricey Italian nitro engine or sizzling modified electric motor isn't much good without the right gear ratio transferring it to the wheels. In fact, with the right gear ratio, you can actually beat more powerfully equipped opponents if their gear ratio's all wrong. If they don't gear high enough. While they're struggling with throttle control to maintain traction you'll be smoothly accelerating out and away.

Calculating Gear Ratios: The obvious first step in optimizing gear ratios is calculating them. If you've spent any amount of time at a race track, you've probably heard veteran electric racers referring to the gear setup as "22/88" or "23/90" or similar numerical combinations. What they're talking about is their pinion/spur gear combination. For instance, a "22/88" setup is the combination of a 22 tooth pinion gear with an 88 tooth spur gear. Using this combination, one can easily calculate the gear ratio by dividing the number of teeth in the spur gear by the number of teeth in the pinion gear.

With nitro vehicles, the ratio is increased or decreased by changing the size of the clutch bell gear. Onceyou've calculated your gear ratio you now have a point of reference to begin making adjustments.

Tweak, Time, Repeat . . . Once you've determined your car or truck's current gear ratio, you can begin the quest for the perfect one. Finding this magical matching of gear combinations is a lot easier than you might think, too. Often car and truck manufacturers provide gear ratio suggestions with their kits. Other more experienced racers can usually help you as well. Ultimately, though, the best way is to simply head out to the track and try a bunch of different ratios till you find the one that suits your particular driving style and powerplant.

One of the simpler methods involves using a series of 5 minute runs at different gear ratios until you consistently achieve the best lap times. Start with a low gear ratio and then time your laps. After a few laps, come back to the pits, incrementally increase the gear ratio and run again. Repeat this process until your performance falls off, then revert back to the ratio you had just before the performance drop.

You may have to repeat this process anytime you race at a new track or install a new motor as these factors will effect which ratio is best. Keep a log of all your lap times with different ratios on different tracks and with different motors. Before long you'll have compiled enough data to know which ratio works best for a particular race environment without a test run.
"There's hills in that thar' dirt!" When racing off-road on tracks with lots of jumps and moguls it's best to choose a lower gear ratio. This allows you to keep your acceleration when heading up hill and clear the jumps with ease. A lower gear ratio also lets you scramble over moguls without loading up the engine and bogging down.

The Power Factor: Regardless of whether you race on-road or off, a regularly used gear ratio can cease to be the right one if power isn't taken into consideration. A typical scenario would be after upgrading to a more powerful engine. The gear ratio that had worked so well with the previous less powerful motor may not be near high enough to harness the awesome power of a new one. This can create all kinds of handling problems coming out of turns or on the straightaways because a car can't get enough traction. Anytime you switch powerplants, remember to check the gear ratio, too.

Track Conditions: Differing track surfaces can create the effect of an increase or decrease in power. A nitro engine or electric motor that did just great on a stickier surface, may find itself suddenly "overpowered" on a slick surface and unable to get any traction if the gear ratio isn't adjusted. Simply increasing the gear ratio should solve this problem and provide smooth stable acceleration without getting loose when coming out of the turns.