Today I pick something from the net which is very useful

.... it's about RATIO

This 90T spur gear and 20T pinion equal a 4.5:1 gear ratio. The pinion must turn 4.5 revolutions to turn the spur gear 1 revolution.

Gears: every car and truck has 'em (yes, even the belt-driven ones), but not everyone understands them?or knows how to get the most out of them. Let's take a look at gear ratios, gear pitch and gearing your car or truck to win.

WHAT IS A ?GEAR RATIO??

Here are two sets of Robinson Racing steel clutch bell/spur-gear combos for the Traxxas T-Maxx 3.3 (top) and Revo 3.3 (bottom). The T-Maxx pair is an 18T/44T combo; the Revo set is a 16T/40T combo. Despite the different tooth counts, the ratios are very close: 2.44:1 and 2.5:1, respectively.

Any two gears will have a ?ratio.? If your car has a 20-tooth (20T) pinion and a 100-tooth (100T) spur gear, that's a 20:100 ratio; however, we need to go one step further and divide the number of teeth on the spur gear (100) by the number of teeth on the pinion (20). It's easy: 100 ? = 5

The gear ratio is 5:1 (say ?five to one?), and that means that the pinion must make five revolutions to turn the spur gear one revolution. It's that simple. Once you know the gear ratio you want, you can use any combination of tooth counts to get it. For example, a 5:1 gear ratio could be achieved with a 100T:20T, 105T:21T, 110T:22T, or 115T:23T combinations.

CALCULATING TRANSMISSION & FINAL DRIVE RATIOS

TRANSMISSION RATIO

Here's an Associated B4/T4 transmission. The top gear has 20 teeth and the bottom gear has 52 teeth for a ratio of 2.6:1.

Any car with a gearbox has some kind of internal gear reduction; most 1/10-scale cars and trucks use a 3-gear transmission that features a top (input) gear, an idler gear and a bottom (output) gear that spins the drive axles and incorporates the differential. To determine the transmission ratio, just divide the number of teeth on the bottom gear (diff) by the number of teeth on the top gear. For example, the gearbox shown here has a 20T top gear and 52T diff gear: 52? = 2.6

FINAL DRIVE RATIO

To keep the math simple, let's say this is a 20T pinion and 100T spur gear. The pinion and spur have a 5:1 ratio, and as illustrated previously, the B4's gearbox has a 2.6:1 ratio. Multiply the two ratios, and you get a final drive ratio of 13:1. So, to drive the rear wheels one complete revolution, the pinion must turn 13 revolutions.

This is the inside of an Associated TC4's gear-box; the diff gear has 40 teeth, and the input gear has 16 teeth for a 2.5:1 ratio.

In addition to the transmission's internal gear ratio, we have the external gear ratio of the pinion and spur gear to contend with. By combining the internal and external ratios, we arrive at the final drive ratio. This ratio shows how many times the pinion must turn to make the transmission's output gear (and with it, the car's tires) complete one revolution. The math is simple; just multiply the internal and transmission ratios. If we use a 100T spur gear and 20T pinion gear for a 5:1 ratio and a 2.6:1 transmission ratio, we get 13:1 for a final drive ratio: 5 x 2.6 = 13

ROLLOUT: PUTTING IT ALL TOGETHER

10.7 INCHES

Mark, roll and measure. B4's Hole Shot tire travels 10.7 inches with each rotation.

Although they have tires instead of teeth, the drive wheels of your car or truck can be thought of as gears in the transmission. Think about it: if you take two identical cars and put small-diameter tires on one and large-diameter tires on another, the vehicle with larger tires will travel farther with each rotation of the wheels and is effectively geared taller than the small-tire car. This distance is called the car's ?rollout,? and it shows the true effect of gearing changes. Here's how to measure rollout:

STEP 1: MEASURE THE TIRE'S CIRCUMFERENCE

The distance traveled by the tire in one rotation is its circumference. To measure it, mark the sidewall of the tire, roll it until the mark makes a complete rotation and then measure the distance. Or, you can calculate circumference by multiplying the tire's diameter by pi (3.1416).

STEP 2: DIVIDE THE CIRCUMFERENCE BY THE FINAL DRIVE RATIO

Let's say our example car has a final drive ratio of 13:1 and a wheel circumference of 12 inches. 12 ? = 0.92. This car travels 0.92 inch with each revolution of the pinion.

OK, so we went all this way for a fun fact? No! Now we can see how gearing changes really affect a car. Let's say we install a 22T pinion in place of the 20T. The external ratio becomes 4.54:1 (100 ? = 4.54). Multiply that by a transmission ratio of 2.6:1, and we get 11.8:1 for the final drive ratio. Divide 12 inches by 11.8, and we arrive at a roll-out of 1.01 inches?that's an increase of 1/10 inch. That doesn't sound like much, but think about it: if the motor spins at an average of 20,000rpm for the entire 5-minute race, and the car goes an extra 1/10 inch for every one of those 100,000 total revolutions, that means your car will travel an extra 833 feet (100,000 x 0.1 = 10,000 inches; 10,000 inches ? = 833 feet). In a 20-lap race, that's an extra 41.7 feet per lap! As you can see, small gearing changes can make a big difference.

GEAR PITCH

A gear's ?pitch? refers to the size of its teeth: the larger the pitch number, the smaller the teeth. That seems backwards, but it makes sense when you consider how pitch is determined. Pitch refers to the number of teeth that can fit around a 1-inch diameter disc; for example, a tooth size that permits 48 teeth to encircle a 1-inch disc is said to be 48-pitch. To squeeze 64 teeth onto a 1-inch disc, the teeth must be smaller; that's why 64-pitch gear teeth are finer than 48-pitch. The pitch of the gears in your car has no effect on their ratio; a 100T-spur/20T-pinion combination is a 5:1 ratio whether you use 32-, 48-, or 64-pitch gears; however, certain gear pitches are better suited to certain applications. For example:

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so see the below table

So you have pick a ratio carefully, not any number, coz that may heart your car.

Every car has it own ratio, you can't put ratio for truck in on road car, and when you want to increase or decrease the ratio, you have to change it with small numbers.

sea Yea