[scalelover]

Compliments of ESPEEFAN he posted this on another forum: it will give you a lot of help in figuring them out


The motor turns really has no effect on the final drive ratio, but it does effect the speed of the vehicle. Basically the lower the turns, the faster the motor will spin. The more turns a motor has, the slower it will spin, but it will have more torque. While the motor turns does not directly effect the gear ratio, you should consider your motor turns when you pick a pinion gear for the motor, as well as the cell count, or input voltage.


You can calculate any gear ratio using a simple formula. The formula is based on two gears. The driven and the driver. You are basically calculating the difference in the tooth count of each gear, which tells you how many revolutions the drive gear will turn for every one revoltion of the driven gear. The ratio ends up being X to 1, or also expressed as X:1. X being the calculated number.

To solve for X, what you do is take the driven gear's number of teeth and divided that by the number of teeth on the drive gear. Formula below.

Teeth on the driven gear
_____________________ = Ratio to 1

Teeth on the drive gear


So lets do a quick example. Lets say we have a pinion gear on a motor with 9 teeth. That gear is driving a spur gear on the input shaft of a transmission. The spur gear has 32 teeth.

We need to take 32 divided by 9 to figure out the gear ratio. The ratio is
3.5 to 1, or can also be written 3.5:1. That means for one revolution of the 32 tooth spur gear, the 9 tooth pinion gear will have made 3.5 revolutions.

Now you can use the formula to calculate a final drive ratio, working your way through an entire truck, starting at the pinion gear of the motor, working your way through the transmission, and even the differential gears, which gives you a final drive ratio. In order to do that, you just work your way through each set of two gears, write down the ratios and then multiply them altogether to arrive at the final drive ratio.

A simplified example of that would be this.

Lets say we have a motor with a 9 tooth pinion gear, driving a 32 tooth spur gear. The shaft that spur gear is mounted to, drives the input shaft of a transmission. On that input shaft, we have a 10 tooth gear, which then drives a 30 tooth gear on the next shaft. On the same shaft of the 30 tooth gear, we have a 15 tooth gear. The 15 tooth gear drives a 45 tooth gear on an output shaft, which spins the drive shaft going to the differential of our axle. Lets say the axle has an input shaft with a 9 tooth pinion, that drives a spur gear for the differential, which has 24 teeth. We can calculate the final drive ratio for the truck by figuring out the ratio of every gear set seperately, and then by multiplying all the ratios together to arrive at the final ratio.

Here we go

32 tooth spur gear divided by 9 tooth motor pinion gear = 3.5

Inside the trans, the 30 tooth driven gear, divided by the 10 tooth gear that is on the input shaft (same shaft that the spur gear is mounted to). 30 divided by 10 = 3.0

The second set of gears inside the trans. The 45 tooth gear on the output shaft, divided by the 15 tooth gear that is on the same shaft as the 30 tooth gear above. 45 divided by 15 = 3.0.

Now the driveshaft goes to the differential. The differential spur gear has 24 teeth and the pinion gear on the diff input shaft has 9 teeth. 24 divided by 9 = 2.6

Now we need to multiply all the ratios together to get the final drive ratio. So we take 3.5 times 3.0, times 3.0, times 2.6. That gives us a final drive ratio of 81.9 to 1. That is very steep, but it's just an example of numbers I randomly choose. Most final drive ratios will not be that low (numerically high).

So what does this mean? Well, if you have a motor that spins 6,000 RPM on 7.2 volts, by the time the reductions have all done their job, the axle shaft, which drives your wheels will be spinning a mere 73.26 RPM.

Also, when we talk gears, a lot of times someone will mention low gearing for rock crawling, or for heavy pulling. While the gearing is low (and slow) we are actually running a really high final drive ratio. Numerically high. What do I mean when I say that? Well, looking at the final drive ratio of 81.9:1 that I figured out above, the 81.9 is very high numerically. If we compared it to a 35:1 ratio, we would know that the 81.9:1 ratio is going to give us a slower truck then one with a 35:1 ratio. But not always, as tire size plays a factor in the speed of a truck too. This is why people change differentials gears when they put larger tires on their 1:1 trucks. Larger tires cover more distance when they roll, effectively lowering the final drive ratio (taking away torque and adding more speed at the same engine RPMs). To get the power back, they run a numerically higher diff ratio, which slows the truck down and gets the engine back into the powerband at highway speeds. It works the same with our RC trucks. Bigger tires give us more speed too, if all else stays the same.

To counter that, we can gear our trucks lower (but numerically higher) by changing the pinion gear of our motor, or by running a motor with higher turns. Say we go from a 35 turn to a 45 turn. We'll get back some torque and keep the speed similar to when the tires were smaller. Usually the easiest thing to do is just get a smaller pinion gear. Not necessarily change the motor, but it can all be tuned together, and we can even bump up the input voltage to the motor, if we want to run really low gearing, yet keep wheel speed. So there are some variables to consider!

Hope this doesn't confuse you more! If you have any questions, just ask.

EDIT- I should also mention that in addition to being able to change gearing by swapping out motor pinions, you can also change the spur gears in many trucks too. By changing spur gears and pinion gears together, you can often get finer adjustments of final drive ratios, then if you simply change the pinion gear by itself. This mostly being the case with the race cars and trucks. Think Slashes, buggies, your E-Revo, ect. I don't know that the crawlers and scale trucks have the options to change spur gears, but it is still something to consider.

Don't know what way to go, when you gear your truck? Smaller pinions will slow the truck down and make it more torquey. A larger spur gear will have the same effect. Just think in terms of a ten speed bike. Small sprocket on the pedals and a larger sprocket on the wheel means it's easier for you to pedal. Or a larger sprocket on the pedals makes it harder to get up to speed, but you'll have more of it.

Don't be afraid to try different combinations of pinion gears and spur gears (when you have the option) to tailor your gearing the way you like it.

Also of interest is this. A lot of our trucks run 32 pitch or 48 pitch gears. Pitch refers to the angle of the teeth on the gears. You'll have more options for adjustment in final drive ratios with finer pitch gears. Usually because there are more teeth available in the finer pitch gears. A 32 pitch gear has fewer teeth then 48 pitch gear of the same diameter. What is the difference in 32 pitch or 48 pitch and why do we have them? Simple. 32 pitch gears are stronger. They have more beef to them. 48 pitch gears offer the finer resolution for adjustment and run quieter. You are more likely to see 32 pitch gears on the larger and heavier trucks. Monster trucks and nitro machines. 48 pitch gears are popular on lighter models and especially the racers. On-road machines, electric buggies, or lighter trucks, and crawlers. Many times a quick look at your model's manual will give you all the possible gear ratios, with the combination of pinion and spur gears that you can swap out, along with some recommendations of what gearing to run with what type of motor.

Also one more little tid bit to remember. You don't have to recalculate all the ratios, when you change a pinion gear, to figure out your new final drive ratio. If you already know what the overall ratio is of JUST the trans and the differential, you only need to multiply the new ratio of the pinion and spur gear to the ratio of the trans and differential.

Example -

Using the same transmission and differential numbers I showed above, we know that the trans and diff together have an over all ratio of 23.4:1. That does not include the pinion and spur gear (which we can change). If we remember 23.4:1, we just need to multiply any new pinion to spur gear combination we want to run, to the 23.4:1 ratio, which gives us the final drive ratio.

Okay so with the 9 tooth pinion gear and the 32 tooth spur gear we had a final drive ratio of 81.9:1. (3.5 times the trans and diff's ratio of 23.4:1)

What if we change the pinion to a 10 tooth but leave the spur gear the same? 32 divided by 10 = 3.2. We then take 3.2 times 23.4 (our trans and diff ratio) to get a new final drive ratio of 74.8:1. See how much difference one tooth can make? That's why it's often best to make small adjustments when you swap pinion gears. I'd suggest going probably no more then two teeth at a time. It's often handy to keep a stock of pinion gears in many sizes, just so you can fine tune your gearing when you like.