Motor Gearing

How To Gear Your Motor

Changing the stock motor in the Stampede is one of the first things many people do. Changing the motor can provide more bang for the buck than about any other modification so it's an excellent item to modify. However, many people don't understand how to gear their new motor and this can quickly turn any motor into nothing more than a paperweight. Sure gearing that brand new Speed Gem 2 Onyx at 24/87 may provide you with some killer speed, but have you tried touching it after a run? You may be surprised how hot it gets. That's bad for the internals of the motor and could possibly damage the speed controller as well.

Below you will find some suggestions and "rules of thumb" for correctly gearing the Stampede. These guidelines are intended for the Stampede only. Other RC vehicles will have vastly different transmission ratios and tire diameters, so these guidelines will be different from what could safely be used on one of those.

You may notice that after you get lower than a 13-14 turn motor it's going to be impossible to start with a pinion that has the same number of teeth as the number of turns on the motor. This is why I recommend that you consider your purchase carefully before buying a motor with less than 13-14 turns in the Stampede. You may have motor overheating problems, since you won't be able to gear it correctly. There are some people who run the Stampede with a very low turn motor and seriously over gear it. However rest assured that their motors suffer the consequences of such actions, and if they are not aware of it they soon will be.

Modified Motors

Modified motors are motors that have adjustable timing and what's referred to as stand-up brushes. Usually from the factory these motors already have 10-20° of timing already dialed into them. This is adjustable and I will briefly discuss this later.

The "rule of thumb" for an initial gearing on a modified motor is to start with a pinion that has two teeth less than there are turns on the motor. I also recommend always using the stock spur until you understand the relationship of spur gears vs. pinion gears, which I will briefly discuss further down on this page. For example, if you had a 17x1 Speed Gems 2 Sapphire, you should begin with a 15 tooth pinion and an 87 tooth spur. Once you've installed the gears and set the mesh between them properly run a fully charged pack through the motor, checking the motor temperature with your finger every few minutes.

When you check the motor temperature with your finger, the motor is considered too hot if you can't leave your finger on the motor for at least 5 seconds. If you find it's too hot, go down to a pinion with 1 less tooth, let the motor cool, and try running another full pack through the Stampede. If after running a whole pack through the Stampede you find the motor is still cool, you can go up 1 tooth on the pinion and try running again. Make sure to let the motor cool off completely so that it's not already starting warmed up from the previous run.

Repeat this procedure until you find a point where the motor will stay cool through a pack or two. Remember that the ambient temperature will have some bearing on how quickly the motor heats up as well. A gearing combo that lets you run a pack or two without overheating in the dead of winter, may overheat the motor in the middle of summer.

After you find the optimum gearing point for your motor you can gear back down if you need more torque (or bottom-end acceleration) for climbing or extremely rough terrain. It may also be possible to gear up 1 more tooth if you run the Stampede on pavement, but this is only if you did your initial gearing runs off-road. Make sure you check the motor often if doing this. Once you scorch the commutator and the brushes the motor will heat up easily and wear much faster.

You can also use the motor timing to control torque and speed, similar to how changing the pinion does. Increasing the timing will add more speed, while decreasing timing will add more torque. Just like when going to a pinion with more or less teeth, the motor will run cooler when the timing is decreased and warmer when the timing is increased. I recommend using this as a fine tuning adjustment and it is even then best left to those that understand what they are doing. Too much timing can quickly ruin a motor, and the benefits of adjusting the timing are often never seen if just backyard bashing. You can find more information on this by going to my section on Motor Timing.

Hand-Wound Modified Motors

Hand-wound modifieds are more tolerant of higher gearing ratios than cheaper machine wound modifieds are. It's possible to be able to gear a hand-wound modified 1-2 teeth higher than what you would be able to gear a machine-wound motor with the same number of turns and winds without over heating it.

Stock Motors

Stock Motors have 27-turns and a fixed timing of 24°. The timing cannot be adjusted and they use what's called a lay-down brush to transfer the power to the commutator. Gearing stock motors is a little trickier than gearing a modified motor. Due to the fixed timing, the proper gear selection can make or break the race using a stock motor. Most stock motors, like the P2K2, run well using pinions ranging from 14 teeth up. Once again you will want to use the "finger method" as you do with modified motors to check for overheating, but the gearing you use will be determined more by the track, or area, you run. However since stock motors are designed to only run in short 5 minute races, you will probably notice it always has a tendency to run hot.

All I can do here is tell you to start low, with the lower tooth gears which will run cooler, and work your way up until you're where you want to be. Always make sure to check your motor temperature during runs and let the motor cool between packs for accurate results.

I also recommend, once again, sticking to the 87 -tooth spur gear until you understand the relationship between the pinion and spur gears.

Using Different Spur Gears

I've mentioned several times that it's a good idea to only use an 87 tooth spur gear. This is to avoid confusion, especially with someone new to motor gearing. The spur gear acts exactly the opposite of the pinion in regards to going up or down in the number of teeth. Also 1 tooth on the pinion makes a much bigger difference than 1 tooth on the spur. For example, a 15 tooth pinion combined with an 87 tooth spur would give us a pinion/spur ration of 5.8. This is calculated by dividing the number of teeth on the spur by those on the pinion. Dropping down to a 14 tooth pinion would give us a ratio of 6.2, while adding a tooth on the spur gear, remember I said it works the opposite, instead of the pinion would give us a ratio of 5.9.

Now you're asking, exactly what does that number mean right??? The pinion/spur ratio tells us how many times the pinion turns if the spur gear turns once. The higher the number the more torque you have, the lower the number the more speed you will have. Now obviously you're not going to find an 88 tooth spur gear for the Rustler because the difference between teeth is so small. The most popular sizes for Stampede spur gears are 93, 87, and 81. You'll can also find some sizes in between those, such as 84 and 90, along with some that are larger or smaller than 93 and 81.

Changing the spur gear is similar to adjusting the motor timing on modified motors. It's used as a method to fine tune. Say for example you are geared 15/87, which gives you a ratio of 5.8, and you decide you want more torque when you are coming out of a corner in the big race. You know from a previous outing that with a gearing of 14/87, which is a ratio of 6.2, causes you to have less top-end speed on the straightaway after this corner than you would like to have. So you want a ratio in between 5.8 and 6.2. So you change the spur gear to an 90 -tooth, which gives you a ratio of 6.0.

Now you're probably wondering what would happen if you went to a 93 -tooth spur instead of the 90 -tooth one in the example above, and kept the 15 tooth pinion. Your ratio would then be 6.2. Are you surprised??? That's right, a 15/93 and a 14/87 You'll find several combinations that are the same with each other. This is something to keep in mind if you are ever running your Stampede and you strip your only 87 -tooth spur gear. If you were running 14/87 and want to keep the same ratio, you can swap out the pinion with a 15-tooth and use the spare 93 -tooth spur you might have in your tool box.

So first read the next section on how to determine gear size, then run to your local hobby store to buy all the gears you can find and go experiment with gearing combos. Just make sure to keep an eye on your motor temperature. If you feel as if you understand the concept of gearing and wish to explore the concept further, or you want to learn how to apply gearing to another vehicle you might have, try reading this article on Understanding Gearing Ratios. This link requires the Adobe Acrobat Reader, which can be downloaded for free.

Part One:	How To Gear Your Motor
Part Two:	Determining Gear Size
Part Three:	Gear Ratio Calculator