Overheating is either caused by overgearing -- too large a pinion or too small a spur -- or excessive friction in the car's drive train.
Gearing is always relative to both the course and the driver.
A course with slow corners and short straights requires a smaller pinion than a course with sweeping turns and longer straights. If you are just playing around with your car, it is very easy to overheat the motor by making tight slow turns and then yanking the throttle. Set up a course that involves some sweeping turns and longer straights. Go easier on the throttle, or use the current limiter if you have one. If this doesn't do it then buy a smaller pinion.
On the same course a better driver can run a larger pinion than a poorer driver, because the better driver carries more speed through the corners and doesn't have to accelerate from such a low speed. The better driver also uses part throttle early in acceleration rather than yanking on the trigger instantly. If you have an adjustable current limiter on your speed control, you can turn the limit down to keep the motor cooler by simulating this driving technique.
Check the friction of your drive train by first holding one rear tire and spinning the other. If the pinion gear is mating too tightly with the spur gear, you'll be able to feel and hear it; loosen the mesh. No spur gear is perfectly round; rotate the gear and make sure there is a little free play even at the tightest point. Next, remove the motor and spin both rear tires. The spur gear should move freely and coast to a gradual stop. If not, it is time for transmission disassembly, cleaning, replacement of bad gears and bearings, and reassembly. Now that the motor is out of the car, remove the motor brush springs. Spin the motor's armature by spinning the attached pinion gear. The armature should spin freely, but not for long because of the magnets. If not you have some cleaning to do. If the motor bushings are badly worn you need a new motor.
Don't forget to check the front wheels for free-spinning operation as well.
One reason for understanding how to compute and use gear ratios is that some motors you buy come with gear ratio recommendations that are a good starting point when dialing-in. Another reason is that you may be new to a track and get a gear ratio recommendation from somebody who is experienced at that track and running a similar motor and similar diameter tires to the ones you are running; if that person is running another brand of car, you'll probably need to run a different size pinion to get the same gear ratio.
To figure your gear ratio you need to know the number of teeth on your pinion gear, the number of teeth on your spur gear, and reduction of your transmission. Here is a table giving reductions for a few common trannies:
Losi XX buggy (XX, XX buggy retro) 2.19 Losi XX truck (XX-T, XX truck retro) 2.61 Losi LRM (Jr-2, Jr-T) 2.18 Associated Stealth (RC-10, RC-10T) 2.25
Your gear ratio is
number of teeth on your spur ------------------------------ * reduction of your transmission number of teeth on your pinion
For instance, a Losi XX comes with an 88 tooth spur gear. If you install a 22 tooth pinion you get a gear ratio of
88 -- * 2.19 = 4 * 2.19 = 8.76 22
Click here to make your own table of gear ratios.
You shouldn't use the same gear ratio on a truck and a buggy; the truck needs a larger ratio because the truck motor must turn a tire with a larger diameter and accelerate a heavier vehicle and a tire with a greater moment of inertia. A rule of thumb to convert between a buggy gear ratio and a truck gear ratio on the same track with the same motor is to multiply the buggy gear ratio by 1.3. So if you are running a 22 tooth pinion on a XX and want to compute the corresponding pinion size on an XX-T, you'd figure it like this:
8.76 * 1.3 = 11.38 88 88 -- * 2.61 = 11.38 so PP = ----- * 2.61 = 20.18 PP 11.38So you might try a 20 tooth pinion on the XX-T. This is only a rule of thumb but it provides a reasonable starting point if you have no other information in hand.
One possible source of confusion about gear ratios: When somebody tells you to "gear up" they mean to use a *smaller* gear ratio, i.e. a larger pinion.
The slipper, used correctly, both improves performance and reduces wear and tear on the transmission:
The slipper is a fussy topic. It takes awhile to learn when to loosen the slipper and when to tighten it. Most beginners set the slipper too loose, which makes the car easier to drive but slower overall.
Before you start adjusting the slipper, you must double-check the diff adjustment. Remove the gear cover. Tighten the slipper all the way so it won't slip. Hold both the spur gear and the right rear tire with your right hand, and try to turn the left rear tire with your left hand. It should be quite hard to turn. If the left rear tire turns with moderate effort, with the transmission top shaft stationary as the left rear tire turns, tighten the diff and repeat.
Now loosen the slipper adjustment a few turns, hold both the spur gear and the right rear tire with your right hand, and turn the left rear tire with your left hand. It should not be extremely easy to turn the tire, but not be extremely hard to turn it, either. Fiddle the slipper adjustment to get it into the right ballpark.
Now put the car on the track pointing toward you. Press down on the car and punch the throttle on-off. Listen to that sound -- that's the slipper slipping. Feel how much forward drive the car has. If the forward drive is weak you need to tighten the slipper.
Keep the car on the track pointing toward you. Back away a couple of feet and punch the throttle on until you catch the car. The slipper sound should last only for about a foot -- it should stop before the car reaches you. (It takes practice to hear the slipper sound among all the other sounds, especially the sound of slipping tires.) If it slips more than this, tighten the slipper; if there is no slip at all, loosen it.
Go out and drive the car. If it is working well, bring it in and feel the adjustment again by turning the left tire. Learn how hard the tire is to turn. This will help you get closer to the right adjustment next time you have to adjust it from scratch.
The Team Losi Hydra Drive slipper incorporates a small torque converter (like the hydraulic clutch in an automatic transmission) in addition to the conventional friction slipper. The torque converter allows the friction slipper to be set very loose, yet maintain good acceleration. The benefit is even better handling on rough tracks and big jumps; the drawback is a slight loss of acceleration with stock motors on tracks with good traction. You can remove the Hydra Drive and use the friction slipper alone if that works best (not likely if you are a beginner.) Hydra Drive is very user-friendly in that the adjustment is less sensitive than a friction slipper. You make major adjustments by swapping the torque converter for another one with lighter or heavier fluid inside. The Team Losi instruction manuals contain a full description of how to adjust the Hydra Drive. Hydra Drive can be retrofitted to Associated cars and trucks; Brian Kinwald used one in his 1993 off-road world championship winning RC10.
On pistol-grip transmitters, you push forward with the trigger finger to apply brakes. This forward push is much less sensitive than the pulling motion used for throttle control. In effect, you have an on-off control for brakes unless you are very talented with the trigger finger. (Stick transmitters are better than pistol-grips in this regard.)
The other adjustment at your disposal is how much braking, if any, is applied when the trigger is at its neutral point (i.e. finger off the trigger.)
My advice is to adjust the transmitter and speed control for little or no neutral braking and moderate to heavy push braking.
The amount of braking you have at neutral affects the way your car enters turns and goes through them. More braking transfers more weight to the front tires and allows you to turn more, which sounds good at first. But too much braking causes the car to spin out unpredictably as it enters turns. Too much braking also slows the car down before it finishes the turn, so you have to get back on the throttle. When you get back on the throttle the weight transfers off the front and the car steers a wider arc. You will be faster with a smoother turn that carries more speed all the way through. So you see that a balance is needed in the neutral brake adjustment, and this balance may depend upon the course you are driving. Most top drivers today dial in no neutral braking at all on a typical off-road course.
Push brakes are useful in order to slow down before turning into a tight corner. You want to brake while going in a straight line, then let off the brakes and turn in. If you turn while braking the rear end will skid, which may get you turned around quickly but is difficult to control lap after lap. Adjust the push brakes to slow the car as fast as possible without locking up the rear wheels.
Brake adjustment depends on the motor you are using, because motors have different amounts of inherent braking. A stock motor may have a lot of inherent braking (due to its large timing advance, heavy springs, and bronze bushings) so you get significant braking without dialing in any brakes. With a modified motor you might have to dial in a some brakes to get the same effect. If you run the same car in stock and modified and forget to make this adjustment, you are likely to be disappointed at the results.
Secondly, make small changes when gearing up. If you make a big change you are likely to cook your motor. Add one tooth to the pinion, two only if the motor was *really* cool the last time. Don't change the spur gear unless you have maxed out the pinion (e.g. 26 teeth for 48 pitch gears).
Thirdly, get somebody to time your laps with a stopwatch. Change the gearing and do it again. Compare the typical laps from each run -- not the fastest laps. Choose the gearing that makes your typical lap the fastest, while not overheating.
What I've described here is a procedure for a perfectionist. Usually you pick a gear ratio that's easy on the motor and gives competitive performance, and spend most of your time working on other things, like tire or suspension tuning and getting enough practice time to master the course.