Ball Diffs

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Ball diff set ups

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BUILDING THE PERFECT DIFF

The best way to have a silky-smooth diff—one that splits the power evenly between wheels—is to give it a good start. Here are a few choice tips to keep in mind when you’re building a brand-new diff. These will also be of help when you’re rebuilding a worn-out diff with new parts.

• Remove residual oil—Manufacturers have spent years developing special lubricants for ball diffs. Most diff lubes are silicone-based, and must not only protect the diff’s parts from wear, but they also must provide just the right amount of grip. This grip is the key to a good diff. A lubricant that was too slippery would cause slippage no matter how much you tightened the diff. For this reason, it’s important that you remove from the diff parts any oil that may be left over from the manufacturing process. Any solvent such as motor cleaning spray will do the trick—just take care not to get any on your skin (we recommend using rubber surgical gloves).

• Flashing is bad. Several diff parts will likely be of molded plastic, and as such, it’s possible that they have some flashing—the additional plastic leftover from the molding process—that can muck up the works if not removed. It’s common for flashing to exist on the main diff gear’s center hole; we will remove flashing using a sharp hobby knife, but take great care not to remove too much material from the parts! We will usually form a slight bevel on both sides of the diff gear’s center hole, as this will also help the gear to seat perfectly on the diff outdrives.

• Compression obsession—If your diff uses a spring instead of a series of Bellville washers, it’s a good idea to compress the spring before you install it to the diff. Use a pair of pliers to fully compress the spring and it will then be “seated.” What this does is protect your diff from loosening and slipping as the spring takes its natural “set.” You will still have to re-check your diff for its first few runs, but by pre-setting the spring, you’ll avoid a situation in which the diff’s setting can loosen to the point of causing damage.

• Work it—The final step to building a diff is to tighten its adjustment screw. Sounds simple, right? Not really. If you tighten the diff all at once, i.e., tighten the screw all the way to its final setting, you can risk damaging the diff washers and rings. Instead, you should tighten the screw until you begin to feel increased resistance. At this point, work the diff halves back and forth to seat the parts, then resume tightening. Repeat this procedure of working the diff and then tightening the screw a bit until you reach your desired setting.

BREAKIN’ (NOT POPPING AND LOCKING)

Like a brand-new nitro engine, the diff works best when it’s been carefully broken-in. There are a few methods for break in—the most popular technique is to simply run the car for a few laps, pull it in and then re-check the diff’s setting. It’s more likely than not that you’ll have to re-tighten the diff (you’ll want the diff to feel as tight as it did before you ran the car).
If you want to go right out and race without having to worry about your diff, you can also speed up the break-in process without leaving your workbench. Here’s how: with the car’s wheels off the ground, hold one wheel and apply a small amount of throttle. The diff will spin the other wheel in the opposite direction. Do this a few times, but don’t hold the throttle for more than five seconds as a time (as doing this can overheat the diff). After you’ve finished, you will probably need to re-tighten the diff screw.

WHEN YOUR DIFF NEEDS YOU

The telltale sign of a lunched diff is a gritty feeling. If the diff doesn’t work smoothly, it will need a rebuild. The worst cases of diff neglect will feel both too tight and gritty! Sometimes, you’ll be able to get away with a cleaning of the diff’s parts and a re-lubing. Usually, you’ll be better off if you replace the large drive washers and the small washers used in the diff’s thrust bearing assembly. Unless your diff has been slipping severely, the balls should last through at least two rebuilds before they’ll need to be replaced.

HOW TIGHT SHOULD THE DIFF BE SET?

The general rule of thumb with ball diffs is that they should be set so that they cannot slip, but like most rules, there are exceptions. In the old days of off-road, before slipper clutches, the diff had to be set to slip in order to shield the transmission’s gears from damage and to control wheelspin. The problem is that any time the diff slips (when the main diff balls are allowed to slip against the large washers), grooves are worn into the washers and the balls can develop flat spots. With the addition of the slipper clutch, the diff could be run tightly and the slipper would reduce wheelspin and wear. On-road cars, however, don’t have slipper clutches, yet because they are usually run in very high-grip conditions, wheelspin isn’t an issue and you can therefore set their diff or diffs tightly.
How do you know if your diff is set correctly? Hold the spur gear and one of the driven wheels and try to turn the opposite wheel—it should be very near to impossible to do so. If your ride has a slipper, tighten it all the way before you test the diff.

RC-Drift.com™:
How to Set your Diff & Slipper Clutch


Avoid R/C’s most common pitfall...

Ever hear the term “Something’s got to give”? It applies well to the transmission in your car or truck. When you have a power source at one end of the transmission (your electric motor or nitro engine) and grippy tires at the other, the “stuff” in the middle ends up taking all the strain. In the earlier days of off-road, the ball differential was the go-to guy, slipping just enough to prevent damage to the rest of the gearbox caused by monster-horsepower motors and the shock of landing from large jumps. Problem was, all that slipping caused the ball diff to wear out quickly, or worse, melt! As the diff is the heart of the transmission—the thing that allows your car to hook up better in the turns and accelerate nice and straight—you can see where there would be a problem!

Fortunately, along came the slipper clutch—a relatively simple device that inherited the slipping duties from the diff. Not only did the slipper clutch greatly increase the life span of the rest of the transmission, but it also allowed for precise control of wheelspin—just the thing for slippery tracks! But just when everything seemed its rosiest, a problem had developed: many people were unable to discern between the sound and feel of a diff slipping versus a slipper clutch. As a result, there were lots of locked-up slipper clutches and burned-out diffs! How can you tell which part of your tranny is slipping? And how do you set each component correctly? Well, that’s what we’re here for.

A Little Diff Basics

What is a diff? It’s a clever little device that takes up the difference in rotating speed between the left and right side driven wheels. Unless a four-wheeled vehicle is traveling perfectly straight, its left and right side wheels travel on different paths, at different speeds. The diff compensates for this while providing drive force to propel the vehicle. Without a diff, one or both of the driven wheels would be dragged across the road surface, resulting in a loss of traction. Some on-road race cars actually have no rear diffs, which enables the driver to accurately slide the car through the turns. But these cars are raced on perfectly smooth surfaces. Whenever there are bumps—even small ones—a diff is needed to keep the vehicle’s driven wheels in good contact with the ground to improve traction, stability and acceleration.

What Happens When Your Diff Slips

A typical ball diff has one large set of diff balls and washers and one small set. The larger balls and washers carry most of the load, while the smaller set primarily keeps the diff working smoothly. When the diff is allowed to slip, the large balls are dragged against their washers rather than rolling against them. The resulting friction not only causes uneven grooves in the washers, but also creates immense heat. The grooves make the diff gritty and less efficient, while the heat can actually cause the diff gear to melt!

How Tight Should the Diff and Slipper Be Set?

Even if you’ve built your diff and slipper perfectly, the only way to really set them correctly is to check them after the first few runs and make any necessary adjustments. When the diff is brand new, its setting will loosen—sometimes significantly—after one or two runs. This is because the diff’s parts will become seated, or worn-in against each other, as the diff operates under load. To compensate for this, you can either set the diff very tightly for the initial run, or, as we prefer, you can check the diff after every few laps and make adjustments as needed. The same applies to the slipper clutch; it too will become looser as its pad or pads are worn-in against the slipper plates. Fortunately, the worse that can happen if your slipper is too loose is that your car won’t go anywhere!

Checking the Diff and Slipper Settings

This is what you’ve been waiting for, right? So how can you tell whether it’s the slipper or the diff that’s slipping? There is one dead giveaway, and you’ll need to use your ears. When a slipper clutch is slipping, it will make a very subtle whirring noise, but when the diff slips, you’ll hear a pretty nasty sounding metallic shriek! That shriek should be your cue to stop running, or else something’s going to detonate! But if your goal is to catch a loose diff before damage can occur, then all you need to do is follow these simple steps.
n Step 1: Remove your car or truck’s gear cover. With the front of the vehicle pointing away from you, hold the spur gear and right rear wheel with your right hand. If you have a truck, you may require an assistant to hold the right wheel. Using your left hand, try to turn the left rear wheel. If everything is correct, only the slipper should be slipping. How can you tell? Look at the slipper adjustment nut; if it turns as you turn the left wheel, you know that the slipper is set so that it slips before the diff can. If the nut doesn’t move, yet you can still turn the left wheel, you know that the diff, not the slipper, is slipping. In this case, you’ll need to re-set the diff.

Step 2: To set the diff, the first thing to do is lock-down the slipper clutch by tightening its adjustment nut until the spring is nearly fully compressed. Now, perform the same procedure outlined in the first step. It should be next to impossible to turn the left rear wheel. If not, stop and tighten the diff about an eighth turn, then re-check. Continue this step until the diff is sufficiently tightened.

Step 3: Now that the diff is set, it’s time to reset the slipper clutch. Begin by loosening the slipper nut a little at a time, checking its setting each time. If you run a stock motor, your slipper should be set more loosely. If you use a mod motor, you’ll need a tighter slipper to deal with the additional power. Set the slipper so that, when holding the spur gear and right wheel, it takes a moderate effort to turn the left rear wheel. As a rule, your final slipper settings should be made on the track. Generally, you should set the slipper so that it slips for about a foot or two as the vehicle accelerates from a dead stop. For high-traction conditions, a tighter setting will provide better acceleration (some pro drivers even lock their slippers completely). For slippery and very bumpy surfaces, a loose slipper setting will provide more traction and will allow the vehicle to work better through the bumps and ruts.

DIFF TIPS

l Make it a habit to check your diff’s setting after every two or three runs. If your diff does begin to loosen, you’ll be able to catch it before any damage is done.

• For best performance, you should rebuild your diff when its action becomes gritty. Usually, all that’s needed is a good cleaning and re-lube. You may also need to replace the large and small diff washers. As long as the diff isn’t allowed to slip, the balls should last through several rebuilds before needing to be replaced.

• Fight the urge to experiment with lubes. Use only lubes that have been developed for use in ball diffs. The silicone-type lubes have special friction qualities that allow the balls to grip the washers to reduce slippage. L

WHAT ABOUT 4WD CARS?

l Four-wheel drive off-road buggies and touring sedans often have two ball diffs—one in back and one up front. You can check the tightness of their diffs just as you would for any ball diff. Generally, you can set the front diff a bit more loosely, as most of the drive force comes from the rear of the car. You can also use the tightness of the front diff as a tuning aid. To make the car easier to drive, run the front diff tighter. This will smooth out the steering and make the car pull out of the turns better. For more responsive steering, especially in tighter turns, try running the front diff more loosely.