Differentials Explained: Gear, Ball, Spool and Locked
What a diff is actually for, the four kinds you'll meet, exactly what thicker oil does at the front, center and rear, how to fill one without trapping air, and how to spot the classic "diff is out" failure.
Updated Jul 22, 2026 ยท RC Crash Crew
A differential is the part that lets your wheels turn at different speeds. It is also one of the most powerful tuning tools on the car, and one of the least understood โ largely because the adjustment happens inside a sealed case and you cannot see what it is doing.
This page covers what a diff is for, the kinds you will meet, what changing the oil actually does at each end of the car, and how to tell a diff problem from a suspension problem.
What a differential is for
Drive a car round a corner and the outside wheels travel further than the inside wheels. If both wheels on an axle are locked together, one of them has to slip.
The differential solves that: it splits power between two outputs and lets them rotate at different speeds while still both being driven.
The trade is built in. A diff that lets wheels turn independently also lets a wheel with no grip spin uselessly while the one with grip does nothing. Every diff decision in RC is a point on the line between "turns easily" and "always puts power down".
The kinds you will meet
Gear differential
A small case containing bevel gears, usually filled with silicone oil. The oil is the adjustment: it resists the internal gears turning relative to each other.
- Thin oil โ the diff acts freely. Wheels take very different speeds easily.
- Thick oil โ the diff resists differentiating. The two outputs behave more like they are locked together.
Because the resistance comes from fluid, it is speed-sensitive: a gear diff resists more the faster the two sides try to differ. That is why it feels progressive rather than like a switch.
Ball differential
A ring of small ball bearings squeezed between two plates, with adjustable spring pressure. Common on 1/10 on-road and touring.
- Adjusted by tightening or loosening a screw.
- Under an extreme load it slips, which acts as a built-in shock absorber for the drivetrain.
- Needs more regular maintenance than a gear diff โ the balls, plates and thrust bearing are wear items.
Spool
Not a differential at all โ a solid connection between the two outputs. Both wheels always turn at exactly the same speed.
Used in some racing classes on high-grip surfaces, usually at the front. Maximum forward drive, at the cost of a car that resists turning.
Locked diff
A gear diff filled with extremely thick fluid, or physically welded/pinned solid. Standard practice in rock crawling, where the ability to keep driving a wheel that is in the air is worth far more than the ability to turn neatly.
On any grippy surface a locked diff makes the car push wide, chirp its tires in corners, and load the drivetrain hard. That is the correct behavior for a crawler and the wrong behavior for almost anything else.
One-way
A diff that drives forward but freewheels on deceleration, so the front wheels are free to coast into corners. A racing-specific part with real drawbacks โ you lose engine braking at that end entirely โ and not something to fit casually.
What oil weight does, end by end
This is the useful part, and it changes depending on where the diff is.
| Diff | Thicker oil does this | Thinner oil does this |
|---|---|---|
| Front | Less steering, more push (understeer) โ especially on power | More steering, more turn-in, more chance of the inside front spinning |
| Center | More locked, more direct, more traction out of corners; less rotation | Front and rear act more independently; easier rotation, more freedom |
| Rear | More forward drive, more stability, more push | Rotates more easily, less drive on hard surfaces, more wheelspin |
Two rules that make this usable:
- Front thicker = less steering. Rear thicker = more stability. Almost everything else follows from those two.
- Change one diff at a time, and change it by a meaningful amount. Going from 5,000 to 7,000 cSt and declaring nothing happened proves nothing.
The catch with the numbers
Diff oil is sold in cSt (centistokes) โ 3,000, 7,000, 30,000, 100,000 and up. Unlike shock oil weight, cSt is a real measured unit, so the numbers do compare across brands. That is genuinely helpful and worth knowing, because shock oil "weight" does not.
What does *not* transfer is what a given number means on *your* car. A 7,000 cSt center diff on a 1/8 buggy and on a 1/10 touring car are completely different propositions. Start from your manual's stock figure, then move from there.
Filling a gear diff properly
The most common diff problem is not the oil weight. It is air.
- Strip and clean everything. Old oil, old gasket, old sealant. Inspect the gear teeth for chipping and the case for cracks.
- Check the O-ring or gasket. It is a consumable. A diff that has leaked will leak again with fresh oil in it.
- Fill to about 80โ90% and rotate the gears slowly by hand to work trapped air out from between the teeth. Let it stand a few minutes.
- Top up, then close it slowly. Excess oil should be squeezed out as the case closes โ that is what you want.
- Do not overfill to bursting. A diff filled solid with no air at all builds pressure as it warms, and pushes oil past the seals.
- Check for leaks after the first run. Oil on the outside of the case, or on the inside of the gearbox, means it is going somewhere.
Both diffs on an axle pair should be built the same way. A front and rear built to different fill levels are not comparable, and any tuning you do afterwards is chasing your own inconsistency.
When something is wrong
| Symptom | Likely cause |
|---|---|
| One wheel spins, the other does nothing | Diff has lost its oil, or is empty โ the classic "diff is out" |
| Clicking or grinding on power | Chipped or stripped internal gears, or a stripped ring/pinion |
| Oil inside the gearbox or on the chassis | Failed seal or cracked case |
| Car suddenly turns much worse than yesterday | A diff has thickened up (contamination) or one has failed |
| Car suddenly turns much *better* than yesterday | Usually not good news โ a diff has emptied |
| Notchy or stiff to turn by hand, cold | Overfilled, or a bearing has failed |
| Ball diff slips constantly under power | Needs adjustment or rebuild; plates and balls are worn |
"The diff is out" is the single most common failure and it is easy to confirm: lift that axle, hold one wheel, and turn the other. It should turn with a smooth, consistent resistance. If it spins freely with no resistance at all, or if the opposite wheel does not counter-rotate, open it up.
The maintenance nobody does until it is too late
- Gear diffs are sealed but not immortal. Oil degrades and seals harden. Check them at the same interval you rebuild your shocks.
- Ball diffs need attention far more often โ they are a wear-based mechanism by design.
- Outdrives and driveshaft cups are where the wear usually shows first. Sloppy, rounded cups make a clicking noise and eventually round out the driveshaft pins.
- After any deep water run, diffs need checking. Water past a seal emulsifies the oil into a pale sludge that does not damp anything.
Where to start if you have never touched one
Leave them alone until the car does something you can name.
Stock diff settings from the manufacturer are a reasonable, tested baseline, and diff oil is the kind of adjustment that will confuse you badly if you change it while you are also changing shocks, springs and tires. Get the mechanical basics right first โ there is a guide on this site specifically about the mechanical check that comes before any tuning โ then come back here when the car has a specific, repeatable behavior you want to change.
When you do start, write down what was in there before you drained it. More diff-tuning afternoons are ruined by not being able to get back to the starting point than by any wrong choice of oil.