The contents of THE Guide is copyrighted © by JQ Products, but we thought it was rather brilliant and informative.

Parts: [1 Intro] [2 Electrics & Clutch] [3 Shocks] [4 Diffs] [5 Geometry] [6 Ride height] [7 Down travel] [Quick Reference Tips]

4. Differentials

1:8 buggies have three differentials, front, centre and rear. They can be fine tuned by filling them with different viscosity silicon oils. They affect the way the car handles drastically. The diffs affect the way the car turns, handles the bumps on and off power, and how it accelerates. All cars come with what has become known as standard diffs, gear diffs with 4 small gears, and 2 larger gears. They are sure to work well on all tracks. There are a number of special diffs on the market which are designed to lock, more than the standard diffs. They are normally used to increase steering.


A basic differential. Gears shims and pins. Pretty simple.

4.1 Building a Differential

Most diffs nowadays are durable, and the oils don’t usually leak out. However it is a good idea to put grease on the outdrive shaft. It reduces wear as well as helps to keep the oil in the diff. When filling a diff it’s important to remember 2 things:

  1. Do not put too much oil in. The pressure in the diff will increase too much and the oil will leak out.

  2. Fill it in a way that you can always put the same amount of oil in. This way your set up will be consistent.

One way to fill the diff a suitable amount, is to fill it so that the diff cross-pins are covered by about 1mm of oil at the centre. It’s a good idea to rotate the outdrive so trapped air gets out, and if needed, more oil can be added.

Another good way to fill the diff, is to fill the diff a bit more than in the previous example, and then place the last big gear in the diff, and turn the outdrive a few times. Then when the oil has settled, simply wipe the excess oil away. This is a good way to do it so you can repeat the process consistently.


One good way to avoid overfilling, and to be consistent, insert the last gear and wipe excess oil off.

4.2 Set Up

A good starting point that will work for all cars is, front-centre-rear, 5000-7000-3000. This is a popular basic set up and if your car is terrible, and you have this diff set up, you can be sure that it’s not the diffs. It will be decent everywhere. However, on THE Car, I tend to like to run a thicker diff in the front, to smooth out the aggressive steering, and a thinner in the middle, to smooth out the acceleration and punch, so 7000-5000-3000.

Diffs can make you fast or slow. Everyone has their own driving style, and it is good to try different diff combinations for oneself, to see what feels best, and what is the fastest combination. Normally thinner oils will make the car easier to drive. On slippery, bumpy tracks thinner oils are better. On smooth high traction tracks thicker diffs will be better. Thicker diffs give more acceleration, more cornerspeed, and help to make the car more stable on a high traction surface. On special tracks, that are really smooth and high traction, asphalt like, the car will be a lot more stable and fast in the corners if all diff oils are made thicker. Front and centre by about 10000 compared to the normal set up, and rear maybe 5000. Because the track is so smooth you can get away with it.

The front diff mainly affects steering on and off power, and acceleration, the centre diff affects the way the car handles bumps, and acceleration, and the rear diff affects rear traction and steering. Next I will try to explain what each diff does when tuned separately.


4.3 Front Differential

Using thicker oil will make the car turn more on power, out of the corner, and accelerate faster. The car will turn less into the corner, as off power steering is reduced. It will feel more stable, and can be easier to drive in bumps. If the car is twitchy and feels inconsistent on a rough track, a good idea is to try a thicker front diff. A thinner front oil will have the opposite effect, less on power steering, more off power steering, less stability. Usually the oils used in the front diff range from 3000-15000. 5000-7000 is a safe bet on all tracks, and that’s what I use most of the time when I’m using a standard front diff. With THE Car it is also possible to use thicker oils as it has so much offpower steering, so 10-15k will work on most tracks.


4.4 Centre Differential

Using thicker oil in the centre diff makes the car accelerate a lot faster, but it can be harder to drive in bumps and on slippery tracks. It gets a bit confusing though, because if the track is soft, and it gets really bumpy, a thicker centre diff can actually make the car skip over the tops of the bumps, and thus it will actually be better and faster. But most of the time, thinner centre diffs are used for blown out tracks. A thick centre diff oil will also make it easier to get on power steering, as the rear of the car can be made to slide out when on power. The centre diff usually has the thickest oil of the three diffs, or the same as the front. The oils used normally range from 3000-20000. A safe bet is 5000-7000. I normally never go below this, because I feel that I loose too much acceleration, specially the first ”snap” when I get on the gas. And I don’t either use thicker oils than 10000 because I’m so aggressive on the throttle, I would fly off the track. Normally, if a thicker centre diff oil is used, the front diff also needs to have thicker oil, so the car still remains stable under acceleration.


4.5 Rear Differential

The rear diff set up is the one that varies the most between different drivers. This is because the rear diff has a big effect on rear traction, and each driver’s driving style determines what oil should be used. It is vital to get the right rear diff oil that suits your driving style, otherwise you will only be fighting your car. Some years ago it seemed like everyone always used the thinnest possible rear diff oils. I think it was because people were used to electric offroad. A thin rear diff oil will make the car have a lot of steering into corners, the car is easy to control when accelerating, even if it’s bumpy. So it seems like the way to go right? Yes and no. One drawback is that the rear can lose its traction suddenly. It will have traction, and then lose it all at once when entering a corner. A thin rear diff suits drivers that brake before a corner, keep a tight line around it, then accelerate hard after the corner, towards the next obstacle. This is not how I drive. I like to control the car a lot with the throttle. A thicker rear diff oil will make this possible, it will be possible to steer the car with the throttle. You can enter the corner aggressively because the rear is very stable, and won’t suddenly lose its traction, it will start sliding in a controllable way. After braking, it is possible to drift on throttle, like a rally car through the corner and onto the next straight. I’m on the gas before the corner, and halfway in the corner, or sometimes even before halfway I’m already on the gas hard.

The thick oil makes the rear tires pull more evenly. On a slippery or bumpy track the car can be too hard to drive. The rear diff oil is normally the thinnest of the three diffs, or the same as the others. The oils used range from 1000-7000. 3000 is a safe bet, no matter what your driving style, or what the track conditions. I nearly always use 4000-5000.


4.6 Last Words

Diffs are a major tuning aid. Anyone that wants to become a better driver or just wants to understand their car better should start trying different set ups and writing down each change and its effect in a notebook. As for the special diffs, they are expensive, and good, but definitely not something you HAVE to have to do well. You can become a World Champion by running normal diffs, 5000-7000-3000 or something similar, there’s no doubt about that!