RC diff oil tuning is very important. Here’s a guide to help you to understand how to set your diffs and how they work.
How diffs work:
Differentials are part of all buggies, truggies and monster trucks and their designs vary slightly from one vehicle to another. However their action and setup still the same. The center diff is the most important because it transfers power to the front and rear diffs. The front and rear diffs receive the power from the center diff and distribute the power to the left and right wheels.
Diffs are also very important when the vehicle is turning. When turning, the outer wheels spin faster than the inner wheels. Without the front and rear diffs, both wheels will be spinning at the same speed making cornering harder and increasing dramatically turning radius.
Most of the diffs are composed of four spider gears and two cross pins
Understanding what happens:
Basically, diff transfers power to one of its outdrive. By design, the easiest outdrive to spin (with less resitance or traction) will receive the power. This also means that the wheel or the end of the vehicle with less traction will receive most of the power. This is where diff fluids are important. You can compare diff action to a lightning. The lightning finds the shortest route to hit the ground. In our case, the power generated by the engine will find (through the diffs) the shortest (and easiest) route to spin the wheels.
Differentials can be filled with different weight of silicon fluid. Depending of the manufacturer, the weight (viscosity of the fluid) varies from 1000wt to 100 000wt. The greater is the number, the thicker the fluid is. Some RTR vehicles come with grease filled diffs. Grease is equivalent to 1000wt diff fluid. The diff fluid “locks” or “retards” the diff action. A diff filled with 1000wt fluid will spin more freely and will transfer power to the “looser” outdrive faster than a diff filled with 50 000wt diff fluid. This will make the “looser” wheel or end spin and loosing traction while the opposite outdrive receive no power.
Some manufacturers have released different types of differential. At the opposite of the conventional diff, the Torsen diff transfers power to the wheel that has more traction. Losi recently released the SmartDiff, a system that “locks” the diff when accelerating and “frees” the diff when off power or braking. Xray also offers a similar diff called Active Diff.
Important note: Because diff action is all about traction, make sure you have the right tires for your track surface before experimenting with your diffs.
The center diff:
When accelerating, weight is transferred to the rear end of the vehicle. The rear wheels get more weight and by consequence more traction than the front wheels. The center diff will transfer the most of its power to the front wheels because they are easier to spin and will make the front tires ballooning. This reaction is also call, diff unloading. To avoid this situation, thicker center diff fluid will be the way to go. But going with thicker fluid will not completely stop the diff from unloading during hard acceleration. The idea behind center diff adjustment is to find out which fluid thickness will give you the best acceleration all around the track without scarifying performances.
The fluid selection must be thin enough to transfer more power to the front than the rear when accelerating. Why? Because you need your front wheels to have more power to help your buggy to exit corners and to track straight under acceleration. The fluid must also be thin enough to not send too much power to the rear wheels which will make your buggy over steer when exiting corners because the rear end will spin and loose traction. A basic center diff fluid weight for buggies is 7000wt and for truggies 10 000wt.
Thicker fluid in the center diff will make your rear wheels push the buggy while thinner fluid will make the front wheels pull the buggy.
To get the best from your center diff, don’t nail the gas. Go progressively with the throttle, this will prevent traction lost.
The front diff:
Changing front diff fluid directly affects on and off power steering. Thinner fluid will increase initial cornering and off power cornering. On another hand, if you go with thicker front diff fluid you lost initial cornering and off power cornering but gain when exiting corners. This is due by the fact that the inner wheel turns slower than the outer wheel. Thinner diff fluid “frees” the diff action and makes it easier for the diff to turn its two outdrives at different speed.
See what happens when you nail the gas while the front wheels steer left or right. The inner wheel will get the most of the power and will balloon. Thicker diff fluid will “retard” the front inner wheel ballooning effect. A good basic weight is 5000wt for a buggy and 7000wt for a truggy.
The rear diff:
The rear diff is the easiest one to tune. It should be tuned to be “free” enough to not decrease cornering but it should also be tight enough to prevent the inner wheel from ballooning. At low speed or off power, a rear diff with too heavy fluid will decrease steering because it is harder for the diff to rotate the inner wheel at a different speed of the outer wheel. A too heavy fluid will also make the vehicle fishtail when exiting corner causing over steer.A good basic weight is 3000wt for buggies and truggies.
If you want more steering entering corners, use:
- Thinner front diff fluid
- Thinner rear diff fluid
If you want more steering exiting corners, use:
- Thicker front diff fluid
- Thicker rear diff fluid
If you want more forward traction, use:
- Thicker center diff fluid
For better bumps handling, use:
- Lighter front diff fluid
- Lighter center diff fluid
Starting diffs setup for buggy:
- Front: 5000wt
- Center: 7000wt
- Rear: 3000wt
Starting diffs setup for truggy:
- Front: 7000wt
- Center: 10 000wt
- Rear: 3000wt
How much fluid to put into the diffs ?
If you overfill your diff, it will eventually leak. To make sure my diffs are filled properly, I first begin by filling the diff half way and then I rotate the diff gears to be sure fluid goes inside the gears. When fluid has made its way into the spider gears, I fill the diff until the fluid reaches about 1mm of the top.
There is no perfect setup. One setup will increase forward traction; another will increase off power steering while decreasing speed when exiting corners.
It may take you many tests before you find out the good setup that works fine for you. Take your time and tune only one diff at the time. Begin with the center diff, followed by the front and by the rear ones. The harder is to understand what diffs do and how they react to changes. Once you get familiar, diffs setup will have no secret for you.