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- Ball Diff Vs Gear Diff in 2wd -
I have received a question from Scott (see image).
To explain this question a recap on diffs: the purpose of the diff is to allow the left and right wheels to turn at different speeds while navigating corners (where the inner and outer wheels travel different distances) but more importantly for a race car the diff controls how much torque (and hence drive force at the tyre) is sent to each side. Limited slip differentials create some resistance when the left and right wheels turn at different speeds, and both a ball diff and an oil filled gear diff are types of limited slip diffs.
'Open diff' with no resistance (eg gear diff with no oil) -> allows the wheels to turn easily at different speeds, and sends equal torque left and right under power / brakes. This gives very free cornering and very good stability on power as both tyres push equally, but traction can be limited as if one tyre is lightly loaded the diff will send all torque to that side (diffing out).
'Locked diff' eg spool -> both left and right turn at the same speed, and the torque at each wheel will depend how much traction each tyre has. These will generally have slow turn in as the diff resists turning, and under power can power oversteer as the diff will send more torque to the outer than the inner wheel, but will give the most forward acceleration as each tyre will drive with its maximum traction.
(There is a detailed tech talk on this in the book by Dave B Stevens)
A ball diff creates drive by having the two drive rings pressing into the diff balls with a silicone fluid that has very high friction under pressure. There needs to be enough pressure for the diff to drive without slipping, and by tightening them up the resistance and limited slip action also increases. The ball diff is very smooth and the limited slip action is quite tight even when you turn the wheels at low speed. The advantage of this is good traction on a lower grip surface : the limited slip action gives enough drive sharing to give forward drive, but does not increase with Rpm. It also gives the car more stability on corner entry. The disadvantage in high grip is that you need to run them so tight to stop slipping that the car feels to have a lot of understeer on turn in.
An oil filled gear diff uses fluid viscosity to provide the limited slip action. Viscous fluids increase resistance with shear speed,so the diff feels very free when you turn the diff slowly but when you spin it faster the resistance increases. There is also more mechanical binding of the gears in the diff under load so they are not as smooth as a ball diff. In lower grip they can make a 2wd feel less stable under power as they allow the car to rotate very quickly off power, then send more torque to the loaded wheel under power and if you get wheelspin on the inner tyre they very quickly transfer torque to the outer wheel (which also makes them feel unstable on bumpy surfaces). However on high grip they have the advantage they do not need to be run tight (the gears provide direct drive and can't slip like a ball / plate) so the diff can be run more free than a ball diff allowing better cornering, and on power steering is generally needed on high grip so you tune the viscosity to balance corner entry and corner exit feel.
In my experience (and talking to those around the world) I would run a ball diff until the traction has reached a point where the tightness of the ball diff is very high just to stop it slipping under power. A gear diff is used on smoother / high grip surfaces like carpet (and some smooth astro conditions). I'm told that in the UK outdoor bumpy astro tracks for example most still use ball diffs. A halfway setting is to only use 2 of the 4 gears in the diff - this gives a more free diff action for better rotation. For a sealed surface like Chargers I believe gear diffs are used as the grip is very high and corner speed will generally be better with the gear diff.
Sorry for the long answer:) Hope this helps to answer the question!!
Ray
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