Originally Posted by Verndog
I was thinking of trying that myself. But as I understand it, cranking down the ball diff and trying to run it hard that way will reduce the life you get out of it. Not sure just where, but at some point you reach a point of diminishing return when your replacing parts too often.
It took me a long time (years, actually) before I got the hang of tuning a ball diff by feel.
The main thing to feel for is as you're cranking down the diff screw is to know when the diff spring has bottomed out. Once that has happened, you can't tighten any further. Keep going and you'll break the diff screw. Ask me how I know.
Assuming the diff is assembled and working properly, tighten the diff screw slowly and you should feel a very linear resistance as you turn the screw and the diff spring compresses. Once you feel the resistance ramp up suddenly, Stop! That's the spring fully compressed. Now back up the diff screw 1/4 turn. That leaves a tiny bit of slack in the diff spring to cope with shocks and other stresses that pass through the diff.
Note that as 99% of diff screws tighten into a nylon locknut, the ramping up of resistance to be felt as the spring hits full compression can be disguised by the resistance required to turn the screw into the locknut. Only experience can guide you here.
You can run the ball diff at maximum tightness with the diff spring absolutely, fully, compressed, but there is the risk of drivetrain shocks breaking the diff screw and causing your diff to come apart mid race. Again, ask me how I know. That's why I advocate releasing the spring tension by 1/4 turn from maximum. That's the absolute maximum safe limit of tightness on a ball diff.
Ultimately, a really tight ball diff needs to be built up using extremely thick, gummy, goopy grease to provide the tightness. Tamiya Anti-wear grease does the trick for me. It is very resistant to being flung off. I pack the trust bearings full of it, and also the main diff balls and rings too. yes Anti-wear grease does work on the diff rings. The thick grease also offers better wear protection for the trust bearings/washers and diff balls because you are squeezing them so tight. Weak, runny grease will not protect from wear when the components are under so much compression stress. From my experience, on a tight diff, the trust bearing washers are even more prone to wear than the actual diff rings.
Furthermore, Anti-wear grease seems to have a funky property that works well in a tight front ball diff application. In suspension damping terms, it seems to have soft slow bump damping but hard fast bump damping. Even though the diff is very tight, rotating it slowly will still allow reasonable diff action. But if you try to apply a quick shock load to the diff (i.e., rotate it very quickly and suddenly), the grease seems to 'bind/pack up' and become more resistant to movement.
That means when cornering off throttle, the front diff works reasonably well as the left and right wheels are allowed some degree of difference in rotation. But once you apply throttle, the diff binds up. Not hard enough to lock up like a spool, but just tight enough.
I haven't had any experience with other types thick grease, but if you do, I would like to hear of your experience.
This is how I do it, and it works. No sense in getting back to geared diffs. That feels like tech from the early days of RC, fit for the Tamiya Racing Master 1/12 on road cars. Remember how in the late 1980s ball diff were all the rage and if you had a geared diff (sealed or otherwise) in your RC car you were a dinosaur? Why are we going the dinosaur route again?