I know this topic has been beaten to death. I understand the general advantages and disadvantages of both, and I know everyone with experience prefers one or the other for certain applications. My question is very specific.

One of my Tamiya XV-01s is a rear-motor conversion, meaning the motor directly drives the rear gearbox, and there's a belt-drive that sends some power up to the front wheels. Basically the same as the TA06, but with the motor behind the rear axle instead of in front of the rear axle. Functionally speaking the only difference is the weight balance, the drivetrain otherwise works the same.

Currently I'm running a tungsten-carbide ball-diff in the rear, but after just a few runs I'm starting to notice some roughness in the ball-diff when I rotate the rear wheels by hand. Since this particular chassis configuration forces the rear wheels to do most of the work of moving the car, *and* there's no "give" in the driveline between the motor and rear diff (whereas with most belt-driven cars there's at least a tiny amount of stretch in the rear drive belt) I'm wondering if maybe I should use a gear diff in the rear instead.

Are there any flaws in my logic? Is there some reason I should I keep the ball diff in the rear anyway? Also, are Tamiya's plastic diff gears good enough, or should I spend the extra money to get metal diff gears?

Well, alot depends on the conditions you're driving in, if traction is at a premium, then the ball diff might be your best choice(& you'd just have to bite the bullet about rebuilding it on a regular basis). But if there's enough grip consistently, then yes, I think you might be happier with the gear diff. I've had my TRF417's rear gear diff in there since I bought it over 3 years ago, & I've never felt a need to mess with it at all(it hasn't even had any significant leakage in the 3 years I've been running it), when built right, it's VERY durable & consistent, & smooth. but about metal or plastic gears in it, again, it depends, if you're sticking with spec motors(say 17.5's or milder), then you may be ok with the plastic ones(I have the plastic ones in my TRF211 buggy's gear diff with a 17.5, & so far, it's held up very well), but if you suspect it'll get more abuse(from either the conditions, or from using more motor), then the metal gears will certainly give you more peace of mind(& I have those in my 417 & my TRF503 4WD buggy, I like those too, very well made & very tough)....

I'm running a 4000KV 2-pole motor in that car. It's geared for a top speed of 40mph. Traction is always at a premium on the unswept pavement I drive on, but I'm slowly learning which tires work best for me, so traction is slowly improving.

Generally I go with metal gears whenever possible, but since Tamiya is apparently much better at making things out of plastic than a lot of RC companies are, I find myself tempted to see whether the plastic gears can handle the load. But since this is a custom conversion, getting into the gearboxes to perform repairs is more of a pain in the ass than usual.

I know, I'm overanalyzing things. That's part of the hobby for me, apparently. There is absolutely no reason I shouldn't go with metal gears except to see if the plastic gears can handle the load, and also to see if having 6g less weight in the rear actually makes a damn bit of difference.

Why do you say the ball diff would be better for traction? I thought their main advantages were light weight and external adjustability for the limited-slip feature. Do they do something else to improve traction that I don't know about?

Ball diff = quicker to adjust, but not good in dirty environments
Gear diff = slower to adjust, but good in dirty environments.

The diff is contained inside a gearbox, so I'm not sure dirt will get in either way.

That's just been my observation between a good ball diff & gear diff. I've noticed it in both my touring cars, & in my offroad buggies. With a ball diff, my cars tend to be more forgiving to drive(& of course, they offer the ability to adjust how tight they are very quickly, to get it right for the situation), while the gear diffs make my cars feel more aggressive(definitely seems best in high-grip conditions). To me they seem to accelerate harder than with a ball diff(ball diffs might still have a wee bit of natural "slip" to them, might explain the difference in feel), & of course, when set up right, the gear diff will be fine for far longer than a ball diff can....

Interesting, thanks. I haven't run ball-diffs in high-power vehicles for long enough to be able to compare between the two.