can someone plez explain what diffs do and why do they have to be tight or loose [8D]

They serve the same purpose as they serve in full size vehicles. When either the inside or outisde wheel is turning at a different speed or is in a different state of traction, the differential allows for torque to be appropriately distributed.

This is a copy paste thing, and the general idea of a diff. (I not great at explaining things)

The differential provides the means for allowing the outside wheel to spin faster than the inside wheel on corners or curves. The two side gears are splined to the axle shafts, while the two pinion (spider) gears act as idlers to transfer the power from the carrier to the side gears.
The pinion (spider) gears also balance the power load between the side gears while allowing unequal axle speeds when the vehicle is turning.

Sorry also to explain the second part of your question.... A locked diff will make it so the wheels spin at the same rate and is usualy best applied in a climbing/crawling situation.

When a car turns, the wheel on the outside of the turn needs to move a greater distance than the one on the inside, as when turning left, the right side wheels will travel the greater distance compaired to the left side. This is acomplished by the diffrential.
A car can still turn without a diff, but it is difficult.
As far as the loose/tight thing, that can be used as a tuning tool. It is best to keep the diff as tight as needed without locking it up compleatly. In the days before slipper clutches, the diff could be adjusted to accomidate diffrent track surfaces. Loosened to help tracktion on a slipery track.

What everyone said here is true, but I found a very good video with an explanation:

http://www.youtube.com/watch?v=K4JhruinbWc#t=1m50s

The video was made in the 1930s.

Fantastic video! That's the best explanation I have ever seen or heard. I wish it went further. "amazingly simple"

Haha! That really is a brilliant video. I've known how a differential gear works for a long time but I've always had a hard time explaining it to people, this helps.

Nice link gex

...They should have that stickied in the rc drifting thread

Man, I loved that vid. It was the same kind that Joel and the bots commented on in Mystery Science Theater 3000. As Crow would say,"Brought to you by Jam Handy; reminding you keep your preserves in a convenient location.".

That was a hilarious yet very informative video. I think that should clear things up for you. If your wondering about limited slip, or locked differentials, it's because if one wheel is in mud (or something else) and can't turn the other wheel will just spin, like a 1 wheel drive car. locking the differential, or puting in a spool will make that problem go away.

You should only lock the diff if you are going 70+mph doing high speed runs on asphalt. Locking the diff for anything else is just plain stupid.

You have a ball diff right? If you have a fast motor, you want to get the diff screw tight. You can check this by puttings the car on the ground and hold the chassis down. The diff will make a barking sound if its too loose. If it is correctly adjusted, the slipper will slip a little and the car will try to go foward.

Or crawling, or driving on ice, or driving in really loose dirt, or drag racing, or pulling, or etc

wow didnt expect all the replys and that video was FANTASTIC!!!!!!!!!!!!!! but anything old like that is always the best!![8D]

Not to be rude, but my jaw dropped when I saw this. How old are you?

Im an oldfart when things back then made sense[8D]

I am going to disagree with that statement. I run a solid spool, as in no diff, on the rear of my Custom Works Dominator dirt oval car and it works better than it did with a diff. Between the spools both front and rear, and the one way bearings in the front hubs, makes it handle great and it's much more predicitable in the corners.

I have to run locked diffs all around on my crawler, makes sense since if the wheel jammed up against the rock is getting no power, ya got issues . I would agree with some cars running a spool helps, Most on-road and 4WD buggy guys run a super tight, if not spool front diff since it allows for STUPID fast cornering. locking the diff works good if you can sacrifice (on some cars) the steering it would give you [8D]