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Gear diff in theory

Gear diff in theory

Old 01-06-2013, 10:49 AM
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Default Gear diff in theory

Talking just in theory so I can learn, suppose my 1/10 scale 2wd SCT with a gear diff is going down a long straightaway on carpet with max traction. How much of the "power" of the motor thru the transmission is going to each rear wheel? 50% each?

Suppose I start a 90 degree left turn and slow up a little. The diff I believe allows the left wheel to slip, meaning it may get less than 50%. How much less? 0%?

I've made the turn and once again on the long straightaway. Is power gradually or quickly added to the left wheel so it gets its its 50% and the right wheel is also at 50%?

Now in the real world, what are these %?

Is it different going from a gear diff to a ball diff?
Does this % change from carpet to outdoor clay or dirt which have different traction?
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Old 01-06-2013, 04:29 PM
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I'm not an expert, but I'll give you some ideas to think about.

Gear diff, straight line and no wheel slip, will have no power loss, as the diff won't be working, it'll just be spinning as if it was a solid gear.

Ball diff, straight line and no wheel slip, if the diff is too loose it will have some power loss (like a slipper), but generally will be about 0 loss as well when adjusted appropriately with the slipper.

When you turn, you don't really "give power" because we don't have smart diffs with traction control. Really the better way to think about it is to think of how easily the diff allows the wheels to spin at different rates (rather than how much power each wheel is getting).

The looser the ball diff, or the lighter the oil in the gear diff, the easier it is for the wheels to spin at different speeds. This is why when you have problems spinning out accelerating down the straight, you tighten the diff or thicken the diff fluid, so you lock up the diff more for less wheel spin on just one side making you squirly (often called "diffing out"). If you can't turn tight enough because the rear kind of drags rather than the wheels rotating at different rates, you can make the turn more easily by loosening the diff or going with lighter diff fluid.

Both diffs will work and be adjustable. A ball diff is easier to adjust (usually just turn a screw a little), and is often considered smoother in the diff action. But they also don't handle huge power as well (such as 4wd sc or 8th scale), and need to be rebuilt more often to stay smooth. Gear diffs tend to last for a long time, but just changing the temperature you're running in will change how well it works (as will warming up the diff by using it a lot when it is cold) with the fluid you have in it (much like shock oil changes in temperature).
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Old 01-08-2013, 11:14 AM
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justpoet is spot on. The only thing I have to add, is this video. It's ancient, and in black and white, but is still THE BEST video on how diffs work. EVER.

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Old 01-08-2013, 06:01 PM
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Great info justpoet, i personally am considering sticking with the gear diff in my xxxsct. I know everyone says the ball diff is the "be all end all" but I have like 3 spare diffs(bought truck used with a ton of parts) that I want to seal and fill with different weights.
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Old 01-08-2013, 06:25 PM
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Great info . That video is way too cool
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