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Old 12-15-2003, 02:10 PM   #1
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Default Ball bearing differential explaination

Could someone please explain to me how one of these work. I can't seem to understand the concept. I even spoke with Associated's R&D today and still don't get it. Everyone I know adjusts them so the when you hold one wheel (front or rear) and the spur gear, then attempt to rotate the other corresponding wheel (front if holding front or rear if holding rear), the wheel will not slip. So how do the two wheels turn at a different rate (aspecially the fronts when cornering) if one does not slip? There is a pretty descent write up on rctek but they don't quite go into enough detail about the ball differential.

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John
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Old 12-15-2003, 02:29 PM   #2
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Hmmm... how to put it in words..

ok, you understand that when the spur gear turns, both wheels turn the same direction. The diff rings have friction against the diff balls.

When you rotate one wheel by hand, you see the other wheel spins the opposite direction. That is because you are turning one diff ring by hand, it is spinning the balls, which then turn the other diff ring which, since it is on the other side of the balls, gets turned in the opposite direction.

So: balls rolling means differential. Balls not rolling means forward acceleration.

How get the entire thing going forward like you are driving straight. When the car is turning, the inside wheel has a little more friction (tighter radius) so it puts a little bit of a different torque on the inside diff ring.. which causes the balls to roll a little bit which causes the outside wheel to go a different speed.

does that help at all?
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Old 12-15-2003, 02:36 PM   #3
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Somewhat, yes. But, althought the inside tire MAY have more friction, I beleive most of the load would be on the outside tire. Please clarify a little more. I still don't see where the slip comes from if the wheels are spinning at different speeds.

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John
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Old 12-15-2003, 02:49 PM   #4
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The Diff doesnt slip it spins. It allows the inside tire to go a lesser distance than the outer tire without scrubbing the tire. If there were no diff the outer tire has to go farther than the inside tire, the diff just transfers some of the revolutions of the inside tire to the outside tire so that 2 tires trying to get to the same place with one(outside) having to go in a longer arch get there at the same time. so it slows the inside and speeds up the outside. it would be easy to explain if someone could show ya on an actual car.
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Old 12-15-2003, 03:26 PM   #5
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Well, it hit me on the way home from work. Tell me if Iím right. Lets start with a stationary car. If you spin one tire 1000 rpmís the other tire goes 1000 rpmís the other direction or technically 2000 rpmís less. So if the diff gear is spinning, say 25000 rpmís and the inside tire slows to 24000 rpmís (1000 less) that the outside tire speeds to 26000 rpmís (2000 more). Itís relative to the speed of the diff gear. I got it now.
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Old 12-15-2003, 03:29 PM   #6
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Sounds like you do.
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Old 12-16-2003, 07:09 AM   #7
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J-maybe the part thats not clear is that when coming out of a corner the car is accelerating and both tires have a tendency to slip from lack of traction. The diff keeps the two wheels and the shaft locked if they are stationary. When moving it's a slightly different story. When moving if one wheel slips faster, then one of the other members (wheel or driveshaft) can move to compensate. Coming out of a corner the inside tire will slip the most from load transfer to the outside and accelerates losing traction, the drive shaft accelerates faster under motor power to accomplish this and compensates for this slip. So two of the members end up rotating faster.

If you watch a full size car stuck in the mud or on ice you can see this in the extreme sometimes, one wheel can be spinning wildly, the other can be perfectly still and the drive shaft of course must be turning to spin the wheel that is loose.

A one way differential in the front is a device that solves this problem of slipping the inside tire.

Last edited by John Stranahan; 12-16-2003 at 07:18 AM.
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Old 01-04-2004, 10:37 PM   #8
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Default Gear Diff Question

This question applies to both EP and GP cars:

I'm using gear diff on both front and rear axles.

On my rear diff, when I rotate one tire, let say, right side (RHS), both tires rotates in the same direction (i.e. forward direction). If I lock the front wheel by holding it still with my hand, while rotating the rear wheel (RHS), then the other rear wheel (LHS) finally rotates in the opposite direction. What does that mean?

After breaking it by running the car a few times, the rear wheel rotates in the opposite direction, but still feels sticky.

Did I build the diff wrongly, or put too much grease?For info, I spread grease inside the bevel gear casing (the conical gear housing) to lubricate it.

My car is TG10R.

I don't have that problems with gear diff on TA04, nor with ball diff on MR4-TC.

For ball diff, the way I set up is by tightening it all the way, and back off 1/8 turn. For gear diff, obviously I don't have this option.

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Old 01-05-2004, 01:53 AM   #9
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the transmission has to be locked to feel the diif action [ie a motor or your hand has to hold the transmission.
otherwise your turning the wheel as if its going straight and all wheels have equal power.
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Old 01-05-2004, 02:28 AM   #10
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Thanks trf racer,

Nevertheless, in my belt drive cars (TA04 & MR4), without holding the transmission, I can rotate one rear wheel, while the other wheel is rotating in the opposite direction.

I had seen people in my track, just simply gave a simple push on the wheel, and the other wheel also rotated in the opposite dir, and he was holding up the car.

Their rear diff looked so 'smooth'
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Old 01-05-2004, 02:32 AM   #11
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just to add...

I didn't remove the motor and pinion/spur. Probably those holding up the transmission?

In that case, if both wheels rotate in same direction, there's something that's not holding the transmission...?
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Old 01-05-2004, 02:43 AM   #12
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A gas car has less drivetrain drag than an electric when you do a manual test of the diff. The gas car's clutch is not engaged, whereas the electric motor magnets cause resistance. So this could partly explain why the diff doesn't seem to rotate freely.

Also, some gas cars will run with heavy grease in the diff, which stiffens its action - this may be the case with your Tamiya.

Or it could have been built wrong or have a faulty part - is it a lot stiffer than the front diff?
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