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Ball Diff vs Gear Diff

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Old 05-03-2019, 10:26 AM
  #31  
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Originally Posted by waitwhat View Post

Your ball race analogy is erroneous because in a typical bearing the force is coming from one of the races, whereas in a ball diff the forces are coming from the "cage".


Newton's third law of motion proves that a ball diff resists differential action as differential action increases.
Fair point, but the reason that a ball diff has less bind is because the only pressure/load sensitive part of the assembly is the contact between the balls and the diff gear. Everything else experiences the same load regardless of how much power is fed into the system so it is more free under load.

In a gear diff, as load is increased, friction increases between the spider/sun gears and the gears and the backing surfaces of the diff case. Everything inside of the assembly is binding and attempting to separate from everything else. Silicone diff fluid isn't a very good shear lubricant so while it feel smooth unloaded in our hands, it's different under load in the car. There are performance benefits to this in some conditions, and it's why most high performance full size cars have some kind of locking differential.
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Old 05-03-2019, 12:05 PM
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The coefficient of friction between a highly polished hardened metal sphere and the plastic of the diff gear, with a bit of silicone grease, is lower than metal on metal with a viscous oil.

Going from the link you posted, the normal force in this case would be the transfer of torque from the diff gear to the diff out-drives through the diff balls. The more torque, the more force, the more force from friction that resists diffing action. The instant one wheel looses traction, the torque drops and so does the friction. That's what ball diffs excel at: instantly letting diff action take place if one tire goes over a pothole or over a slick spot, preventing all the torque from being applied to the other tire and causing it to break traction.

We aren't saying this friction that 'locks up' a ball diff doesn't exist. Just that it does so to a lesser degree than a typical spider gear diff with common diff oils. Another part of this is the location of the diff balls relative to the center of the diff. The further away from the center of the diff, the less force there is on the components to transfer the same amount of torque. The balls in a ball diff are further away than the faces of the gears in the gear diffs by a good 30-40%. This reduces the force (normal force, to tie this into the previous paragraph) which reduces the friction force.
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Old 05-03-2019, 12:57 PM
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Originally Posted by waitwhat View Post
You seem to misunderstand how a ball diff works.

The diff gear transmits the driving force into the balls and the balls roll along the rings. The increase in resistance comes from the fact that the balls do not roll along the diff gear, but rather slide against it. The more differential action there is, the more resistance there is because the balls are fighting against the diff gear, not the diff rings.

Your ball race analogy is erroneous because in a typical bearing the force is coming from one of the races, whereas in a ball diff the forces are coming from the "cage".


Newton's third law of motion proves that a ball diff resists differential action as differential action increases.
Ok Pal, I see asking you any form of question results in the standard response: 'you don't understand how ball diffs work...wah wah'. Whatever.

Ball diffs are very, very simple to understand and you're the one that's got things all arse about face. Posting irrelevant 'Physics for stupid people' links is just going yank people's chains.

It wouldn't have been so bad had you posted links to do with friction co-coefficients, or other open diff theory. But Nada.

The only part of the diff we need to be concerned with are the parts that we can do something with. Your equal and opposite reaction nonsense is not going to turn a ball diff into a limited slip diff. Ball diffs are the crudest of all diff options.

The diff screw is what we adjust to control diff action. The friction in the diff arises from the cage, yes, but also the diff rings, the thrust race and the tension applied to them.

The diff action you described in your earlier post would have us believe that a ball diff has more of a limited slip effect than a viscous differential. Making a mistake is one thing. But once this has been pointed out and the folly of such reasoning has no answer - then it becomes willful ignorance.
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Old 05-03-2019, 01:20 PM
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Pick the car up, by the rear tire. Goose the throttle. Feel which car pushes more force into the held tire.

It's that simple. Ball diff and gear diff are the same thing. The balls are just acting as tiny spider gears.
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Old 05-03-2019, 01:36 PM
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Originally Posted by Zerodefect View Post
Pick the car up, by the rear tire. Goose the throttle. Feel which car pushes more force into the held tire.

It's that simple. Ball diff and gear diff are the same thing. The balls are just acting as tiny spider gears.
Yes that is a given and not being disputed by anybody. It's more about the subtle difference in how they drive and why is what is being discussed.

I always thought it would be skmple to prove by running a buggy with one drive shaft taken out and compare how each diff accelerates at different throttle levels.
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Old 05-03-2019, 01:53 PM
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So subtle you can duplicate all of them with either diff.

Until you start running really thick diff fluid in the gear diff. Ball diffs can't do that well.
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Old 05-03-2019, 02:19 PM
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.

Last edited by trf211; 05-03-2019 at 02:51 PM.
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Old 05-03-2019, 02:30 PM
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Originally Posted by Zerodefect View Post
Pick the car up, by the rear tire. Goose the throttle. Feel which car pushes more force into the held tire.

It's that simple. Ball diff and gear diff are the same thing. The balls are just acting as tiny spider gears.
Simply put, yes in a nutshell + factor in the fact that you can put oil of your choice into a gear diff.

Last edited by Horatio; 05-03-2019 at 07:38 PM.
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Old 05-03-2019, 02:44 PM
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Originally Posted by Zerodefect View Post
Pick the car up, by the rear tire. Goose the throttle. Feel which car pushes more force into the held tire.

It's that simple. Ball diff and gear diff are the same thing. The balls are just acting as tiny spider gears.
It isnít that simple. Thatís still testing the diffs in an unloaded state. The balls are doing the job of the spider gears, but as power & load are added, friction between them and the diff rings doesnít increase. Friction does increase in the pasrts of a gear diff under load. They do the same job, but not exactly the same way.
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Old 05-03-2019, 02:55 PM
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Originally Posted by Zerodefect View Post
So subtle you can duplicate all of them with either diff.

Until you start running really thick diff fluid in the gear diff. Ball diffs can't do that well.
I totally get where you're coming from. Within the useable range of a ball diff (which is a very small adjustable range) lies the range where 99% of gear diff users will get their diffs.

Running really thick oils though - ball diffs can't do that at all. Does super gluing the balls to the diff rings count? >;p

Lol!
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Old 05-03-2019, 03:21 PM
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There is a school of thought that suggests adjusting a ball diff by bringing it up to tension - rather than cranking it up then backing it off.

I agree with this. I'm a firm believer of not running a ball diff any tighter than is absolutely necessary.
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Old 05-03-2019, 05:37 PM
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Old 05-03-2019, 07:40 PM
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Originally Posted by waitwhat View Post
And?
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Old 05-03-2019, 07:57 PM
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Originally Posted by Horatio View Post
And?
There is your limited slip differential that operates like a ball diff.

Maybe you guys are not understanding exactly what I'm saying. The ball diff does gain mechanical impedance as differential action is increased (up to a point)...

If your car is diffing out you are overdriving the car, or perhaps your car needs some suspension adjustments. I run my 2wd buggy (equipped with a laydown gearbox and a ball diff) on my loose dirt back yard track. Holeshot type tires are the tires which work best, and my car has no problems pulling wheelies. I don't overdrive my cars, and I am very acutely aware of their performance.

Tell me Horatio, when was the last time you set a lap record?

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Old 05-03-2019, 07:59 PM
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Also, unless you change the fluid very frequently, your gear diff performance changes so drastically that the car can go from good to shit in the course of a few runs.

I can get a whole season out of a ball diff between services. I can't get a whole season out of a gear diff between services.
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