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Ball Diff vs Gear Diff

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Old 05-14-2019, 11:28 AM
  #106  
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Originally Posted by dowj28 View Post
How about make a special diff nut that is closed at one end that way the long diff screw will bottom out when it hits the end. A dummy cant over tighten the diff. Then back it out 1/8- 1/4 as needed...problem solved. AE and TRL and others get to work on this!!
That concept will not work. If you set the diff screw to bottom out near the "correct" setting with new parts, then the diff will be too loose once it's broken in, and can never be tightened sufficiently. You would also be relying on the rest of the parts keeping the same tolerances over numerous production runs.
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Old 05-14-2019, 02:42 PM
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Originally Posted by dowj28 View Post
Someone please invent a tool like a torque wrench so we can tighten the diff ball tight enough without having to guess. Call it a Torque Hex Driver..save us all the hassle.
That's a really good idea!
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Old 05-14-2019, 02:46 PM
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Originally Posted by fyrstormer View Post
That's why you have to be careful.
I thought I was! ha ha!

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Old 05-14-2019, 07:53 PM
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Well...all I can say is I haven't personally had any problems with black grease leaking out from behind the thrust washers and contaminating the diff balls. Meanwhile, when I test my ball diffs by jamming both outdrives and trying to turn the input gear, I can see the thrust washers turning against the outdrives, rather than feeling that distinctive grinding sensation of slipping diff balls. As a method for preventing a ball diff from being overloaded with torque and damaging itself, I think greasing the backs of the thrust washers is effective and doesn't cause any problematic side-effects that I could notice.
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Last edited by fyrstormer; 05-14-2019 at 08:40 PM.
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Old 05-14-2019, 08:41 PM
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Well, oil is liquid and has the capacity to flow and seek surfaces, so itís easy to see why I eventually had a problem. The black grease should stay put so long as care is taken to remove any and all excess, like you described.

Likewise, Iíve only very rarely ever had a diff get worn or loose enough to slip, but that does sound like a good safeguard. Iíll remember to do that next time I do my diff.
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Old 05-14-2019, 11:39 PM
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Originally Posted by fyrstormer View Post
Well...all I can say is I haven't personally had any problems with black grease leaking out from behind the thrust washers and contaminating the diff balls. Meanwhile, when I test my ball diffs by jamming both outdrives and trying to turn the input gear, I can see the thrust washers turning against the outdrives, rather than feeling that distinctive grinding sensation of slipping diff balls. As a method for preventing a ball diff from being overloaded with torque and damaging itself, I think greasing the backs of the thrust washers is effective and doesn't cause any problematic side-effects that I could notice.
I'm with this guy. Silicone on the ball side, moly on the outdrive side.

I've been running the same Schumacher/MIP diff for several years now and I've never skidded a ball. It's on the 3rd diff gear (replaced the last 2 when they were shark toothed) and each rebuild I have re-used the same balls, washers (grooved and un-sanded) and thrust assembly. Butter smooth.

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Old 05-15-2019, 12:09 AM
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Originally Posted by fyrstormer View Post
The difference is, the friction that occurs between the tires and the ground is useful, while the friction that occurs between the diff balls and the ball carrier wastes energy.
I would agree, but again, so little energy that it's insignificant. If you've got the gist of this thread, the main topic of conversation was how the balls allegedly start to progressively lock up because of the way they interact with the cage/diff gear/pulley. That was the context behind my post.
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Old 05-15-2019, 12:14 AM
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Originally Posted by dowj28 View Post
Someone please invent a tool like a torque wrench so we can tighten the diff ball tight enough without having to guess. Call it a Torque Hex Driver..save us all the hassle.
They have such things as torque screwdrivers.

But as someone who messes with torque wrenches all day, every day, I can tell you it is very far from a perfect solution, especially at low torque values. Reason being a ball diff is all about the pressure between the balls and the diff rings, which means compressing the spring. A bolted joint usually does not have a spring. A torque wrench usually relies on a calibrated spring inside it. This would probably cause all hell to break loose.
The most reliable way of setting the tension therefore, is the angular rotation of the screw.
An even more accurate way would be to measure the protrusion of the diff screw from the diff nut, but that would be deeply impractical and too easily affected by tolerances of the various diff components, which are usually not precise enough.

I suppose you could get angle type torque screwdrivers if you can find one......
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Old 05-17-2019, 03:29 PM
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To put it differently: A M3 screw with 0.5mm threads has a leverage ratio of ~19:1. However much torque you apply to the screw, the pressure exerted by the screw is ~19x higher. Since the amount of pressure needed to make a ball diff work correctly is pretty small, the amount of torque needed to exert that pressure is so tiny it would be almost impossible to measure using a torque driver.
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Old 05-17-2019, 09:21 PM
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Originally Posted by fyrstormer View Post
To put it differently: A M3 screw with 0.5mm threads has a leverage ratio of ~19:1. However much torque you apply to the screw, the pressure exerted by the screw is ~19x higher. Since the amount of pressure needed to make a ball diff work correctly is pretty small, the amount of torque needed to exert that pressure is so tiny it would be almost impossible to measure using a torque driver.
Not only that, the pressure also rises with a *mild* exponential. It's not perfectly linear. A good ball diff, however, is very close to linear.

When torqueing a conventional bolted joint, the pressure is definitely *highly* exponential to the torque value.
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Old 05-18-2019, 09:51 PM
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Originally Posted by fyrstormer View Post
Regarding Delrin plastic: The self-lubricating property comes from the fish oil used to protect the polymers from oxygen. (why fish oil? I guess because it's cheap and plentiful.) The polymers release the fish oil slowly as the plastic wears down, similar to the way bronze Oilite bushings work.

Having said that, a friction coefficient of 0.2 is definitely not nothing. Rubber on tarmac has a friction coefficient of 1.0, whereas Teflon on Teflon has a friction coefficient of 0.04. So the friction coefficient of diff balls on Delrin is a factor of 5x lower than rubber on tarmac, but a factor of 5x higher than Teflon on Teflon.

Regarding a ball-diff as an "expensive single-use slipper clutch": If you lube the backs of the thrust washers with black grease, the thrust washers will slip against the outdrives before the balls gouge the thrust washers. If the thrust washers are keyed, you can grind-off the keys with a Dremel. Then you can set the torque limit by tightening or loosening the pressure screw. I have all of my ball diffs set up this way -- and since all of my ball diffs seem to be installed in vehicles that don't have slipper clutches, it's a very convenient way to get the same functionality.
Some Delrins also feature Teflon within the plastic mixture itself, thus making it a very slick plastic. Violent Paintball Products made Bolts for paintball markers that didn't require any oiling because they were naturally slick due to the type of Delrin he made his stuff from. I made Bolts for a Marker using Molybdenim Disulphide filled plastic and it was a tad harder but slicker than regular grade Delrin so the only thing you had to oil/lube were the O-rings.

Where you a Gear diff is far inferior is in the viscosity of the oil because depending on conditions you need different thicknesses and when track conditions mandate a more viscous oil, resistance within is greatly increased as well. With a Ball Diff you can tune resistance (firmness or looseness) on the fly and adjust the slipper according to track conditions and go. That's the case on Center diffs for Wheelers and 2WD and I don't care what BS any "Pro" says because in 2013/2014 Gear diff was the new best thing and al cars and pros ran them, and then ball diffs and slippers were back in a prominent way.

In my case I bought the Hobby Pro SB401 and since Gear Diffs were all the rage, I never got the Slipper that was supposed to be included with the kit. When I called to complain I was told the slipper setup was scrapped because no one wants them anymore, so why incur the cost? I was upset and lived with it and I hated every lap I turned without a slipper. Before that I ran the DEX410 and to this day it is IMPO the best wheeler ever made, with the best features, and its a car that was a dream to tune and would respond to changes unlike any other car and in far less time than anything else. A year later HP released the damn slipper and I installed it and the car was suddenly smooth and predictable without ANY of the twitchiness it had with the center diff.
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Old 05-20-2019, 12:15 AM
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Originally Posted by the incubus View Post
Some Delrins also feature Teflon within the plastic mixture itself, thus making it a very slick plastic. Violent Paintball Products made Bolts for paintball markers that didn't require any oiling because they were naturally slick due to the type of Delrin he made his stuff from.

Delrin 500AF Grade Nylon here is lubricated with 20% teflon - it's fibre reinforced and very slick. Ideal for gears etc,


Where you a Gear diff is far inferior is in the viscosity of the oil because depending on conditions you need different thicknesses and when track conditions mandate a more viscous oil, resistance within is greatly increased as well.

This is the thing - gear diffs clearly aren't inferior - otherwise we'd see ball diffs in full size autos - they're just different.


With a Ball Diff you can tune resistance (firmness or looseness) on the fly and adjust the slipper according to track conditions and go....

The 'tuneability range' with a ball diff is very small. The lack of back lash and smoothness of a ball diff is generally far more relevant.

Last edited by Horatio; 05-20-2019 at 01:03 AM.
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Old 05-20-2019, 12:50 AM
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The reason you wonít see a ball type diff in real cars is because they require more maintenance to keep running in top form, just as is required for the RC variety. The adjustment window IS indeed very small, but the most minute adjustment can make a world of difference. I started in this hobby since before the very first stealth transmission was even a thought, and I still own the very first one bed released in my original RC10, so I have decades worth of tinkering with both systems and I can say with all certainty that a ball diff is the superior system, and the adjustability is the crown jewel of the system. A gear diff does not allow for ANY adjustment and therefore it is a closed systems that performs just one way unless you take it apart and clean it, and then change the viscosity of the oil. And while a gear diff can not replicate the performance of a ball diff, you can fine tune a ball diff system to mimic the performance of a gear diff quite closely. So while youíre absolutely right in that they are different, one is superior.

If you take the same exact kit and drop a transmission of each type into it, you should find that the gear diff will allow for a tighter turn radius at medium to slow speed, and the ball diff canít quite manage to turn quite as tightly, but not too far off. On the other hand, on high speed cornering you should notice that if properly tuned to track conditions, the ball diff car will be easier to drive whereas the gear diff car will have a tendency to have heavy understeer, and when decelerating through slower tighter sections, it will then have oversteer so youíre battling both ends of the spectrum. People who have committed to driving gear diffs wonít notice it much if at all simply because theyíre used to it, but hand that car to a person who runs ball diffs and theyíll have a tough time driving it at speed.

If you look back at all the championships won since the advent of the ball diff, you should notice a significant commonality in the winning cars.
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Old 05-20-2019, 02:18 AM
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Ball diffs were "invented" in the 1950's for light-duty applications such as self-propelled lawn equipment. ("invented" is in quotes because it was almost certainly in-use before the 1950's but no patent applications had been filed, so there were no official records of the design.) From a manufacturing perspective, ball diffs are less expensive to make in small quantities, because you only need a couple flat washers, a spring, a bag of BBs, and a pulley with holes drilled through it to act as a bearing carrier. Gear diffs, on the other hand, require special machinery to cut right-angle gear teeth.

Aside from maintenance, the other big concern with ball diffs is the non-interference nature of the components, meaning the components don't interlock and thus are capable of slipping when overloaded. For obvious reasons, nobody would want an automobile diff that dissipates excess torque in any fashion other than spinning the wheels -- not only would rapid overheating and premature wear be a problem with highly-loaded diff parts slipping like that, but in certain applications (trucks, tractor-trailers, off-roaders) it would render the vehicle unusable for its intended purpose.

I'm guessing The Incubus only races. That's the only explanation I can come up with for why he would say gear diffs are inferior because they can't be tuned with a simple twist of a screwdriver. In the real world where the rest of us live, driving surfaces aren't perfectly smooth and clean, and ball diffs wear-out much much faster from grit intrusion and drivetrain shock when wheels skip over bumps. In outdoor driving, the superior tunability of a ball diff is vastly outweighed by their inferior durability. Furthermore, in real off-road driving as opposed to "off-road" racing on hardpacked clay tracks, one or more wheels are in the air often enough that very thick gear-diff oil is the only way to ensure torque gets distributed to the wheels touching the ground. Try driving a T-Maxx or a Savage with shock-oil in the diffs. (there are no ball diffs for those vehicles, so thin gear-diff oil is the closest analogue to a ball diff's performance.) See how many seconds you can stand to drive it before you have to rebuild the diffs with thicker oil to try to improve the torque distribution. Even if ball diffs were available for off-road trucks, they wouldn't be used because they would slip too much and diff-out too much. So no, ball diffs can only be considered "superior" if you narrow your scope to exclude everything except on-road track racing.

Anyway, I have some of each diff type. I make my RC buying choices based on which vehicles look like fun, not just on what their drivetrain specs are. I have a couple buggies and touring cars with ball diffs, and assuming I get them set-up juuuuuust right the first time, they will work fine. If I mess-up and let them slip a few times before I realize they need to be tightened further (or if the thrust washers and BBs are undersized and can't handle the motor's torque), I'll pretty much have to suck it up and buy a rebuild kit to get the diff working correctly again. Gear diffs aren't vulnerable to sudden damage from improper setup, and they can be tuned over a much wider range of operating conditions by using oil as thin as water or as thick as molasses, depending on my needs. Gear diffs are a little more tedious to set up due to the need to disassemble, drain, and refill the diff multiple times until the correct oil viscosity is discovered, but after that it "just works" for months or years. There is something satisfying about the feel of a freshly-built, well-tuned ball diff, but that satisfying feeling wears off pretty fast when it has to be readjusted after every couple runs.
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Last edited by fyrstormer; 05-20-2019 at 02:33 AM.
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Old 05-20-2019, 02:32 AM
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Originally Posted by the incubus View Post
The reason you wonít see a ball type diff in real cars is because they require more maintenance to keep running in top form, just as is required for the RC variety......
No, the reason is that a ball diff cannot take the same loads a full-size Torsen or Gear diff. They only work on comparatively low torque applications, which is why we see them in 1/10th electric classes, rather than any of the other high powered 1/8th, 1/5th, 1/4 scale I/C or brushless applications.

The adjustment window IS indeed very small, but the most minute adjustment can make a world of difference.
I agree that small adjustments to a ball diff can be beneficial to racers that know exactly what they're doing. The benefits are rather small though compared to other chassis, suspension, tyre related tuning options. Otherwise, the over-tightening or over loosening of a ball diff can just be yet another thing to get wrong.

so I have decades worth of tinkering with both systems and I can say with all certainty that a ball diff is the superior system, and the adjustability is the crown jewel of the system.
Frighteningly, I have about 40 years worth of RC Car, Heli and Aircraft experience. Which means I'm officially an old git now.

A gear diff does not allow for ANY adjustment and therefore it is a closed systems that performs just one way unless you take it apart and clean it, and then change the viscosity of the oil.
A gear diff allows for a much greater range of adjustment than a ball diff, but like you mention (somewhat contradictorily I might add) it involves disassembly first and the adjustment is determined entirely by the weight of the silicone oil utilised. But it's adjustment of the diff never the less!

And while a gear diff can not replicate the performance of a ball diff, you can fine tune a ball diff system to mimic the performance of a gear diff quite closely. So while youíre absolutely right in that they are different, one is superior.
Nonsense. In the hands of an experienced racer, both diffs can be made to work very well indeed. A tight ball diff can be made to feel a bit like a gear diff with medium weight diff oil. A gear diff with low to medium weight oil can be made to feel a bit like a ball diff. A ball diff can never be made to feel like gear diff with heavy oil though - it simply isn't possible. You clearly like ball diffs and I do too. I've used them happily for years in 1/0th racing on and off road, with some degree of success at local, regional and even national level. In stock and super stock classes, I felt ball diffs offered an advantage of lightness and smoothness that gear diffs at the time simply didn't have.

But this is referring to just 1/10th scale racing. In all the other forms of racing, it was gear diffs that I used and none of them were deficient in any way when properly built.

It's quite different now though, especially since 1/10th classes are using much higher powered brushless motors/lithium batteries. Gear diffs are getting lighter and power/duration challenges that people had in the 80's and 90's is no longer a concern. Getting power to the ground reliably though is more the brief in 2019. I suspect that we'll see modern gear diffs utilising delrin variants like Zytel and AF500, with the diffs getting lighter yet having more volume in the diff cases for oil.

If you take the same exact kit and drop a transmission of each type into it, you should find that the gear diff will allow for a tighter turn radius at medium to slow speed...
Thus, by your own admission, gear diffs might well be 'superior' given a certain kind of twisty/technical track layout/condition.....

.......and the ball diff canít quite manage to turn quite as tightly, but not too far off. On the other hand, on high speed cornering you should notice that if properly tuned to track conditions, the ball diff car will be easier to drive whereas the gear diff car will have a tendency to have heavy understeer, and when decelerating through slower tighter sections, it will then have oversteer so youíre battling both ends of the spectrum. People who have committed to driving gear diffs wonít notice it much if at all simply because theyíre used to it, but hand that car to a person who runs ball diffs and theyíll have a tough time driving it at speed.
And there we have it - some racers may actually be quite happy to have their car possess oversteer on slow speed corners - so long as they can keep it on the island - it may well mean that the more skilled amongst us can get on the inside of a more neutrally set up car. It's horses for courses.

If you look back at all the championships won since the advent of the ball diff, you should notice a significant commonality in the winning cars.
Ball diffs were a master stroke by Cecil. Light, cheap, adjustable - everything an electric racer needed when packing a 540 sized motor with 1200 SCR cells. But times are changing, requirements are different, plain and simple.

Ball diffs have won precisely zero 1/8th off road world championships.

In 1/10th 4WD offroad, we start seeing gear diffs winning the world championships with the team Durango car in 2009. From 2015 onwards in 4WD, it's gear diffs that have won the world championships in 2 different makes of shaft drive cars, because of the development in motors and lithium batteries. Belts and balls struggle with all this extra horsepower.
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