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Old 04-25-2019, 01:31 PM
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Originally Posted by Davidka View Post
Does anyone have experience with TLR's planetary gear diff on dirt? It seems like that design would have less bind under load (which may be why they've gone away from it on their carpet spec car..), lending itself to better dirt performance. I think the tuneability of different fluid weights would be valuable on dirt.
I've driven my tlr 22sct on dirt with a gear diff with 3K oil in it the last 3 years. It performs quite well actually. I had been racing the truck on carpet and then switch to outdoor clay and just left the gear diff in to see how it would perform. I just left it in. I still got the ball diff from the kit in an unopened bag sitting in a drawer somewhere at home. That bag will stay unopened as far as I'm concerned. I like the way the truck drives with the gear diff
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Old 05-02-2019, 09:27 AM
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Switched over to ball diffs on astro, reason being diff access is a complete non-starter on the YZ4 and I wanted the adjustability. Think I barked the rear as I set it too loose, so that feels gritty. But despite that, it makes the car feel very "smooth". Much smoother than the gear diffs. It makes the car feel very light-footed and just sails on astro, no issues whatsoever.
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Old 05-02-2019, 11:03 AM
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Originally Posted by oldernoob View Post
Ok, i searched and found nothing conclusive.

Gear vs Ball diffs in 2wd, why would a ball diff drive better on dirt? They both allow the 'differential ' in speed to allow cornering, ball diffs shouldn't be slipping-that's what the slipper is for. Gear diffs have resistance to rotation (like a ball diff) by adjusting fluid. So what is the actual 'diff'erence?

Not just 'more side bite' or 'more forward traction'-there must be a reason for the feel differences, so does anyone know?

Back in the day, a gear diff would diff out onto the unloaded wheel in a corner-this has been countered with sealed fluid filled diffs, this is why everyone preferred ball diffs.

What i don't understand is why a ball diff should be preferred on dirt nowadays?

Asking for a friend ;-)

Ok, so this goes back to the OP 2yrs ago. Lol.

A ball diff and a gear diff feel different on track and can have benefits on a given surface as a result of their function.

A ball diff and a gear diff have very much the same function off power and we all know that during a bend the outside wheel will need to rotate a quicker speed than the inside. This off power cornering and the cars “rotation” through a bend will be dependent on the viscocity of the oil in the gear diff, and the tightness of the thrust race spring on a ball diff. These can can be tuned accordingly.

The difference in their function comes when they are “on power”. During the bend, both diffs will bind on power BUT the ball diff will bind a great deal more.

The harder the vehicle accelerates the greater the ball diff will bind, effectively preventing the ball diff from a differential action.

The result is the reason that a ball diff is preferable on low grip. As you add throttle mid corner the ball diff binds, prevents a diff-ing action and drives the car forward. This will have the feel of weight shifting to the rear and potentially the loss of on power steering and helps prevent losing rear traction exiting a bend.

Alternatively the gear diff will still have a free (considerably less binding) action no matter how hard you accelerate and this can favour carpet or high grip tracks. Mid corner acceleration will have little effect on steering and the vehicle can turn much more freely. This can help you hold the inside of a sweeper at a greater speed than you could with a ball diff. The common phrase is “greater corner speed”

It should be said, as the OP had commented, the gear diffs action under acceleration of staying free, can be the reason why a vehicle with a gear diff can “diff out” compared to that of a ball diff.

Hope you find this helpful.

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Last edited by Piles; 05-02-2019 at 11:43 AM.
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Old 05-02-2019, 11:14 AM
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Originally Posted by Piles View Post



Ok, so this goes back to the OP 2yrs ago. Lol.




A ball diff and a gear diff feel different on track and can have benefits on a given surface as a result of their function.




A ball diff and a gear diff have very much the same function off power and we all know that during a bend the outside wheel will need to rotate a quicker speed than the inside. This off power cornering and the cars “rotation” through a bend will be dependent on the viscocity of the oil in the gear diff, and the tightness of the thrust race spring on a ball diff. These can can be tuned accordingly.




The difference in their function comes when they are “on power”. During the bend, both diffs will bind on power BUT the ball diff will bind a great deal more.




The harder the vehicle accelerates the greater the ball diff will bind, effectively preventing the ball diff from a differential action.

The result is the reason that a ball diff is preferable on low grip. As you add throttle mid corner the ball diff binds, prevents a diff-ing action and drives the car forward. This will have the feel of weight shifting to the rear and potentially the loss of on power steering and helps prevent losing rear traction exiting a bend.




Alternatively the gear diff will still have a free (considerably less binding) action no matter how hard you accelerate and this can favour carpet or high grip tracks. Mid corner acceleration will have little effect on steering and the vehicle can turn much more freely. This can help you hold the inside of a sweeper at a greater speed than you could with a ball diff. The common phrase is “greater corner speed”




It should be said, as the OP had commented, the gear diffs action under acceleration of staying free, can be the reason why a vehicle with a gear diff can “diff out” compared to that of a ball diff.




Hope you find this helpful.

It's 100% the opposite. A ball diff binds less on power than a gear diff which is critical for stability on a low traction surface.
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Old 05-02-2019, 12:11 PM
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Originally Posted by Krio View Post
It's 100% the opposite. A ball diff binds less on power than a gear diff which is critical for stability on a low traction surface.
You might want to listen to this. Tristram Neal, Schumachers designer and the first manufacturer to put a ball diff in an RC car.

http://astateofrc.com/wp-content/upl...Trish-Neal.mp3
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Old 05-02-2019, 12:45 PM
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Originally Posted by Krio View Post
It's 100% the opposite. A ball diff binds less on power than a gear diff which is critical for stability on a low traction surface.
I can 100% relate to what Piles wrote, on astro ball diff wants to either straighten up or drift under power out of the corner. Gear diff hardly changes the steering radius coming out of the corner - as a result of this i find I can get back onto the power with a gear diff much sooner, initially it felt I was behind the car.
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Old 05-02-2019, 02:40 PM
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Originally Posted by Piles View Post


You might want to listen to this. Tristram Neal, Schumachers designer and the first manufacturer to put a ball diff in an RC car.

http://astateofrc.com/wp-content/upl...Trish-Neal.mp3
Tristram Neal is a new pup - was barely even a twinkle when Cecil Schumacher designed the ball diff for model car use! Cecil applied for a patent, but was denied, because the ball differential had already been designed and utilised in self propelled lawn mowers (of all things! Not very 'racey' Lol!).

There is no technical reason why ball diffs should start to lock-up or 'diff-out' less as the torque increases than gear diffs. In fact, fluid filled gear diffs have to over-come the friction of the silicon oil, the thickness governing the action.

Ball diffs might be considered adjustable, but in my experience the useful range of this said 'adjustability' is very small. Too little tension on the diff screw and the diff will squeal and lose power. Worse, the diff screw could back up completely! This also damages the balls, flat spotting them. Too tight, the diff won't operate smoothly and the thrust race and/or balls will be damaged. The main and real technical advantage is lightness and cheapness.

All the theory in the world won't change people's hands on experience with either type of diff. Both types can be setup to work brilliantly, but for higher powered applications, geared diffs are preferable.

For electric classes, in the 80's right through til about 10 years ago, ball diffs were king - due to their lower rotating mass. A well maintained and set up ceramic ball diff can be silky smooth and consistent, regardless of temperature - an aspect that can negatively effect fluid filled diffs if race day temps fluctuate appreciably.

In 1/8th Buggy, Torsen diffs could be utilised to good effect on blown out tracks - usually front and/or centre. Too big, clumpy and expensive to use in 1/10th scale though, despite being actual limited slip differentials.

Last edited by Horatio; 05-02-2019 at 02:45 PM. Reason: Reasons....
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Old 05-02-2019, 02:57 PM
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On low grip tracks if you put more power down to the outside tire you lose grip, not get more. Think about driving your real car around a dusty corner, you cannot apply more throttle or else you will lose traction and spinout. So when your buggy is searching for traction on a low grip track you want the buggy to be able to diff out more easily so the outside tire does not get overloaded and spin you out.

This is why ball diffs are chosen for low grip tracks. They provide a more linear build up of differential action that is very predictable and helps not disrupt the tires and break traction. A gear diff has a more progressive build up of resistance on throttle which makes it easier to overload the outer tire if used on low grip.

On high grip artificial tracks there is no shortage of traction, the buggy does not benefit from finding more traction and splitting power. Putting power down to both tires while retaining a similar low speed driving demeanor/rotation is more important.

I tried this back to back several times with a 2wd and I found the ball diff was more predictable at low and high throttle, the gear diff was predictable at low speed and looser at higher throttle. In order to get the gear diff to feel similar under heavy throttle I had to soften the oil and then it suffered a lot with over rotation and looseness off throttle or at low speed. For this reason I stand by what Krio said.
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Old 05-02-2019, 05:27 PM
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Originally Posted by Piles View Post



Ok, so this goes back to the OP 2yrs ago. Lol.



Some subjects survive the test of time better than others. Now that people actually get to choose between ball diff or gear diff as a tuning option, you can see why people will be Re-visiting threads like this one.


A ball diff and a gear diff feel different on track and can have benefits on a given surface as a result of their function.

A ball diff and a gear diff have very much the same function off power.......
Well that depends on what end of the car we're talking about AND what weight fluid is in the said gear diff. Gear diffs with a greater volume of oil surrounding the planetary gears can be set with enough black mugen super grease so as to effectively lock them out - so there are significant variables.

This off power cornering and the cars “rotation” through a bend will be dependent on the viscocity of the oil in the gear diff, and the tightness of the thrust race spring on a ball diff. These can can be tuned accordingly.
But on a ball diff, you can't just lock the diff down with as much - or as little - diff screw action as you like. There's a fine operating envelope that the diff screw must remain within for the diff to work.

With gear diffs, the limits are down to whatever range of diff oils the racer has in his/her pit box.

The difference in their function comes when they are “on power”. During the bend, both diffs will bind on power BUT the ball diff will bind a great deal more.
[
How is this so? The ball diff uses balls instead of gears that are squeezed together between plates using a screw, spring and a thrust race. Planetary/spider gears are replaced with balls. How is it more likely to bind when more torque is put through it vs a gear diff? Again - a gear diffs performance under power is entirely dependent on the torque being transferred through a viscous fluid, selected by the user. If the fluid is thin, the diff will be more likely to diff out, or if the fluid is thick - less likely to diff out and feel more like a solid axle or spool. Depending on which end of the car we're dealing with, this can be used to increase or decrease on/off power steering.

The harder the vehicle accelerates the greater the ball diff will bind, effectively preventing the ball diff from a differential action.
Again, the ball diff's balls have no choice but obey the laws of physics here. If the torque passing through the ball diff exceeds the pressure/friction between the diff rings and the balls, it ceases to function as a differential and becomes a really expensive form of slipper clutch! :/ One that knackers your diff balls! Otherwise, it performs just like a ball diff would at any other time - the only diff limiting element being the tightness of the diff screw, which merely serves to put pressure between the diff rings and balls.

The result is the reason that a ball diff is preferable on low grip. As you add throttle mid corner the ball diff binds, prevents a diff-ing action and drives the car forward. This will have the feel of weight shifting to the rear and potentially the loss of on power steering and helps prevent losing rear traction exiting a bend.
[
Well setup ball diffs, with nice round balls and diff screws that don't back out, can be a pleasure to use on slippery tracks. But what you describe here infers that ball diffs behave like limited slip diffs, which in my experience, they don't. Nor should they in theory, either.

Also, a diff that 'binds' - or to put it another way - 'starts to lock up', will tend to break traction on that axle. So in other words, on the rear diff, that would result in on-power over-steer. Or on the front, on-power under-steer.

Alternatively the gear diff will still have a free (considerably less binding) action no matter how hard you accelerate and this can favour carpet or high grip tracks....
Well again, this would depend entirely on the choice of fluid used in the diff. The thicker the fluid, the more like a solid axle it would feel.

Mid corner acceleration will have little effect on steering and the vehicle can turn much more freely. This can help you hold the inside of a sweeper at a greater speed than you could with a ball diff. The common phrase is “greater corner speed”
[
Only if the car is balanced properly on the way into the corner and on the correct 'trajectory'. This is largely down to correct car set up and driver skill more than diff choice. A gear diff with the wrong choice of fluid will not be carrying greater speed through a sweeper than ball diff.

It should be said, as the OP had commented, the gear diffs action under acceleration of staying free, can be the reason why a vehicle with a gear diff can “diff out” compared to that of a ball diff.
Diff out occurs when the power being applied by the motor/engine exceeds the traction of the tyres being driven. The tyre with the least traction will spin. With a ball diff, besides a little tweak on the diff screw, there's little you can do with the diff itself. Tyre choice is obviously the first thing to look at and also setting the slipper clutch to prevent wheel spin and diffing out in the first place.

With a gear diff, diff out can be tackled by going up in weight with the diff fluid.

With 3 diff cars, 4WD can very quickly become 1WD! Lol. That's why Torsen diffs were desirable on the centre diff on really loose, dusty tracks in 1/8th.

As with most things, it's all about finding the right balance between being able to put the hammer down in a straight line without losing power to wheel spin, whilst having enough differential action to negotiate corners well enough to get good lap times.

Hope you find this helpful.
It's very helpful to discuss things like this if it yields fruitful results at the track.
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Old 05-02-2019, 06:16 PM
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@trf211
You and Krio are right. With gear diffs, when they are diffing out, the gears have to shear the viscous fluid (silicon diff oil) inside the diff case, thus will still put some torque to the weighted wheel - even if the un-weighted wheel is spinning. The thicker the oil, the more torque the weighted wheel/s is going to get.

Like you say, on very dusty, slippery tracks, I could understand why some might find the ball diff easier to drive.

It's all down to driving style and good setup to suit said driving style.

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Old 05-02-2019, 09:05 PM
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Very simple test since some of you can't seem to understand how ball diffs and gear diffs perform under load.

Take a car with a ball diff hold one of the drive wheels and punch the throttle. Make note of how much the force exerted on the tire in your hand.

Do the same test with a gear diff equipped car.

​​​​​​A gear diff transfers less torque as the differential action becomes greater. This is because the fluid can't fill back in between the gear teeth as the gears start spinning faster and faster.

A ball diff has a set resistance to differential action, and that resistance to differential action goes up as differential action increases, basically acting more and more like a solid axle. A ball diff doesn't experience less resistance to differential action, physics won't let that happen.
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Old 05-03-2019, 02:36 AM
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Originally Posted by waitwhat View Post
Very simple test since some of you can't seem to understand how ball diffs and gear diffs perform under load.

Take a car with a ball diff hold one of the drive wheels and punch the throttle. Make note of how much the force exerted on the tire in your hand.
Firmly holding one wheel, opening throttle, unloaded wheel spins. The more I open the throttle, the faster the unloaded wheel spins. There is little difference to the amount of force required to keep the one wheel from turning when increasing the throttle because only the friction between the diff rings and balls determines it.

Do the same test with a gear diff equipped car.
Done. Though I used 2 different machines. 1st machine was a nitro buggy, with 5000 weight oil on the rear diff. It was easy to feel the slight viscosity of the chosen weight oil which was selected specifically for the rear diff. The held wheel had a slight amount of torque, which as the throttle was blipped, you could feel was trying to turn. The unloaded wheel was just waiting to give the unwary a friction burn. The front diff has thicker 8000 weight oil. The force required to keep one wheel from turning was slightly higher.

Next up, a monster truck. This rear diff has just 50,000 weight oil to prevent diffing out on obstacles and to ensure good wheelies. With a gloved hand, it required significantly more force to hold the wheel, as the diff has to overcome the shear forces of the silicone 50,000wt oil. With the diff filled with Mugen Super Grease (black) it virtually locks the diff completely.

​​​​​​A gear diff transfers less torque as the differential action becomes greater. This is because the fluid can't fill back in between the gear teeth as the gears start spinning faster and faster.
This is a terrible generalisation to make, because gear diffs come in low volume and high volume versions. A low volume diff with light fluid will behave like the above statement. A high volume diff, correctly filled with a higher viscosity fluid will behave nothing like the above statement.

A ball diff has a set resistance to differential action,
Yes it does, which is very marginally adjusted by setting the diff screw with slightly more or less torque.

and that resistance to differential action goes up as differential action increases, basically acting more and more like a solid axle.
The resistance is set by the diff screw, but the smooth, shiny, round diff balls are still very free to roll. They have nominal friction, the same as balls inside a ballrace would have, so as long as the input torque is greater than this resistance, the diff spins (diffs out). In other words, the only torque being supplied to the captive wheel would be from nominal friction from within the ball diff itself, marginally controlled by how the user has set the diff screw.

A ball diff doesn't experience less resistance to differential action, physics won't let that happen.
Less resistance to differential action compared to what exactly? Ball diffs follow the same laws of physics as every other type of diff.

First, for clarity, resistance to differential action can be expressed as limiting slip. Under load, compare the function of a healthy ball diff with a high volume gear diff packed with 100,000wt oil.

Which of the 2 is going to experience 'less resistance to differential action'?
Answer: Ball Diff.
Which of the 2 is going to have the greatest limited slip?
Answer: Gear Diff (with 100,000wt oil)

It's all relative.

Now consider Torsen diffs. When these start to 'diff-out', they mechically and progressively start to lock themselves. So, when the going is good, they act like a gear diff with very light fluid. But when a wheel starts to slip, the diff behaves like it has a very thick fluid. Despite the apparent 'best of both worlds' functionality, for driveability, Torsen diffs were not desirable on the rear of the car - only the centre and front. They cost about $200 A piece, so on a buggy that's a $400 investment 8/ No wonder they aren't common place. Some people swear by them though - it's all a matter of $$, taste and driving style.




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Old 05-03-2019, 06:30 AM
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The oil can't really be evacuated from between the gears unless you don't put enough oil in, or the diff casing has a lot of air space for the oil to run off into. The centrifugal force of the diff casing spinning should ensure that the gear teeth remain submerged with oil so it doesn't revert to being an 'air filled gear diff'. I guess if you want to avoid that you can 100% fill the diff casing with oil as long as leaking isn't a problem.
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Old 05-03-2019, 06:33 AM
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Originally Posted by nbTMM View Post
I guess if you want to avoid that you can 100% fill the diff casing with oil as long as leaking isn't a problem.
I've found there's a limit to this. I once overfilled my front diff on my B64. There was no leaking and the diff would come off the track feeling like it had 50k fluid in it (was 10k) from expansion. There needs to be a little air to compensate for that.
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Old 05-03-2019, 07:32 AM
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The resistance is set by the diff screw, but the smooth, shiny, round diff balls are still very free to roll. They have nominal friction, the same as balls inside a ballrace would have, so as long as the input torque is greater than this resistance, the diff spins (diffs out). In other words, the only torque being supplied to the captive wheel would be from nominal friction from within the ball diff itself, marginally controlled by how the user has set the diff screw.
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.
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