I came across this thread on rctech.com, and thought it was an very excellent read. So much so, I thought I'd put it on my site for everyone to read. The original author is a guy with the user name dinorider. I sent him a PM last week trying to find out who he is so I can give him credit for the post, but have yet to hear back from him. As soon as I find his real name, I'll add it to this post.
Enjoy!
[/Tim Sanderson]
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Gear Diff vs. Ball Diff - written by dinorider
Remember in the 1970s, the infancy of modern RC racing, where pretty much everything was 2WD offroad, cars came out of the box with a rear spool, or open bevel geared diff.
The rear spool on a 2WD buggy was fine on loose dirt, but on higher traction surfaces the diff was the better option. But an open bevel geared diff allowed the inside wheel to diff out and spin uselessly should it lift off the ground while cornering. Because of the mechanics of a bevel geared diff, once the inside wheel goes into the air, the opposite wheel will get zero drive. So the car looses forward propulsion.
In high grip 1/12 pan car carpet racing though, the open diff worked fine because the wheels rarely ever lifted off the ground.
Then the ball diff started appearing on the scene at the end of the 1970s/start of the 80s. It's worth noting that the ball diff is a RC car specific piece of equipment. 1:1 cars don't have such a component. Perhaps the closest thing on a 1:1 car is the Limited Slip Differential, a.k.a LSD. The ball diff was a revolutionary idea in that it stopped wheels from diffing out in corners, and it also worked as a slipper clutch (before the separate slipper clutch was invented) to reduce gearbox damage in the rough offroad racing environment.
Back then, like now, everybody wanted in on a piece of the action and went nuts 'upgrading', but without the benefit of the internet and whatnot, much misunderstanding was had about the ball diff. Remember, in those days, pretty much all RC info came from magazines and via word of mouth. It didn't help that many RC car magazine writers back then didn't fully understand the function of the ball diff either.
In short, people 'upgraded' without understanding the real benefits of that piece of tech.
Essentially, the ball diff worked by squeezing ball bearings between two diff outdrives, and the friction/pressure from that gave the whole unit a degree of tightness while still allowing the two wheels to rotate at different speeds when tracing a line through a corner. At this point, I assume you know why a diff is fitted to a car. If not, there's howstuffworks.com.
This tightness in the diff ensured that the wheel in contact with the ground would still get some drive power should the inside wheel ever lift off the ground.
However, early ball diffs did not have keyed diff rings. The diff rings were designed to slip a little on the outdrives, a function that the slipper clutch eventually took over.
This did mean that the early ball diffs were really meant to last 3-5 runs before a complete rebuild. Expensive, and a heck of a chore trackside. The rush was on to make harder diff balls because the soft balls were slipping and flatspotting. Then the quest for harder diff rings as the harder diff balls bit into and gouged the diff rings. Another component that people often overlooked until the thing self-destructed was the trust bearing in the diff. This tiny ball race, or bunch of balls in modern ball diffs, was under the same compression stress as the main diff rings and diff balls, but because RC hop-up makers didn't really focus on it, people overlooked it. The trust bearing washers sometimes wore out faster than the diff rings because so much compression stress was put on so small an area. It never ended. Eventually, people got pissed off from what seemed like excessive maintenance time and cost on the ball diff.
And there was born the spur gear mounted slipper clutch. This took the shock protection function off the ball diff, allowing the whole unit to be built more solidly and also allowed keyed diff rings that would not slip on the outdrives. This of course made the ball diff more durable, though it didn't make them bombproof.
Throughout all this, there were some interesting and innovative configurations in cars through the 1980s. Tamiya's 4wd Avante buggy (amongst a few others) sported front and rear geared diffs with a centre ball diff. It must be noted here that the tamiya centre ball diffs were not adjustable. They used conical spring washers that where either correctly tight or uselessly loose. No halfway settings.
The theory was that the centre diff allowed the front and rear wheels to rotate at different speeds, while being a ball diff, it did not allow the front or rear end to completely diff out should either end lift off the ground. The Avante's geared diffs were also unique. Instead of the bevel gear design, it had unitised planetary gears rotating around a sun gear, all sealed up in a drum not unlike the current 'new' geared diffs.
The geared diff unit in the Tamiya Manta Ray (originally released in 1990) is pretty much identical in design to the current TA-06 rear diff. With the exception that it wasn't designed to be filled with oil, so had no o-ring seals at where the outdrives entered the unit and no sealing gasket either. It obviously didn't occur to RC manufacturers that an oil filled diff could be a good thing.
The challenge in those days was to have a diff that allowed a reasonable amount of limited slip while still allowing each wheel to rotate at different speeds. I did try to pack the diffs in my Manta Ray with grease, but as heavy grease was still not a normal part of the RC lineup yet (i.e., not in regular use), the result was only moderately successful.
And so the manufacturers ding-donged between geared and ball diffs for awhile before settling on the ball diff as the standard thing to have. To illustrate the success of this, the original Manta Ray ball diff hop-up part released in 1991 is today used in the M-03 as a hop-up. It's the exact same part.
Then sometime in 1992-93, the touring car craze took off, once again launched by Tamiya. The original TA-01 chassis was really a Manta Ray buggy chassis slammed to the ground, with short suspension arms and a road car body.
Ball diffs became de rigueur in racing, partially because of the fact that manufacturers gave up on developing a decent limited slip geared diff. They'll say that the ball diff is lighter and more efficient and that's why, but realistically, part for part, the geared diff is a much simpler mechanism with less parts.
They also continued to parrot that ball diffs are adjustable. But long years of experience has shown to all that cranking the diff screw does very little except accelerate diff wear/ break diff screws.
It's all marketing hoopla, folks.
Adjusting a ball diff in an early 1990s buggy/touring car was no easy task either. The whole thing was shut up in the gearbox and had to be dropped out, fiddled with, put back in, out onto track to test, then the process repeat if the setting or feel was not right.
Sometime during this period, A one-way bearing was fitted to the front pulley of the Tamiya TA-03, which was the first belt driven touring car design. Tamiya called the bearing a Torque Splitter. Note that during those times, the simple two-pulley, two belt and one spur design seen on current touring cars where not yet standard. So in the TA-03R, the motor drove a gearbox, which drove the rear diff, and a separate layshaft in the gearbox took power to the front gearbox via a belt. The one-way bearing was fitted to the front gearbox layshaft, which meant that the front diff was still a regular ball diff.
This was the commercial birth of the front one-way, but in a roundabout way really. In theory it was designed to allow the rear end of the car to push the front through a corner. The front end could freewheel like a bicycle coasting along should it be spinning faster than the rear end while cornering. The idea was that besides the difference in wheels speeds on the left and right compensated by the diffs, the one-way bearing compensated for the difference in front and rear wheel speeds.
But, like now, the one way bearing made braking very very unstable.
Then also, HPI introduced the original RS4. The car was a legend in its own time and pretty much slayed all the tamiya touring cars. It was also the car that pioneered the current 2-belt, 2-pulley touring car drivetrain. The pulleys were now finally integrated into the diffs for much improved drive efficiency.
And so the tamiya-style one-way bearing became obsolete, to be replaced by the one-way diff. The front one-way allowed each front wheel to freewheel like a bicycle coasting along independent of the rear, and also each other. But once power was applied, the front wheels locked up to the drivetrain and delivered power to the ground. The amount of lockup was tunable in some units.
For a long while, the front one-way and front diff fought for club dominance, until just a few years ago the front spool became popular. In theory, the front spool seemed like a stupid idea to people who liked to fettle with their cars. Why replace the fun and complicated arrangement of bearings, balls and rings with, of all things, a cheap looking fixed length of rod? Isn't a spool something you would find in a cheap child's RC toy with no diff?
But this was because somewhere along the way, drivers started to realize that touring cars were not fast if being pushed round the track. They needed to be pulled, with some assistance from the rear pushing end. This is another departure from 1:1 cars, and required some mindset adjustments for many. A car being pushed by its rear wheels is great for drifting and all other malarkey, but in a 4WD touring car set up, the front end has to be dominant because it steers and if you force the car to go where the front end drags it to, the rear has to follow in an efficient a path as possible. The only efficient way to do this in a 1/10 RC car is to lock the front wheels together with zero loss of drive, so enter the front spool. In all fairness, a one-way basically acted exactly like a spool when throttle was applied, but braking was non-existent with the one-way. Try using only your car's handbrakes to slow down from 120mph. That's what braking with a front one-way was like, and thus usable only by the best RC drivers.
The spool had the benefit of keeping the front end very stable under braking as it applied even braking force across both wheels regardless of how they are turned.
But now there lies another problem. Accelerated wear on the front prop shafts and outdrives. It cannot be helped because as the car corners, the outside wheel has to run a longer distance than the inside wheel. The diff allows for this speed difference, while a spool doesn't. So at any corner, the inner front wheel is always being dragged and scrubbed across the surface. All that chatter and vibration in a car as it corners comes from this issue.
But granted, in the faster racing categories, a front spool is the quickest way.
In the midst of all this, notice that manufacturers have stopped developing new ideas for diff units. There are ideas to integrate different ideas into one (like the X-Ray multi diff), but in general, development stopped and went on to other parts of the RC car.
So everyone experimented, and soon found that a front ball diff packed with thick grease allowed some of the stability of a spool while reducing wear on the propshafts and outdrives. This worked fine for the slower spec racing classes, but is of no use in the very fast classes.
Now, I believe that the current, new generation of geared diffs come from the fact that Lipos and modified brushless power have now pushed the grabbing capability of the ball diff to the limit, and people are also tired of fiddling with tiny balls during diff rebuilds.
So the old style, drum type geared diff units have been resurrected, but now sealed and filled with oil to provide the limited slip capability. Admittedly, the nitro cars had a head start on this new fangled filled geared diff thing, but we're talking electrics here.
Ultimately, all the adjustability of the ball diff that manufacturers continually propagandaed us with in the ball diff's heyday was found to be more trouble than its worth. How do you remember a diff tightness setting? There are no gradiations on the diff screw, no markings, nothing. You have to do it by feel. And 'feel' is impossible to replicate day to day. Also, not all ball diffs are built the same. In fact I found that the designs of the ball diffs in the TA-01 to TA-04 are sturdier than the current touring car chassis ball diffs. The old ball diffs were built into a drum like the current new style geared diffs, with a big, chunky diff screw. They were very hard to get to if you wanted to adjust something, but they were also less likely to fail.
The infinite adjustability of a ball diff is more a hindrance than help these days. And in almost all cases, the tightness of the screw has very little outcome on the actal overall tight feel of the diff. The grease used on the ball diff itself plays a much bigger part.
Also, for guys new to RC trying to build ball diffs, how smooth is smooth enough?? How tight is tight enough? How do you know when a ball diff is built just right? These were the same questions and problems I grappled with for years when learning to set up ball diffs. They really have to be learned by hand feel and experience. No amount of reading can teach you how to set up a ball diff right. Might as well learn how to swim by reading a book on swimming. I still see newbies making the mistakes I made 10 years ago with ball diffs. New guys who think that ball diffs must have some slippage in them, or crank the diff screw down until the diffs implode/break. The actual detailed learning curve of a ball diff is quite steep.
At least with a sealed geared diff, the oil used makes it easy to retain the same, consistent feel absolutely everytime and there is much less room of mistakes.
However, for those chasing the gadzillionbergilliontrillionmillion weight diff locking oil to fill the front geared diff with, I would suggest don't bother. Just put the front spool back on. Why try to seize up the front diff with oil when the spool is exactly that with no additional parts?
[/dinorider]_________________________________________________________________
