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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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RCX 2011 Setup: Differential (2WD)

Posted 05-27-2011 at 07:30 PM by color0
Next up in this series of articles is the differential setup. The differential in a 2WD Mini-Z is super important -- while a bad diff or a badly maintained diff will not make your car undrivable (as bad tires would), a bad diff will hamper your car's ability to carry speed through a corner, and hamper its consistency in handling your inputs to the car. So you ALWAYS want a smooth diff; you can leave it 100% open and free or you can adjust its LSD effect using diff tension, thick greases or even the spur gear design nowadays, but when you turn one of the rear wheels the diff must always feel smooth and not grainy or notchy. This means the wheels always get the same treatment from the diff, which means consistent handling.

A quick rundown of how these differentials work (if you're not familiar already). You can see the spur gear easily in the first picture: this is driven directly by a pinion gear attached to the motor shaft. Looking at the second picture, you can see that this spur gear drives 6 black balls. These balls are sandwiched between two plates (first picture), and these plates are connected to the left and right wheels, the left plate going through the diff shaft to the rear left wheel, and the right plate going through the right-side diff hub directly to the rear right wheel. When you apply tension to the plates, they push on the balls and the friction allows the spur to drive both wheels. Yes, the crude old ball diff is indeed a friction-drive device. When differentiating between left/right wheel speeds is required, the balls will turn relative to the spur gear, so the plates then can rotate at different speeds, whatever's required to keep the rear wheels happy on the road.

For RCX, I put Qteq, Kyosho, Reflex and PN parts together into a hybrid diff; it's been very dependable and has only failed me once when the thrust bearing died on me mid-qualifier. Actually, I didn't even notice the thrust bearing failure when driving, I was bench-testing it after rebuilding my diff when I finally noticed.

Up top in the second picture is the PN lightweight diff shaft, which to me is the ultimate diff shaft until somebody figures out a good implementation of the 1/12 carbon axle and hub clamp. You can see that the Qteq diff hubs (grey, 6-pointed stars) are super-miniscule and definitely very lightweight looking. The hubs don't have a whole lot of traction with the Kyosho diff plates so you're supposed to glue the plates to the hubs; as you might guess, then, the left side glue has come apart while the right side is still glued tight. I will fix that, haha... For their part, the Kyosho pressure plates have held up very well, this is the only pair I've ever bought, never replaced it once this diff was finished. I sanded the plates first with 600grit paper to flatten them out, but since that first time I've never touched them again. Notice that there is an inside and an outside groove on the left Kyosho plate -- standard spur gears like Kyosho, Reflex, Atomic spur gears cause that groove, whereas the newest PN spurs space the balls out WAY far, to the extents of the plate, and hence you get the outside groove. All the way to the right is my replacement thrust bearing, now a Reflex high-grade bearing originally in my rear axle.

A quick note about the E-clip at the end: I'm using Qteq E-clips so this may not apply to everyone, but always note which side of the E-clip is facing outwards when you remove or reinstall it. On many E-clips the sharp outer edge (a byproduct of the E-clip stamping process) can interfere with the thrust bearing, making the diff feel notchy. I myself was having this problem for a few minutes till I flipped my E-clip around and that fixed everything.

As for spur gear choices, I like the latest PN 126p spur the best: although the 126-pitch teeth are fragile, they also reduce drivetrain losses and the spur design itself helps, it's super-light (only 0.1g off from the benchmark Reflex V2 spur). Unlike some other spurs it holds the bearing just tight enough that I don't need glue but also doesn't wobble at all. Here's the spur in detail:

Yes, I left the balls (TOP Racing ceramics, 3/32") in the LSD-action holes. This spur has the option of non-LSD holes but I've really come to rely on the limited-slip effect, especially for getting away with tuning more turn-in steering into the car (the locking effect balances out the additional steering on corner exit, it's perfect!). I did not use any special shims or washers to align the spur gear; I pretty much inserted the spur gear bearing, made sure it sat flat, popped it on the axle and made sure it spun straight, and that was it! As long as it's centered on the balls, I don't think there's anything special to the installation of this spur gear. For their part, the balls themselves were the best deal I found at the time, very round unlike some normal steel balls, and the hardness of ceramic gives excellent longevity. They've lasted some three years now and I see no reason to switch out yet! They're each held in place by a small film of Kyosho Ball Differential Grease, this is hands-down the best grease for Mini-Z ball diffs as it's light, SHEDS dust and grime rather than attracting it, and the diff gets freer as you run it! Just don't ever blow dust into my diff, that definitely introduces more crap than the grease can handle and the only option then is to rebuild the diff entirely.

So about diff tension: some people run loose, some people run tight, some people don't care, but diff tension can have a fairly large effect on your car's performance and it's good to know how you can affect it by changing diff tension. A tighter diff (increased tension; plates clamp the balls harder) will resist the left and right wheels differentiating, so the car will not like to rotate as much off-power, but it will rotate more on-power as both wheels drive the car fully and no power is lost through the inside rear wheel slipping ("diffing out"). A looser diff (reduced tension) will not resist differentiating, so the car will love to dive into corners, but on-power it will be prone to diffing out and you may lose some propulsion. In the past this used to be a matter of compromise, but for me at least the latest lineup of PN spur gears is the perfect compromise. These have a specially shaped pocket for the diff balls that will grab onto the balls on-power; essentially, increasing diff tension on-power while keeping diff tension unchanged off-power. What this means for me as a a racer, then, is that I can leave the diff loose, and get good rotation off-power, but have a tighter diff on-throttle, and get good rotation on-power too! This is the only way I setup my diff nowadays, and the PN spur (126p, 106t) is definitely my favorite spur gear yet, it matches up well with my weight-shifting, no-braking driving style.

If your diff tension is changing from run to run, I would first suspect your O-ring: mine is from a Kyosho diff and seems to provide consistent tension still after 4 years, but if yours is collapsing (you'll notice lower diff tension on subsequent runs) it's definitely time to replace it.

One last word about diff tension: in certain low-traction conditions, you may find yourself fighting hard to get traction. A useful way to get around the problem is simply loosen the diff so much that the entire rear axle slips a bit on-power -- it's almost like traction control, the diff is purposely set such that any forces it can drive the wheels with are guaranteed not to exceed the maximum force the tires can handle on the surface. The result is that your car won't be strictly as fast as one where the tires can slip and propel the car just that tiny bit more, but with a much larger safety envelope than that said car, yours will be much more consistent, and hence be more competitive in a race environment. This was NOT necessary for Stock 80t racing at RCX, however, if I was running Mod I probably would've considered doing it as traction was definitely poor, even on the PN 6 radial tires, if you had sufficient power.

So to sum up this section, here's the list of things I'd go through in my mind when building a new diff:
- Shaft weight
- Shaft straightness (check this after a hard hit!)
- Diff hubs weight
- Diff hubs' traction with pressure plates
- Pressure plate flatness (sanding, etc.)
- Pressure plate longevity
- Pressure plates' traction with balls
- Balls' roundness
- Balls' material (affects hardness, roundness after time)
- Spur gear weight
- Spur gear teeth (finer = faster, weaker)
- Spur gear ball hole design (circular vs pocketed (PN))
- Thrust bearing condition (replace when worn!)
- O-ring condition (replace if collapsing)
- Diff grease
- Diff tension

I think that should about cover it for differential setup, the trick is to keep it light, smooth, and consistent -- if you can get everything put together properly you're pretty much guaranteed a good diff (at least till it breaks!).

Next week, rear suspension.
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