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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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Drifting Goodies: Grease Diff

Posted 12-15-2009 at 01:55 AM by color0
Updated 03-08-2011 at 09:31 AM by color0
So I've been doing the RC drifting thing ever since I started this hobby, but one of the things that has always irked me is the fact that the throttle control "patterns" are completely different between RC drifting and real-car drifting. Part of the reason is because RC drifting is almost always done with AWD cars, with the front wheels helping pull the weight around. This just doesn't feel right especially when transitioning, if you can just wail on the gas and the car straightens out rather than pulling into a tighter line. One of the ways to get the RWD feel back without sacrificing AWD stability is to bias power delivery to the rear axle. In real cars, this can be done with a viscous coupling, so why not do something similar for RC cars?

The grease diff is the crude semi-equivalent of a front viscous coupling, built simply by taking the planetary gears out of the diff and replacing them with thick grease. Depending on thickness of the grease you use, you can tune in different power biases, from around 40/60 with REALLY thick greases, to nearly RWD using the thinnest oils. The trick is mainly in the type and amount that you use. The weight of grease required for a certain behavior is dependent on the amount of grip the car has: when the car is really slippery (Scotch taped tires on waxed hardwood floor, etc.), a lighter oil like damper oil (1000wt, for example) will feel generously RWD-like, while 20000wt diff greases will make the car feel no different from an AWD car with a front spool. In contrast, when there's actually grip (RCP drifter with rubber tires, etc.), 1000wt oil will make the car feel like pure RWD. You'll need that heavy 20000wt diff grease to transfer enough power to the front wheels for stability. It often takes a couple tries to get right, and it's fun to experiment with different power biases.

For RC cars like the Mini-Z AWD and Xmods (original generation), where the diff is a closed assembly, building a grease diff is pretty trivial. Just take it apart first:



As you reassemble, stick the outdrive into the side diff casing, and apply your grease of choice to the inside, where the diff's side gears will contact them. Use liberal amounts of grease here, you want to have the grease cover the entire mating surface with no air bubbles. Let that grease squeeze right out when you pop the side gear on.



Repeat for the other side. This side of the diff casing may have breather holes in it (take a close look, because mine does). If it has holes, you should seal them up from the outside with a dab of superglue (small dab will do!) as to prevent the grease from leaking out.



Fill 'er up with a little more grease, and then pop the entire diff assembly, without the planetary gears, back together. That's really it!



You should feel a difference right away when using a grease diff up front: throttle input will cause the car to start drifts and gain more angle instead of straightening out. To link drifts, you'll actually have to lift off and let the car's inertia swing itself around, rather than just force-powering through the transition. The end result won't look too different on a video camera, but the driver -- you -- will notice the different technique(s) required to drift the car properly.

Sidenote: I suppose that if you desired a non-50/50 power bias in your racing AWD's as well, you could apply this tutorial just the same way, except you'd be using VERY heavy grease in order to transfer the torque to the wheels. I don't really think it'll improve your laptimes though, any grease heavy enough to transfer the torque would probably lock the diff more tightly than would be optimal.



Anyways, hope you guys and gals enjoyed my last article for the year 2009 -- have a Merry Christmas and happy holidays! And drive safely over the holidays: drunk driving is not cool, unless it's with Mini-Z's.
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