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Consistency Wins: Using TH. SPEED to Tune Gear Diffs

Posted 02-19-2012 at 08:59 PM by color0
Updated 07-29-2012 at 04:09 PM by color0
One of the curiosities I've encountered after making the switch in my car from ball diff to gear diff is how the instantaneous power delivery causes a lot of problems with my setups -- setups that have been honed very carefully for the smoother power delivery of a ball diff. What happens? Well, in my case, I get a bunch of spin-outs on power:

[Wall] <-- [My car]

Since my setups tend to emphasize turn-in oversteer and powering out of the corner before the apex, I tuned quite a lot of steering into the overall setup so that even with some ball diff slip, the motor power would still break the rears free just a tiny bit to rotate the car around the apex and fire it out. This setup strategy relied heavily on PN Racing's Limited Slip Spur Gear in the ball diff to lock the rear axle on power, so that the rear tires would actually slip a bit, but not too much. If only one tire unloads, there wouldn't be any rotation, but if both tires slip too much then the oversteer will just put my car into the wall.

So I've had my setup pretty well-tuned for a while when along comes the new PN Gear Diff, and the moment I put it in I notice that the power delivery is both instantaneous and strong. The acceleration off the line is insane, but if I had any semblance of motor power (say, a PN 33t or an Atomic Chili) then all of a sudden, corner exits became pretty hairy. Thanks to the design of the teeth on the gears inside the diff, the PN Gear Diff locks quite strongly when you hit the gas, and that's the reason why.


(Wikipedia image)

When you apply a torque to the spur gear (blue), it transfers to the spider gears (green); the spider gears then apply that force to the side gears (red/yellow), but there's a catch:


(Wikipedia image)

Because of the shape in the gear teeth, when you mash two gears together they actually apply a force that wants to push their axles apart. The blue arrows show the force/reaction force directions quite clearly.

Now look back at the blue diff and the green spider gears -- if we apply force to the the green spider gears in the blue diff, they will actually force the red/yellow side gears outwards into the housing! This creates a significant amount of friction, generating a limited-slip effect that is quite a bit stronger than that of the PN LSD spur gear. The same thing that happens in the blue diff applies to the PN Gear Diff: the spider gears mash the side gears into the restrained sides of the diff assembly (Delrin nut on left, E-clipped bearing on right) and create friction, though of course a bit less since we do have a bearing in there.


So the PN Gear Diff locks strongly on-power and also happens to lay down all the power you give it immediately: this combo was killing the rear tires and killing my consistency, thus I reduced the Throttle Speed setting on my EX-10 Eurus in an attempt to gain back the forgiving feel that I used to have with the ball diff, and hopefully start being able to flog the car again instead of babying it around the track.

As I've written about before in "Transmitters Explained", the Throttle Speed setting available of many high-end radios pretty much limits the speed at which the actual, transmitted signal (that the car receives) matches the input from your trigger finger. This isn't to say the actual signal is slowed down -- this means that when you yank the trigger, the radio will tell the car something like,

"Hey dude, go 25%, now 50, now 75, now 100!"

rather than

"Hey dude, go 100% now!"

Basically, it emulates you having a smoother trigger finger than you actually do. The slight reduction in instantaneous power delivery approximates the initial slip that you get with a ball diff, thus the car WON'T burn up the rear tires at the tap of the throttle and I can avoid spinning out so much even with the stickiest tires money can buy.

Does it work? Absolutely. I took it to Kenon for a quick club race last night and for testing purposes, tried out three different TH. SPEED settings on my Eurus -- 100%, 75% and 65%. 100% is unfiltered throttle input: i.e. spin out city. At 75% I started getting the ball diff feel back -- I could hit the gas early without spinning out, the rear would rotate a bit before pinning itself down and firing the car out of the corner. And finally, at 65% the car was just too smooth -- the throttle was too dull at this setting to actually cause throttle steering, and so the car would have to wait till after it cleared the apex to apply power (I will admit it was quite consistent though).

From this quick experiment I would say that for KO radios, TH. SPEED ranges between 75% and 85% are about right for gear diff use as it seems the closest to replicating the slight slip and slight lag in acceleration that you get from a ball diff (there's still an advantage of course -- don't forget that the freer gear diff increases corner speed overall). At 65%, as I started to get used to the feeling I also noticed that it felt a little too laggy to be quick. Very smooth and very consistent, but a little too slow to be competitive. So that's the result I'm reporting today, that 75-85% TH. SPEED makes a gear diff feel a lot more like a good ball diff and generally combines the pros of the gear diff (corner entry, mid-corner speed) with the pros of the ball diff (consistency, smooth exits). I will be continuing tuning my Mod MR-03 to race with a gear diff as I love the indestructibility and tunable power delivery of the PN gear diff, now that we have this trick in our tool belts.
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