RCBS #3: “Gearing”

This question comes up all the time. It’s a perpetual newbie wail on internet forums. “What gearing should I use?” or “What pinion do I need?” And I understand the rationale behind it; gears are expensive, up on $6 some places! You only want to buy one and be done with it.

People often answer with a gear tooth number, which is a little helpful, I guess… but it could be better. The pinion gear tooth count doesn’t really matter. The important factor is final drive ratio, the actual ratio of motor revolutions to wheel revolutions. Different cars will have different transmission ratios, 2.6 for Associated, 2.43 for Losi, which means that “24/69” on one car will act much differently on another.

On-road guys have got this down already. For the rest of you, it’s a simple calculation:

( Spur Teeth / Pinion Teeth ) * Transmission Ratio = Final Drive Ratio

For example, on my DEX210, which has a transmission ratio of 2.6, I’ve got a 69 tooth spur and a 34 tooth pinion. My final drive ratio then is (69 / 34) * 2.6 = 5.28. When you’re trying to match a FDR, it’s just some simple algebra to figure out what gears to buy:

• Spur = (FDR / Trans) * Pinion
• Pinion = Spur / (FDR / Trans)

The second problem with this question is that finding the right drive ratio is not a one-and-done thing. It’s a process. To do it properly, you need an assortment of gears and a temp gun. You can get by with your finger in a pinch, but a temp gun is a cheap investment and will make everything better. Put a pinion on there, using a suggestion or an educated guess on the right tooth count. Run the car for a short bit, six or ten minutes, and check the temperature. If it’s too high, above 150F, use a higher FDR (smaller pinion/larger spur). Repeat until you’re happy with the temperature and speed.

RCBS #2: “CVD”s

Do you know what CVD stands for? You might think “Constant Velocity Drive”, but if that’s the case, it’s a misnomer. It’s no more “constant velocity” of a joint than a regular U-joint, or a dogbone flopping around in an outdrive. A real “constant velocity” joint is much more complex.

Constant velocity implies that, regardless of the angle, angular velocity at one end is always equal to the other. A regular Cardan joint, two pivots perpendicular to each other, will produce oscillations at an angle. Two cardans at 90 degrees of eachother will cancel this oscillation out, thus why dogbones are “flat”. But this only works if the angle at both sides are equal. For the rear wheels, toe and camber will mess that up slightly, but the front wheels have it worse, since they’ll be turning at much sharper angles.

Functionally, “CVD” axles perform identically to U-joint axles, dogbones, and slider shafts. So if not for “constant velocity”, what does “CVD” stand for? Costs Volumes of Dollars? Captured Vampires Dangle? Craving Violet Drapes? Nothing. It’s just a name now.

RCBS is a column where I rant about things in R/C that don’t make sense, but everyone accepts them anyway.

RCBS #1: Battery Numbers

A battery’s “capacity” is listed in “milliamp-hours”, which is a measurement of dialectic flux, not energy. This means a 7.4v 4000 mAh battery and a 14.8v 4000 mAh battery have the same “capacity”, even though the latter can store twice as much energy as the former! For racers who generally stick to one voltage, this isn’t a big deal, but for bashers who will experiment with 2s, 3s, maybe even 6s packs, a capacity measured in watt-hours, an actual unit of energy, will be more useful for comparing packs.

“C” ratings are ridiculous as well. You know “C” is a measurement of? Frequency. 1C ≈ 0.277 Hz. The number you actually want out of a battery is its discharge current, which should be measured in amps. Sure, you can calculate this by multiplying the C rating by the “capacity” in amp-hours, but why not just cut out the middleman! And as for the charging current, normally stated at “1C”, why not let the manufacturer suggest a current instead? They probably know best, since they made the damn thing.

Before: “7.4v 4200 mAh 65C”. After: “7.4v 31Wh 270A”

I’m not even sure why we bother with discharge rate anyway. What we really want to know is internal resistance…

RCBS is a column where I rant about things in R/C that don’t make sense, but everyone accepts them anyway.

RC Mini-tip: Get good tools

Just bite the bullet and get a decent set of screwdrivers. This Dynamite 1/16" driver's tip has worn down and even twisted while trying to unscrew a screw.

I'm not sure whether to be disappointed with the screwdriver, or impressed by the screw, which did not strip.

RC Mini-tip: Name decals

Print your own custom name decals with inkjet sticker paper. $0.50 per sheet at my nearby Fedex Kinko’s.

Generating the image is up to you.

RC Mini-tip: Shock stand

Four holes make a stand for letting air bubbles settle.

A body reamer does the job on this plastic toolbox.

RC Mini-tip: Even out tire wear

If your truck uses the same tires all around, rotate them frequently to have the tires wear evenly.

On most tracks, the tires on will wear faster on one side than the other. Be sure to rotate them in the same pattern each time. I do "back across".