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.
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