RC10Talk - The Net's Largest Vintage R/C Community

Ok so I consider myself a bit of a numpty when it comes to figuring this stuff out but I would like to work out what ratio suits what motors etc... I have a few aftermarket 48dp and .6 module spurs and would like to better understand gearing and what underdrive and overdrive relates to on this car.
I intend to run both 540 silver can and at the other end low turn modifieds in some of my cars.
Thanks in advance-Jason

I went through that about a year ago. It sure wasn't easy to find it online but here's a thread I started at RCTech.net that should give some insight. I didn't want to just know the number, I wanted to know how the number was actually calculated. Fredswain, a member here and there was helpful and put me on the right track. In short, it's about 3.02:1.

http://www.rctech.net/forum/electric-off-road/509220-help-kyosho-optima-gear-ratio.html

This post in particular is where I figured it out mathematically, which is important if you want to mix & match gears or change anything from stock.

Coelacanth wrote:Fred, your numbers are right on for calculating the FDR, and those numbers precisely match what was printed on the box. I'm not following the calculation that results in an internal gear ratio of ~3:1. How is that number calculated?

The limiter gear is indeed 28T; the inside of the spur gear is 18T. All other gear numbers seem correct.

EDIT: I think I just figured it out; correct me if I'm wrong.

(Spur gear / Limiter) + (Limiter / Inside of Spur) = (41/28) + (28/18) = 1.464 + 1.556 = 3.02

Optimal and Ultima w/ stock gearboxes were 3.03:1. 37 diff/19 final pinion= 1.95. Outside, h counter gear28/the small gear o the cluster 18= 1.555. 1.55*1.95=3.03

The UD set I think had the same overall ratio. The OD set had 26/20 on the outside. 26/20=1.3....1.3*1.95=2.54

On my ultima, I have he stock 26 outer gear with a 20t spur adapter for a final trans drive ratio of 2.73

You're exactly right, Wes. To get your final drive ratio (FDR), multiply your: (Spur gear / Pinion gear) by your transmission gear ratio. My calculation was off, and quite the fortunate coincidence that it resulted in almost the exact same number.

The stock Optima came with 12T and 15T pinions and a 41T spur gear. The ratios listed on the box are 8.28:1 / 10.35:1.

41 / 15 = 2.733 x 3.03 = 8.28
41 / 12 = 3.417 x 3.03 = 10.35

All else being equal, increasing FDR results in quicker acceleration at the cost of lower top speed.

Hi guys.

This is the Scorpion setup, so help me guys with the math

5.348:1Lars.

Spur divided by pinion x5.348 = final drive ratio.

20\16 = 6.685

24/12 = 10.696

Thanks Wes, but I’m a bit confuse, I was ones told that 24/12 would be around 8.2 which means that the gearbox should be 4,1:1. There is a huge difference between 10,696 and 8,2, but I’m sure you are right.

Not sure where I made the mistake before, but I was on the ipad, and switching screens between the calculator and the browser... ANyway, I'm now getting 5.36 for the "internal"ratio. I'm not all that familiar with the scorpion series. I'm gathering that there were two spur clusters available, and only one set of motor mount holes? Seems to me an adjustable motor plate/drilling out th stock one for more pinion options would be a MUST DO hop up. I'm a big Ultima fan, and I hate that cluster gear design... I finally happened on the adapter I'm using on my ultima and made it work (it wasn't an Ulitma part ) and now I can put any std spur on it... wish I had bought the other two he had at the time.

You are right, original there were only two spur clusters and with that two pinion’s. So CRP mad an adjustable motor plate to fix that problem. Otherwise the gear-box layout is the same as the Ultima.