Ballpark-guessing pinion gear ratio

[Coelacanth]

Since I was unable to find any exact info on an old Optima/Turbo Optima final drive ratio, and since I have one that is performing fantastic with a certain motor/pinion combo and tire size, I wonder if it's possible to ballpark a similar gear ratio with another car with smaller wheels & tires? Here's where I'm at, feel free to point out any issues with my guesstimation.

-Zebra Optima has a brushed motor pushing 30K rpm and is running 2.2" wheels & tires and an 11T pinion gear. The car performs beautifully with this gearing; ridiculously fast and no excess heat.

-CYANide Optima has a BL 2500 kV but the important detail is that the Zebra's tire diameter is 40% taller than the CYANide's HPI vintage tires. Therefore I estimated that increasing the approximate number of teeth by the same factor would get me in the ballpark of a suitable pinion gear.

When I multiplied the pinion size (11T) by that 40% factor, I got 15.4; I had a 16T pinion lying around so I decided to try that one first.

Is this a workable estimate on selecting a tall enough pinion to offset the significantly smaller tires?

[scr8p]

this is from a gear ratio chart in temples tech:

Optima/Ultima - 3.00:1
MID-Optima - 2.64:1

[Coelacanth]

this is from a gear ratio chart in temples tech:

Optima/Ultima - 3.00:1
MID-Optima - 2.64:1

Thanks scr8p, I'll be able to do some more accurate calculations now.

[Coelacanth]

I found some useful RC gear ratio/rollout ratio calculators and even a speed estimator on this website:

http://www.scriptasylum.com/rc_speed/rollout_ratio.html

When I enter 3.0 as my internal gear ratio and my other knowns (3.425" tire diameter, 11T pinion, 41T spur), I got a rollout ratio of 0.96--which is apparently almost perfect (1.0 being ideal).

I re-calculated with the smaller 2.52" tire diameter, blanked the pinion box, the resulting pinion calculation was 14.92...so my half-assed guess using percentage of height difference was pretty close. A 16T pinion gives a rollout ratio of 1.03.

I'm unsure how the 3.0:1 internal gear ratio is calculated, though. The terminology for the different names of gears tends to vary from site to site. My understanding is the "spur gear" is the one that meshes with the motor pinion. This I believe is also referred to as the "center gear". But how exactly is the internal ratio calculated? My diff ring gear is 37T, the number of teeth of the chain-side of the diff is 18T. 37/18 = 2.056...?? Does the final pinion gear inside the gearbox that meshes with the diff ring gear factor into the equation (it's 19T)?

I again searched all over the 'net for a gear ratio for the old non-Mid Optima and again drew a blank...

[Coelacanth]

This is an old topic but the question was never answered as to how the Optima 3.03 internal gear ratio is calculated. I've since figured it out and will update this topic in case anybody else has the same question. Here's the math:

Gearbox gear ratio = 37T diff gear / 19T final pinion = 1.555
External gear ratio = 28T counter or limiter gear / 18T cluster* gear = 1.947

*The smaller gear on the inside of the stock Optima spur gear is the cluster gear, which meshes with the counter gear; a.k.a. the limiter gear if it's a Turbo Optima.

1.555 x 1.947 = 3.03 (rounded up from 3.0276)

The Optima was sold with 12T and 15T pinion gears. Final gear ratios (as shown right on the Optima box) are as follows:

(Spur gear / Pinion gear) x 3.03

12T pinion: 41 / 12 = 3.42 x 3.03 = 10.35
15T pinion: 41 / 15 = 2.73 x 3.03 = 8.28

Those 2 numbers match what's printed on the Optima box. Updated my image for clarification. (Note that the pic shows an 11T pinion, not the stock 12T or 15T.)

[marlo]

I found some useful RC gear ratio/rollout ratio calculators and even a speed estimator on this website:

http://www.scriptasylum.com/rc_speed/rollout_ratio.html

When I enter 3.0 as my internal gear ratio and my other knowns (3.425" tire diameter, 11T pinion, 41T spur), I got a rollout ratio of 0.96--which is apparently almost perfect (1.0 being ideal).

I re-calculated with the smaller 2.52" tire diameter, blanked the pinion box, the resulting pinion calculation was 14.92...so my half-assed guess using percentage of height difference was pretty close. A 16T pinion gives a rollout ratio of 1.03.

I'm unsure how the 3.0:1 internal gear ratio is calculated, though. The terminology for the different names of gears tends to vary from site to site. My understanding is the "spur gear" is the one that meshes with the motor pinion. This I believe is also referred to as the "center gear". But how exactly is the internal ratio calculated? My diff ring gear is 37T, the number of teeth of the chain-side of the diff is 18T. 37/18 = 2.056...?? Does the final pinion gear inside the gearbox that meshes with the diff ring gear factor into the equation (it's 19T)?

I again searched all over the 'net for a gear ratio for the old non-Mid Optima and again drew a blank...

A few years ago, while I was doing a scrap run, looking in other people garbage. I found a box full of RC parts. That gear you have pictured ( for the chain drive ) was in there. I'm looking at it right now, I had now idea what it was for. I just put it in my junk box. If you want it you can have it.

[Coelacanth]

A few years ago, while I was doing a scrap run, looking in other people garbage. I found a box full of RC parts. That gear you have pictured ( for the chain drive ) was in there. I'm looking at it right now, I had now idea what it was for. I just put it in my junk box. If you want it you can have it.

Sure, but what gear, exactly? Does it have sprockets for a chain?

[marlo]

It kooks just like the gear you showed with that green spacer, would the tooth count be marked some where.

[Y'ernat Al]

Interesting So just to confirm Coel', if you put your optima on a stand, mark a tooth on the gear the motor pinion meshes with, spin this gear around exactly three times, the wheels went around once right? Sorry but there was a lot of math up there.

For the fdr, you can also mesh up a pinion and watch the set screw, slowly rotate the wheels around one full revolution and count how many times you see the set screw. That is a "ball park guess" method for the fdr I've used, before I had a phone with a calculator on it And now that I'm old I can't read the damn micro writing on the gears anyway.

[Coelacanth]

It kooks just like the gear you showed with that green spacer, would the tooth count be marked some where.

If you mean the diff gear with the yellow plastic ring in the middle, that would be a diff for an Optima Mid--sure, if you don't have a use for it, I'll take it. Just let me know what the shipping costs and I'll reimburse you. PM me and we'll take it from there. Thanks!

Yes Brian, that's about it. For most people, knowing your FDR and rollout ratio isn't really important, but when you're custom-building a car that's a long way from stock--i.e. CYANide or Barney--the different sizes of the tires, much higher power of the modern electrics, and so forth make these calculations very important. You want to end up with a rollout and FDR that are close to 1:1, ideally. I'm putting together a spreadsheet to make the calculations as easy as inputting some numbers, I can then take a screenshot of it and if anybody wants it, I'll email them the spreadsheet. It would work for any car, as long as you know the variables. It will help you choose the best pinion and/or spur gear just by playing with some numbers. I've seen some online calculators but the explanations are unnecessarily confusing and you can't usually tell what the formulas are behind the results.

[marlo]

alright, I'll post it out on sat, along with any other non rc10 parts that I found in that box. all small things

[JK Racing]

this is from a gear ratio chart in temples tech:

Optima/Ultima - 3.00:1
MID-Optima - 2.64:1

scr8p - a wealth of Kyosho knowledge

[Coelacanth]

this is from a gear ratio chart in temples tech:

Optima/Ultima - 3.00:1
MID-Optima - 2.64:1

scr8p - a wealth of Kyosho knowledge

Interestingly, the 3.00:1 isn't quite correct, it's actually 3.03:1. ...at least according to the math. As for the Optima Mid ratio, I'll take his word for it, of course scr8p is the forum's closet Kyosho expert. But, I forgot to write down the rear gearbox diff & final pinion gear numbers so I can't be 100% certain. Too late now to disassemble the gearbox.

Anybody here have the stock Optima Mid internal gears and can update me on exactly how many teeth they have? I'd appreciate that, and update my spreadsheet I've been working on the past few days.