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For my Optima gear ratio & rollout calculator, I'm missing some information to make it complete. I have all the details for the Turbo Optima, but I'm missing some specs for the original Optima and for the Turbo Optima Mid SE (I believe all the Mids have the same rear gearbox gear specs, but please correct me if I'm wrong).

What is:

-The outside diameter of the original Optima spike tires, from one spike-tip to the opposite one?

-The tooth count of the stock rear gearbox Optima Mid diff gear? (Is this the same for all the Mid series?)

-The tooth count of the stock rear gearbox Optima Mid final pinion gear? (Is this the same for all the Mid series?)

I don't need specs on that top shaft idler gear for the belt, though.

Thank you for your help!

Coelacanth wrote:
-The tooth count of the stock rear gearbox Optima Mid diff gear? (Is this the same for all the Mid series?)

Thank you for your help!

37! I checked both a gear diff and a ball diff so that means all the six Optima Mid models have the same.

Good work!

According to manuals for at least four Optima Mid models the stock pinion gear is W-5090 (20T).

j0pp3 wrote:According to manuals for at least four Optima Mid models the stock pinion gear is W-5090 (20T).

Thanks for the info, j0pp3!

Now, do you know how many teeth the OTHER gear inside the rear gearbox has (not the idler gear on the shaft)? If the manual's picture is correct, I'm guessing it's 18T.

And you've just shown me that I'm also lacking another spec--the tooth count of the stock Mid spur gear. Do you know it? Basically, I need to know how the resulting 2.64 ratio is calculated.

Once I have this spreadsheet finalized, I can send it to anybody who's interested.

A stock Mid spur is 74 teeth.

The counter gear is 14 teeth, so 37/14 = 2.64 for the gearbox. The idler/center gear tooth count is irrelevant, but for completeness it's 28 teeth.

So, with stock gearing, a Mid has a final gear ratio of (74/20)*(37/14) = 9.78.

ROH73 wrote:A stock Mid spur is 74 teeth.

The counter gear is 14 teeth, so 37/14 = 2.64 for the gearbox. The idler/center gear tooth count is irrelevant, but for completeness it's 28 teeth.

So, with stock gearing, a Mid has a final gear ratio of (74/20)*(37/14) = 9.78.

Right, the idler gear (or belt pulley) is irrelevant for the gear ratio calculation. Thanks for filling in those blanks Rob, you da man! I believe the Optima Mid's tire diameter is the same as the Turbo Optima, as they used the same wheels. So, the rollout can be calculated properly and is within 10% of 1:1, or 1.075, specifically. So knowing that, if you're upgrading to an aftermarket spur gear or changing tire size--for example if you're doing an onroad or monster truck conversion--you can simply enter the values for the new tire size and pinion gear to get a rollout that pretty closely matches the stock rollout and therefore maintain the same gearbox efficiency, without resorting to trial & error and the risk of undergearing/overgearing, motor/ESC overheating, etc.

EDIT: Thanks also to j0pp3 for the diagram, I totally overlooked the gear attached to the belt-sprocket gear, that's all-important.

If Marwan's tires are faithful reproductions, then the diameter of brand-new ones is 3 1/4 inches, or 82.5 mm, tip to tip.

Thanks, Mark! Updated my spreadsheet. Those engineers who designed the cars must've known what they were doing, because the rollout ratio of the stock Optima with the 12T pinion gear is almost bang on 1"; 0.986 to be exact. Within 1.5% of "perfect". The only one of the Optima series that was geared below optimal is the Turbo Optima, but I bet they undergeared those because the stock MSC resistor would've overheated and fried if they used the usual 12T or taller pinion gear.

Coelacanth wrote:Thanks, Mark! Updated my spreadsheet. Those engineers who designed the cars must've known what they were doing, because the rollout ratio of the stock Optima with the 12T pinion gear is almost bang on 1"; 0.986 to be exact. Within 1.5% of "perfect". The only one of the Optima series that was geared below optimal is the Turbo Optima, but I bet they undergeared those because the stock MSC resistor would've overheated and fried if they used the usual 12T or taller pinion gear.

Turbo Optima was also made for running with a 7 cell pack so it got the speed anyway. I remember my Turbo as freaking fast and hard to control for my young age back then. The voltage dropped fast though

Coelacanth wrote: The only one of the Optima series that was geared below optimal is the Turbo Optima, but I bet they undergeared those because the stock MSC resistor would've overheated and fried if they used the usual 12T or taller pinion gear.

Your 1" 'optimal' rollout only works if you are using the standard kit 27T motor. The Turbo needed to be geared lower for the 240S 19T motor that came with the kit. Running it at the same gearing as the standard Optima will bog down the motor and overheat it. What rollout you use depends on what the motor is, although brushless seems to throw out all the old techniques. I've had brushless systems overheating and geared up get them to run cooler.

My main purpose for understanding rollout was primarily due to 2 of my cars using tires that are quite far from stock diameter, and wanting to know which is the best pinion gear to start with. From there, as long as you know "you're in the neighborhood", it's easy enough to gear up or down. Motor too hot? You're probably overgeared and need to choose a smaller pinion. ESC too hot? you're probably undergeared and should try a taller pinion.

The interesting thing is, when you compare a lot of cars' specs, how close they all end up being to a 1" rollout for 1 turn of the motor pinion. (Modern brushless cars, that is.)