Optima Mid Custom gearing help.

[Bormac]

It's time I got my Mid Custom Special out and on to the race track. I currently have a 14 double bolted in and the car has an 87 tooth 'tufnol' spur mounted. I have a selection of nice old brushed motors to choose from but after trying to research the Kyosho manual I can't find any gearing suggestions other than via the stock spur and some Kyosho motors listed in the manual.

Can anyone here offer me some assistance in a good place to start? I'd like to stay with the 14 double but will be fitting up one of Jake's B4 slipper shafts shortly and need to buy some spur gears.

Could use a good place to start on this one. Cheers-Jason

[Percymon]

I asked the same question a few months back and didn't get any where with gearing suggestions. There is plenty of scope for different pinion sizes with the stock spur, which should accomodate most motors unless you want to go insane with a low turn brushless.

Kyosho spurs are getting in shorter supply, not helped by a lot of the aftermarket ones (like Robinson) being designed to work with bushings / adaptors or different shafts). Using the B4 one gives you many more options but you'll need to be wary of the max size spur that will fit under the gear cover and not go too small that the pinion/motor adjustment isn't enough for sufficient mesh (as I've just managed with a B4 rebuild DOH !)

In the last 2 weeks I've manged to buy
a Robinson spur that won't work with the Kyosho shaft (too big inside diameter and 9mm bearing isn't snug enough to ensure concentricity in use - first DOH !)
Associated spurs and pinion that don't allow mesh, even though in theory the combo is in their gearing charts - second DOH !)

Gives me an excuse to buy more RC parts though

[Coelacanth]

I've brought this up several times on the forum, I had to research it in-depth myself because I didn't just want answers, I wanted to know HOW they were calculated. The info I learned & shared can be found here:

http://www.rc10talk.com/viewtopic.php?f=30&t=31186

And here:

http://www.rc10talk.com/viewtopic.php?f=30&t=21530&start=145

More discussion here:

http://www.rc10talk.com/viewtopic.php?f=40&t=21525

So to summarize, to approach a 'good' rollout ratio of close to 1:1, you need to know your (1) tire diameter; (2) spur gear; (3) pinion gear and of course (4) transmission gear ratio.

I made a spreadsheet for the Optima series which calculates rollout provided you know the aforementioned variables, then you just have to plug in your own numbers to see what brings you close to 1:1, give or take no more than 10%. It includes samples of stock Optima/Optima Mid so you can compare aftermarket gearing results with OEM specs, which can be helpful.

Jason, I thought I emailed you that spreadsheet already? If not, PM me your email address and I'll send it. Same for Percymon.

Anyway, when I input Jason's knowns, assuming he has stock-diameter tires, I would go with a 22T pinion gear along with that 87T spur, that would get you to a rollout of 1.006:1. A stock Optima Mid's rollout is 1.075:1; a 23T pinion would get you very close to that, at 1.052:1. A 24T would be too tall, IMHO, and you'd be close to 10% over an "optimal" 1:1 rollout, being 1.097:1.

[Bormac]

Thank you for taking the time to assist me Marc. I will go back to the links you have provided and spend some time understanding the specifics.
I have driven the car (on pavement) as it is and it is close to the way I think it will need to be. Plenty of punch and great top speed with no lag. Still, I want to know it is right and will go back and run the numbers in the way you have suggested.
Thanks again mate. Your help is always appreciated.
Cheers-Jason

[Coelacanth]

You're welcome...also what I've read is that for electric cars, if you want to play it safe, it's better to under-gear the pinion than over-gear it. Using a 21T pinion instead of 22T will give you stronger acceleration at the cost of a little less top-end, but it'll be easier on the motor, all else being equal.

[Bormac]

Thanks Marc, this I definitely knew. Luckily my guess was a 20T pinion. I'll pop in a 21T pinion and see how it runs at the track.
Did the Optima Mid have genuine 48dp or .6 module gearing?

[Coelacanth]

Did the Optima Mid have genuine 48dp or .6 module gearing?

.6, I'm pretty sure. I chose to combine an 85T Robinson Racing 48dp gear in OptiMutt with a 21T pinion gear, for a final rollout of 0.996:1. An 87T spur and 21T pinion seems like it would be a nice, reasonable 0.973 rollout, a little more torquey acceleration than the combo I'm starting with, but should get a bit more top speed than what you got with the 20T. Just for information's sake, I'd really appreciate it if you could run your 21T hard and get back to me with your motor & ESC temperature findings. If they're not too hot to the touch, then it would help validate my theoretical spreadsheet results.

[Bormac]

Will do Marc.
Although I'm no state or national champ I have (over time) learnt how a race is won. It's never the fastest car down the straight but rather the quickest through the infield. This was a lesson learnt early on and the aim for me is to get a car to perform at its strongest in this area. I've seen a mate running a silver can 540 out drive others with CO27 tuned stock motors on their home track. Funny thing is he had never driven that track before.
I want my cars to be quick through the infield so I'll go with your recommendation for the 21T.

As for the B4 slipper arrangement, I might as well stick with the spur size I currently have.

Again I really appreciate your input and value your opinions. Thanks again- Jason

[Coelacanth]

Your feedback would be much appreciated. I'm no racer by any stretch of the imagination, I probably couldn't beat a Frog driven by a good driver with my Optimas! On the other hand, I love understanding the theory and calculations behind-the-scenes. I'm sure those top-of-the-hill racers who effortlessly win races no matter which car they use, are able to combine that kind of theoretical knowledge with great racing technique. Since I utterly lack the latter, might as well bone up on the theory.

[aip47-2008]

I asked the same question a few months back and didn't get any where with gearing suggestions. There is plenty of scope for different pinion sizes with the stock spur, which should accomodate most motors unless you want to go insane with a low turn brushless.

Kyosho spurs are getting in shorter supply, not helped by a lot of the aftermarket ones (like Robinson) being designed to work with bushings / adaptors or different shafts). Using the B4 one gives you many more options but you'll need to be wary of the max size spur that will fit under the gear cover and not go too small that the pinion/motor adjustment isn't enough for sufficient mesh (as I've just managed with a B4 rebuild DOH !)

In the last 2 weeks I've manged to buy
a Robinson spur that won't work with the Kyosho shaft (too big inside diameter and 9mm bearing isn't snug enough to ensure concentricity in use - first DOH !)
Associated spurs and pinion that don't allow mesh, even though in theory the combo is in their gearing charts - second DOH !)

Gives me an excuse to buy more RC parts though

I have several universal spur gear adapters that I have, I will send you one to fix that problem. Just give me your address!!!


Aaron Evans

[Bormac]

Marc there's nothing quit like wheel time to make you a better driver/racer. Understanding how to get the most out of your car mechanically will only go so far. The good guys usually put in a good amount of time behind the wheel and this will usually net good results. Understanding how your car performs on a given circuit is paramount. You begin to find your groove so to speak.
Being away from racing for a good while I'd at the very least like to get trackside with a mechanically sound car so it will only be my poor driving to holding me back.

[Coelacanth]

I doubt there's any chance of me ever being a competitive racer; in fact, the thought of taking my cars to the track actually intimidates me. What *does* intrigue me, though, is letting a guy who's a kick-ass racer--a guy I can trust not to ram the car headfirst into a wall--test-drive one of my builds to see how well the theory translates to performance. I'm sure a pinion change here or there and choosing the right set of tires would optimize performance, but I've tried to put all my logic into my builds and it would be awesome to see someone who can really drive, put them through their paces.