Final drive ratio

[allaircooled]

Ok, I understand the math to figure this out. What I don't know is what I do with the number I get. 78 tooth spur/24 tooth pinion X 6 gear 1.87=6.08 Now I have this number, what do I use this for or how do I use it?

[craigc8791]

Now you use diffrent spur/pinion combos and see what give you the best final ratio

[allaircooled]

I guess my question now is, how do I know what the best final drive ratio is?

[Eau Rouge]

It all depends on the motor you are using for any given application. A 12-turn brushed motor will be completely different from a 17.5 brushless or a 27t brushed stocker. The final drive ratio is only useful when comparing different cars with different gearboxes, really.

For example,

A 6-gear transmission with a 78/24 (3.25:1) has a FDR of 6.08. To get that exact same ratio for your exact same motor in a Stealth, you take the 2.25 internal ratio of the Stealth and divide from 6.08 to get 2.70:1. To get that same ratio with a Stealth gearbox and the same spur gear (87), you divide out again, and get a number of 32. So to make those two cars identical, you need a 78/32 in the Stealth to equate to a 78/24 in the 6-gear.




Now, if someone says, "for a 17.5 brushless motor, at this track, I use about a 5.5 FDR," you with a 6-gear and your buddy with a Stealth should both be able to get the same FDR with that motor, and get the same speed and performance for that application.

17.5 brushless, 5.5 FDR =

6-gear: 5.5 ÷ 1.87 = 2.94 : 78 ÷ 2.94 = 26.53 (which you would use a 27t pinion). 78/27 on a 6-gear is close to 5.5 FDR (5.40)
Stealth: 5.5 ÷ 2.25 = 2.44 : 78 ÷ 2.44 = 31.97 (which is a 32t pinion). 78/32 on a Stealth is close to 5.5 FDR (5.484)



Tire circumference also has something to do with the final speed and output of any given car, but most buggies all use the same size tires. If you are comparing cars with different size tires, and trying to get the same FDR for a particular motor, you use a mathematical calculation known to foam tire racers as "rollout" which uses the circumference of the tire to show how far the car will move with one rotation of the motor.



Hope that's not too complicated.



doug

[Eau Rouge]

I guess my question now is, how do I know what the best final drive ratio is?

I guess the best way to answer that is to say, there is no one answer. The FDR is essentially useless for you unless you are comparing it to someone else, or a motor manufacturer gives you a suggested starting point. Work towards the FDR to get the proper gear ratio, not the other way around.

What motor?
What wind?
What battery?
What gearbox?
What tires?

And are you racing or playing in the street?

[craigc8791]

Eau Rouge you are a beast.

[allaircooled]

Thanks, thats all easy to understand. I guess I never knew how people came up with the gear combinations that they did, I just used what they used with the same gear box (no stealth back then) and same motor. Don't remember seeing any recommended starting points with my motors back then but I could have easily missed them I guess.

[Eau Rouge]

Yeah, there really weren't "starting points" back then. That was something that really came about later on. Sometimes, you would get a gear chart with a car's instruction manual, or maybe even with a motor if you were lucky, but it was rare. Usually, it was a lot of floundering around with burned brushes or trial and error (one tooth up at a time). Mostly, it was asking someone else who ran the same thing as you, or at least knew what they were doing at the race track.

With brushless today, you can still do trial and error, with a temp gun and a stopwatch. It's just that the internet has made most people really lazy (including me) and it's easier to just ask what everyone else is using as a starting point. You can usually get within 1-2 teeth from perfect by getting a general consensus starting FDR. That's how you adapt between car to car and get the relatively same track speeds.



doug

[allaircooled]

Cool thanks, I guess I was just over thinking this.