Yevins

iTrader: (1)

Here is a formula to find your right gearing:

http://www.rcuniverse.com/forum/m_3255405/tm.htm

try it....worked for me

superdave2

iTrader: (3)

cool that is a lot of reading but it should work

Drifting101

iTrader: (8)

Cool, thx for that

bprocket

iTrader: (2)

I'm not sure I agree with the formula used. If my FDR should be equal to the roll-out (tire diameter * pi), then my FDR should be around 12.57 (4 * pi). With an IDR of 2.43 and a spur of 78t, my pinion would have to be 15t. I'm running a 17.5t BL motor and with a 15t pinion I get blown away. I'm geared with a 32t pinion and my acceleration and top end are competitive, and I'm not overheating the motor.

I'm wondering if the formula is more oriented toward brushed motors because the pulish date is 2005.

edseb

A formula is only what it is. It isn't biased, but like math just the equation for input. What you do with it is how you input. FDR and rollout are 2 different things.

bprocket

iTrader: (2)

I may have mis-understood the document referenced in the web link, but it looked to me like it was suggesting that FDR and roll-out should be equal. Please feel free to correct me.

JeromeK99

iTrader: (3)

Well.. I read the article and understood what they are trying to say.. but there are so many other factors involved...

Using that article basically says that every "stock" motor has this magic number based on the FDR and rollout of the tire.. ok... What about motors being different.. even the same can/arm brand of motor is usually a bit different in rpm and torque.. not to mention efficiency.. what about different cans? What about track and traction conditions? You gear taller for tracks that are bigger or have lots of straights... You might gear taller for tracks that are loose or have lots of ruts... What about time of year? Where I race its around 100 deg during the summer... and the winter it's 60deg or less...

Most offroad race cars/trucks have a 2.4 to 2.6 internal drive ratio... I have found that you need to have your FDR anywhere from 9.0 to around 11.5.. This is a big range to choose from..

Bottom line is you need to run your motors on the track.. with good tires and test.. and test.. until you get the results you need.. Then with that.. you are pretty much on for the same motor/can for the next motor.

If you can keep your motor under 140deg for a brushed stock after 5min of runtime with the power you need for racing.. your good.. The higher the heat though the faster your motor will wear and will require more maintenance.

Now this is way off for brushless... A lot of folks are now trying 17.5bl motors and the gearing is nothing close to what we use for brushed. They are gearing in the neighborhood of 5.0 - 7.0 FDR. It seems to me that these motors need to spool up in the RPM before they have any power. Running tall gearing allows the motor to reach this "powerband" quickly without a heavy load. Unfortunately most of the current motor plates and gear covers can barely fit the required gearing. I bet that will change soon.

From what I've seen in the stock classes, the guys with 17.5bl systems, that have it right do very well. The folks with the wrong gearing are in the back. I think the 17.5BL with the right gearing will be the great equalizer! Before, only the folks that could really tune and maintain brushed motors had speed and power on the track.


Jerome

edseb

FDR is final drive ratio which means gearing for the car. The problem is that it doesn't account for the size of the tire. Now that's ok if everyone is running the same size tire (like rubber tire touring car) since if someone gives you their FDR then you will be able to gear the same since you don't have to worry about differences in the size of tires. But, in foam tire racing, like 1/12th scale or foam touring car, the diameter of the tire is different from car to car since there are many factors that affect the size like wear, etc. So in order to accurately gear your car, you also have to take in to account the sizer of the tire since gearing will be much different on a car with 62mm tires vs. 53mm tires. You can do rollout in rubber tire, it just isn't usually needed since the diameter is always relative.

wingracer

iTrader: (24)

That is the dumbest thing I've read all week. If your final drive ratio and tire circumference are the exact same number that does NOT mean the motor turns once for one turn of the tires. The numbers are two totally different things. To prove this just think about this. Let's say you have a gearing combination that does result in one turn of the motor equaling one turn of the tire and your numbers were the same. Now put a bigger tire one the car. One turn of the motor will still only turn the tire one time but the numbers will not match anymore.

Also, the difference in the two numbers has nothing to do with efficiency.

wingracer

iTrader: (24)

Quote
"By a 1 to 1 ratio, I
am referring to one revolution of the tire to one revolution of the motor. In
doing so, we do our best to maintain the overall efficiency of the motor by
keeping it in the ‘Sweet Spot’."

So according to this theory a 1/12 pan car should have 50 tooth pinion and 50 tooth spur resulting in one revolution of the motor equaling one revolution of the tire. Trust me, this car would be slow and the motor and/or speed control would fry in no time.

katfish317

iTrader: (6)

I tried this out and looked at my car using the gearing for this. I noticed that my motor goes two times around to the tires one. Plus I am too old for all this math my head started smoking out my right ear.

mauromj

iTrader: (26)

12th scale doesnt have a tranny though

wingracer

iTrader: (24)

So? That just means for the internal drive ratio calculations you use 1:1. So you just leave that step out.