dragnse7en

<div>Hello again people and friends</div><div></div><div>This time I want to cover gear ratios and the final drive ratio. I will help you find what your RC vehicle&rsquo;s gear ratio first, and then apply it to the final drive ratio. Lastly, if you like to use rollout to calculate the size pinion gear you need.</div><div></div><div>I do want to note that I will not cover RPMs or speed in this article. I will cover that in a separate article.</div><div></div><div>Gear Ratio:</div><div>To calculate the car&rsquo;s gear ratio, you need to be familiar with the type of drivetrain it has. 2WD RC vehicles typically have a single one-speed transmission. 4WD vehicles also usually have a one-speed transmission, and are gear and propeller shaft driven or belt and pulley driven. Multiple speed transmissions can also be calculated for gear ratios, if attention is paid to the things mentioned below.</div><div></div><div>If the vehicle&rsquo;s manual didn&rsquo;t provide a gearing table in its contents, then you&rsquo;ll have to take apart the transmission to count the number of teeth on each gear and write it down.</div><div></div><div>Here are the names of the gear you will need to count:</div><div></div><div><span>1.<span> </span></span>Diff Gear (diff pulley for belt drives)</div><div><span>2.<span> </span></span>Counter Gear (center pulley for belt drives)
</div><div>The counter gear/ center pulley counts as any component that runs on the same shaft or axis as the spur gear. Note that some shaft driven cars won&rsquo;t have a counter gear close to the spur; they would be the gears that mesh directly with the diff gears.</div><div></div><div>Now that the number of teeth on the gears or pulleys have been counted and written down, we can plug the numbers into an easy formula to get the gear ratio of the car&rsquo;s transmission.</div><div></div><div>Gear Ratio Calculation:</div><div></div><div>X = diff gear/ pulley</div><div>Y = counter gear/ center pulley</div><div>Z = Gear Ratio</div><div></div><div>X / Y = Z</div><div></div><div>Gear ratios are typically expressed in Z:1 (2.055:1 for example).</div><div></div><div>For example, one of my cars has a 36T diff pulley and a 16T center pulley. I should get a gear ratio of 2.25:1</div><div></div><div>36 / 16 = 2.25</div><div></div><div>For the sake of simplicity, we&rsquo;ll use Z above to get the car&rsquo;s Final Drive Ratio. It&rsquo;s another simple formula, using the difference between the spur and pinion gear, and getting a product from Z, the car&rsquo;s gear ratio. Again, count and write down the number of teeth for both the spur and pinion if you don&rsquo;t know what they are.</div><div></div><div>Final Drive Ratio:</div><div></div><div>Spur gear = A</div><div>Pinion gear = B</div><div>Power output ratio = C</div><div></div><div>A / B = C</div><div></div><div>Finally, multiply the power output ratio by Z above, the gear ratio:</div><div></div><div>C * Z = FDR</div><div></div><div>The FDR (final drive ratio) is also typically expressed as a :1, or to one ratio (4.75:1 for example).</div><div></div><div>Rollout:</div><div></div><div>To calculate the rollout of the car&rsquo;s gearing, you will need to measure the tire&rsquo;s diameter in standard inches. Once that&rsquo;s written down, calculate the circumference of the tire by multiplying it by Pi (3.14). One example would be a tire with a 2.44 inch dia. would have a cir. of 2.44 inches.</div><div></div><div>Divide the circumference of the tire into the car&rsquo;s gear ratio, and you will get the potential rollout of the car.</div><div></div><div>Tire circumference = A</div><div>Gear ratio = B</div><div>Rollout = C</div><div></div><div>A / B = C</div><div></div><div>For example, using the numbers we already have:</div><div></div><div><span>2.44</span>/ 2.25 = 1.08</div><div></div><div>To convert mm to inches, divide the mm by 25.4 (for example, 62mm = 2.44 inches, or 62 / 25.4 = 2.44).</div><div></div><div>That&rsquo;s it! I hope these formulas help out. Please leave any comments, problems with the formulas, or questions here.</div>

redfisher1974

Neat little program with pre-set gear ratio's etc to find top speeds .. [link=http://www.scriptasylum.com/rc_speed/top_speed.html]Give it a try[/link]

redfisher1974

Here is my storm with a fully charged 3s lipo and just over a 1:1 roll out

Differential Ratio: 1
Transmission Ratio: 2.6
Other Ratio: 1
Spur Tooth Count: 87
Pinion Tooth Count: 19
Total Voltage: 12.60
Motor KV: 4600
Tire Diameter (inches): 4.1
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor Coil Resistance: 0
Spur/Pinion Ratio: 4.58 : 1
Total Ratio: 11.90526 : 1
Tire Circumference (inches): 12.88 inches (327.17 mm)
Rollout: 1.08:1
Total Motor Speed: 57960 RPM
Vehicle Speed: 59.38 mph (95.39 km/h)
Effective KV Value: 4600
KT constant: 0.29 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

dragnse7en

That's much better than both of the Android apps on my smartphone! Thanks for the link - bookmarked.

redfisher1974

Your welcome, the site has alot of other good info too.

Haddi Taha

thanks this info is very helpful

Casper06

Welcome back Red!!

Nice write up too Dragon.

Haddi Taha

my tb03 goes 76km/h

redfisher1974

Thanks Dude

dragnse7en

Thank you You can get an app for this on the Android Market, and I Imagine they've got one on the Apple app store as well.

Haddi Taha

actually my tb03 goes 79kmh. How much more speed will i gain if I put the timing on 22 degrees?

dragnse7en

About 3- to 5% more power at the cost of heat. Can you plug in an extra fan into the BATT slot on your receiver?

ozzie-crawl

i have gearit 2 on iphone works great.
for any thing else i like this program. http://dhrc.rchomepage.com/RolloutCalc.htm

dragnse7en

Thank the gods that iphones have it - I thought I was blowing smoke lol - nice to see that you got it hoss sweet

ozzie-crawl

yea its great, i use it all the time with my pan car and now on a drag car project.
i have found some of the calculated speeds very close to gps numbers,until you add boost/turbo timing then it all goes out the window
but there still great especially if running foam tyres so you can redo your roll out as the tyre wears.

great job on writing out them formulas,im sure it will help many people out

dragnse7en

Thanks hoss. I was thinking of strapping my Andoid smartphone and turning on its GPS speed app and go insane with my 7700Kv motor, but then again I would probably never be able to talk on that phone ever again.

Haddi Taha

i am already using the reciever power for my esc fan. Need an adapter

ozzie-crawl

seen that done a few years back,brand new camera phone strapped to roof of monster truck.they wanted some onboard footage of the race. it would have been good if the camera/phone lasted more than 2 laps

lucky for me a mate has data logger/gps and onboard cameras

dragnse7en

You can splice the same wires into the esc's receiver wires. That's the trace for the BEC circuit.

If not, Great Planes/ Horizon Hobbies has been selling a volt split harness for about two decades now. Goto tower hobbies and you'll find a receiver power supply splitter/ y harness adapter plug for a futaba style receiver plug, JST plug, anything you want and in between. They sell a lot of y harnesses for rc airplanes, but I've seen em before.

Haddi Taha

cool, i should get a parallel style one?

dragnse7en

lol they're all parallel if they are a y harness. If you need more voltage to run two fans, see if you can go into your esc's software and bump up the BEC voltage to 6.0V.

jrowles91

Sorry if this is a stupid question but why do we need to know the roll out and such to pick a pinion? Could I just revert back to this http://www.hpiracing.com/graphics/in.../sprint_19.jpg as that is the car that I have? Are street tires different diameters and such?

jrowles91

Maybe I did the math wrong but according to this my Sprint will only do 15mph with that calculator. I think I did something wrong.

dragnse7en

That speed sounds about right if you're running it stock. Yeah, disappointing when you see the numbers, because it looks like it's going faster, right?

I've currently got a 17.5T/ 2200kV bl motor, a 7.4V 2S lipo battery, and my gear ratio is 2.055:1. I'm running a 100T spur and 40T pinion in it. I get barely over 23mph for all that.

Remember - it's a basic estimate or calculation, so those numbers could never be accurate all the time the car's running.

Haddi Taha

you can gear that sucker more dragnes7en. Alot of people iv'e seen aim for the 3.5-4.5fdr range for 17.5t motor
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