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Scat(\/)an
03-11-2001, 11:17 PM
Calculating Speed:

A while back, someone on this forum poseted a question about the speed of a box-stock 4-Tec. I had been wondering the same thing for a while and had just recently come across the missing piece of the puzzle - when RC Car Action published an article about increasing performance in closed end-bell motors. Their test subject was a Traxxas 20-turn Stinger and the article included dyno results - including maximum rpm. This information was vital in determining speed.

The calculations to determine speed are simple, you just need to know a few things:

1. Spur and Pinion sizes
2. Tire Diameter
3. Engine RPM

We'll use the stock 4-Tec as an example.

STEP 1.
84 / 28 = 3
Our gear ratio is 3:1.

STEP 2.
Divide the maximum motor RPM by the gear ratio.
23,912 / 3 = 7970.667
How many times the layshaft turns in one minute.

STEP 3.
Divide the layshaft speed by the gear reduction. The differential pulleys are different sizes than the layshaft pulleys, further reducing the gear ratio.
7970.667 / 2.1 = 3795.556
How many times the differential turns in one minute. Since the differential directly drives the wheels, this the RPM of our wheels.

STEP 4.
Calculate tire circumference. This will tell us how far the car travels after one tire revolution. My tires measured out at 2.5" across (diameter).
2.5" X 3.14 (pi) = 7.85"
For every revolution, the car moves 7.85 inches forward.

STEP 5.
Multiply the results from steps 3 and 4.
3795.556" X 7.85 = 29795.115"
In one minute, our car will have traveled this far.

STEP 6.
Now we need to figure out how many inches in a mile - since a mile is 5280 feet, and a foot is 12 inches:
5,280' X 12" = 63,360" in a mile

STEP 7.
Divide the result from step 6 by the result from step 5.
63,360 / 29,795.115 = 2.127
This is how long (in minutes) it would take for our car to travel 1 mile.

STEP 8.
Divide 60 minutes (one hour) by the result from step 7.
60 / 2.127 = 28.2mph

It's that easy. The hard part is finding what your motor's maximum rpm is. The pro level motors come with a sticker with this information, while the less exspensive ones don't. Luckily, RCCA published an article with the specs of 50+ modified motors in August 2000. These numbers are not absolute. Your motor may turn faster or slower than what is listed, and the quality of your batteries will effect rpm too. Even so, we can calculate the car's actual speed to within a mph or so...

Here's a copy of the important info from the article if you ain't got it.
http://www.geocities.com/scatman_1776/motors.doc

[This message has been edited by Scat(\/)an (edited 03-11-2001).]

super4-tec
03-12-2001, 12:36 AM
Hey scat thanks for the formula. I did my calculations on my car and this is what i came up with..56.07 mph

Motor 15t speed gems by trinity rpm is 31,774
Gears are spur=78/pin=27
tires=2.5
I am running a sanyo super cell 2400 pack is this wrong did i miss something help please

Scat(\/)an
03-12-2001, 12:48 AM
I came up with 38.94mph.

78 / 27 = 2.889
31774 / 2.889 = 10998.269
10998.269 / 2.1 = 5237.27
5237.27 X 7.85 = 41112.578
63360 / 41112.578 = 1.541
60 / 1.541 = 38.94

pooycow
03-12-2001, 03:14 PM
I came up with 360mph!!!

(only joking) i need to find the RPM of my motor!

JZTraxx
03-18-2001, 11:11 PM
Any body have the speed for a 12x4 D3.5 with stock gearing?

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JZ