 # Making dragster with cortex

In school, we are making dragsters with the cortex I want to win so do any of you have tips that would be much appreciated thanks.

Make a chin dive and make sure that all wheels are moving. I did this today and that is what I did and I won. I went to 84 gear and 12 gear. I hope you win.

If you don’t have to worry about turning, then you don’t have to worry about turning.

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You want good acceleration and top speed. This can be obtained via an ideal gear ratio or a transmission.

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Compound gear ratio…60/36 and 60/12. Mine just ran for 5 seconds with the motors set to “coast” and went 125+ft and 250+ in 10 seconds. 2 200rpm motors.

You could also do some interesting things with using rubber bands to store energy to make your dragster go faster than using only gears.

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Do you have a limit on how many motors you can use? How long is the course? With this information and the estimated weight of the robot, you can calculate the ideal gear ratio to maximize the acceleration and velocity for the given track length.

I would highly recommend messing around with the Team Tentacle Torque/Amp-Hour Calculator on the Ask Arron combat robotics Combat Robot Design Tools - Ask Aaron (runamok.tech)

Here, you can simply input the values for the 393 motors (assuming that is what you are using) where the operating voltage is 7.2, stall amperage is 4.8amps, voltage constant is 13.8rpm/volt (because no-load speed is presumed to be 100 rpm, and kv is simply rpm/voltage), and kt (the torque constant in oz-in/amp) = 1352/kv (you can read why this is on the page linked below: Ask Aaron - Converting basic electric motor specs to motor constants (runamok.tech)).

Now just plug in how many motors you intend to use per drive-side, the wheel diameter, and whatever coefficient of friction you want to calculate for (don’t worry about current drain too much for this; its a drag race so you likely wont deplete your battery. just don’t go over the stall current and you’ll be good) and then put in some arbitrary gear ratio. After that, you can go to the page labeled “Acceleration Calculation” where you will see all kinds of useful information about your robot’s motion. By setting the ‘arena size’ to whatever straight-line distance your drag-race track is, you can now simply tune your gear ratio from the previous page until you minimize your “Time to side”

If you want to be even more thorough about this, you can calculate the static coefficient of friction between your wheels and whatever surface you intend to race on by taking the tangent of the angle of whatever angle your wheel starts to slide at (test this with the wheel of the robot and then input that info the torque calculator instead of the default value. You can read about why the tangent of the angle yields the coefficient of static friction with this link New Page 1 (pstcc.edu).

side note- I just noticed this was a vex iq topic so youre probably using iq motors lol but the same principles apply. the kv for the iq motors is 16.6 rpm/volt (since the operating voltage is still 7.2v and the no load speed is listed as 120rpm) and the kt is just 1352/16.6=81.4 oz-in/amp. stall current is just stall torque (in oz-in) divided by kt, so for the IQ smart motors it should be 58.627/81.4=0.72amps (the stall torque is listed as 0.414 newton-meters on the products page, so I simply converted to oz-in to work with the 1352=kt*kv equation). if you plug these values in instead of the ones I gave for the 393 motor, it should all work out

Good luck

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2 motors are the maximum we are allowed to use and 20 feet in 3.5 seconds motors are vex 2 wire 393 motors.

Assuming you use 2 motors, 4” wheels, weigh around 1 lb (lighter is better ofc but this configuration was optimized for 1lb) and have a tire coefficient of friction of 0.7- a 1:10 gear ratio would be ideal. With this configuration, you’d theoretically be able to go the full 20 feet in 1.8 seconds. Whether or not you can actually achieve this in practice though is up to your build quality

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