# Vex EDR Robot Speed Comparison

My Robot Can travel 55inches per second straigh foeward distance. I was wondering what speeds your all’s robots were operating at? Thanks

If you have 4” wheels:
I think your wheels are going 525 RPM so either you have 600 RPM with friction or you have a 3:7 gear ratio.
Regardless, what you have is overkill.
Edit: Scroll down, I was incorrect

Interesting. I’m running 4 300 rpm Mecanums.

Let me double check my maths, Im probably wrong now that I realize I likely made a mistake.

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My apologies… Yea I was way off. This is assuming you had 4” wheels. If you had 4” wheels this would equate to similar performance, but this doesn’t include torque and acceleration difference which may play a big factor for mecanums.

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the way you measure the speed as well as factors like friction will make a difference compared to the theoretical ft/s speed. if you start your robot not moving, you’ll have a much slower speed then theoretical due to acceleration times. But if you start measuring after your robot has already driven up to speed it will be closer to the theoretical value.

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We can see in the results vs what was responded. The motors are 300 RPM and the mecanums are 4”. If we had 4” regular wheels it would be 300 RPM easily, but as you can see the mecanums would equate to 262.5 RPM. That being said, if the torque is the same between regular wheels and mecanums we would have roughly 88% efficiency as compared to regular wheels.

What i did to measure speed is that i stared a timer. when the timer started i started the robot when the timer stoped i stopped the robot and measured that distance. I also know that @Xenon27 you have a pretty competitive tank drive would you give me your robots speed please? Thanks For the info!

If that is the case then I am hypothesizing mecanums to have an efficiency of around 90-92% of regular wheels efficiency. So, I think it may be a good note to understand that mecanums are likely 8% less efficient as compared to regular wheels, which is not a bad tradeoff.

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that’s not a great way to measure actual speed, because your robot will be accelerating differently based on a variety of factors, and you won’t be able to perfectly start driving when the timer starts, or perfectly stop driving when the timer stops. what you should do is lay out a tape measure or a meter stick, and take a video of your robot driving past it. then pick a starting time (after the robot has already accelerated to max speed) and a stopping time (anywhere before the robot drives off screen or slows down.) and then find the time between those points as well as the distance (by looking at the meter sticks/tape measure).

I haven’t measured actual speed of my robot yet, but the theoretical speed is 56.6"/second.

I think losses in efficiency are probably more heavily influenced by things like friction and weight than the wheel type.

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Ah. Thanks A lot!!!

i wouldnt recommend measuring linear velocity that way, for reasons the others said

use encoders on the wheel shaft (tracking wheels would be ideal) to get the rpm the wheel is spinning at. make a graph of it and once you finish accelerating it should be pretty easy to see because the angular velocity will be relatively flat. after that, multiply the angular velocity (the flat line value) by the circumference of the wheel (pi * r^2), and you’ll be able to see your linear velocity. then just compare it to your theoretical linear velocity (using simple gear ratio math to find the theoretical rpm at the wheel, multiplied by the circumference of the wheel) and you can get a rough idea of how close you are (note that dividing the actual and theoretical velocity won’t give you your efficiency value; efficiency deals with power, and to find the comparison between power input and power output you have to take the acceleration into account. i can guide you through the math involved in that step, but it’s a bit longer to find and doesn’t matter a whole lot in vex)

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not sure what you did to find this value, but you can’t just divide the real and theoretical velocity to find the efficiency. as i mentioned in my previous post, efficiency is a comparison between power input and power output. gotta take into account acceleration, something mr timmy did not do (remember, power is angular velocity times torque. you can’t solve it with just one of those variables)

My response was just a hypothesized guesstimation, where if the guestimation is right then the tradeoff is not bad. It’s a hypothesis. 1 Like