How can you move very fast and be able to still turn very good without omni directional wheels. Also what is the best gear ratio?
I’m guessing maybe a swerve drive would work. About the gear ratios, it depends on what you want to accomplish, more torque or more speed. Here’s a motor speed chart, it should help (
). It has all kinds of combinations depending on your needs.
Hope it helps! Take care!
The best way to turn at high speeds without omni wheel would be a swerve drive or a 2 wheel drive (with programming to prevent the robot from turning too fast).
The fastest that my team got a robot was with a 5:1 ratio, 4 motors, and two 5 inch wheel. Typically we use a 5:3 ratio on 5 inch wheels and 4 or 6 motors. But these robots are typically pretty heavy.
If you are trying to get a robot go as fast as possible, I would suggest a swerve drive, with all of your motors geared 7:1 or 15:1 (depending on how many motors you have) on two 5 inch wheels.
With a short wheel base and wide track, you can turn reliably with a medium-weight robot (15 pounds-ish) that is geared for 3 feet per second or less.
The Exothermic Elevation robots (which all have omni wheels) weighed between 8.5 and 11 pounds. Even with the omnis, the robots that were geared for 3.6 to 4 fps had turning problems. Most of the Exothermic robots added two extra motors to the rear, which had the effect of making turning MUCH better, and improved acceleration to boot. When one of our teams tried a geared speed of 5 fps, they could not turn reliably.
The factors are:
- The weight of the robot. Lighter is better.
- The tires/wheels chosen. I would rate the three standard wheels as 4" easiest, then 5", then 2.75"
- The geared speed from the “VEX speed chart.” Slower is better.
- Wheelbase (distance between wheels front-to-rear) Shorter is better.
- Track width (distance between wheels from side-to-side) Wider is better.
Making a fast robot turn well is a trick. Generally speaking, the slower the robot, the easier it is to turn.
This is geared for nearly 11 fps. Could the robot even turn? We tried this on a very light robot, and it still didn’t work right.
That’s a useful ratio. That was the gearing 419 used this year with 4" wheels (10.9 pound robot), and it was very nimble. Once you start playing with speeds over 4 fps, you get real design issues.
I’ve been playing around with an idea for a robot faster than the VEX dragster, but it would require a transmission to get the thing rolling. Anything geared at 15:1 with big wheels would accelerate at glacial pace, use lots of current (it would eat batteries), and have poor turning, which is why the swerve is recommended. It would be a fun design to play with, but it would not make a useful competition robot. And, yes, I know not everyone cares about competitions!
is there another way to drive and turn really fast with out swerving wheels and without omni directional wheels? were using 4 motor drive to add more info
Run it on half rims. What I mean is take the rubbery outer layer off whatever wheel you’re using. The plastic underneath has a lower coefficient of friction than the rubber. Use that on half your drivetrain, front/back half depending on which one you prefer. It’s not as good as an omni-wheel but it’ll do for the most part. On the other hand, a triangular bot with two wheel drive and one free spinning hub at the front could also work
I would also play around with your drive code. The exothermic bots used a cubic drive code which let us run fast when going straight but also make nice smooth turns.
What wheels in the kits are good for turning? Is it the small, medium, or large wheels?
If we are using 4 of the same wheels from the kit not omni what is the last gear ratio that will make it move really fast and turn really good before it has turning problems? (4 motors we are using)
Is this the same wih six wheels or diffent what is the gear ration then if its not the same?
Also is it possible to turn with six wheels better?
Team 254’s FRC robot uses a 6 wheeled drive with the center wheel dropped and each side is chained together. The robot would lean forwards or backwards, but it always had 4 wheels on the ground and the lean was negligible. This allows us to turn on only 4 wheels, but have a stable 6 wheeled drive. Another advantage of this is that the wheels that are touching on the ground are wider apart relative to the length, which allows it to turn better.
So, yes typically 6 wheels is better than 4 wheels when it comes to turning, if the centered wheel is dropped.
what dou mean dropped?
Something like this:
https://vexforum.com/gallery/files/6/4/5/1/robot1.jpg
Notice how the center wheel is lower than the outside two, or “dropped.”
Dropped means that if you rested the front and back wheels on a surface, the middle wheels would rest BELOW that surface, making a very very very shallow “V” shape.
When the robot is placed on a flat surface this “V” shape means the robot is either resting on it’s front and middle wheels and the back wheels are in the air or it’s resting on it’s back and middle wheels and it’s front wheels are in the air. During a run the robot may “rock” where it switches from front to back wheels or from back to front wheels.
It may seem tempting in such a case to make the middle wheel the drive wheels but when the robot encounters an incline (such as the middle pad in elevation) the middle wheels may leave the ground.
All that being said, there is no “right final answer” which is what you seem to be looking for. There are a lot of factors and many things at cross purposes.
For any given number of motors:
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The heavier your robot is, the slower it will go or the slower it will accelerate.
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The faster your robot is, the slower it will accelerate.
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The wider the wheels the slower it will turn. Note that robots that turn easily tend to turn easily even when you don’t want them to turn. When you build something that’s fast with heavy acceleration and will turn on a dime you can bet the dang thing isn’t very precise in it’s movement.
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The further the front wheels are from the back wheels, the more friction you’ll have to overcome when turning.
For an example of #4 take a pencil and imagine it as a dragster and then slowly rotate the pencil. See that the ends are moving completely against the direction of the wheels. Now imagine it as an extremely wide short car and slowly rotate it. The ends should be moving along the wheel track.
The dropped wheels (above) are an example of fixing this. Basically, by doing this the distance between the front wheels (that are on the ground) and the back wheels (that are on the ground) is halved. -
You can change this friction by not having your wheels straight. In such a case the robot will turn faster but it’s acceleration and top speed will be hampered. Wheels shaped <> will turn on a dime but will be fighting friction to go forward. Then again, they’ll go sideways as well as they go forward. Tilting them slightly (instead of 45 degrees) will leave the turning slightly improved, the forward movement slightly hampered, and the side movement possible but not as good as forward. (Assuming 4 wheel drive)
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You can change this friction by modifying the wheels to reduce friction.
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By allowing the front wheel(s) to rotate in any direction and not attaching a motor to them, the robot will not have to deal with any of that friction.
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You can build a robot with steering, much like a car. The complication pretty much exceeds the value though.
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Go back and look at the speed chart juankiCIEM linked to. I personally tend to do the math, but the speed chart is a great place to start and I tend to use it to figure out if I messed up my math before I’m done. Gear ratio is only half of the equation when it comes to acceleration and speed. Wheel size is equally important because you’re converting revolutions per minute (the motor’s output) to feet per second (the wheel on the ground).
A Tip: When a really heavy robot/machine causes the 4" (or any size, but especially the 4") Vex tires to sink into the foam tiles of a Vex competition field, the ridges on the outer edges of the tires will dig into the foam and make it very hard to turn the robot.
It turned…a little too well - see it here. It was uncontrollable, even when programmed to go straight. If I ever get time, I’ll recreate it with a third unpowered wheel on a servo.
I recall suggesting a solution to a similar question just a little while back… was that thread deleted?
If we add a wheel on the back of are robot that is like this - that is powered by a motoer and the other 4 wheels are like this | will are robot turn.
CAnt think need help
At this point I don’t understand the last question. It sounds to me like you want a 5 wheel robot with an extra wheel in the back all facing forward. WHY you think an extra forward facing wheel will make it easier to turn is a lesson in physics I don’t understand.
For simplicity sake try this:
- Put the wheels on the robot
- Leave the gears, motors, batteries, controllers, etc off.
- Roll it with your hands.
If it’s easy to turn, it will be easy to turn. If it’s hard to turn it will be hard to turn.
or if you want mank 2 wheels at te front
and one wheel at the back hooked up to a motor so it steers like a car
or have two wheels at the front and make a castor at the back
there would almost be no friction at all
Bon u know what dont reply to this thread. U are being kind of mean. This is what i’m asking one more time!!
To see what i am asking look at team 2467d in the video.
http://www.youtube.com/watch?v=ONrpe1aYJwM
If we have 1 or 2 wheels powered by a motor in the back will that help are robot turn. We are asking because we want are robot to go fast but its having problems turning we want are robot to move really fast and turn good.(remember we are not thinking of swerving or using omni wheels since we think swerving is a bad idea for are robot and omni wheels are to expensive)