Re: How can you make turning smoother?
 

Not trying to be Al, but yes your bumpers have to stay in the bumper zone.

You do not have to lift much to get the desired effect.

Re: How can you make turning smoother?
 

The bumpers must stay within the bumper zone when the robot is flat on the floor.
4.1.6.4 R25
BUMPERS must be located entirely within the BUMPER ZONE, which is between 2 and 10 in. from the floor, in reference to the ROBOT standing normally on a flat floor.
Any robot using the six wheel (or more) dropped center, will rock and so will the bumpers. When climbing,pushing or interacting with the pyramid, the robot is expected to change attitude ans so too the bumpers. Also bumpers do not need to be parallel to the floor if that helps in your design.
Does that help?

Re: How can you make turning smoother?
 

It doesn't cause normal force to technically decrease. When driving forward or backwards, there are at least four wheels on the ground. During this time, the amount of normal force on the dropped wheels as well as the normal force on the set of outer wheels is evenly distributed and the same. When the robot is pivoting on the dropped wheels, the normal force pushes in tangential to the place of pivot, this case being the dropped wheels. Inertia therefore causes the outer wheels to lift and balance which in turn means the dropped wheels are the only wheels touching the floor at this moment. I got this information from A.P. Physics B where I am currently getting a high B in. I learned of centripetal force last year and asked my mentor who also happens to be my A.P. Physics teacher is this what caused a dropped 6wd to turn smoothly. He said yes.
Study centripetal force and you'll know what im talking about.

Re: How can you make turning smoother?
 

You need to go back and read Jason's post more carefully:

Dropping the center wheels has exactly the effect described above by Jason, and this is the main reason why dropping the center wheels slightly makes the robot easier to turn.


Could you please politely ask him to send me a PM?

Re: How can you make turning smoother?
 

Sure but he isn't on this website. Can you PM me your email? That way he can get in touch with you. He's the one that founded our robotics team and not really an engineer, but helps us determine how things should theoretically behave.

Re: How can you make turning smoother?
 

Jibri, what Ether is saying is that when you have weight centered on the robot and you add two dropped wheels, the weight is more distributed over the center wheels than the outer wheels, causing a higher normal force on the center wheels than either the front or back set of wheels. Your point about the centripetal force is still valid though.

Re: How can you make turning smoother?
 

I just talked to him. He said that the centripetal force does cause the outer two wheels to lift. Its the same force that NASA is currently studying to create "artificial gravity" in space stations. Centripetal force (in the form of static frictional force in this case) pulls inward and the outer wheels are pulled out parallel two the floor. As the robot begins to pivot, the centripetal force is out of balance. If you have ever done work using vectors, you can tell that one of the outer wheels on the ground and one wheel floating is what causes this imbalance. The centripetal force in turn forces a balance by making the forces on the two outer wheels equal a.k.a. set their vectors parallel to each other.

Think of it like a spinning top. When the top isn't spun, it is laying sideways on the table right? But when you spin it, it balances on a single point that touches the table, almost as if defying gravity. The force that is causing the top to balance is the same force that is causing the outer two wheels to lift.

My mentor also said that this observation is not as observable with an FRC 6wd because the dropped wheel is not dropped enough to notice, the FRC field carpet is too high to notice, FRC robot weight normally isn't evenly distributed on both sides of the robot, etc. so its hard to notice and may not come fully off the ground, but the wheels do lift.

He also said that this only occurs when the robot is pivoting. When the robot is driving forwards or backwards, four wheels are normally touching the ground and the weight (or as you said normal force) is even on all four. One set of wheels will be off of the ground and none of the weight will be on those wheels.

To put it plainly, since the wheels are probably touching the carpet when pivoting, normal force is decreased on the outer wheels when pivoting but not because of the dropped wheel. It is because of the centripetal force acting on the robot by tangential friction. Because a lot of Newtonian mechanics use a lot of the same measurements like Newtons, seconds, meters, etc.., it isn't always easy to figure out exactly what causes what. Even so in this case, it is centripetal force. The dead giveaway is the circular motion.

Btw Ether, love the Einstein picture. I love how he's the father of quantum mechanics yet dedicated his life to proving it wrong!

Re: How can you make turning smoother?
 

While centripetal force exists, I'm not sure it does a lot for helping you turn on an FRC robot.
Also a test to show centripetal force is causing the better turning (or isn't) would be to decrease velocity of the turns and see if you notice more "scrubbing" due to a lower centripetal acceleration.

Also, I don't see a way that centripetal force/acceleration could be shown to be the reason an eight wheel drive with four dropped center wheels works.

Do a little search for "wheel base" on here and I'm sure you can find the reason drop center 6wd/8wd works the way it does.

Re: How can you make turning smoother?
 

The spinning top is an interesting example. What happens when the top is imbalanced and spinning slowly?

Re: How can you make turning smoother?
 

While decreasing the velocity, it would skid more.

Looking at equations, (Fc=centripetal force, ac=centripetal acceleration)
Fc=m*ac
ac=lvl^2/r
So looking at this as circular speed decreases, so does the acceleration and in turn the centripetal force. Less centripetal force means less gravity it can counteract and balance.

As for the 8wd, the reason that works well is because the wheels that are pivoted on are close to the pivot point of the robot. This means that the frictional force they provide produce less torque on the robot than wheels that are further away from the pivot point. An example is something you all probably have used which is an allen wrench. If you tighten a screw with it with the long way in the screw, it gets hard to turn. You then turn it to put the short way in and it gets easier to tighten. This is the same thing happening here.

Re: How can you make turning smoother?
 

Centripetal force decreases due to friction on the top by the surface and maybe some air resistance. Eventually, gravity begins to start winning again and begins pulling a side of the top down.

Re: How can you make turning smoother?
 

Let's assume you have a robot frame with electronics and the center of mass is directly in the middle of the robot, a few inches from the floor. Let's say it weighs 30lbs. Now, you add the battery (assume 15lbs), which is equidistant from the two center wheels and the two rear wheels.

Let's say you're spinning it slowly such that centripetal force is negligible. There is more normal force on the center wheels than the outer wheels because the center of mass is somewhere between the battery and the center of the robot (and more toward the center at that). This means there is less frictional force applied to the outer wheels and they will effectively "skid" across the floor, allowing you to turn on a dime.

If you increase the turning rate and take centripetal force into account, you are applying a force on the frame that is opposing the centripetal force on the battery.

Imagine you have a heavy weight on the end of a string that is attached to a vertically mounted pole. When you swing the weight around the pole (think tetherball), the weight indeed rises. However, the robot isn't the weight in this scenario. The robot is the entire system, and the weight is the center of mass. The pole tends to lean in the direction of the weight because of the force opposite centripetal force that is being applied to it. The same thing happens for FRC robots.

I believe it's much less noticeable because the dynamic friction is less than static friction in the case of wheel tread and with a robot weight mostly centered but off to one side, the center wheels will never start slipping. I have seen Vex robots with six wheel drive behave like four wheel drive robots, because the center of mass is either equidistant from the four wheels in contact with the floor or more toward the outer wheels.

Re: How can you make turning smoother?
 

We had that issues in our 2008 season overdrive game. The way we fixed it was getting omni
Wheels and kiddie corner them from each other so we could turn at full speed and it worked be very good. But before we got them in we had a comp so what we did was zip tied the the front left wheel and back right wheel so it have the same effect as the omni wheel would have. I hope this helps and good luck.

Re: How can you make turning smoother?
 

Jibri,
While centripetal force may exist (on an FRC robot), I do not believe it is the dominant force in turning at the velocity we encounter on the field. Simply acceleration or deceleration cause the robot to tip towards the front or back wheels and generally halves the wheelbase in doing so. Teams have found that the distance the center wheel is dropped has significant effects on turning friction dependent on several factors such as wheel tread width, tread type and robot weight. In these designs, it is arguable how much weight is transferred to each wheel but it is unlikely that each wheel will ever receive equal force. In rare cases where the team has managed to balance the robot weight over the center wheels and keep the COG low, it is possible for all the weight to rest on the center wheels. This leaves the front and back wheels (in a six wheel robot) to act more as training wheels than actually supporting the robot. In these robots, turning is accomplished almost exclusively on two wheels minimizing the friction as discussed above. High speed, arcing turns also bring other forces into play shifting significant robot weight to the outer wheels.

Re: How can you make turning smoother?
 

Are you using victors? If so, make sure that the dynamic brake is turned off. If it isn't, when you let go of control your robot will stop abruptly.