Are six wheels better than four?
 

well... the title was mainly b/c i could not think of anything better... but i wanted to know if people had done experiments to prove weather or not having more wheels creates more traction. form what our team has found, it seems to come out in a wash, but i wanted to see what other people thought. any help would be appreciated :)

Re: Are six wheels better than four?
 

In theory, more wheels will not give you more traction.

More wheels will give you the ability to redistribute your weight to change the the robot's handling on the surface. So sometimes more wheels will give you better handling and other times more wheels will give you a handicap.

Re: Are six wheels better than four?
 

The interaction of the wheels on the floor is $@#$@#$@#$@# close to two ideal surfaces.

Therefore, surface area in no way will increase traction.

the coefficient of friction is said to be .05/.06 for static, therefore all robots that are 150 lbs can only push with 7.5/9 lbs of force, regardless of the number of wheels.

Re: Are six wheels better than four?
 

so what you are saying is that it may give better handling, but not better traction?

Re: Are six wheels better than four?
 

Nothing can legally be done to get you more traction that a robot at full bumper and robot weight. You can only reduce the traction by having a robot weigh less.

Now, the wheels slipping provide less traction then not slipping, you won't increase the maximum traction by solving that issue, but you'll reduce the time you have minimized traction. This would probably solve the same issue, as if you are trying to push another robot and have traction control to keep your wheels from slipping, and they don't, you can push ~20% harder.

Re: Are six wheels better than four?
 

Six wheels are better than four, if the middle wheel is some type of idler wheel used to provide feedback in the programming portion, to attempt to create some sort of traction control system. ;)

Re: Are six wheels better than four?
 

Unpowered idlers of any type are a horrible waste of normal force.

Re: Are six wheels better than four?
 

To me it seems that the most power you can create is when your on the verge of doing a burn out. Its a very very fine line.

Re: Are six wheels better than four?
 

Keep in mind that most of the discussions concerning friction and traction assume a flat surface contact. The FRP material is going over carpet. If the FRP was placed on a hard surface like concrete or a gym floor the 2 D assumptions would be valid. Because of the carpet the FRP may be deformed changing the contact area. This puts the physics into a 3 D problem. Does this change things? Do fewer wheels cause more defection and better performance or does the deflection make it worse? Is the deflection enough to have a significant effect? I don't know the answers. Not sure about the math either, but it needs to be looked at. This is a case where one needs to consider the assumptions of a math model to determine if the model is valid for the problem.

Re: Are six wheels better than four?
 

True, you won't really notice any substantial difference in traction but you may notice under certain situations you will have more possible traction like when turning or making certain maneuvers but you won't raise your maximum traction value.

True that. Anything that touches the ground spreads should be powered... IMO

Re: Are six wheels better than four?
 

We are actually going to use one idler wheel. As long as the benefits outweigh the inefficiency created, why not? ;)

Re: Are six wheels better than four?
 

We've had a lot of debate about this on my team, and this seems like an appropriate thread to ask for clarification:

What sort of difference, exactly, is there between adding extra driven wheels versus adding extra undriven to our robot, and why?

I've seen snatches of conversation on the issue, but nothing really substantial enough for me to draw a conclusion from.

Re: Are six wheels better than four?
 

True, very true. If you can make it work then more power to you.:D

Re: Are six wheels better than four?
 

Good question. The robot's weight is supported by every wheel that touches the ground, driven or undriven. This weight corresponds to the normal force, which represents how much frictional force each wheel could generate (mu*Fn=Ff). However, it can only generate this force if it's being driven. Make sense? So say you had a 120lb robot and 4 wheels evenly supporting the weight--not likely, as robots usually aren't perfectly weight-symmetrical, but nonetheless. Each wheel supports 30lbs, so each can generate a maximum force of mu*30. So if all the wheels are driven, it's 4(mu*30). If only 2 are driven, it's only 2(mu*30), significantly less.

EDIT: For those wondering, "mu" is the coefficient of friction (I imply it's static in this case), which is basically a measure of how difficult it is to move two materials against each other, with larger numbers (usually around 1) being more difficult than smaller ones. As you can imagine, the mu's in this game a very low, between .15 and .05, depending on who you ask. Also, the reason spinning wheels generates a frictional force in the first place basically goes back to Newton's third law, the old action-reaction one.

Very true. In fact, if you've seen the FRP, it itself is bumpy. I can't explain the math, but I think what it comes down to is that contact area is actually important. Because of the contact area changes, the more surface area you have, the more likely you are to hit a higher mu. (Note this conclusion is based solely on dynamic contact issues, I'm not sure about deformation.) To at least quasi-demonstrate this to your team, I might recommend an incline test. That is, slap together a chassis, put the wheels on the FRP and tilt the plastic until the chassis starts to slip. The terminal angle can also give you static mu values if you run the force-sum equations, but it's worth it just to see wheels get stuck on a bump, causing the angle to change slightly.

Re: Are six wheels better than four?
 

Extra undriven wheels take weight off of your driven wheels. In short, you get less traction to your motors. Also, it will create some drag. More driven wheels would be great if you ever get up to the point that you are using the full power of the usual two powered wheels. However, you probably will never get to that point on such a surface. To be honest, I don't see much of an advantage of an unpowered wheel. They are going to slow you down. Even with the data they could collect, it would still slow you down too much. As far as extra powered wheels, it is just a waste of your weight because you will never need that much power on this game given the surface and the size of the field.

Re: Are six wheels better than four?
 

In most cases, yes. However, this is a special case, in that the wheel does not need to support any of the robot's weight, it is being used only for instrumentation. If the speed sensing wheel is attached to a hinged arm, and carries only it's own weight, then it should not affect robot performance noticeably.

Re: Are six wheels better than four?
 

Driven (motor is powering the wheel) wheels will provide forward force.
Undriven (not powered by motor) wheels will distribute the weight more.

Traction depends on pounds per square inch of wheel that is in contact with the ground.

For example: you have 4 wheels, one in each corner. Robot = 120lb. Assuming that the robot is perfectly balanced, each wheel is carrying 30lb.

Example 2: 6 wheels, three on each side (left/right) with a 120lb robot gives 20lb per wheel.


More drive wheels still decrease weight/area ratio, but make up for it (possibly) with the added power.

Re: Are six wheels better than four?
 

Fat lot of good it will do as an encoder if it isn't pushing on the floor with significant force. If it slips, it won't be sensing anything meaningful.

Re: Are six wheels better than four?
 

I suppose someone who wants to use an extra wheel for instrumentation could calculate how much force it needs to contact the floor with, to prevent it slipping under maximum robot acceleration conditions (it would slip if it takes more torque to overcome it's own inertia, than will be transfered to it by the frictional force of it's contact with the arena). I expect it won't take (much) more than it's own weight. I haven't done the calculations....I'll leave that for the students :)

Re: Are six wheels better than four?
 

The point being, any normal force used to creating friction between the instrumentation wheel and the ground is normal force that's not able to be used to create friction between the drive wheels and the ground.

On a different note:

Our team is still considering different drivetrain options, but we've nearly decided on a six-wheel, dropped center wheel method if we follow a differential drive design, as the ability of the robot to rock slightly will allow the robot to not be as affected by transverse friction when trying to turn. We've tested a 4-wheel differential drive and while accelerating was not as difficult as one might think, turning was not as responsive as we would like. We're waiting for two more rover wheels to test out the 6-wheel drive.

--Ryan

Re: Are six wheels better than four?
 

my thinking is that it will just give you more speed.
meaning, you may go sliding more on the crater floor.
now i have no clue if this is true or not, this is just my opinion from what i've heard.

Re: Are six wheels better than four?
 

Yup. Making this decision might involve comparing the control advantge to be gained by implementing a good traction control algorithm, vs. the loss in tractive force by putting a small part of the robot's weight on the unpowered sensor wheels.

Re: Are six wheels better than four?
 

squirrel: The only thing I want to know is why you need normal force to create the traction control algorithm? You can do it without such a reading. All you really need is to do a little bit of math (calculus at most) and you should be able to come up with it as a simple formula. A little programming should allow you to use this formula for the traction control.

Re: Are six wheels better than four?
 

I would normally agree with this, but in this game there is so little friction available you might be talking about the difference between nil and .95(nil). I think comparing the rotational rate of an idler wheel compared to that of the powered wheel might be useful in managing wheelspin.

Re: Are six wheels better than four?
 

There are different ways to implement traction control. Having a sensor wheel is one of them. What type of system you want to use is up to you..maybe the sensor wheel type could work better or be easier to program, or eliminate the need for other sensors. If you can do it a different way, great!

Re: Are six wheels better than four?
 

If you had a six-wheel drive with a lowered center wheel, wouldn’t the weight of the trailer only allow the center and rear wheel touch the ground? If the center and rear wheel are the only ones touching the ground, the front wheel would be doing nothing to help drive the robot.

Re: Are six wheels better than four?
 

Another thing to think about is that the trasverse coefficent of friction is greater than the inline so not only dose it matter how many wheels but if they are turned.

Re: Are six wheels better than four?
 

It would not give you more traction. In fact it would do better to have less wheels so that you could get more weight onto your motorized wheels.

Re: Are six wheels better than four?
 

I think encoders in the drive train are good enough to see slippage in the direction you are trying to go (just look for unnatural acceleration of a single wheel). Detecting slippage normal to the direction of travel is much tougher. I'd try a gyro before an optical mouse kind of thing. This is a tough surface to get an optical mouse to work well.

A couple of posts talk about the increase in traction if the floor is not smooth. This might be true but it is not because there is more surface area or a change in the coefficient. The reason is because there is now something to push against (bump/s in the material). There is an additional force term in the direction of travel.

Re: Are six wheels better than four?
 

According to this link, you shouldn't have problems with instrumentation slippage done in this fashion, due to the fact that you can use a higher COF wheel.

Re: Are six wheels better than four?
 

Yeah, I just saw that. Interesting....makes the traction control thing easier

Re: Are six wheels better than four?
 

yeah we figured that six wheels is better

Re: Are six wheels better than four?
 

To answer the original question: Theory states that any number of wheels will be exactly the same. The reason is that each wheel takes a portion of the weight, that portion gets smaller as the number of wheels increases.

Reality is not quite so clean. More wheels give you a very, very slight advantage, possibly so small as to be difficult to measure by a FIRST team. The reasons for the advantage mostly relate to your ability to maintain more of your grip even if one wheel hits a defect in the surface that momentarily reduces friction (think dust bunny), but also include some effects related to reduced deformation of the surfaces involved.

That being said, I really do not believe that anyone will measure a difference between 4 and 6 wheels that can justify the extra 2 wheels.

Six wheels have other advantages, however, that may convince some teams to use such a drivetrain.

One thing is clear - no wheel needs 2 CIM motors driving it...

Don

Re: Are six wheels better than four?
 

this question came up when we were designing some of our robot and we figured out that haveing 4 wheels is better because this year, surface area isnt going to do anything. what we found is that if you have 4 wheels, more weight is going to be distributed to each wheel while when you have 6, less weight is going to be in each wheel. if you have 4, when you meet maximum weight, each wheel is going to be getting 30 lbs each. when you have 6 wheels, each wheel gets 20 lbs each. it is a 10 lbs difference in all.

Re: Are six wheels better than four?
 

How does the momentum of the robot relate to this? Maximizing the weight maximises the friction, but could there some weight less than maximum that would be the optimum relationship between traction and momentum?

Re: Are six wheels better than four?
 

If you add wheels to the outside of the robot that will help create traction against themselves too.

Re: Are six wheels better than four?
 

please everyone keep in mind that when you are making design decisions you take into account that in the game you will ALWAYS have a trailer attached to your robot in the matches.

this will add an extra 40 pounds to your robot AND an extra set of wheels.

be sure and design your robot to include these because it will make a difference how your robot handles on the field.

hope this proves useful to the teams that read this ^_^

-Avinesh-
-Team 1778-