Are six wheels better than four?

[cziggy343]

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

[thefro526]

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.

[AdamHeard]

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.

[cziggy343]

Originally Posted by thefro526

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.

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

[AdamHeard]

Originally Posted by cziggy343

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

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.

[waialua359]

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.

[=Martin=Taylor=]

Originally Posted by waialua359

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.

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

[sporno]

Originally Posted by waialua359

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.

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.

[Gdeaver]

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.

[thefro526]

Originally Posted by cziggy343

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

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.

Originally Posted by Hachiban VIII

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

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

[waialua359]

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

[pogenwurst]

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.

[thefro526]

Originally Posted by waialua359

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

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

[Siri]

Originally Posted by pogenwurst

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.

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.

Originally Posted by Gdeaver

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.

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.

[Molten]

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.