Traction Questions

So I’ve heard a lot of people talk about increasing traction/friction between their robot’s wheels and the carpet by stacking extra wheels on top of each other or using wheels wider than 1". Both of these methods sum up to increasing the amount of surface area in contact with the carpet. This is confusing to me. IIRC, I learned in physics that the force of friction is dependent upon the coefficient of friction and the normal force, and surface area has nothing to do with it. Given, that equation only holds true for hard surfaces, not “squishy”, yielding surfaces like carpet and roughtop tread. Round wheels on FRC carpet will “sink” into the carpet slightly. The only effect I see of adding more or wider wheels is to spread the weight of the robot out more evenly, and reduce/relieve the “pressure points” in the carpet. This might have a positive effect on wear characteristics, but that’s beside the point. So, with that in mind, I pose these questions:

-Is there a difference between traction and friction?

-If so, what is the difference?

-Does adding wheels/width increase traction, friction, or both?

Thanks!

*While you are waiting for a response, you might try searching the forums. There are scores of posts about this very topic.

*

This is a difficult question to answer, but I can try to shed some light.
There is a difference between friction and traction; friction is the resistive force to motion while traction is putting friction to use. (it is mostly semantics, however.)

Adding wheels or increasing wheel thickness really does two things. Like you said it increases the life of the tread, but that is not the whole answer. The amount of horizontal force that you can produce with a given coefficient of friction does in fact increase as the normal force increases, however it does so at a diminishing rate, i.e., it is not a linear relationship. For this reason, wider tread is more efficient because the relationship between normal force and frictional force is more linear.

Also, think of the grooves in your tread as teeth and the carpet as another set of teeth. As your wheels try to spin the teeth on the wheels will dig in and push against the teeth in the carpet which will move the robot. So the more teeth you have pushing against each other, the more “traction” you have. Lets say there is only one set of teeth, i.e., one groove in the tread and one in the carpet. Now, since the tread is a compressible material, it will not take much load to deform the tread and cause it to slip. If there were 100 pairs of teeth it would take a lot more to deform all of the teeth and slip the wheel.

Hope to makes sense.

This is actually pretty close. To summarize previous discussions:

There is a small-scale interaction between the tread of the wheel and the carpet, think of it as “mini-Velcro”. Nominally, the wheel has a certain coefficient of friction, thus a certain maximum force that can be exerted either by the wheel or on the wheel (which we generally think of as “traction”, F=mu*N). This small interaction will slightly–oh-so-slightly–increase the apparent coefficient of friction, and thus increase the traction very slightly.

Here’s where it gets tricky. As you add more wheels, N goes down per wheel, something about more points for N to act on. But… you’re also adding more interaction points, which will slightly increase mu overall… There is a limit, I think, to how useful it is to add width and area, but I’m not sure where it is.

Someone managed to kill the discussion entirely by noting that in 2009, where traction was limited, their team got better traction by doubling up some or all of their wheels. Anecdotal, to be sure, but still worth considering.

In Lunacy, we did measurably (but only slightly) increase tractive force by using a total of 12 wheels on a 6-wheel drivetrain.

It is absolutely true that increasing the contact area does not affect traction if the weight stays the same. But to explain our measurements we theorized that if any small area of the floor (or wheel) was allowing more slip (say, due to some dust on the floor) the larger contact area would allow for a smaller percentage of the available traction to be lost.

We made our decision based on empirical data, so not entirely anecdotal, but in hindsight we probably would use only 6 wheels in the same situation.

I’m not sure if Lunacy is the best evidence for this. The way wheels interact with the Lunacy field and the way the interact with carpet is pretty different. Because the treads “dig in” to the carpet, there is a normal force applied which makes the traditional friction model…well, not really fall completely apart, but it’s not as accurate as it could be.

I fully support Ether’s suggestion: those 10^4 lines of code don’t get nearly the workout that they should.

Also, it has been my experience that aggressive (read: winning) drivers tend to wear down wheel surfaces in roughly inverse proportion to surface area. Prolonging tread life by doubling the wheel surface can prevent losing a critical pushing contest during the second of two back-to-back matches. This is an important consideration for district teams – we need to keep our robots performing well in all twelve qualifying matches.

I think you’ve answered your own question. Classic newtonian friction calculation doesn’t work for irregular, soft, non smooth surfaces. The best way to figure this stuff out is test it yourself with tread and carpet. Try driving your robot into a wall and measuring the motor’s current draw with different size wheels, this should give you a pretty accurate predictive model of the relationship between wheel width and effective traction.

At the risk of being alienated, since I am new, I will offer my empirical data for this discussion. My time as a racer, motorcycles.

Look at any funny car, drag car, drag motorcycle. They ALL have WIDE tires. Even with rubber compounds being different they all have one thing in common…WIDTH. They would not be able to get the TRACTION needed to propel their vehicles if they were not WIDE. Skinny tires would not do it. If skinny tires worked they would use them as they are cheaper and weigh less. For these type of sports, motor sports, it is all about power to weight ratio. The problem is how to get all that power to as large as surface area, TRACTION, as possible to get down the track the fastest. The same applies for tractor pulls or 4wd trucks. You don’t see the skinny tires of a tractor on the back for a reason. It is reasonable to say the sport of robotics, in this case, is no different. We have a fixed weight and yes the force per tire goes down with more tires but that is also dependent on how the weight is distributed.

Of course turning is a different issue all together.

It seems like a monthly thing that this problem shatters the illusions of a physics student.
Increasing contact with the carpet does increase traction in certain scenarios. But until someone does an empirical study across several FRC-relevant scenarios, we will all be left to our own intuitions and experience as to what the nature of the relationship is.

You seem to be trying to pick a fight, so my suggestion to you would be to make sure your preconceptions are well grounded before you bury yourself in a hole. I think Mark McLeod’s signature might be relevant here:

deduction is limited by knowledge, and knowledge is limited by preconceptions

With that in mind, Are you confident that the sole reason these vehicles use large wheels is to gain traction? Because that is what your post implies.

I like these answers as to why dragsters use wide tires.

Smith and Peterson, not so much.

Not trying to pick a fight at all. Just bringing in a discussion about a subject which there seems to be much passion.

I have a mechanical engineering degree for starters so I have been through all the proper training/schooling for this discussion.

I also know that racers of any kind, or even car manufactures who make high powered vehicles, always spec their tires to be wide to allow for better traction. Also any type of belt driven sheaves on high powered machines are wide for two main reasons. Strength and the larger the surface area the less likely to slip. The diameter of the sheave has a direct correlation to this as well.

I agree about the responses to the question. The first two are silly.

Since our robots do not do burnouts prior to a match the section about negative static friction coefficeint does not apply. This section also speaks to the type of material being used. In our application this is not a concern as well. We dont leave rubber down the playing field

A couple of things I got from the posted article

Friction is surface-area independent in only a few ideal examples. The real world is more complicated. Especially for tires that are made of rubber

“You want to choose a width, height, and tire compound that gives the best friction for the duration of the race.”

“Increasing tire diameter and tire width increases the contact area.”

It does speak to downward force due to a wing attached yet this does not apply either as we have weight limits and cannot go fast enough in the limited space even if we did have a mechanism to give downward force.

So the only parts of this article which directly apply to us is the width, height, compound and contact area.

I know I could never win a race if I had a skinny tire on my motorcycle.
If even everything else were the same, the burnout, the compound, the track conditions, same powered vehicles. The one with the RIGHT width tire will always win.

Numerous real world examples are before us as to why width, using ruber wheels, does in fact increase traction.

I guess one more way I can put it is this. If you had to move a piece of 4x8 plywood and could not carry it and the only two options you had were to

1. Lay it on the ground flat and try to push it?
or
2. Lay it on its edge and push it?

Same amount of weight. one has a bigger contact patch then the other. Which one will be easiet to move?

I would choose #1, the edge bites in the ground when try to move it. The only thing that is harder about #1 is having to get low on the ground, but I imagine trying to move it on a tabletop since the height is the same. It slides easier on its flat side in my experience, but that is subjective.

If I’m pushing on a carpet, number 2 is much easier than number one. If I push on a smooth floor, then the second may be easier. It depends on the surface you’re pushing on.

On a rough surface I’d stand it on edge lengthwise and pull it, lifting slightly so the corner does not catch.

I’ve always wondered about the traction of a larger diameter wheel compared to a smaller diameter wheel of the same width. I’d speculate it is more, since the curvature of the wheel approaches a straight line as the diameter approaches infinity. On squishy surfaces such as rubber and carpet, I can definitely see this making a difference.

It would be cool if AndyMark and VEX would step up to the plate to put this debate to rest once and for all, with some real scientific testing and published results

In the end though, the real limiting factor in pushing is the internal resistance of the battery.

Precisely, it depends on the surface, hence why modeling this can get complicated, different surfaces interact in unique ways.

Here’s some reading that went way over my head that may help understand friction between rough surfaces: https://workspace.imperial.ac.uk/medynamics/public/Akay%20A%20numerical%20model%20of%20friction%20between%20rough%20surfaces.pdf

Why make them do it? Most teams have the resources to test this ourselves. Perhaps we’ll do a study of this and publish our results.

You can simulate this without buying huge wheels, just use a pneumatic wheel and test it at varying levels of inflation, the under inflated wheel will likely have better traction in accordance with your theory.

Because aside from banebots and colsons, they have all the wheels.