Robot Pre-View

[samboydh]

http://www.youtube.com/watch?v=39dXQXmhNH8&feature=channel_page

oh yea look at them wheels.

[sdcantrell56]

I hope those wheels are a joke. If not then adding wheels does not increase traction. Traction is only influenced by the normal force and the coefficient of friction. Also it looks like the front wheels are unpowered which would decrease your total force you can apply as well. Basically. The amount of wheels doesn't matter this year, and all of the wheels should be powered for the maximum usage of available traction.

[guakam0li1089]

but doesnt having less wheels work better? i think so because with less wheels you are distributing more wieght per wheel making the friction due to gravity greator. (not sure if its right though, just something me and a few other students were disscussing)

[sdcantrell56]

but doesnt having less wheels work better? i think so because with less wheels you are distributing more wieght per wheel making the friction due to gravity greator. (not sure if its right though, just something me and a few other students were disscussing)

This is partially right. If all of the wheels are driven then it doesn't matter. Each wheel will have less traction per wheel due to a lower normal force per wheel but the total traction will be the same as the more wheels will add up to the same value. Either way having 16 wheels does not improve traction at all and having unpowered wheels even with 16 wheels means this drivetrain will have less traction than a simple 4wd design.

[samboydh]

all the wheels are powered. That clip dosnt do justice to how well it preforms.

[sdcantrell56]

At least all the wheels are powered. You are wasting a large bit of the weight budget with those wheels though.

[nathanww]

Are you using any sort of traction control or steering augmentation? If so, I'm going to have to rethink my plans...

[Jonathan Norris]

I'm sorry to burst your bubble... but more surface area will have next to no effect on the Glasliner surface. Do the math/physics (http://www.chiefdelphi.com/forums/showpost.php?p=790707&postcount=18) you may want to think about saving all that weight for something else.

[RyanCahoon]

Not at all arguing the physics of friction, but one thing that may change with the extra wheels is how the robot performs while turning. While with this robot, you will certainly not get any more friction when driving in a straight line, distributing the frictional force of the wheels around the robot means that when turning, the amount of torque that each wheel is supplying will be different as its distance to the axis of rotation, for both the linear and transverse friction of the wheels.

While I have not done any actual math, playing with vector diagrams a little yields this:
http://lenkirma.googlepages.com/16wheelfrictonforces.png

So it seems that not only are more wheels hurting your weight, they're actually decreasing your ability to turn.
all the wheels are powered. That clip dosnt do justice to how well it preforms.
Interesting; have your tried to do comparative testing with only four wheels? Or someone correct me if I'm wrong.

--Ryan

[Doug Leppard]

Some of you missed the point of the video. Watch again and listen carefully to the video. First run is without traction control, second run is with traction control. Notice the obvious difference in traction and acceleration.

This obviously is a proto-type. Any team smart enough to pull off what they did know about wheel configuration.

Good job team 702.

[seandherbert]

http://www.youtube.com/watch?v=fClOU0yYuCA

[Wayne Doenges]

Another thing to condsider with multiple wheels. Unless each wheel is EXACTLY the same diameter, only the larger wheels will make contact with the floor. On carpeting this wouldn't make much of a difference, but since we are playing what is essentially a flat floor it should make a difference IMHO.

[jamie_1930]

We originally had ideas to utilize multiple wheels to increase traction but after review our physics a little realized that that would just further distribute weight and traction would remain the same.

[dtengineering]

As much as I agree with the statement that "in the simplified theory of friction as taught in high school physics class, extra wheels should not increase traction", it is important to remember that the simple relationship between normal force and resultant force makes some assumptions that may or may not hold true in a practical environment.

For instance Formula One race cars evidently find that having a larger contact patch improves traction. This does not contradict the theory taught in high school physics, but does mean that some of the assumptions made in that theory are not valid in the race track environment.

For instance the "high school" theory assumes that neither surface deforms due to the normal force (remember that the regolith lies over top of carpet, and is a fairly thin material...), nor fails under the resultant force. Given the "white powder" produced by spinning wheels as reported on CD, there is definite surface failure during dynamic friction of highly loaded wheels. Perhaps this is less of an issue with lightly loaded wheels....

Perhaps they are on to something with this design, perhaps not. I suspect not, but if the team has tested multiple wheels and found them to be an improvement then they have my congratulations for not being bound by the assumptions of a simplfied theory.

[thefro526]

That's So Awesome.

How many wheels did you have in the back? 8? 10?

In theory more wheels will not increase traction but it will change how your robot handles with the traction thats given. So maybe having that super awesome wheel thing in the back is actually helping them turn with the trailer......

[Woody1458]

For all those that are confused about the physics of friction and why in some cases surface seems to help and in others it does not, http://www.chiefdelphi.com/forums/showthread.php?t=71243&highlight=friction+physics that thread will answer many questions. Specifically notaPINKtruck's (College student from 233) post:

This is based on my understanding from freshman physics at Cal. One of the reasons explanations tend to be so poor is that (a) this is not generally taught in an engineers' physics classes and (b) physicists are just now beginning to fully understand the nano-scale forces at work between two bodies in contact.

The basic f = uN model only applies to very hard/rigid materials on an incompressible surface. Therefore, the model is broken by treads/tires in several ways.

(1) Treads are rubber, and thus compress (changing the orientation of their atoms and thus the coefficient of friction.

(2) Carpet is also "squishy" to a degree, and thus violates the model in the same manner as above.

(3) The pile (fibers) of a carpet form a small layer ABOVE the "main" surface of the carpet, meaning that your static friction is dependent on how far you have "sunk" into the carpet and thus how long you've been sitting there.

(4) The rubber that tread is made from is molecularly composed of VERY long hydrocarbon chains (think sort of like a pearl necklace). These chains then are tangled into an "elastomer" structure, like a big bundle of cords thats bunched up (and stretches out, thus the strechiness). Because of the girth of these molecules and the uneven electron density between the carbon "backbone" of the chain and its outer hydrogens, significant Van der Waals (VdW) forces occur between the molecules of the two surfaces at contact point, essentially forming very weak "temporary bonds" which require energy to break (i.e. move). See "dispersive adhesion" on Wikipedia.

We now understand VdW forces to be the reason that geckos can stick to surfaces so well. Their feet are coated in thousands of hairs (which then split into even smaller sub-hairs like a tree) called setae. These hairs' miniscule size and sheer number allow the gecko to exploit the same polar/VdW forces mentioned above, and stick to a surface using even just one toe.

I know this is long, but I hope it helped. I guess the moral is that friction is one thing but "sticktion" is another. Gratefully, this years' playing surfaces are essentially ideal as far as the "standard model" for friction goes, so you won't have to worry about making gecko wheels for your bot until next season .

Later in the thread he was asked to back up his theory with tests:

Already did it yesterday. The result was almost exactly the same. If I recall correctly, it took about 35lbs of impulse and 25lbs continuously to move the bot transverse. I don't have any numbers on the force required to make the wheels slip (this wasn't really easy to determine with our kit chassis + wooden board + light team member + fish scale test rig). We had them sit in between the wheels and then rest all of their weight on the back wheels. This is not surprising considering the near-ideal surfaces we're dealing with.

[JesseK]

Hmm. The ol' surface area vs friction debate.

We still have to remind ourselves that the glassliner surface is NOT smooth, and is probably further from the ideal case than carpet since each 'pebble' is rounded. We also have to remind ourselves that the wheels are infact (at a molecular level) a bumpy lattice, hence there will always be localized slippage. The molecules will eventually break away from their structured bonds, which forms the white powder that has to be cleaned off (I've noticed this after every practice drive on our glassliner floor). Even in minute amounts, this powder causes lower traction.

Hence, I argue that since the weight is spread out across many more lattices with more wheels, there is less chance in a given split second that the molecules will break off to become powder, thus resulting in more traction overall. From an engineering perspective though, I postulate that in a 1-on-1 race across the 48' 'regolith' field, without traction control, the difference in traction is very negligible. This is because the wheels will always be in a noticable slip condition.

Either way this could be argued, it's my opinion that even the traction gained with extra wheels and traction control isn't worth the weight of that many additional wheels for 1885's design. Perhaps' 702's is different...it certainly looks impressive.

[rcflyer620]

OK so ignoring all the arguments about whether or not this will get more traction, my question is... What else is the robot going to do. It might drive like a Farrari but the frame configuration shows me nothing about a collector system or where a shooter/dumper might go.
Very interesting prototype though!:D

[Jared Russell]

So you are why AndyMark ran out of wheels!

[Daniel_LaFleur]

As much as I agree with the statement that "in the simplified theory of friction as taught in high school physics class, extra wheels should not increase traction", it is important to remember that the simple relationship between normal force and resultant force makes some assumptions that may or may not hold true in a practical environment.

For instance Formula One race cars evidently find that having a larger contact patch improves traction. This does not contradict the theory taught in high school physics, but does mean that some of the assumptions made in that theory are not valid in the race track environment.

For instance the "high school" theory assumes that neither surface deforms due to the normal force (remember that the regolith lies over top of carpet, and is a fairly thin material...), nor fails under the resultant force. Given the "white powder" produced by spinning wheels as reported on CD, there is definite surface failure during dynamic friction of highly loaded wheels. Perhaps this is less of an issue with lightly loaded wheels....

Perhaps they are on to something with this design, perhaps not. I suspect not, but if the team has tested multiple wheels and found them to be an improvement then they have my congratulations for not being bound by the assumptions of a simplfied theory.

While I do not disagree with the jist of what you've posted here, I do want to correct a common misconception. F-1 cars use wide tires to dissipate heat, not to gain additional traction. Additional traction in F-1 is generated by the wings on the car providing additional nominal downward force. At speeds above 100MPH that additional downward force is greater than the weight of the car and the car could drive upside down on the ceiling (If it could get there).

[dtengineering]

While I do not disagree with the jist of what you've posted here, I do want to correct a common misconception. F-1 cars use wide tires to dissipate heat, not to gain additional traction. Additional traction in F-1 is generated by the wings on the car providing additional nominal downward force. At speeds above 100MPH that additional downward force is greater than the weight of the car and the car could drive upside down on the ceiling (If it could get there).

The additional contact patch also serves to distribute the shear force in the outer layers of the tire across a broader area, as well as dissipate heat, but we agree on the main point... that the simplified theory of friction is fine.... in theory!

Jason

[domoarigato]

No matter how you look at it, frictional force is just the coefficient of friction times the normal force. coefficient of friction between the robot and the floor is not changeable, as per the rules. The Fn, however, can be altered within the scope of the rules...

Might the extra wheels be apart of a larger scheme? Maybe this team isn't showing us the whole picture...

MAYBE the extra wheels are a means to distribute weight rather than gain traction. Perhaps this team plans to add a downward force to increase their apparent weight (using a fan, vacuum, etc.) and needs a bit of extra support. A snow shoe effect.

...or maybe not. It was just a thought.

[Vikesrock]

The Fn, however, can be altered within the scope of the rules...


See <R06> and the Q&A posts regarding this rule. Other than holding more orbit balls you can't really increase Fn either.

[Sean Raia]

Our team is having a weight problem with just 4 wheels! You better have one lightweight upper mechanical configuration or your in trouble when it comes to the weigh in. And if the upper mechanics are lightweight, make its stable enough to survive a crash!