Has 6 wheeled designs surpased tank treads
 

I am a fan of six wheeled bots and have been reading the lengthy post here on that topic. :o I am a fan becuase our team can afford to add a third wheel to each side without the need for a lot of extra engineering. :rolleyes: Many of my students however are still fans of tank treads. We have never gone that way, we've only had four or two wheeled bot, because of the expense of pulleys, buying extra timing belts for treads...

My question is, How many teams still design and use tank treads on a competition robot :confused: What advantages do you think you have over the latests trend of six wheel robots :confused: I still don't see us going with treads and this is a general curiosity of mine.


APS

Re: Has 6 wheeled designs surpased tank treads
 

Hello Mr. Schuetze,

I have seen a lot of 6 wheeled robot and tank thread ones for the past 4 years. both of them are effective. but if i was given a chance i would go with threads, just because of my experience. In reality, tank thread bots has a lot of traction and at the same time, no turning problem.

if you do need to know about tank threads my suggestion would be Contact few teams who has done it for a while. For example, S.P.A.M., Cyber Blue. You can sort their members out on cd by going into the member sectioin and doing an advance search with their team number. good luck... :)

Re: Has 6 wheeled designs surpased tank treads
 

Andrew,
If you ever saw the current demand of a tank tread turning on carpet, you wouldn't even think of using it. You can't beat the traction moving forward or back but you will eat a battery trying to turn in a single match. Those that work have some method of raising the track so only a small portion is in contact during turns. Even four wheel and six wheel drives suffer from high current demands unless they employ some method of raising some wheels in turns or use omni wheels on some of the wheels (i.e. front wheels).
If you were to tell me that in driving your robot draws less then forty amps total current but you wonder why your battery runs dead in less than two minutes, I would be able to tell you are using tank drive without ever having seen your robot.

Re: Has 6 wheeled designs surpased tank treads
 

I have always had this question, it always looks like that a robot with tankthreads on turns so much easier and this past season i have seen teams struggling to turn with wheels on. How is it that the robot looks like its turning to easily and according to Mr. Skierkiewicz when the robot turns (which has threads on) drains battery? I know our battery used to drain very fast every match. I have never worked on a tankthread system before. any advice will be appreciated... thanks. :)

Re: Has 6 wheeled designs surpased tank treads
 

One of the most interesting things I saw at nationals last year was a compromise. There were a few teams who ran 6 wheel drive robots, but each wheel had a notch in it to accept a timing belt loop, on each side of the robot one loop ran around all three tires. During normal operation I image the current leaching effect of tank treads wasn't a problem because the treads barely touched thr ground, but when climbing stairs these robots worked as well as ones with a full set of treads.

Re: Has 6 wheeled designs surpased tank treads
 

^That would be us, team 25. The treads i personally think were only good for getting up onto the ramp, and were basically pointless and more of a hassle during the matches. If you use the same design that we had this year, make sure you give the treads enough room to move around in. Whenever we took a good solid hit this year, the treads would pop off, and be near useless in helping us move. One round they actually completely prevented us from moving. They also were a problem when hanging, if they came off, we might touch the ground and i would have to get them tangled in the wheels in order for them to not touch.

We are definitely going back to just 6 wheels next year, and would probably never think of using the same set up as last years, unless the game required it. If you can improve upon what we had this year go ahead, but from my personal experience it is pretty darn rough. My suggestion is convince your team on the 6 tire set-up, and if you want any help, contact team 25 members, we have had some good experience with it.

Re: Has 6 wheeled designs surpased tank treads
 

980 also had a good 6-wheel drive system.

Re: Has 6 wheeled designs surpased tank treads
 

We have used tank treads the past two years and really like the traction BUT..... We always had a low center wheel to help turn, treads are EXPENSIVE if you get them from Brecoflex, not to mention the lead time on getting your order, lastly there are plenty of pneumatic tires that can easily hold their own against treads. This year most likely we will be going to something other than treads, or maybe not depending on the game. If your teams is going with treads make sure to get your order in immediately at the begining of the season.

Re: Has 6 wheeled designs surpased tank treads
 

We've been using tank treads for about 5 years now, and we plan to continue using them as long as possible. There is no competetion when it comes to traction.

As stated above, many teams have trouble turning with tank treads. This is because instead of 1" of non sticky rubber material per wheel touching the ground, there is 30+ inches of very sticky rubber touching the ground on each tread. We have used a system of idlers with our treads to overcome this. If you look at the picture below from our 2003 bot, you can see how in the middle of the tread there is an idler which lowers the tread about 1/2" from where the hubs would normally touch the ground. If you balance the weight on your bot the right way, you can cut down the surface area of the tread on the carpet from 30" to 1 1/2"



There are many pros to using a system like this. Because of the idlers, the current required to turn is cut down by a large amount. Also, you can turn "on a dime". But my favorite part of the whole system is when you are trying to pull or push something.. it forces the bot to rotate backwards on the idler, and you can put the entire power of your transmission into the pushing/pulling match. (This is much like putting sand bags in the bed of a pickup truck to increase traction.)

If you are interested in this, I am willing and glad to post pictures and diagrams of this system that has worked so well for us.

Re: Has 6 wheeled designs surpased tank treads
 

Arefin,
The simple explanation is that tank treads for all their driving friction also have enormous side friction when turning. The tighter the turn the higher the current. We have seen many robots over the years run near full stall currents on the drive motors. Take one of your treads and drag it sideways to see for yourself the laod you are placing on your drive train.
Take a very close look at Tom's picture above and note that the center wheel is lifting the robot so that only half of the tread is in contact with the floor. If you were able to place this wheel at the center of weight distribution then the turns would only be using a small portion of the tread. Picture the differences in your mind as your robot or Tom's would turn. Then imagine what you are doing to the carpet.

Re: Has 6 wheeled designs surpased tank treads
 

IMHO, 6 wheel drive will never surpass tank treads, nor vice versa. The main reason why is that each game is different, and as such, each drive system has it's benifits. Until the day FIRST forbids a certain type of drive system they will remain co-equal, although 6 wheel drive is really hot recently.

Re: Has 6 wheeled designs surpased tank treads
 

Wow, these days like every other post on these forums talks about how great six wheel drive is. It's going to be so funny when every robot on the field next year has six wheel drive. :D

Re: Has 6 wheeled designs surpased tank treads
 

I love to see some more pictures of those treads. They look really impresive. What width belts do you use? are they from Brecoflex?

Thanks,
Ben Van Selous

Re: Has 6 wheeled designs surpased tank treads
 

With the advent of pneumatic tires in the competition, wheeled robots are now competitive with treaded designs when it comes to traction. I say competitive, not superior. Some timing belt backings can deliver a coefficient of traction of 2.0. That means the pulling force is twice the weight of the robot. I know that this is true because we measured it many times with our highest traction robot, Fluffy. They are competitive because they are more simple to design and build.

As you may have noticed in the past years, those teams with pneumatic tires have a hard time (harder than properly designed treaded systems) turning. The 4-wheel drive systems bounce around like Mexican jumping beans. It is quite fun to watch.

The trick to making any high traction configuration turn is to have what we have termed a "boogie" wheel midway between the front and back rollers on the treaded system. This boogie wheel is a little bit lower than the front and rear rollers. What this does is roughly cut you turning moments in half since you are supporting the robot on the boogie wheel and either the front or rear roller. In a 6-wheel configuration, just lower your center wheel a little bit.

Therefore, I would say, no 6-wheeled designs have not surpassed tank treads in terms of traction. But they can give them a good run.

Re: Has 6 wheeled designs surpased tank treads
 

I dont think I have any good pictures of them on my computer, so I will take some at the next team meeting on Tuesday and post them then.

Re: Has 6 wheeled designs surpased tank treads
 

I would have to say that 6 wheel drive systems are much better for rookie to intermediately skilled teams for many reasons. The first being that 6 wheel drive systems are much easier to design and build. For a efficient 6 wheel drive system, you don't need any complex tools. All 3 six wheel drive robots were built on a low budget, with only a band saw and drills. You don't need complex custom made mounting brackets displayed in Tom Bottiglieri's picture that look like it was made with a CNC Milling machine. The design of a good six wheel drive system is a very open-ended one. You can easily design a simple 6 wheel drive system on your first try that may weight a considerable amount or have other draw backs but over the years you will figure out ways to cut weight and increase efficiency.

Treads can probably give better traction in pushing situations and although I have no experience with them, there are probably many other pros to treads.

So, I guess it depends on the contest it self and the experience/tools/budget of the team and neither are superior to the other.

Re: Has 6 wheeled designs surpased tank treads
 

Historically 134 has used tracks most years. We've worked with two diffrent situations with tracks.

In 2004 we used a tread that spanned the length of the bot. Due to the placement of the center of mass and the raised center wheel we were able to turn like a bot with two wheels in the center. This allowed use tremendous pushing power while also allowing us to turn on a dime...literally. We also never had much of a problem with the batteries (granted we only drove with CIMS). We would start a match with 12-13v and never ended with less then 11v. We also never snapped a belt either in 1 regional and three offseason competitions with this drive train.

The second solution that we have attempted is similar to the old German half track. About half the length of the robot would be track and the other side (or if centered both sides) would have casters. This worked well but was not efficient as the 2004 machine.

I prefer the track drive if its designed right.

-Pat

Re: Has 6 wheeled designs surpased tank treads
 

psst...don't tell these guys that treads require complex tools...



the truth is that a 'bot is only as complicated as you make it. granted, it may be easier to over-complicate a tank-bot, if you design with simplicity and machinability in mind, i expect it could be built with band-saws and drills.

Re: Has 6 wheeled designs surpased tank treads
 

In the above robot, what advantage does that have over a good wheel? It is a nice attempt but I don't see the benefit.

Re: Has 6 wheeled designs surpased tank treads
 

look at the evolution of the competition, apparently tank treads were popular when we had ramp style competition. where as 6 wheeled designs have come into play mostly with the new start style.

Re: Has 6 wheeled designs surpased tank treads
 

Tank treads can be complicated, depending on what your recourses are, for instance a simple bot as shown above is easily made, but wouldn't stand up against last years competition. one of the teams at our regional last year used treads, their goal was to climb the steps, but weren't able to and ended up just pushing around balls. I personally favor treads for their easy drivability. And Ken, i could easily make the mounting brackets if you drew them for me, being i have a maxnc cnc milling machine in my basement, which everyone on my team seems to forget. :yikes:

Re: Has 6 wheeled designs surpased tank treads
 

maybe you and i don't see the advantage, but apparently they did, else they wouldn't have built it.

but that wasn't the point i was trying to make. i was trying to point out that complexity lies in the design, rather than the type of drive train.

Re: Has 6 wheeled designs surpased tank treads
 

116 used a tank design in 99 to climb the puck. It was used again in 2001 to be able to go over the 4x4 but under the bar.


Tank drive was used as a function of the design.
Last year was four wheel drive, with large wheels to get up to the bar. Function of the design.



Stratagy drives design, not features.
Wetzel

Re: Has 6 wheeled designs surpased tank treads
 

small little story:

my legoleague team #2883 competed in a competition with there very complex robot with tank traads. the thing is awesom but it never goes straight no matter what you do and the kids get nervous and due to its complexity they wasted alot of time durring the match in setup.

now they've changed to a 6-wheel drive with the center wheels lowered. and only 1 moterised moving part beyond that a very simple thing. and it GOES STRAIGHT! i think in their next competition they are going to do much better

Re: Has 6 wheeled designs surpased tank treads
 

As has been stated before, designs are just as complicated and efficient as you make them. In my teams case, we have been successful with treads. Other teams have been successful with 6 wheel drive.. I dont believe either one is "better".

Tytus brings up a good point.. In his Lego league team's undertakings, 6 wheel drive has been superior to the treads. This is not because treads are a horrible idea and 6 wheel drive is much better, it is simply because that teams design and method of construction may have been different from that which would make "the perect system"

Re: Has 6 wheeled designs surpased tank treads
 

and also in the case of lego, most tires you would want to use have flat surfaces. the rubber lego tread has a taper (is that what you call it?) on the sides, so it will turn better on carpet but on hard surfaces will not help the tread stay flat to the floor. the some lego tires are wide too. which have more scrub when turning so helps keep the robot move in a straight line.

Re: Has 6 wheeled designs surpased tank treads
 

This year team 234 used the tank design mostly because it allowed us to fly over the six in. steps. again, the game dictates the design.

Re: Has 6 wheeled designs surpased tank treads
 

All you tank tread lovers, educate me on the following things:


1. Show me the math that shows tank treads have better pushing force (traction) than a wheeled design.

2. While doing number 1, assume whatever tread material you use for the tank tread can be wrapped around a wheel.

3. Leaving the obstacles out of the equation (stairs, ramps, etc.), why do certain vehicles have tank tread designs (tanks, excavators, snow mobiles, etc.)?

4. Again, leaving obstacles out of the equation; what advantages are there for FIRST robots to use treads? If you say pushing force, or traction; then prove it with math/physics.

I look forward to the range of answers.

Re: Has 6 wheeled designs surpased tank treads
 

i have a question about the idlers. about how much lower should the idlers be so that they keep a high efficiency but also don't draw all the current?

Re: Has 6 wheeled designs surpased tank treads
 

Well, this depends on the material of the wheel mainly. If you are using solid rubber wheels, then you only need to lower them a very very small distance, just to take pressure off the outer wheels. If you are using pneumatic tires, the amount that you need to lower them is greater because they will squish down under the weight of the robot. This is just a general rule, the actually distance depends greatly on the individual robot with its many factors, such as weight distribution about the robot, where the idler is located on the robot, etc.

There is also a sort of balance between current and controllability. If you lower the middle wheel a lot, then you will maximize current saving, but you may encounter problems such as your robot drifting from one side to another, or teetering back and forth (which can be annoying). But if you don't lower them enough and all 6 wheel are touching the ground but pressure is off a set of them, you will eat up current trying to turn but you will have more controllability. So, I guess its best to experiment with your design to come up with what your team is comfortable with.

Re: Has 6 wheeled designs surpased tank treads
 

ok here are the pictures as promised. I forgot my camera so we took them on a camera phone.. so they are not the best quality.

Attached are pics of me holding the tread and looking like an idiot, how the hub connects to the transmisson, and heres a link to a video of me playing with the tread assembly

Re: Has 6 wheeled designs surpased tank treads
 

Anyway I think we have established that the equations you use in physics are only part of the story because every single explanation I have seen for tank treads (and even some explanations for race car tires) use the surface are explanation.
Actually thats an easy question. I know the reason why snow mobiles have treads. It's because of the decrease in pressure upon the snow. It analogous to walking in snow with high heals to walkin in in snow shoes.
Found another dicussion

Re: Has 6 wheeled designs surpased tank treads
 

Adam,

Great link! I almost forgot about that thread (2001-too long ago). Again, the surface area does not change your available tractive force UNTIL THE CARPET SAYS UNCLE. In all cases that I have seen practical uses for tank treads, there has been one common denominator: A not so hard surface that needs the load spread over more area so the surface doesn't fail (your high heel snow shoe example is perfect). In most cases (except for 2002) the load need not be dissipated over such a large surface area to save the carpet.

The blanket statement that tank treads get more traction due to the surface area is just not correct. I ask again to show me the math and get as complicated as you like. The bottom line is the math assumes a stable surface and when the surface becomes unstable (melting carpet), then all bets are off (surface area must be used to dissipate the load).

-Paul

Re: Has 6 wheeled designs surpased tank treads
 

Though most of us have been taught that friction is independent of the area of contact, according to Bowden & Tabor [The Friction and Lubrication of Solids (Clarendon, Oxford, 1950)], there is an area dependence, but on the very small scale. This is because on a microscopic scale, there are no true, continuous surfaces, so two surfaces pressed together are actually touching at a very large number of microscopic points, at different pressures, to boot. Nature had a good article (sorry - subscription required) which made the analogy to Velcro, where the number of points of attachment (velcro hooks) is proportional to the applied pressure, and where some points come apart sooner than others (since they don't all see the same load).

Can this be extrapolated (scaled up) to our question about wheels vs. treads to show that for very uneven surfaces, compliant treads offer more points of contact, and less likelihood of losing contact altogether? Or, as Matt B. said in the linked thread, "because the wheel treads are actually 'hooking' the carpet."

Re: Has 6 wheeled designs surpased tank treads
 

3 years ago in the zone zeal competition, 322 initially used a tread design. during our first regional (GLR) we soon discovered many problems with our design, including: treads falling off the wheels during tug o war matches and high speed turns, and huge amounts of stretching. whenever the treads fell off (which was often), we were left immobile. we ended up finishing this regional close to last.

at our next regional, all we did was take the same tread material and wrapped it around each individual wheel (as PAUL suggested earlier), and had tremendous success. as a result, we were actually picking for the finals (which we eventually made).

now i don't know the proper calculations for calculating traction for this situation, but we were able to find out the pushing power of both bots the old fashioned way. we simply put a heavy duty bathroom scale against a wall and pushed against it with our robots. i don't remember the actual pushing power of the robots, but i do remember that the 4 wheel design pushed just about the same amount as the treaded design (and this was with the same exact transmission, just different ways of contacting the carpet).

BOTTOM LINE: even though 322 hasn't gone back to a treaded robot since zone zeal... STICK WITH WHAT WORKS FOR YOU!!! IF TREADED BOTS WORK FOR YOU... stick with it! IF 6 WHEEL BOTS WORK FOR YOU... stick with it! IF 4 WHEEL BOTS WORK FOR YOU...stick with it! just improve upon what works for you from season to season, and surely you'll have a competitive robot at the end of the six weeks!!!!

Re: Has 6 wheeled designs surpased tank treads
 

Paul, the kinematical physics say that surface area does not matter. A wheel and a track will slip once the coefficient of friction is broken regardless of surface area. I find that tracks handle vertical obsticles(step, bump, etc.) better than a six wheel bot because the track is always in contact with the edge of the obstacle. I guess all and all I believe that both have the same basic physical advantages. It's more a matter of preferences.

(What I meant in my previous post by pushing power before was that the bot itself was a very strong machine)

-Pat

Re: Has 6 wheeled designs surpased tank treads
 

Pat,
Re-read Paul's most recent post.
He is not disputing kinematics, just taking things to another level of complexity.

To be truly in touch with things, we must analyze the interactions between the wheels/tracks and carpet.

John

Re: Has 6 wheeled designs surpased tank treads
 

Ok after reading Paul's second post (sorry amidst studying for finals is causing me great confussion) I've come up with this (This could be a repeat I'm not sure)

On a solid surface (i.e. concrete, etc.) a wheel and a track essentially have the same principle, the weight gravity force is "pulling" down on the bot and the surface is "pushing" back against the wheel/track that is in contact with the ground. The difference between a wheel and a track is the distribution of the wieght force of the robot. This doesn't matter however in the case of the hard floor because the way a wheel/track works is that static friction is applied in the same direction as whatever the wheel is attached to is moving. Friction does not include surface area in any way shape or form.

However on the other hand, while moving through sand or snow, a track has an advantage over a wheel because of the mass distribution. I guess the high heel and a snowshoe is a good analogy. I guess another would be a stick and a sand pail, but w/e. I pretty sure that there is math to prove that h/o i dont know it yet.


If I remember correctly from physics class, the interaction between the surface and the wheels/tracks is as follows.

No matter what the surface is made of because of the natural tendency of the weight force "pulling" down on the mass above there will always be the smallest depression. For an example of this I use a ball bearing and a sheet. If you hold a sheet taught and place a ball bearing on it the ball bearing will create a small depression.

Please correct me if I'm wrong or repetitive.

-Pat

Re: Has 6 wheeled designs surpased tank treads
 

ok i wouldnt say you are wrong, but missing a couple things. first, yes a snowshoe may be easier to walk over snow than high heel shoes, but high heel shoes if they actually puncture all the way through the snow into the ground, they will provide superior grip. seen in the high narrow wheels some serious mudgoing vehicles. so no you cannnot say tracks will always have an advantage over wheels in snow or sand. and if the ground was completely flat, yes what you are saying is completely correct, but if the ground was more not as smooth, wouldnt more in contact with the ground sometimes provide more grip? for example. velcro. more contact equals more grip. so well... i guess its not fair because it will be called "hooking into" the ground.

okay. im gonna stop because im just gonna make a fool of myself trying to disprove the laws of physics :p
so ill just correct you on the one thing im sure about.
that would be holding the sheet "taut" not "taught"
:D

Re: Has 6 wheeled designs surpased tank treads
 

Good catch ;)

-Pat

Re: Has 6 wheeled designs surpased tank treads
 

We have been using tracks now for over 10 years.

1) When done right the tracks system are easy to make and easy to fix if something goes wrong.

2) tank system has a better chance of having a lower center of gravity than the wheels system. ours CG is about 3" off the ground

3) when it comes to pushing other robots I would say we are in the top 5% in doing this. We have not come across any wheel robot that could push us in a pushing contest but we also use 4 motor drive system.

4) very costly $200+ per track $140+ for pulleys

5) tracks can overcome any obstructions.

6) under extreme stress track will rip even with the best tracks. ( but robot can still move without tracks.)

7) We have worked with Breco Flex for the past 10 years and had some good tracks come out of there company.

8) go to our web site to see all of our past robots and see the track system
http://www.valleytech.k12.ma.us/robo...photos2004.htm

Re: Has 6 wheeled designs surpased tank treads
 

Well, treads will actually provide a better traction system than wheels on very high torque low speed robot simply because they distribute the weight. Asuming coefficent of fricition is constant for any wheel/tread in contace with the ground, if the treads are distrubuting the pushing force on a greater surface area, the tread system will allow the drive train to provide greater overall force before the treads slip than a wheeled design. This is not because surface area affects friction, but because the force is distributed.

The decision to go with treads as opposed to wheels is depenent on the force exerted by the wheels/treads on the carpet. If in fact the force of the drive train will cause the wheels to slip, than a tread design may be preferred.

Re: Has 6 wheeled designs surpased tank treads
 

ShadowKnight,

The force being distributed along the tread has absolutely nothing to do with the ability to push, unless... the surface that your treads contact fails due to the contact. If the wheel design is wide enough such that the force imparted on the carpet does not make the carpet fail (i.e. tear apart like in 2002), then it will provide the exact same pushing force as a tread made of the same material provided the robot is the same weight.

Mike Norton's post is very accurate, but notice that the advantages of the tread design (disadvantages, too) really are not related to pushing force. Regarding Mike's point #3, I guarantee that if his team put a six wheel design with the same tread material they would still be in the top 5% of pushing robots.

Now, specifically regarding 2002: treads had a much better chance of not tearing the carpet than wheeled designs because they distributed the load. Tearing the carpet leads to less traction. The carpet of 2002 related to the possible weight of a fully loaded robot (around 500+ lbs) was like sand to a construction vehicle. Treads helped dissipate the pressure on the carpet to a point where the carpet would no longer fail. Wider wheels would provide similar functionality.

-Paul

Re: Has 6 wheeled designs surpased tank treads
 

Yes, but that is assuming ideal conditions. As others have said, you can have materials like Velcro, or rubber, that will "stick" to the surface. If you were to measure mu between rubber and asphalt, and used two blocks with the same weight but a different surface area, you'd get a slightly different result for mu (would it be linear with area?). Try it with Velcro and carpet and the difference would be huge.
For instance, extrapolate and loctite a block to a table. Loctite 158184 will hold 1300 N per square centimeter. Take a 1x1x1 (cm) block that weighs 100 g and pull it till you manage to rip it apart from the table. Would you consider your mu to be 1300 (Force/Normal)? Obviously not, because friction is not the only issue here. Same with the situations I discussed above.

Now I have two questions I'm not sure of the answer:
a) If surface area really doesn't matter, why is it much harder for a treaded robot to turn on a dime than it is for a robot with wheels? Also, why does it get much easier if you add an idler pulley? (I haven't put a lot of thought into this one yet, but I think it's not entirely dependent on the surface area, but also because the treads dig into the carpet and you have to overcome the fibers in order to skid laterally)

and

b) (not that it really makes much difference, but...) We all know that a rolling wheel has v=0 on the contacting point and v=2wr at the diametrically opposed point. Now, do we have static of dynamic friction? :ahh: I would say static, considering if you "break traction" the force you apply to the floor will be reduced, and as the wheel won't be translating anymore, the relationship I mentioned above won't hold true.

Re: Has 6 wheeled designs surpased tank treads
 

I'm not prepared to say whether or not an increased surface area given the same normal force yields more traction. However, anyone who is arguing that it makes no difference whatsoever has both not actually tried it for themselves, and has not read the whitepaper on the subject.

Re: Has 6 wheeled designs surpased tank treads
 

jimFortytwo,

Was that aimed at me? If so, then I am ready to put my money where my mouth is. Are you?

-Paul

Re: Has 6 wheeled designs surpased tank treads
 

I would trust what the UFH Engineer has to say about this.

Thats just me though.

Re: Has 6 wheeled designs surpased tank treads
 

No, I wasn't trying to single you out. Though I imagine you have not yet, in fact, read the white paper in question, and have not done the experiments yourself. Or are you holding out on us? :)

I suggest reading the whitepaper. Following your challenge, I reread it, and the results still somewhat surprise me. I won't bother summarizing, as it would spoil the surprise.

the paper:
http://www.chiefdelphi.com/forums/sh...threadid=15719

I guess the other part of "putting my money where my mouth is" should be to mention the traction tests I did for my team last year. We were looking at 8" hard rubber tires with an aggressive crosswise tread pattern. I did tests for kinetic friction with 30 pounds and 20 pounds of pressure on the tire. I was working with the risky approximation that in a 4-wheel drive robot each tire sees 30 pounds (30*4 =120), and in a 6-wheel design tires see 20 pounds (20*6 = 120). I can't remember the exact numbers, but my recollection is that I extrapolated a 10-20% benefit in the case of distributed load. Obviously a better procedure is needed to give any meaningful results, however.

I remain convinced that the traction advantage of treads over wheels is so highly dependent on the carpet and material that you can not generalize it. If I can put together materials to do a proper investigation of the subject, I will. Unfortunately we're on winter break, so that might have to wait until January.

Re: Has 6 wheeled designs surpased tank treads
 

We have been useing treads since be for I joined the RoboDawgs. We do not have problems with dying batteries. We've only had problems going straight.

Re: Has 6 wheeled designs surpased tank treads
 

That's actually somewhat surprising. It seems to me that a tread system would "naturally tend" to go straighter than a wheeled system since it is harder to turn. But perhaps this was just because of the bias in your drill motors. Maybe you could elaborate a little.

Re: Has 6 wheeled designs surpased tank treads
 

Hmmmm.... Did you attach the motors backwards. I don't have a drill motor handy but Im almost certain that there is a positive and negative lead on the motors. Attaching one motor one way and the other one the other way will result in such a situation since the motors go faster in one direction than another.

Re: Has 6 wheeled designs surpased tank treads
 

Jimfortytwo,

The basic flaw in the distributed load assumption is that in a six wheel drive configuration that lowers the middle wheel (and in treads that have a lowered middle in order to pivot), the center wheel sees most of the weight load. This is true if the CG is close to the center of the robot. In general, the wheel closest to the CG will take most of the load from weight. In many of the six wheel designs you see today, the outboard wheels (all four of them) are simply powered casters. If balance wasn't such a big deal, then the two wheel drive with the wheels directly under the CG would be the most maneuverable and most efficient in transferring load to useful pushing force.

I read that white paper, but the results were not as convincing as the discussion leads us to believe.

Yes. I have done numerous tests and I guess I am holding out on everyone. Not because I want to, but I have not had the time to reduce all the data and put it into a nice format. The results I saw convinced me enough to proceed. I promise I will get my results out soon.

The thing I want the readers of this forum to take from this is that the selection of wheel (tread) material is most important and the differences (if any for our game) between area and friction are negligible and definitely not noticeable on FIRST carpet.

-Paul

Re: Has 6 wheeled designs surpased tank treads
 

Paul

Yes, weight distribution is the problem with the distributed load assumption. Of course in our 6-wheel drive robot last year we didn't drop the center wheels, so it wasn't quite as egregious an omission.

Presently my hypothesis is that there is a crush load (pressure) that distinguishes wheel surfaces that just ride along top of the carpet and those that mash down into the carpet. Before reaching the crush zone, I would imagine that our basic AP physics friction rules apply, and above the crush zone I would imagine they apply too. I'm not convinced the transition zone between one and the other will be linear like the rest, however. Thats just my present thinking, however.

One problem with the white paper is that the scale of it (smallest test patch 150 square inches) makes it hard to apply with any certainty to wheels.

The main thing I'm suggesting is that tread material choice is most important, and that you can't assume either a positive or inverse relationship between surface area and friction without empirically testing your materials. (sounds roughly familiar)

Re: Has 6 wheeled designs surpased tank treads
 

just to throw something interesting into the mix....we did a 6 wheel drive bot last year, and those beadlok wheels from skyway are much heavier than one might think. So if you do chose to go 6 wheel i recommend a small wheel or you are going to be fighting to get that weight back.

Friction, tread material ... can six drive like a track
 

It is interesting how this thread has revived itself and has also digressed to the much debated coefficient of friction between a track and a wheel.
If I can sumerize the last few posts to clarify my thinking. The physics of tracks verses wheels as applied to FIRST class robots and the carpet does not present an appretiable difference between the two.

If one is looking to gain traction, one needs to focus more on the tread material of which ever system they employ.

Track systems are more expensive, require a bit more enigneering expertise, draw more battery power but still don't kill the system in the short time frame of a round. One needs to have spare tracks and design the system to keep the track on during turns. They do provide greater obstacle drivability.

A six wheel system will drive straighter and draw less power. One can gain back some manuverability by lowering the center wheels 1/4 inch or less.

If we can put those issues to bed, I am still interested to get opinions or tested data for the following: :]

Do six wheel designs exist that can approach the obstacle drivability of a track system? :yikes:

Re: Friction, tread material ... can six drive like a track
 

again, this matters on tread material and width. more width means more scrub which means it tracks straighter. and in the designs with center idlers or lowered center wheels, so you cannot necessarily say a 6wd will track straighter. espescially if it the frame gets tweaked a bit.

the mars rovers? in a independent suspension view i think 6wd will have better obstacle drivability. but with zero suspension i think treads may have the advantage of being able to angle their treads at the front as in to climb over higher objects. but if the wheel for 6wd and pulley for tread were the same, id say it matters on what obstacles you want to clear. as in sand tread will distribute weight more but in rockcrawling and w/e you probably want a bit more stuff to actually "claw" at the surface. meh. i dunno

Re: Friction, tread material ... can six drive like a track
 

Andrew,
I need to weigh in here from an electrical standpoint and add some other observations.
Drive systems that cannot steer, eat batteries when turning on carpet. The side loading is incredible and many teams have reported to me that they can barely last one match on a fully charged battery. (I have observed low battery conditions on several robots at the end of matches.) The data we collected with current monitoring suggests that most multi-wheel and tread drive systems regularly run near stall currents on all drive motors during turns. Slow, tight turns are obviously worse than higher speeds. The degree of battery drain depends on driver strategy and operation. A driver who only drives forward and back with little steering should not experience any greater battery drain than other designs. Due to the additional friction in the drive components, (pulleys, drive trucks, etc.) a treaded design generally runs higher electrical demand. Tighten up the belt to prevent it slipping off and those frictions increase. Only low friction bearings can help on these problems. When asked, I recommend that a robot should be able to run two to three matches on a single battery as a rule of thumb. This margin insures enough electrical power be available for a single match.
Although the discussion on treads and carpet friction is very interesting, I think there are too many variables (assumptions) for this to be a general discussion. Designs that have no way of transferring loads to the tread except at the wheels have to be analyzed differently than those that spread the load across the full length of tread in contact with the carpet. Even those designs that use center trucks that are lower than the either the front or back pulleys require a different analysis. Turning friction varies with design as well. Often in turns, belts tend to go "up on edge" so that the full width is no longer in contact with the carpet.
I am not saying that tank designs are bad but there obvious problems and tradeoffs. Teams that have successful tank designs do exist but further investigation into their design criteria is warranted.

Re: Has 6 wheeled designs surpased tank treads
 

My team has been around for 3 years this year and each year that we've been competing we've used treads. We've also used the same transmissions as well and hopefully we can use them again.

Re: Friction, tread material ... can six drive like a track
 

Al,

I remember back in my BattleBots phase that one builder suggested running your robot against a curb for three minutes (in their case) to check and make sure you absolutely have enough power to last a shoving match. Would such a method work for testing FIRST robots as well?

Re: Has 6 wheeled designs surpased tank treads
 

Bill,
That seems like a big overkill since our robots won't sit there shoving against another player for the full two minutes. You are not only stressing the battery but the drive motors and controllers as well. I can tell you that there is a huge list of teams that have asked this question, admitting that as little as 1:30 of heavy driving killed the battery. Driving in practice should give you a fair indication of battery use. Teams when pushed to the limit will try to devise faster methods of battery charging (to keep up with demand) which starts that downward spiral of early battery failure. The KOP battery can be safely charged at 6 amps maximum, which would require at least 3 hours to fully charge the battery if it was fully discharged. Remember that the backup battery only hides the real problem. A dead primary battery will keep you from driving whether the RC is functioning or not.

Re: Has 6 wheeled designs surpased tank treads
 

The problem we had with the treads last year was a slight warp of the frame and one of the drive gear boxes was mounted crooked. This caused a lot of friction on the left side that really became noticeable about half way though the match. We redesigned our tread system to eliminate that source of friction and made it easier to adjust the treads tension.