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.