pic: Concept 8wd Drivetrain

[s_forbes]

Originally Posted by JVN

The question is, why would you?

People ask me that all the time when I ride my unicycle...

[sdcantrell56]

Originally Posted by JVN

I bet you can connect 10 more pairs of wheels, 20 pairs even. The question is, why would you?

Without justification (benefit for the albeit minor cost) my team would not do it.

The justification to have 8 wheels is for the added ability of climbing an obstacle without having to raise the cg. I completely agree if the field ends up being completely flat, or has a minor obstacle then I am sure we will be going with a 6wd design as usual and probably direct drive the center wheel. This drivetrain is merely one that we would go with if we need to climb obstacles. Who knows what the actual game will be.

[Andrew Schreiber]

Originally Posted by JVN

I bet you can connect 10 more pairs of wheels, 20 pairs even. The question is, why would you?

Without justification (benefit for the albeit minor cost) my team would not do it. If you find that justification, great. I'm just surprised that people would argue "we can, therefore we should". The fact that it is easy doesn't mean that it is smart.

-John

This may be something you need to ask 703...
http://www.chiefdelphi.com/media/photos/27823

http://www.chiefdelphi.com/media/photos/26312

And I can attest to how SCARY that drive train was. RUSH had their arm sheared off at the base when the decided we needed to get out of their way in 2007. Granted, their bot weighed the MAX legal amount that year but no one shoved them and the time they climbed our ramp they ripped our graphics clean off.

And John, if the students on the team decided they wanted to do an 8 wheel drive just because it would be fun to do and they would learn something would you do it? I realize FIRST is about inspiring but is it not also about teaching? My personal opinion is that if the students say they want to do something after a mentor has explained to them the problems then it is not our job to over ride their decision, instead we should support them. Yes it may fail but then we have a good lesson and can show them how to learn from mistakes. If it works, we learned something. "Because we can" may not be a good reason but, "Because we want to" is.

[sdcantrell56]

As well as asking 703 how about we ask 25 or 1270 both of which no one wanted to get in a pushing match with. I also believe both teams were extremely successful in 2007 with 8wd. Also If we can pull off 8wd without sacrificing anything else then why not do it. It does have advantages over other drivetrains, which assuming we can execute it properly will help us be more successful.

[JVN]

Originally Posted by Damien1247

This may be something you need to ask 703...
http://www.chiefdelphi.com/media/photos/27823

http://www.chiefdelphi.com/media/photos/26312

And I can attest to how SCARY that drive train was. RUSH had their arm sheared off at the base when the decided we needed to get out of their way in 2007. Granted, their bot weighed the MAX legal amount that year but no one shoved them and the time they climbed our ramp they ripped our graphics clean off.

Does anyone have any quantitative measurements of their performance, or are we basing everything off anecdotal evidence (as usual)?

an-ec-do-tal - adjective
based on personal observation, case study reports, or random investigations rather than systematic scientific evaluation: anecdotal evidence.

Yep, 703 was great at pushing in 2007. They had an incredible defensive machine. Yet... I bet that NO ONE made any quantitative measurement of it's performance. Here is what I would looooooooove to see:

• Step 1 - Measure it's linear pulling force while on carpet (use a fish-scale).
• Step 2 - Remove all but the 4 outer wheels.
• Step 3 - Throw the extra wheels on top of the frame, so the overall robot weight doesn't change.
• Step 4 - Measure it's linear pulling force again, while on carpet.

I speculate that these numbers will be extremely similar. Within 5%.
Maybe I'm wrong. I would love to be wrong. I would LOVE for someone to prove me wrong with some solid engineering analysis beyond the usual "703 pushed really hard, they pushed against truck town and everyone knows that nobody pushes truck town..."

Anyone? Anyone? I'd be genuinely happy being proven wrong. I don't have the time myself to do the kind of testing I'm talking about.

And John, if the students on the team decided they wanted to do an 8 wheel drive just because it would be fun to do and they would learn something would you do it? I realize FIRST is about inspiring but is it not also about teaching? My personal opinion is that if the students say they want to do something after a mentor has explained to them the problems then it is not our job to over ride their decision, instead we should support them. Yes it may fail but then we have a good lesson and can show them how to learn from mistakes. If it works, we learned something. "Because we can" may not be a good reason but, "Because we want to" is.

Absolutely. If the students decided they wanted to do something "just because they want to" even if it doesn't pass our cost-benefit analysis, I would need to respect that decision.

However I'm chuckling to myself, because I don't really believe our students would ever force me to back up that statement. I guess I'm lucky that our school district's values, our sponsor's values, team's values, our student's values and my values are all pretty much in line.

Then again... maybe it is a pied-pipe type of thing...
Moral of the story, you'd have difficulty finding a Robowrangler who has a bad experience, and our program keeps our sponsors, parents, school, and community happy; I can't really ask for anything more.

-John

[JesseK]

I think it boils down to the same thing everything else in life (such as politics, relationships, etc) does: some things you really just have to try yourself. Sure, physicists can scream that surface contact area has nothing to do with dynamic friction forces, but that's hard for some people to visualize. You can throw an equation at them as a proof, but they may find it hard to see the effects of equations balancing themselves when certain values go up or down. You can even give them fantastic spreadsheets that show the numbers going up and down based upon input values, but the end result will be the same.

I decided I wanted to do a c-channel H frame for our prototype and refused to use anything but the CNC to mill out the wheel holes. Sure, there were simpler ways, more efficient materials, and simpler designs (though I'll argue that one after I post the design). But the point is, now that it's all said and done I truly understand why the manufacturing process using c-channel is complicated and will seek to improve it.

Let's not forget, we're all supposed to be stubborn engineers who take nothing for granted and will argue to the end about an idea until proven right or wrong... correct???

[Andrew Schreiber]

Originally Posted by JVN

Does anyone have any quantitative measurements of their performance, or are we basing everything off anecdotal evidence (as usual)?

Yep, 703 was great at pushing in 2007. They had an incredible defensive machine. Yet... I bet that NO ONE made any quantitative measurement of it's performance. Here is what I would looooooooove to see:

• Step 1 - Measure it's linear pulling force while on carpet (use a fish-scale).
• Step 2 - Remove all but the 4 outer wheels.
• Step 3 - Throw the extra wheels on top of the frame, so the overall robot weight doesn't change.
• Step 4 - Measure it's linear pulling force again, while on carpet.

I speculate that these numbers will be extremely similar. Within 5%.
Maybe I'm wrong. I would love to be wrong. I would LOVE for someone to prove me wrong with some solid engineering analysis beyond the usual "703 pushed really hard, they pushed against truck town and everyone knows that nobody pushes truck town..."

Anyone? Anyone? I'd be genuinely happy being proven wrong. I don't have the time myself to do the kind of testing I'm talking about.

Absolutely. If the students decided they wanted to do something "just because they want to" even if it doesn't pass our cost-benefit analysis, I would need to respect that decision.

However I'm chuckling to myself, because I don't really believe our students would ever force me to back up that statement. I guess I'm lucky that our school district's values, our sponsor's values, team's values, our student's values and my values are all pretty much in line.

Then again... maybe it is a pied-pipe type of thing...
Moral of the story, you'd have difficulty finding a Robowrangler who has a bad experience, and our program keeps our sponsors, parents, school, and community happy; I can't really ask for anything more.

-John

John, I was by no means suggesting that you wouldnt take students into account, merely showing a valid reason to do something like this. I apologize if my statement was taken that way.

Now, about the evidence, after reading the threads about their drive train their goal was to get the benefits of a tread without using tread, did they meet their goals? For that answer you would most definitely have to get one of them. From my perspective they achieved their goals, 1) eliminate the ability of breaking a tread if pushed sideways. 2) They had an incredibly low cg and still did not bottom out climbing obstacles. They proved that a large number of wheels gives you those benefits. They also proved that the draw back was complexity and weight. Did they gain any additional pushing force out of it? Probably not. Would they have been able to shove us around without 14 wheels? Most likely, but the fact remains that they DID gain an advantage from doing that many drive wheels.

What teams need to take from this discussion, in my opinion, is that there are situations in which 8wheel drive would be effective, there are times when 3 wheel swerve would be effective. You wouldn't try to tighten a bolt with a hammer would you? Always weigh the costs with the benefits and use the appropriate tool for the job.

[AndyB]

Originally Posted by Damien1247

Now, about the evidence, after reading the threads about their drive train their goal was to get the benefits of a tread without using tread, did they meet their goals? For that answer you would most definitely have to get one of them. From my perspective they achieved their goals, 1) eliminate the ability of breaking a tread if pushed sideways. 2) They had an incredibly low cg and still did not bottom out climbing obstacles. They proved that a large number of wheels gives you those benefits. They also proved that the draw back was complexity and weight. Did they gain any additional pushing force out of it? Probably not. Would they have been able to shove us around without 14 wheels? Most likely, but the fact remains that they DID gain an advantage from doing that many drive wheels.

I think John's point was that you can't tell if having the extra 8 wheels made any difference. If they were just as effective with 6 wheels, why use 8 more.

They would have had VERY similar traction, and in that game, 14 wheels could handle the ramps just as well as a 6wd could. I don't really understand your argument. If they didn't gain any pushing power, but gained weight and complexity, how is that an advantage?

The only advantages I see are played into tread not breaking (not really a big deal if attached properly, nor worth the weight), and CG (wheels aren't the only way to get your CG lowered). I would argue that you have more risk in more wheels than tread breaking risk.

[gorrilla]

Originally Posted by AndyB

I think John's point was that you can't tell if having the extra 8 wheels made any difference. If they were just as effective with 6 wheels, why use 8 more.

They would have had VERY similar traction, and in that game, 14 wheels could handle the ramps just as well as a 6wd could. I don't really understand your argument. If they didn't gain any pushing power, but gained weight and complexity, how is that an advantage?

The only advantages I see are played into tread not breaking (not really a big deal if attached properly, nor worth the weight), and CG (wheels aren't the only way to get your CG lowered). I would argue that you have more risk in more wheels than tread breaking risk.

exactly my thinking

[Andrew Schreiber]

Originally Posted by AndyB

I think John's point was that you can't tell if having the extra 8 wheels made any difference. If they were just as effective with 6 wheels, why use 8 more.

They would have had VERY similar traction, and in that game, 14 wheels could handle the ramps just as well as a 6wd could. I don't really understand your argument. If they didn't gain any pushing power, but gained weight and complexity, how is that an advantage?

The only advantages I see are played into tread not breaking (not really a big deal if attached properly, nor worth the weight), and CG (wheels aren't the only way to get your CG lowered). I would argue that you have more risk in more wheels than tread breaking risk.

First off, I am NOT an engineer, so I can only offer my observations and my basic understanding of Physics. I apologize for this shortcoming on my part and am working towards fixing it

Now, their decision was to go with wheels instead of treads. They felt that they wanted to use many small wheels linked together with gears instead of chain. Were these decision what you would have done? Maybe not, but they chose them anyway, why? Instead of trying to explain it myself I will link you to two posts:
http://www.chiefdelphi.com/forums/sh...1&postcount=35
http://www.chiefdelphi.com/forums/sh...9&postcount=36

I do not feel that 14 wheel drive is necessary, they probably could have done just as well with 8 wheel or even 6 wheel drive. But they are probably a good person to ask WHY one would go with more than 6 wheels.

[AndyB]

Originally Posted by Damien1247

Did they gain any additional pushing force out of it? Probably not. Would they have been able to shove us around without 14 wheels? Most likely, but the fact remains that they DID gain an advantage from doing that many drive wheels.

My argument is not why they did 14wd, I'm only asking you to reconsider this statement. Because apparently, I'm missing your point.

[JVN]

Originally Posted by Damien1247

This may be something you need to ask 703...
http://www.chiefdelphi.com/media/photos/27823

http://www.chiefdelphi.com/media/photos/26312

And I can attest to how SCARY that drive train was. RUSH had their arm sheared off at the base when the decided we needed to get out of their way in 2007. Granted, their bot weighed the MAX legal amount that year but no one shoved them and the time they climbed our ramp they ripped our graphics clean off.

Originally Posted by Damien1247

I do not feel that 14 wheel drive is necessary, they probably could have done just as well with 8 wheel or even 6 wheel drive. But they are probably a good person to ask WHY one would go with more than 6 wheels.

Right... I agree. I'm not questioning 703's design process, or the results of their design process (again, it was a great machine...)

What I AM questioning, is why we let people get away with loosely referencing a machine like this as proof that "more wheels = more pushing force" or a similar argument. ESPECIALLY if "the physics" doesn't support such an argument.

Anecdotal non-quantitative arguments are driving me batty... That was my point. I'm past the point where I'm debating the merits of an 8WD.

-John

[Andrew Schreiber]

Originally Posted by AndyB

My argument is not why they did 14wd, I'm only asking you to reconsider this statement. Because apparently, I'm missing your point.

Incorrect wording on my part, They gained what they perceived as an advantage. They did 14 wd because they thought they gained something by it. Thats all I'm trying to say.

Also, John, I also mentioned they had a weight advantage over many other teams. (For anyone who doesn't recall, 2006 had weight classes where a shorter robot could weigh more) I never said the wheels were what gave them additional traction, in fact I know that they cannot. Force due to friction is equal to normal force (in this case mass * gravity) times the coefficient of friction. So for anyone thinking I was saying that increasing surface area increases friction I am by no means saying that. I am saying 703 had an impressive drive train that was very good defensively.

And as for letting people get away with anecdotal non-quantitative arguments, I would hardly say you are letting me get away with it. Some people, myself one of them, don't know the physics behind things, we can only comment based on our observations. Now if we read things into those observations that are not true I would hope that we could be corrected and would be willing to learn.

[B.Johnston]

Originally Posted by JVN

There are a lot of people throwing out pros and cons for an 8WD that don't make a lot of sense to me. In particular, those people talking about robot turning and traction. Let's talk through this quickly. To me, there are several different configurations that should be discussed.

1. No wheels are dropped, all 4 are in-line.
2. The 3 of the wheels are dropped so only one pair is raised (front or back)
3. The middle 2 wheels are dropped, and the robot drives on these most of the time.
4. For fun... let's also talk about a "standard" 6WD with middle wheel drop.
5. aaaaand... we'll show a 6WD with NO wheel drop.

JVN's editorial:
Yes, I would use an 8WD, but probably only in a situation where we needed to climb a step or something and I couldn't make a 6WD climb it elegantly. To me, there just isn't any compelling reason to go this direction for a flat field. The 6WD designs I've played with have a reasonable amount of turning scrub, and turn just fine (I don't need a longer or shorter support polygon, I'm happy with the balance I have).

Honestly, I love my 6WD for a robot which requires "max pushing force". If we ever had a game where pushing wasn't required, I would consider doing a 2 Traction + 2 Omni drivetrain or a 2 Omni + 2 Traction + 2 Omni drivetrain. (These configurations would have great handling with max stability and still reasonable pushing force.) Heck... I might even do 6WD anyways. The important thing, is that I didn't do a swerve drive.

Remember to ALWAYS use physics in engineering discussions. There is really no room for "feelings" in this sort of thing. I don't care how you feel about an 8WD, or what you "think" might happen. I only care about your physical justifications for how and why things happen.

Not to stir the waters here, but, there are a few benefits to having a few more parts and or a little more weight via an 8 wheel drive that havent been discussed yet.

The 2008 1680 design was 8 wheel drive with a 4 in line (no drop) profile.

Each side comprised 2 separate modules of:
1 - 2.5inch cim motors with a 3.57 reduction (14:50 gearing)
1 - 6inch "First Wheel" direct driven in the middle front or rear position and
1 - 6inch AM Omni Wheel at the outside corners driven by 25 chain from the directly connected center wheels.

This gave us.

1. A long support polygon.
2. Short overhangs for obstacles.
3. No wheels dropped, all 4 in-line traction.
4. A short effective wheel base for tank pivot turns.
5. A moderate increase in resisting being spun vs the (6 Omni-Wheel-Omni combo.

And new to this disscussion ... Fault Tolerance.

Each side could maintain some motive force with the following failures

1. 1 chain = 75% wheels driving (2 Wheels - 1 Omni).
2. 2 chains = 50% wheels driving (2 Wheels).
3. 1 Transmission = 50% wheels driving (1 Wheel - 1 Omni).
4. 2 Transmissions = 0% wheels driving (Okay Dead in the water here).

Granted Multi speed transmissions may have some mechanical advantage here.
But there's no reason you couldn't have different mechanical ratio's and electronically gear the drive pairs.

Imagine...

the rear module geared down for pushing,

the front module geared for speed,

under load (pushing/acceleration)the rear pair could receive max current and the front would operate at a reduced level...
until the rear has reached its "slip" limit...
the current drive would then be redirected to the front (still at a loaded condition)...
this would pull the now unloaded rears to an rpm above that which they have torque at.
Presto.. two separate velocity profiles, no gear changes.

Something to consider.

PS 3.57:1 as a final drive ratio was a little fast last year (34ft/sec)

[AdamHeard]

Originally Posted by B.Johnston

And new to this disscussion ... Fault Tolerance.

Each side could maintain some motive force with the following failures

1. 1 chain = 75% wheels driving (2 Wheels - 1 Omni).
2. 2 chains = 50% wheels driving (2 Wheels).
3. 1 Transmission = 50% wheels driving (1 Wheel - 1 Omni).
4. 2 Transmissions = 0% wheels driving (Okay Dead in the water here).

I believe Fault tolerance is something that can be built into any drive train, and isn't a distinct advantage to an 8 wheel.