pic: Concept 8wd Drivetrain

[JVN]

Long Rambling post... Beware. There may be some decent content. No promises.

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.

Since we're only talking about the effects of the 8WD itself, we don't need to discuss any of the "additional" characteristics of the drivetrain. Examples of some of these characteristics which have NO bearing on the PRO/CON of an 8WD system are: Motor usage, gearbox/transmission, wheel size, drivetrain configuration (widebody vs narrow), robot CG, overall robot weight, etc.
Though these characteristics are all VERY important to the overall drivetrain performance, let us assume "all other things equal" so we can have a good comparison.

First, we will talk about robot turning.
For reference, everyone should immediately go read this whitepaper and learn all the physics behind it:
http://www.chiefdelphi.com/media/papers/1443

Based on this paper, almost everything "cancels out". (If you don't believe me, read the whitepaper again). The main comparison we need to look at, is the length of the support polygon (as support polygon is the polygon you draw between the points where the robot touches the ground at any given time). If you took the time to do your HW and read the above whitepaper, and have a good understanding of the physics involved you will understand why.

As you can see in my attached image (excuse the crude drawing). we can compare the cases listed above pretty quickly.

Longer Support Polygon = more turning scrub = higher resistance to being spun, worse robot turning.

Shorter Support Polygon = less turning scrub = less resistance to being spun, better robot turning.

"But wait... I want to change something to affect this"
Well, we could change the traction material on some of the wheels to drastically alter the drivetrain characteristics, but remember we promised we would do an "all other things equal" comparison". (We promised, and we need to honor that promise.) Because of this "all other things equal" the above comparisons are pretty straightforward.

Next let's talk quickly about overall traction...
I am one of those people who does not believe "more contact patch" = "better traction". Call me old fashioned, but I think we're pretty darn close to a F= mu * N model for robot traction. (More contact patch DOES however result in reduced tread wear, which is nice.) However, if you're one of those crazy people who REALLY likes more grip on the floor, we just need to make one more comparison. "How many wheels are on the floor?" Take a peek at the attachment one more time and make this comparison, go ahead, I'll wait.

More considerations?
These are the straight-forward ones.

• 8WD makes it easier to build a "climbing robot" while still maintaining a low-ground clearance.
• 8WD has more parts which is by definition more complex to build, heavier, and probably less reliable than an "all other things equal" 6WD system.
• Depending on where the CG is, some of these configurations will "rock less" than others. This may be important to you. You can play with your support polygon to further affect this.

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.

For goodness sakes... READ THIS WHITEPAPER:
http://www.chiefdelphi.com/media/papers/1443

If anyone has any questions about this, feel free to ask.

Good Luck!
-John

[sdcantrell56]

I too agree that we are close to the ideal equation for traction. The main reason for the wider wheels on my drivetrain is for the reduced tread wear not for increased traction. I would like to not have to change treads frequently so that is where the 1.5" width came from. Also yes if the game does not include much climbing or if the object to be climbed is not particularly steep then a 6wd layout will most likely be used.

One thing about this particular design is that the overall weight gain from adding 2 extra wheels is not particularly large (>3lbs) and the drivetrain has an inherent cool factor. If we have the extra weight, the cool factor certainly outweighs anything else assuming all things else are equal. Im confident we can run 2 extra chains and wheels without a problem. Something about laser cutting everything tends to have everything line up perfectly.

[gorrilla]

make sure you think throug hall aspects of your descision or else you may be regretting you choice

and cool factor should not be considered

i saw plenty of teams that had cool robots but dident preform very well

[sdcantrell56]

O believe me, we always think through our design. And I seriously doubt there would be anything to regret about an 8wd drivetrain as long as it's executed correctly which of course it will be.

The idea that a design should not be done because it is cool is a ridiculous notion. We had arguably one of the coolest designs this year and I will challenge anyone to argue that it was not brutally effective. We did win the Xerox creativity award at the Championships for it as well as were one of the top pure scoring robots in the world.

This idea that cool is not something to be considered kind of stifles innovative design for the sake of being safe. I caution all teams not to become to scared of a design to give it a try. Of course carefully design it, and thoroughly test it, but by all means try something different.

[Daniel_LaFleur]

Originally Posted by gorrilla

make sure you think throug hall aspects of your descision or else you may be regretting you choice

and cool factor should not be considered

i saw plenty of teams that had cool robots but dident preform very well

I'm not so sure I agree with this.

The goal of FIRST is to inspire. If the 'cool factor' inspires then the goal has been achieved.

[sdcantrell56]

Originally Posted by Daniel_LaFleur

I'm not so sure I agree with this.

The goal of FIRST is to inspire. If the 'cool factor' inspires then the goal has been achieved.

Precisely!!! That is indeed the purpose of FIRST.

[JVN]

Originally Posted by Daniel_LaFleur

I'm not so sure I agree with this.

The goal of FIRST is to inspire. If the 'cool factor' inspires then the goal has been achieved.

Yes. This is 100% correct. However whether or not "cool factor" inspires varies greatly from team-to-team and year-to-year.

Our team uses a weighted-objectives-table (WOT) to help determine what strategies and designs we want. "Cool Factor" is typically weighted significantly less than "efficiency", "elegance", and "effectiveness". This is a clear quantitative assessment of what our team values.

Excuse the expression, but that is just how we roll. Your mileage may vary.

-John

[Cory]

Originally Posted by JVN

Yes. This is 100% correct. However whether or not "cool factor" inspires varies greatly from team-to-team and year-to-year.

Our team uses a weighted-objectives-table (WOT) to help determine what strategies and designs we want. "Cool Factor" is typically weighted significantly less than "efficiency", "elegance", and "effectiveness". This is a clear quantitative assessment of what our team values.

Excuse the expression, but that is just how we roll. Your mileage may vary.

-John

I'm with John, but I'd argue that if you have an elegant robot it's probably pretty darn cool.

[sdcantrell56]

Elegant is probably what I was looking for. Elegance is what I strive for in all of my designs, and nothing says elegance to me like a highly efficient, and light weight 8 wheeled drivetrain that uses the bare minimum of metal and still is stronger than most metal framed robots.

I should also throw it out there cory that your team is one that I strive to learn from and to eventually be like. Your designs are truly beautiful.

[billbo911]

Originally Posted by JVN

Our team uses a weighted-objectives-table (WOT) to help determine what strategies and designs we want.....


Excuse the expression, but that is just how we roll.

-John

Any chance you would want to share your WOT? Noob teams, and even those with years of experience, could learn a lot from a simple tool like that.


BTW, no need to apologies for that expression.

[JVN]

Originally Posted by billbo911

Any chance you would want to share your WOT? Noob teams, and even those with years of experience, could learn a lot from a simple tool like that.

A month later:
http://www.chiefdelphi.com/forums/sh...575#post777575

-John

[Daniel_LaFleur]

Originally Posted by JVN

Yes. This is 100% correct. However whether or not "cool factor" inspires varies greatly from team-to-team and year-to-year.

Our team uses a weighted-objectives-table (WOT) to help determine what strategies and designs we want. "Cool Factor" is typically weighted significantly less than "efficiency", "elegance", and "effectiveness". This is a clear quantitative assessment of what our team values.

Excuse the expression, but that is just how we roll. Your mileage may vary.

-John

While I agree with what you are saying here (If you've ever looked at the last 7 years of robots my teams have built you'll definately see 'industrial machine' written all over them ), I just wanted to point out that the goals of FIRST isn't to win, it's to inspire.

And while winning does tend to inspire others, it's not the only way to inspire ... and we as mentors need to keep reminding ourselves of that (we mentors tend to be a competitive lot )

[AdamHeard]

Originally Posted by Daniel_LaFleur

I'm not so sure I agree with this.

The goal of FIRST is to inspire. If the 'cool factor' inspires then the goal has been achieved.

In terms of design, the goal should be to create the most competitive product for the customer (the customer being FIRST).

[Daniel_LaFleur]

Originally Posted by AdamHeard

In terms of design, the goal should be to create the most competitive product for the customer (the customer being FIRST).

Again I'll disagree here.

Our goal should be to inspire the students and help change the culture of the areas we live in. And our competitive product is the future of these students ... not some piece of whirring metal.

We're not here to win a regional or any other 'event'. We're here to win a future for these kids. If a team wants to build a 8 wheel drive chassis, and are inspired by doing that, then we should encourage them to achieve that goal and let them understand the design strengths and weaknesses of doing just that.

[sdcantrell56]

From all of this discussion, the only true weakness I see to an 8 wheel drivetrain is the added weight. However, with careful planning and material selection, not to mention smaller wheels and sprockets for the same ramp climbing ability, even that weakness can be negated. The extra machining and assembly are so small that it really is not a problem.