pic: Concept 8wd Drivetrain

[s_forbes]

Smaller wheels also let you set your whole robot down lower to the ground (low center of gravity, yay!) and they allow you to make your wheelbase about 2" longer than one with 6" wheels (less tippy, yay!). There's lots of good reasons to use them.

We put 4" wheels on our robot last year since it didn't have to do any climbing, and I didn't notice any difference in performance between it and our previous year's robot (with 6" wheels). Both were set up with 6 wheel drive and wedgetop tread.

I've also been kicking around a wooden eight wheel drive base in my head for a while, but it looks quite a bit different... I like the eight wheel drive concept, it has several things going for it that make it preferable over six wheel drive (IMO). Having four center wheels instead of only two slows down tread wear, and as someone else stated doesn't let your robot "rock" while it's driving around. It also gives you a bigger contact patch on the ground to avoid being spun. By bigger contact patch I don't mean more traction, but traction where you need it. Six wheel drive bots spin pretty easily when pushed on a corner since all their weight is on their two center wheels. But an eight wheel drivebase has four center wheels that carry the weight, and they're all distributed further out from the center.

And the obvious reason for using an eight wheel drivebase is that it allows you to climb over all those ramps on the field without high-centering.

[AndyB]

Originally Posted by gorrilla

well do you think its really worth it i mean 1557 was 119.1 pounds and that was mostly in our frame and scissor lift

and with smaller wheels wouldent you have less traction because of less surface area

No, because the overall robot weight is the same. If the surface area goes down, then the concentration of weight on the area goes up. Traction does not change.

Small wheels provide less cost, less size, and you need less reduction before the wheel. The only thing they lack is provided ground clearance. But that can be worked around.

[gorrilla]

but were would one get four or 5inch wheels? without custom machining

[Andrew Schreiber]

http://www.robotmarketplace.com/prod...on_wheels.html comes to mind quickly, Banebots also has some.

[AndyB]

Originally Posted by gorrilla

but were would one get four or 5inch wheels? without custom machining

IFI has 4", Andymark has 4", 5", and will have 4" plaction soon (as far as I know). Banebots has 3", 4", and 5". Colson makes smaller wheels as well I believe.

[jimbot]

Call me ignorant but wouldn't an 8wd chasis with 4 wheels lowered simply be a 4wd chasis that has a small wheel base? If so what exactly would the advantage of an 8wd have over a 4wd chasis?

[Daniel_LaFleur]

Originally Posted by s_forbes

Smaller wheels also let you set your whole robot down lower to the ground (low center of gravity, yay!) and they allow you to make your wheelbase about 2" longer than one with 6" wheels (less tippy, yay!). There's lots of good reasons to use them.

Ground clearence and wheel size have little to do with each other. Relationship of the frame to the axles has more to do with ground clearence and CoG than wheel size. We've used 6" pneumatic wheels for the last few years, but have had less than 1" ground clearence since our axles have been positioned above our frame.

The Longer wheelbase of a 4" wheel is offset (in the case of this thread) by the number of wheels. The biggest advantage I've seen with smaller wheels is that they are lighter.

[techtiger1]

As a member of a team that has designed, tested, and built and used a 8wd system for a year in competiton here are my thoughts. Look into the game first and see if you want 8 wheel drive these types of systems get heavy and can compromise other parts of the robot that need structure to be effective. 1251 exhibit A our claw this year which continually bent from trying to keep it light. Also make sure you have enough torque in the system to turn the robot. Raiseing the two wheels is pointless your better off using 6 wd in that case. Ours works by having the front wheels offset and 3 motors which allows us enough torque to be able to keep 6 wheels on the floor at all times and still turn. Finally the wheel size of 1.5in tread width is fine. With the direction FIRST is going I don't think the 8 wd drive is necessary that is my final thought for you. Look at it this way too if you don't use this system it was good cad practice.

My two cents,
Drew

[sdcantrell56]

I don't know where to start with all the questions so I'll try my best.
First of all the reason I chose to do an 8 wheel drive design is due to a couple different factors. I will try to outline my thinking. I wanted to use a small diameter drive wheel to reduce the overall cg of the robot and to minimize material cost. I also thought that there is a decent chance of an obstacle being in this coming years game, hence the use of 8wd which equals the ability to climb a much steeper obstacle without bottoming out given the ground clearance. Also note the cuts in the front and rear of the frame to increase approach and departure angles also to help with climbing. 8wd is also inherently more stable as the rock is less dramatic. The 8 wheels in theory does not increase traction however in the world of FIRST it absolutely does. Not so much in the front-back direction but in the sideways direction. It will be much harder to push this robot sideways and to spin it without us wanting it spun. Finally I am trying to minimize the amount of machining we need for transmissions and such. With this design we can use an almost stock andymark 2 speed. The only thing changed is one gear is turned down and then a custom 7075 output shaft machined that is longer. With 8wd I can have 2 separate chains running one to each center wheel which creates a backup if one chain fails.

I am planning on Roughtop tread for the wheels so traction will be very high.

Additionally the issue of extra weight is very moot with this design. The entire drivetrain will be under 40lbs. that includes motors, transmissions, wheels and all the other little things, as well as all the mounting points for our superstructure. This means we have 80lbs to play with for electronics and the superstructure.

As for there not being much point to 8wd if we raise the outer wheels, look at 1270 from 2007. They raised the outer wheels and were one of if not the strongest pushing robot. They were also quite successful, making it all the way to einstein.

[gorrilla]

if your going to run eight wheels wouldent it be easier(to implement not make unbreakable)and lighter to run actual treads? then you would have lots of traction


anyways it just seems unneccesary because if everyone knew we were going to be climbing lots of steep things then i know that i would consider treads before eight wheels simply becuase its more complex(Lots of chains and things to break) and harder to make everything straight and workig funcionally im not saying it would be harder to drive or climb stuff with just considering building and implementing things

[sdcantrell56]

An 8 wheeled robot is actually much easier to make and design. All you have to do is run an extra axle and chain versus a 6 wheel design. For treads you really need at least 4 wheels/idlers per side so there is no reduction in complexity. The only thing easier about treads is the lack of chains. Although if you do a quick search you will see many many negatives about tank treads, namely there propensity to snap under side-loads and there high cost. Each tread will cost around $270 dollars.

The drive cogs for treads are heavy as well and the tread itself is heavy particularly compared to 8 plastic wheels and #25 chain. Unless there is a game that involves sand or other loose material, I as well as most people in FIRST will be hard pressed to consider treads as a viable option given the cost, weight, and reliability aspect.

Also how is it any harder to align 8 wheels versus 6 or versus 8-10wheels/idlers in the case of treads. If there is a problem with alignment then it will render any drivetrain useless, not just an 8 wheel drive robot

[David Guzman]

First of all very nice CAD work. I also like the way you did the base in the front and back for climbing. Here are a few things about 8wd based on our (1251's) 8 wheel drive for 2008.

Pros:

1. Very difficult to get pushed side ways since there is more contact points.
2. If done right, no rocking back and forth.
3. Easier to adjust weight distribution in the design phase.
4. Using shorter loops of chain require less tension adjustments.
5. If done with 6 motors, very hard to be pushed in low gear and maintains high maneuverability at high speeds. (in our case 16.5 fps)

Cons:

1. Heavier base
2. Harder to manufacture (more time consuming)
3. If done with 6 motors, battery dies much faster.

My two cents:

I would recommend the 8 wheel drive to anyone if the have they weight and time to pull it off. Also like Drew said if you have the 4 wheels in the middle lower; you might as well make a 6WD. The best way to do it is to make your front/back wheels a little higher, so you always have 6 wheels on the ground. Make sire your 6 contact points are farther apart in width and closer in length, this will improve turning.

Also you don't need your wheels to be so wide as others have said this does not increase the force of friction. The number of contact points is more important.

[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