6 Wheel Drive vs. 8 Wheel Drive

[dricks]

Team 704 used 8 wheels this year mainly because it was the students design. Our 8 wheel drive was a bit different then most. We used 8" wheels in the front and back and 6" wheels for the middle four. The Front and back wheels were raised .5" Making there center 4.5". this allowed us to cross the center "Hump" easily. The wheels were geared to run the same speed. The four center wheels gave good stability.

Originally Posted by Cory

a well implemented 8WD should have superior maneuverability to a 6WD as the effective wheelbase is significantly shorter.

I don't mean to come off as rude, but from all of the times I've watched Skyfire (that's a lot), I always had this feeling that Slipstream was so much better drivetrain wise. It was more maneuverable, smooth on the field, and didn't rock, whereas Skyfire rocked a bit and didn't drive so smoothly.

Is this done on purpose in the design? Is it because of new drivers? Or do I just love your 2011 robot too much?

[Akash Rastogi]

Originally Posted by SuperNerd256

I don't mean to come off as rude, but from all of the times I've watched Skyfire (that's a lot), I always had this feeling that Slipstream was so much better drivetrain wise. It was more maneuverable, smooth on the field, and didn't rock, whereas Skyfire rocked a bit and didn't drive so smoothly.

Is this done on purpose in the design? Is it because of new drivers? Or do I just love your 2011 robot too much?

I think Cory posted as a facebook comment to a picture of their 2012 base that it had a larger drop center than it needed. Don't know what the drop was last year.

[Cory]

Originally Posted by SuperNerd256

I don't mean to come off as rude, but from all of the times I've watched Skyfire (that's a lot), I always had this feeling that Slipstream was so much better drivetrain wise. It was more maneuverable, smooth on the field, and didn't rock, whereas Skyfire rocked a bit and didn't drive so smoothly.

Is this done on purpose in the design? Is it because of new drivers? Or do I just love your 2011 robot too much?

"Superior" may have been a poor choice of words. a 6WD is already highly maneuverable, so an 8WD would only be incrementally better.

2011 definitely drove better. It only weighed 90 lbs to this year's 119.9 lbs. Keeping the 2011 bot as light as possible for acceleration purposes was a major design consideration. Unfortunately that wasn't possible this year.

[BrendanB]

Very insightful discussion here!

Our team went with a 6wd, 6in, evenly spaced drivebase. Our goals included: fast/manueverable, ability to cross the bump, and high traction so we wouldn't slide on the bridge and have an edge if we needed to play defense. At first we decided to use the kit wheels, they were readily available to us, had great grip on the carper/bridge, and didn't cost us a thing. After much testing when the drivebase was complete we realized we needed a beefier setup as our kit wheels were breaking. We settled with AM plactions as our outer wheels and IFI as our center wheels. *new fan of IFI wheels*

We protoyped wheel spacing using a kitbot chassis, wooden rails, and 6in wheels. Using this setup we could easily adjust wheel spacing/height as well as the angle of our skid plates in the front.

2012 drivbase:




Looking back after the season we could have simplified our design by doubling our drop down intake with our skid plate design much like 33 and 233. Combined with an 8wd with 4in wheels so we wouldn't bottom out on the bump and we could have used a much simpler chassis.

Overall we are extremely happy with our drivebase this year!

[R1ffSurf3r]

it is pretty easy to bottom out our robot and balance it helplessly on the bump if you drive carefully. however, shaking our front manipulator seemed to give the last bit of oomph to upset the balance. then again, so did driving over at full speed

[MichaelBick]

Originally Posted by SuperNerd256

I don't mean to come off as rude, but from all of the times I've watched Skyfire (that's a lot), I always had this feeling that Slipstream was so much better drivetrain wise. It was more maneuverable, smooth on the field, and didn't rock, whereas Skyfire rocked a bit and didn't drive so smoothly.

Is this done on purpose in the design? Is it because of new drivers? Or do I just love your 2011 robot too much?

I felt the exact same way, wether it is because of drop or because it was 8wd. Though, in almost all games, I actually prefer the rocking of 6 wheel, as long as you have a good amount of driver practice.

[EricH]

Originally Posted by JVN

The fun one:
Ever wonder why some teams can hang more of their robot off the bridge than others? CG is only one half of that story... Support Polygon is the other.

Can you think of a situation in which an (evenly spaced) 6WD would be better for balancing than an (evenly spaced) 8WD?

Situation: The 8WD's CG is in the middle of the middle 4 wheels, and the 6WD's CG is in between the middle wheels and one end's wheels. I'm assuming that the end of the 6WD with the CG is on the bridge, and the CG is fairly close to the middle but not exactly there. (There are other reasons to not have the CG exactly in the middle of the 6WD, but that's beyond the scope of this discussion for now.)

I've included my reasoning. Before looking, see if you can figure it out (unless you're JVN, in which case you've probably already figured it out). Use a pencil and paper if you need to.

Spoiler for Why:

We'll assume that the bridge can't leave level configuration first, just to make life a little bit easier.

When the robot is just under halfway off, the 8WD's CG is already causing the robot to think about falling off the end of the bridge, due to the support polygon suddenly shortening (something about one side running out of support, then the frame reforming the support polygon by landing on the bridge). The 6WD's primary support polygon is still fully on the bridge.

Now move the robot to exactly halfway. The 8WD is pretty much toast now; its CG is about to go off the bridge completely (if it hasn't already due to height of the CG). The 6WD is also very close to going off, but still has its full support polygon, if barely.

Now, go just over halfway. The 8WD's CG goes off the bridge (groans from the crowd and a crashing noise), while the 6WD's support polygon just got shorter due to the center wheels going off of the bridge--and the 6WD's CG is getting very close to the edge of the bridge if it didn't just go off of it.

BUT! Now it gets even better. We repeat the experiment with a pivoting bridge, with a 150# robot-sized counterweight fully on the bridge. At just under halfway off, the bridge is starting to tip--throwing the 8WD's CG just about fully off of the shortened support polygon. But the 6WD, while also at risk due to CG placement, probably still has a little bit of support polygon left. At halfway... goodbye 8WD, 6WD is starting to go over but might have just that little bit of support polygon that it needs to stay aboard.

[BJC]

Originally Posted by JVN

It seems to me, there is no harm in re-holding this discussion. ESPECIALLY since this year's game had at least TWO unique (and somewhat subtle) design considerations which come into play here.

The fun one:
Ever wonder why some teams can hang more of their robot off the bridge than others? CG is only one half of that story... Support Polygon is the other.

Can you think of a situation in which an (evenly spaced) 6WD would be better for balancing than an (evenly spaced) 8WD?

The boring one:
Also of course, the number of wheels, and ground clearance play a large role in the bump crossing design challenge.

Perhaps some of the teams who used 6WD or 8WD this year could share their justifications for doing so, and how the unique design considerations of this game came into play in their decision making?

To some of us, the process is the most interesting part of this competition...

-John

Our team found a fairly unique solution to meet all of our drivetrain expectations for this year.

-go over the bump fast and stable
-perform a triple balance (take up little bridge space)
-difficult to push
-easy to push others
-not stupidly complex

We came up with an asmytrical 6 wheel drive with an extra pair of raised wheels. The robot has a very stable 16" wheel base which makes for solid and stable turning in a wide robot. The back wheels are 4" by 2" wide IFI wheels raised 1/8" so when we were going over the barrier or pushing robots up the bridge we would have the furthest back and highest traction point of contact with the ground we were able to. The inner wheels are 4" colson wheels which have excelent bridge traction. When we were only part of the way on the bridge these wheels allowed us to pull the rest of the robot on to the bridge. Because of our wheel configuration we could also hang the back of our robot off the bridge so long as the third pair of wheels were on. This wheel configuration combined with our drop-down fins also allowed us to take advantage of low bumpers which had numerous pluses in this game including making it difficult for others to push you, not riding over balls, and getting under other robot's bumpers when pushing them up the bridge to balance. The raised wheels were only for contact going over the barrier, we would have grounded out on the barrier immobilzing us for the rest of the match at least 5 times this season without them providing that key contact. The front wheels were the 6" older kit wheels which we chose as optimal to first contact the barrier with. These wheels were lathed roundish to provide less turning scrub.

Hopefully someone found that interesting!
Regards, Bryan

(P.S. picture of what I was trying to explain)

[LeelandS]

So, here's my take on 8WD vs 6WD. Now, this is only coming from what I've read perviously and seen out of a 6WD, so take all this with a grain of salt. Maybe a teaspoon.

6WD
Wins:
Lighter and simpler than 8WD, obviously due to one less set of wheels.
Yeah, that's pretty much it.

8WD
Wins
-More maneuverable. With 6WD, the turning radius of your robot, I believe, becomes centered between the center wheels and whatever outer wheels the robot is sitting on. With 8WD, it's in the center of the robot.
-Able to overhang. On a 6WD, the robot is sitting on 4 wheels at a time. The center ones, and a set of outer ones. These outer wheels are set out at the far ends of the robot. When you start to try and overhang, as soon as one set of wheels comes off the surface you're on, the robot is going to start tipping that way. With 8WDs center wheels, the robot still rests on 4, but the weight distribution of the robot along those 4 wheels is more centralised. As long as those 4 wheels are still in contact with the surface, you shouldn't tip over. Weight distribution on both of these is a huge factor, so you saw some teams on 6WD and West Coast able to overhang, but I believe this rule generally holds true.
-With the raised wheel set and a sloped chassis, overcoming obstacles is much easier.

Anyone finds flaws in this, please tell me. Writing down wrong information then having CD correct me is usually how I learn here

[akoscielski3]

Originally Posted by LeelandS

So, here's my take on 8WD vs 6WD. Now, this is only coming from what I've read perviously and seen out of a 6WD, so take all this with a grain of salt. Maybe a teaspoon.

6WD
Wins:
Lighter and simpler than 8WD, obviously due to one less set of wheels.
Yeah, that's pretty much it.

8WD
Wins
-More maneuverable. With 6WD, the turning radius of your robot, I believe, becomes centered between the center wheels and whatever outer wheels the robot is sitting on. With 8WD, it's in the center of the robot.
-Able to overhang. On a 6WD, the robot is sitting on 4 wheels at a time. The center ones, and a set of outer ones. These outer wheels are set out at the far ends of the robot. When you start to try and overhang, as soon as one set of wheels comes off the surface you're on, the robot is going to start tipping that way. With 8WDs center wheels, the robot still rests on 4, but the weight distribution of the robot along those 4 wheels is more centralised. As long as those 4 wheels are still in contact with the surface, you shouldn't tip over. Weight distribution on both of these is a huge factor, so you saw some teams on 6WD and West Coast able to overhang, but I believe this rule generally holds true.
-With the raised wheel set and a sloped chassis, overcoming obstacles is much easier.

Anyone finds flaws in this, please tell me. Writing down wrong information then having CD correct me is usually how I learn here

I dont mean to be rude but some of the things you have said here are wrong. :$ And i dont want people to be miss informed when designing a robot chassis.

I have designed many chassis and have been the driver for a couple years now, so I have experience with the 6WD vs 8WD. Though we have never used a 8WD.

6WD chassis do not spin on the middle wheels and what ever else wheel/s it is on. They spin only on the Center wheels, and (if you picture this it will help) "rock" back and forth from the back and front wheels. Making the middle wheels the "Axis" for the turning. In 8WD it is a little different how it moves. It moves with the 4 center wheels, and eliminates the "rock". And makes it a little smoother chassis. But the rock may or may not be a problem depending on how much it rocks. For example this year a 6WD could have made your shot off, by the slightest little bit. 8WD might (most likely) Eliminate that problem.

You contradicted yourself in the second point IMO. As long as 4 of your 6 (in a 6WD) wheels are on the bridge you should not tip. Depends on your COG though. With 8WD you do not have a center wheel to support your robot on the bridge. When 4 wheels are in contact, however (if COG is in center) you have (if wheels are evenly spaced) more than half your robot off of the bridge, and you know what that means...

(I Know I am terrible at explaining things, so sorry)

[LeelandS]

Originally Posted by akoscielski3

I dont mean to be rude but some of the things you have said here are wrong. :$ And i dont want people to be miss informed when designing a robot chassis.

I have designed many chassis and have been the driver for a couple years now, so I have experience with the 6WD vs 8WD. Though we have never used a 8WD.

6WD chassis do not spin on the middle wheels and what ever else wheel/s it is on. They spin only on the Center wheels, and (if you picture this it will help) "rock" back and forth from the back and front wheels. Making the middle wheels the "Axis" for the turning. In 8WD it is a little different how it moves. It moves with the 4 center wheels, and eliminates the "rock". And makes it a little smoother chassis. But the rock may or may not be a problem depending on how much it rocks. For example this year a 6WD could have made your shot off, by the slightest little bit. 8WD might (most likely) Eliminate that problem.

You contradicted yourself in the second point IMO. As long as 4 of your 6 (in a 6WD) wheels are on the bridge you should not tip. Depends on your COG though. With 8WD you do not have a center wheel to support your robot on the bridge. When 4 wheels are in contact, however (if COG is in center) you have (if wheels are evenly spaced) more than half your robot off of the bridge, and you know what that means...

(I Know I am terrible at explaining things, so sorry)

Don't be sorry for correcting me I put this out as I was informed. I like being told I'm wrong, actually, because it's a chance for me to learn. So, first off, thank you!

Secondly, what I think I was trying to say (and I was kinda in a rush when I typed it out), is when you have a drop center 8 wheel drive, you have the two center sets of wheels, and the 2 outer sets of wheels. The robot sits on the 2 center sets, if I'm not mistaken. So that's what the robot is supported by. As long as those wheels sit on the bridge, the robot will not fall over (again, depending on weight distribution). On a drop center 6 wheel, the robot rocks. Depending on where your weight is distributed, when one set of wheels comes off the bridge, the weight of the robot starts leaning in that direction. Thus, the robot falls over. Is this correct? Maybe I'm mistaking what I'm visualizing as an 8 and 6 wheel drive.

As for the rest, thank you for this info. I love the opportunity to learn from those with experience!

[akoscielski3]

Originally Posted by LeelandS

Don't be sorry for correcting me I put this out as I was informed. I like being told I'm wrong, actually, because it's a chance for me to learn. So, first off, thank you!

Secondly, what I think I was trying to say (and I was kinda in a rush when I typed it out), is when you have a drop center 8 wheel drive, you have the two center sets of wheels, and the 2 outer sets of wheels. The robot sits on the 2 center sets, if I'm not mistaken. So that's what the robot is supported by. As long as those wheels sit on the bridge, the robot will not fall over (again, depending on weight distribution). On a drop center 6 wheel, the robot rocks. Depending on where your weight is distributed, when one set of wheels comes off the bridge, the weight of the robot starts leaning in that direction. Thus, the robot falls over. Is this correct? Maybe I'm mistaking what I'm visualizing as an 8 and 6 wheel drive.

As for the rest, thank you for this info. I love the opportunity to learn from those with experience!

OH! Okay I miss understood what you were talking about for what wheels the 8WD sits on during the bridge balancing. I thought you meant a set of Center wheels and a set of outside wheels. Which would obviously make it fall off the bridge.

I can see where you are coming from for the 6WD. However it is not true. As long as you have your COG "on" the bridge, AND have your middle and 1 set of outer wheel on the bridge you will not fall off. Example. This is our practice bot hanging off the bridge 17". Though we had our COG as far back as possible, we had our front wheel WAY off the bridge and stayed on, without falling.

I have No problem helping you learn, but i feel bad sometimes when I have to tell a person they are wrong :$ I dont want to be a "know it all" or act like one. But I guess thats what FIRST is all about right?

[Gray Adams]

I'm curious, has anyone built a robot that can either be 6 or 8 wheel and tried driving it? Maybe you guys are a lot better at balancing your center of mass, but it seems pretty hard for us to compare wheel numbers and center drops because all of our robots are so different in the superstructure. It gets really hard to tell what is causing the different behaviors.

[PAR_WIG1350]

Originally Posted by akoscielski3

6WD chassis do not spin on the middle wheels and what ever else wheel/s it is on. They spin only on the Center wheels, and (if you picture this it will help) "rock" back and forth from the back and front wheels. Making the middle wheels the "Axis" for the turning. In 8WD it is a little different how it moves. It moves with the 4 center wheels, and eliminates the "rock". And makes it a little smoother chassis. But the rock may or may not be a problem depending on how much it rocks. For example this year a 6WD could have made your shot off, by the slightest little bit. 8WD might (most likely) Eliminate that problem.

This is not always true, it depends on how the robot is designed. Actually, a well designed 6WD will NOT rock while turning. unless it is being driven in a jerky manner. The ideal situation for a six wheel drive is to have the COG off-set to the front or to the back to eliminate the rocking effect since that would interfere with scoring and stuff. Typically, the COG is off-set to the rear so that it won't rock back during sudden accelerations and and to counter balance an arm if one is present, but there is no reason that the off-set couldn't be to the front if it provides some advantage specific to your design. This means that if all wheels are identical and the amount of drop is just enough that the set of wheel "in the air" has a negligible effect on driving, It will rotate about a point roughly mid-way between the two pair of wheels in contact with the floor. This is'n as noticeable as one would think, however and might seem like it is pivoting around the center wheels. In a few case, such as low traction corner wheels or a small enough drop that the "raised" wheels cause some scrub effects, the pivot will move closer to the center.