6 Wheel Drive vs. 8 Wheel Drive

[jwfoss]

This year FRC2168 went with an 8wd, over last years 6wd. Below are some of the factors played into this final choice:

• We wanted to ability to cross the barrier, without an active mechanism, with the smallest wheels possible without the chance of high centering. (resulted in 5" Colson Performa wheels, with a wedge leading edge on the chassis)
• The center to center distance between wheels was all equal, and calculated to ensure that high centering was not possible
• Our base driver commented on the "tippyness" of our 6WD last year and with shooting, we determined that being as stable (and repeatable) as possible was important
• With a 6WD we would have been able to hang more of the bridge, so to counter that we made the entire chassis shorter (only 34" long)

I will say that the drivers and the rest of the team favored the 8WD and we will likely be refining it as we move forward.

You can read some more about it and see some pictures here: FRC2168 - Behind the Design - Drivetrain

[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.

[Brandon Holley]

The NUTRONs went with an 8WD this year. 5" Colson Performa wheels, 1.5" width. The main reason for going 8WD was the same as 2168's, which is to traverse the barrier without any active mechanism.

One unique thing about the drive system was that the wheels along a drive side were NOT of equal spacing. Looking at our drive side with the front being labeled as wheel 1 and the back being labeled as wheel 4; wheel 1 and wheel 2 were quite close, wheel 3 and wheel 4 were not quite as close as wheel 1 and 2, and wheels 2 and 3 were the furthest apart.

We did this because it helped to offset one of the disadvantages of an 8WD in this game, which I believe is what John is referring to. We wanted to try and get a wheel as close to the center as we could so that we could hang as much of the robot off the back as possible. We still need 6 wheels on the bridge to hang, but those 6 wheels + front bumper only require 26" of ramp space.

The entire drive system had 0.5" of ground clearance, and all of the areas that came into contact with the barrier were either wheels or delrin slides. We developed a pretty cool cutout (relief) in our sideplates that made it impossible for us to become high centered on the barrier. This was because as the robot's weight transferred over the barrier, the robot rode on an arc instead of a flat. This guaranteed that the robot could never stop on top of the barrier. You can see those cutouts in this picture:



-Brando

[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

[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

[Mr. Van]

Our 'bot this year used our standard "poor-team's" version of West Coast Drive - 6 wheels center drop, live axles, cantilevered wheels. We used the kit wheels this year after finding that they had the best traction on the Bridge - much better than any of the conveyer belt material we tried.

The axles are evenly spaced at 10" - yes, our 'bot frame perimeter was only 27" front to back (and also side to side). This meant no hangups on the Barrier. Center of gravity was very much on one end which allowed for hanging off of the end of the Bridge. We took only 18" of bridge space including bumpers - something that was crucial for our 3-bot balances.

In most years, we would try to center the cog, but this year we wanted to hang off the Bridge.

- Mr. Van
Coach, Robodox

[JesseK]

With a wide orientation, we determined that there was no difference between 4, 6 & 8 wheels for robots that did not traverse the bump, so we went with 4 wheels. At competition we realized that there was an additional tradeoff we hadn't accounted for: we couldn't be either outside bot on a triple balance.

The purpose of our drive train was to be incredibly simple and sprint from bridge to key. It used a custom frame with 4 CIMs/CIMple boxes through moderate gearing for a top speed of ~9.5 ft/s. It weighed ~21lbs. It was designed by the end of day 1 and the design only had minor modifications as we further specified what the 2-sided intake would do. The drive train was built by the end of week 1. That simplicity allowed us to focus on a shooter for more time, thus getting us further than ever in a build season. We also made it further than ever at champs (SF's). Would we have benefited from additional wheels to be able to hang off or cross the bump? The magic 8-ball is still unclear on that one.

So one cannot put "6WD vs 8WD" up for debate without debating orientation and context. 4WD wide-drive is just as effective as either as long as one accepts the tradeoffs.

[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.

[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!