What are the advantages and disadvantages of a 6 wheel WCD in comparison with an 8 wheel WCD? My intuition says it’s a trade off between less weight as opposed to added stability, but there’s probably more to it and I’d like to know what those with more experience with these drive trains have to say.
This has been asked and answered hundreds of times over the history of CD and FIRST. Nothing has changed since then.
Here’s a place where you can start your search: http://www.chiefdelphi.com/forums/search.php?searchid=4262217
There have been a ton of other threads about 6wd vs. 8wd before. I know. I probably started a lot of them. 
However if you really want more information, or have a question, feel free to PM me about it. I’ve learned a lot about wheels and drivetrains, so I should be able to answer most stuff.
I spent 30 minutes searching for a thread on the topic before posting and couldn’t find one. If one exists and you could point me in the right direction that would be very much appreciated. Many of the threads I found addressed the advantages of 6WD and 8WD in comparison to other drive trains in general, but nothing seemed to be devoted to comparisons between the two.
One I found titled “6WD vs. 8WD” deals with drives involving omni-wheels and ones that aren’t WCDs, so I’m not particularly interested in those, but it still had useful information.
You mean like this? http://www.chiefdelphi.com/forums/showthread.php?t=88272&highlight=6wd+vs+8wd
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
We went with a 8WD 8" Blue Nitrile Tread 0.125" drop because we were worried about the barrier and how well our robot would cross it. We knew the configuration we chose would cross without a problem but after seeing some of the teams at CMP, I think we wish we looked a bit closer at 6WD because of its superior maneuverability (for us anyways). We had a lot of traction and pushing power with the 8WD though, which was useful for the balancing.
Hi Grant,
You said a lot there, but also not a lot. 
You described what configuration you chose, and what challenges you hoped it would overcome. Can you explain what specifically about your chosen configuration you thought would lend itself to overcoming those challenges? Do you have anymore insight into your process which lead to this decision? Was there prototyping involved - CAD or otherwise?
Can you explain why you think 6WD has superior maneuverability?
What characteristics of your drive do you think lended themselves to high traction and high pushing power (which are related)?
-John
2175 went 10 wheel with 4" plaction this year for a few reasons: As mentioned, you can hang more of your robot off with wheels closer together (specifically, with our CG just behind the wheels, you could get as far as half the robot hanging off, though we never did). Also, we need to not bottom out on the bump, but also have as low a cg as possible, so many small wheels did this for us. We dropped the center 3 on each side (6 total), so it behaved much like an uneven 8 wheel in terms of stability, which was nice for shooting, balancing, and just for stability in general. Not really many problems turning (contact rectangle 9" long by 28" wide), though it did stall out when attempting minor adjustments (the driver got good at it). I imagine a 6 wheel would do better there, since it can lift it’s end wheels off the ground in the turn. Also lots of contact surface, so some could argue more traction (though I haven’t seen or even sought data to support this).
We definitely did our share of CADing the wheel layout, checking for clearance, cutting cardboard profiles, making wooden wheels, etc.
Unfortunately, some other poor design (little review, not enough manpower, not enough time, etc.) meant we didn’t actually get our CG low enough to get over the bump (or do a lot of other things).
a well implemented 8WD should have superior maneuverability to a 6WD as the effective wheelbase is significantly shorter.
You described what configuration you chose, and what challenges you hoped it would overcome. Can you explain what specifically about your chosen configuration you thought would lend itself to overcoming those challenges? Do you have anymore insight into your process which lead to this decision? Was there prototyping involved - CAD or otherwise?
Can you explain why you think 6WD has superior maneuverability?
We chose that configuration after some testing with vex (we didn’t have a good test bot for 8wd configuration) models. We did do some testing with other prototypes such as a raised front idler wheel, but we really didn’t want that. CAD was extensively used we developed 4 iterations and how they would interact with the field before we settled on what we thought was the simplest and easiest to implement with the resources we had. We wanted to go with what we knew would work, and the 8" 8WD plaction we were very confident would work.
This was our very first year working with 8WD, so we didn’t have to much to work off of besides what peoples opinion were on CD and some of the robots offered to the public on FRC Designs (which really helped us a lot this year). We had worked with a .1875" 6WD drop for last years game and for our T-Shirt cannon (0.125" 6WD drop), and those are the easiest drive trains for our driver to control. The 8WD he said was a bit sluggish in turning (0.125" drop). That may also have been because of the wear on the center tread.
a well implemented 8WD should have superior maneuverability to a 6WD as the effective wheelbase is significantly shorter.
Yes I do know some teams implement very successful 8WD, such as the one you mentor, this was our first time and first experience with changing variables such as tread wear.
What characteristics of your drive do you think lended themselves to high traction and high pushing power (which are related)?
Nothing really concrete other than our observations.
1: It was very hard to push laterally.
2: It was quite easy to push other robots onto the bridge.
3: Pushing power due to the amount of traction we had & the ratio of our low gear.
4: We based it off of 359’s Breakaway robot which had impressive pushing force on the field.
We used a wide oriented 8wd with four inch wheels, with a slightly smaller gap between the dropped wheels. This system was selected almost entirely for bump traversal reasons, and to keep things simple. It allowed us to use quite a conventional chassis structure, with no cutouts or fancy chain routing to increase ground clearance. We figured that being a wide robot, plus other features of our robot that made us ideal for being the “middle” of a triple balance negated any need to think about hang-off from the bridge.
Our 8wd this year tends to rock very high in the air when direction is changed very quickly. Our 6wd from 2011 stays on the ground much more easily, but I haven’t driven it recently enough (or enough in general) to know if it turns better or worse.
If you don’t mind, why did you guys use a 6wd in 2011 after having used 8wd in 2010?
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.
Why
[spoiler]
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.[/spoiler]
We only used 8WD in 2010 to allow us to cross the bump. We would not have used it this year if there had not been another obstacle to traverse.
It adds a non-trivial amount of work for us to make more wheels, bearing housings, much more involved side rails (something like 8 operations each this year, due to the size of our mill), as well as a third unique shaft for our sponsor to run. There is also a weight penalty.
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
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.
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.
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:
http://sphotos.xx.fbcdn.net/hphotos-ash4/409032_10100353021933029_1814541_49186394_777840569_n.jpg
-Brando