6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

This has been asked and answered hundreds of times over the history of CD and FIRST. Nothing has changed since then.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

Here's a place where you can start your search: http://www.chiefdelphi.com/forums/se...archid=4262217

There have been a ton of other threads about 6wd vs. 8wd before. I know. I probably started a lot of them. :P

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

You mean like this? http://www.chiefdelphi.com/forums/sh...ght=6wd+vs+8wd

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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).

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

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.

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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?

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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? ;)

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

"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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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!

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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)

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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 :)

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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)

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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!

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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?

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

Well, technically our 2007 robot was built with the option to take out the 4th wheel on each side and drive around on 6 wheels.

I think the best comparison between two robots has been our 2011 and our 2007. They were incredibly similar.

We experimented with the 8WD for one season and didn't really like it or think it was worth it, so we went back to 6WD.

Re: 6 Wheel Drive vs. 8 Wheel Drive
 

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.