6 Wheel Drive vs. 8 Wheel Drive

[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

[GRT808]

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.

[JVN]

Originally Posted by GRT808

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

[Aren Siekmeier]

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).

[Cory]

Originally Posted by GRT808

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.

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

[GRT808]

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.

[Joe G.]

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.

[Gray Adams]

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.

Originally Posted by Cory

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

If you don't mind, why did you guys use a 6wd in 2011 after having used 8wd in 2010?

[Cory]

Originally Posted by Gray Adams

If you don't mind, why did you guys use a 6wd in 2011 after having used 8wd in 2010?

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.

[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.

[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!