4 vs 6 wheels

[rytcd]

are there any real advantages to having an extra 2 wheels? pros and cons? we dont see much of a difference in descriptions of their performances. (we are a rookie team)

[O'Sancheski]

team paragon is doing a six wheel drive robot... the two middle wheels are high traction IFI wheels... and our four outside wheels are the 6" AndyMarks that came in the KOP... each side drive is powered by our custom two speed pneumatic shifters... putting the power with two chains to drive the three wheels on each side.

[lbarger]

Four wheel drive, skid steer, robots take a lot of power to turn, especially if it uses high traction wheels. The advantage of 6 wheels is that the corner wheels bare less of the weight, making it easier to turn. Most 6 wheel configurations use either lower friction wheels on the corner or the center wheels are slightly lower than the corner wheels (1/16 - 1/8") to carry a larger portion of the weight. The 'wobble' from the lower wheel is almost unnoticed on carpet.

The disadvantage of 6 wheel robots this year is they must match the 45 degree incline of the bump to go over it (if you choose to do so). A long wheel base 4 wheel robot can cross the bump with less total tilt. You do have to watch you total ground clearance as you can high center if it the frame is too low.

[hipsterjr]

We think it adds some much of an advantage that we doubled it and going with an 8 wheel design! Unless compensated for, rigid 6 wheel designs tend to slam down when coming over the crest of the bump. We tested two of our past 6 wheel robots that were VERY solid, and wound up breaking a axle on one of them coming down the back side.

[Wayne TenBrink]

Aother option for 6WD (or 8) is to elevate all center wheels so that they only come into play when crossing the bump. By having them elevated, you don't get the traction benefit on flat ground, but you can eliminate the "high centering" while still reducing the angle that the chassis must reach while crossing the bump - you go over smoother and are less likely to tip because you don't rock forward so much when coming off the top of the bump.

[joek]

well, it's harder to strafe with 6 wheels, and you will have a harder time getting over the bump W/O crashing while you go over it, and you can't strafe on the bump with 6 wheels.

[Chris is me]

Originally Posted by rytcd

are there any real advantages to having an extra 2 wheels? pros and cons? we dont see much of a difference in descriptions of their performances. (we are a rookie team)

All of this assumes a long robot. A wide robot can get away with 4wd high traction wheels with the ability to turn without having to use omni wheels.

Normal Year: Four wheel drive robots with high traction wheels on all four spots have massive problems turning correctly. Adding a 5th and 6th wheel in the center helps with this a little bit. On top of that, "dropping" the center wheels 1/8" so that at any one time, only four wheels are touching the ground will make turning even easier, since the robot's wheel base will be wider than it is long. This allows robots to turn easily while still maintaining a 100% high traction drive for pushing and dealing with defense, while a four wheel drive would need some lower traction or omni wheels, which makes them vulnerable to defense on one side of their drivetrain.

This year: The debate over four versus more than four wheels is more complex. As the Kitbot is built now, a four wheel drive with big wheels is more able to deal with the bump than a 6+ wheel drive, since half of the robot "falls down" before the other half when 6+ wheel systems have all of the robot "faling down" at once. Barring an unconventional design, you've reached a design tradeoff. If you can't make a cool suspension or something, do you want better, more safe bump climbing ability at the expense of half of your drive having less traction? Or do you want a full, high traction drive that has to deal with more forces when it comes to bump traversing?

This is pushing all active suspensions, etc. aside. This particular tradeoff is one that bothered me for the first whole week of build season, and I hope the decisions made were the best.

[waialua359]

Originally Posted by Wayne TenBrink

Aother option for 6WD (or 8) is to elevate all center wheels so that they only come into play when crossing the bump. By having them elevated, you don't get the traction benefit on flat ground, but you can eliminate the "high centering" while still reducing the angle that the chassis must reach while crossing the bump - you go over smoother and are less likely to tip because you don't rock forward so much when coming off the top of the bump.

This is an interesting post. We are weighing to what degree the focus needs to be on one or the other.
How much time will be spent going over a bump, if any?
In the game a soccer, defense is a huge part of the game.
Will teams spend the majority of the time in contact with opposing robots?
If so, will traction be important?

For us, an 8wd addresses all of the concerns.

[I.U.man_22]

At the beginning of our season we set out to have a six wheel drive. We built it and tested it. We found that the center wheels on the six wheel drives actually pushed us back over causing us to flip over and never actually go over the bump. We assembled the kit bot chassis with four wheels and it went right over the bump with out any problems.

On the both drives we used the andymark traction wheels.

2360 POWER Storm Robotics

[Chris is me]

Originally Posted by I.U.man_22

At the beginning of our season we set out to have a six wheel drive. We built it and tested it. We found that the center wheels on the six wheel drives actually pushed us back over causing us to flip over and never actually go over the bump. We assembled the kit bot chassis with four wheels and it went right over the bump with out any problems.

On the both drives we used the andymark traction wheels.

Were all 6 wheels powered? Where was your center of gravity located roughly?

[andy174drive]

Originally Posted by hipsterjr

We think it adds some much of an advantage that we doubled it and going with an 8 wheel design! Unless compensated for, rigid 6 wheel designs tend to slam down when coming over the crest of the bump. We tested two of our past 6 wheel robots that were VERY solid, and wound up breaking a axle on one of them coming down the back side.

An 8 wheeled robot is pretty sweet and if you work it right really effective. Not speaking from experience or anything. I've seen a few videos of teams with both four and six wheel drive systems that look kinda scary going over the bump. So far our wheels actually work with the bump for a smooth ride. I personally don't wanna shake the robot to pieces flying over the bumps.

[ChrisP]

Generally speaking, 4wd goes over the bump easier, but has more problems turning in place, while 6wd has more trouble going over the bump, but turns fairly well.
Maybe you could find a compromise between the two?

[fuzzy1718]

most teams typically use a 6wd system, assuming the long configuration. If it is the wide configuration 4 wheels are all that is needed. A word of caution though, my team assembled the kit bot in the wide configuration (4wd) unless you mount your battery 1/2 way below the frame dead center you will flip on the bump. As far as long configuration goes there are some tradeoffs:
4wd:
-high CG due to the need for clearence. High CG = more likely to tip
-Less room for kicker close to ground
-Less traction = less pushing power
6+wd:
+Low CG
+More space for kicker close to ground
+More pussing power. Due to the increased traction caused by more high traction wheels
-cant strattle the bump
-more strain on drivtrain do to "crash down" when crossing the bump

I am a little biased because of what my team has kicked around. Anyone else have any pros or cons to add?

[machetemonkey]

our robot uses 4 large (like, very large) wheels with not tons of traction (I'd say a bit less than the KoP grippy wheels), and we use the long-wheelbase configuration. We've had trouble getting the robot to turn at all, however most of this is, we believe, due to our loose chain tensioners which defeated their purpose by skipping whenever the drivetrain was under stress. haha.

Anyways, my point is to not underestimate any given issue you might have. You could say "meh, it may not work well but at least it'll work" when designing and find out it doesn't work at all.

[Grim Tuesday]

I think that both 4 and 6 wheel drive is not as good as tanks treads (what we are using), since with treads, you get enormous amounts of traction, plus easy turning, AND lots of surface area when de-bumping.