6 wheel vs. 8 wheel WCD

[zinthorne]

I am wondering, what are the differences/pros and cons between a 6 wheel WCD and 8 wheel WCD. A 6 wheel WCD is lighter, an 8 wheel probably has more traction, but is there anything else?

[EricH]

6WDs are prone to rocking due to their dropped center wheel (barring other arrangements like omnis or odd CG arrangements). 8WDs are much less prone to rocking because both center wheels are dropped.

The "more traction" argument is one that will depend heavily on the specific drive systems involved. I suspect that some 6WDs have more traction than some 8WDs, as well as the other way around. Think about it this way: The same weight is being spread across more points of contact with the ground, reducing the normal force. OTOH, that's also more points where power can be applied and frictional effects that appear to raise the CoF can happen. The only way to make that claim definitively is to do a full analysis of two systems that are identical other than number of wheels (and associated power transmission).

[bkahl]

In an 8 wheel drive, the center four wheels are usually dropped, so there is always a stable platform. In a 6 wheel drive, the center two wheels are dropped making the robot rock sometimes. Many teams chose the 8 wheel drive design this year so the rocking didn't reduce the accuracy of a shot.

[EricH]

I forgot one other difference. It wasn't an issue the last couple of years, but it has been in the past, and that is traversing obstacles.

A 6WD will generally need somewhat more ground clearance than an 8WD to cross the same obstacle. This is because there is more space in between the wheels to get a "high-center", where the obstacle hits the frame and stops progress. Those aren't exactly fun to get out of, and often need a hit from a partner.

[evanperryg]

Originally Posted by EricH

I forgot one other difference. It wasn't an issue the last couple of years, but it has been in the past, and that is traversing obstacles.

A 6WD will generally need somewhat more ground clearance than an 8WD to cross the same obstacle. This is because there is more space in between the wheels to get a "high-center", where the obstacle hits the frame and stops progress. Those aren't exactly fun to get out of, and often need a hit from a partner.

Of course, this difference was hardly noticeable on the flat field we've had the last two years But it is a good point, if you go back and watch matches from 2010, almost everyone who could efficiently get over the bumps was an 8-wheel drive with some considerable clearance. (also note that I only know of one mecanum drivetrain that could get over the bumps in 2010.)

On a flat field, 6 and 8 wheel drives are almost exactly the same. 8 wheel drives do have the advantage when you look at the details, but the main difference between the two will be the quality of the driver.

[Electronica1]

Originally Posted by evanperryg

Of course, this difference was hardly noticeable on the flat field we've had the last two years But it is a good point, if you go back and watch matches from 2010, almost everyone who could efficiently get over the bumps was an 8-wheel drive with some considerable clearance. (also note that I only know of one mecanum drivetrain that could get over the bumps in 2010.)

On a flat field, 6 and 8 wheel drives are almost exactly the same. 8 wheel drives do have the advantage when you look at the details, but the main difference between the two will be the quality of the driver.

We ran a 6 wheel drive in 2010 and 2012 and got over both bumps just fine.

[evanperryg]

Originally Posted by Electronica1

We ran a 6 wheel drive in 2010 and 2012 and got over both bumps just fine.

Alright, I guess I should clarify: 8 wheel drives need less clearance to get over the same obstacle. In 2010, there were plenty of 6 wheel drives that could do it, but 8 wheel bases didn't need as large of a ground clearance.

[FrankJ]

Bomb Squad's swerve drive (4 wheel) didn't seem to have any problem getting over the bumps in 2012. Our kitbox 4 wheel drive didn't have a issue in 2010. That was with one motor per side. As said earlier, it is about ground clearance & design.

Calculating which drive system has the most friction is difficult. You have to have a model of the wheel carpet interface. Classical friction theory says they are all the same, but that model really doesn't apply completely. If you are running a drop center scheme you mostly have traction on just 4 wheels at a time. Welcome to the difference between science & engineering.

[Travis Schuh]

Originally Posted by evanperryg

On a flat field, 6 and 8 wheel drives are almost exactly the same. 8 wheel drives do have the advantage when you look at the details, but the main difference between the two will be the quality of the driver.

A well balanced 6wd will have better turning response/handling than a 8WD because it will have minimal wheel scrub forces on the outer wheels (due to there being very little weight on them). We find this to be a huge advantage and really helps with having an amazingly driven robot.

6wd also have fewer parts, which helps with weight and manufacturing.

[Joey Milia]

Originally Posted by Travis Schuh

A well balanced 6wd will have better turning response/handling than a 8WD because it will have minimal wheel scrub forces on the outer wheels (due to there being very little weight on them). We find this to be a huge advantage and really helps with having an amazingly driven robot.

6wd also have fewer parts, which helps with weight and manufacturing.

We've found this is the case too. Our recent 6wd have performed much better than out tests with 8wd.