So I am looking to design an 8 wheel drivetrain for the offseason this year. It would be my first time designing a drivetrain and hopefully would be my teams first custom drivetrain (we have been using mainly the KOP drivetrain). I understand with a 6 wheel drivetrain, the centre wheel is dropped slightly to avoid having the drivetrain rock and shake when turning and to keep the wheels from wearing out too much. What I want to know is what would be the “centre wheel” of an 8 wheel drive. do I drop the four centre wheels at two different drop distances, thus still keeping only 4 wheels on the ground at any given time? I should note that all the wheels are 6 inches and the four corner wheels I plan to be omnis.
if the corners are omnis then u shouldnt have to have the center drop i think
If your four corner wheels are Omnis you don’t need to drop
If they aren’t omnis you drop the center 2 wheels on each side.
@GoalkeeperBoss beat me to it…
well, even if the corner omnis are there, I want to center drop for a specific reason. if for some reason we cant get to third HAB platform, but we have time to attempt a dukes of hazard onto the second, I want to have some slight center drop between all the wheels and hopefully get some “scoop” to the jump.
then just drop the center 2 at the same height
The main point of a center drop is to decrease your effective wheelbase. In order for your robot to turn well, you want your wheelbase shorter than your trackwidth (there was a good paper on CD that explained why this is, but I don’t know where it is). So if you use all traction wheels, drop the two center wheels (e.g. robonauts in 2016) and if you have corner omnis you don’t need a drop, but you can use one if you want. It also depends on the exact dimensions of your robot.
We’re using this same setup - 8 wheel WCD with corner omnis. Our center wheels are white HIGrips and they’re dropped about 1/16" below the omnis to bias our weight to the higher traction wheels and allow for some wear before everything levels out. Both center wheels are dropped an equal amount.
Yeah, in 2010 we dropped the centers of our 8 wheel drive by about 1/4" (or maybe 1/2") to help get up the ‘bumps’ running across the field that year. It worked great!
Just keep in mind that your center of gravity also matters… if it is shifted to the back then you’ll ride mostly on the rear two axles, which means that your center of rotation will shift, too. But if CoG is between the two middle axles, then no worries.
We on 1519 have built drive bases with 8WD in most years since about 2011. It has become our preferred drive base design, typically with pneumatic tires.
Presuming that your robot has the center of gravity between the center 4 wheels, yes, you want to have the 4 center wheels be lower in the frame than the 4 corner wheels, if you are using traction wheels for all 8 wheels. Personally, I prefer to think of this as raising the corner wheels, rather than dropping the center ones, but they’re exactly the same principle, regardless of what one calls it.
The amount of height difference needed depends upon the rigidity of the robot chassis frame and type of tires. With the pneumatic tires we use, the air pressure in the tires matters, too. I think we have about 3/8" of difference in wheel heights on our robot this year.
However, by putting omni wheels on the corners of your robot, you can have all 8 wheels in continuous contact with the floor – you do not need to (and probably shouldn’t) use any drop at all if the corner wheels are omnis.
We had 8WD with 4 traction wheels in the center and 4 omni wheels in the corners on our 2013 robot.
One of the things that I like about the 8WD is that at reasonable speeds, the robot basically just drives on the center 4 wheels, making a stable platform particularly during autonomous operations, without the robot tending to shift between tipped to the front and tipped to the back, as can often happen with 6WD robots.