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Unread 02-04-2012, 12:28
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Nathan Streeter Nathan Streeter is offline
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FRC #1519 (Mechanical MAYHEM)
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Join Date: Feb 2008
Rookie Year: 2006
Location: Merrimack, NH
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Re: Moment of inertia and turning performance

From watching a few of your matches on youtube, the symptoms (rocking "hops" when turning, with size of rock roughly proportional to speed of turn) seem very much like those of a drivetrain with too much resistance to turning, the result of having too much traction at the corners of a drivebase with a long wheelbase and narrow track.

Some ways to reduce this issue:
- Reduce traction at corners by:
* Making corner tires less grippy (slick wheels, or something less extreme)
* Making corner tires move sideways easily (90 degree omnis)
* Reducing weight on corners (increasing "rocker" and/or stiffening frame so that sag doesn't eliminate "rocker")
* Reducing weight on corner wheels by centering weight over center wheels
- Shorten wheelbase and/or widen track to get a footprint more advantageous for turning

I wouldn't recommend a rocker greater than 1/8"... If putting the center wheels 1/8" lower isn't sufficient, look at your frame sag. Then consider the lateral grippiness of your corner tires and where your CoG is.

Note that you don't want the robot to turn without any resistance, as it'd be nearly uncontrollable; however, you don't want so much resistance that you're hopping at even fairly low-speed turns... Some teams prefer just using omnis at the corners, while others prefer using rocker, or a more advantageous wheelbase/track ratio. Personally, I prefer a combination of the rocker, CoG, and wheelbase/track ratio to provide a drive base that goes straight naturally but still turns well and won't be spun around by a defender.
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Student: 2006-2010 (#1519)
Mentor: 2011-Present (#1519)