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Unread 03-07-2008, 12:40
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Re: 6WD Chassis Stiffness vs. Maneuverability

Thanks. I really appreciate the great feedback so far.

Here are some observations of our chassis developement. There is a bit of a monkey wrench in the "CG" height thing.

During the build season, we took it for a test drive when it was still a bare chassis and balasted with sand bags up to competition weight. We used the sand bags to change for-aft weight distribution, but the "CG" height could not have been more than 7" off the floor. The same dynamic issues that we see in the fully prepped machine were there with the bare chassis.
The omnis were added to improve cornering. When we had omnis on all 4 corners the chassis predictably had a lot of whip. If the "CG" was not balanced for aft, it was extremely loose (unstable). When I ran the turning delta power calculations it was under 50 Watts (delta side to side) for a relatively high speed tight turn. This can be achieved by just the slightest release of inboard power with a 2 chip per side powertrain. When we had 2 KOP wheels on the back the chassis required a lot of reverse inside torque to turn even at high speeds (approximately 300 watts calculated for the same turn that was 50 watts above). This was correlated with motor temps as example 1 the inner motors stayed cool, while the motors were significantly hotter in example 2. For competition we went with 1 omni (inside rear) and 1 KOP outside rear, high traction wheels in the center and omnis up front. The calcualted turning power (again for similar turns) was about 150 watts. This set up has been good for our machine for this year, but would lack in 2007 games.
As JVN stated, I know it is possible to run with 6 high traction wheels with relatively little rock (I have seen these machines and talked to them about their "rock" or "drop"). It just isn't "working" for us. That is why we were thinking stiffness plays a big role.
("Working" is relative term here in that we were able to repeatably get 5 and sometimes 6 lines in Hybrid on clean runs, so it does handle pretty well. I am really looking for turning good into great refinement.)

Ken,
The 2 traction 4 omni set up performed similar to what the 2002 did (so I am told, I have only 4 years with the Bees). With the high speeds of Overdrive it was just a little too unstable.

Tom- Talk to Jim Z. and he can give you some really cool tips on how to drive straight. With a gyro and some tuning he can help with the straight equation.
 


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