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Originally Posted by magnetnerd
I don't see how the 6 wheel drive would be optimal. If the center two wheels are lower, then only 4 wheels are touching at one time. If this is true, then you actually have power going to the two wheels that aren't on the ground simply being thrown out the window. Why not have a 4 wheel drive system with the wheels very close together. Am I missing something here?
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1. The energy isn't being thrown out the window, because usually the efficiency of a wheel not on the ground is very high. Very little energy is being "lost". There isn't any real voodoo involved, the torque from the gearbox is just transmitted to the wheels under load (the ones touching the ground), and is mostly being turned into pushing force. The inefficiencies of this design aren't any greater or less than a normal gearbox. It is just as easy to make a high efficiency 6WD base, as a high efficiency 4WD base (which is still a tricky thing to accomplish).
Striving for high efficiency is something we've spent a lot of time looking at. Check out any drive team 65 has built. Those pesky Huskies sure know what they're doing. Being able to take the power from the motor, and put most of it to the floor is a huge advantage.
2. Putting 4 wheels close together would work. Except, it is highly unstable, and the robot is vulnerable to flipping (especially if it has a high CG). Think about it. When the robot comes to an abrupt stop, the CG of the robot tries to rotate around the furthest forward point touching the ground. The further forward this point, the less likely the robot is to flip over forwards.
6WD, with the middle wheel dropped allows for excellent turning, while still utilizing high traction wheels for ALL contact with the ground (which maximizes pushing force). It also allows for a looong wheelbase, which as mentioned above increases robot stability.
Problems with this design?
Well, another set of wheels is more weight. The means of driving these wheels is... more weight and more complexity in the drive. Also if the distance the middle wheel is dropped is "too much" then the robot will rock back and forth during driving and "waddle" across the field. 6WD can ve VERY VERY elegant when done correctly, but there are of course pitfalls to overcome. 60/254 overcame them very, very nicely.
I'm a fan,
John