4wd turning

[Dylan Danzeiser]

I've been reading about the turning abilities of 4 wheel drives, including the famous whitepaper, but none of the resources I could find address specifically longer, arcing turns while the robot is moving forwards as opposed to turning in place. Is this easier, harder, or of similar difficulty as turning in place if you happen to have a longer wheelbase than normal?

[Kevin Phan]

What you're describing seems to me is what I all a swing turn. When I was teaching fll kids about programming this turn, I find that the turn is harder since one side is not powered or powered slightly and the other side of the robot is doing all the work. To answer about the difficulty turning with a longer wheelbase, I'm not sure, the bigger factor in my experience is the coefficient of friction on the wheels.

[Calvin Hartley]

What exactly do you mean by easier or harder? In regards to programming? Driving? Stress on the robot? A little more context will do great things in accessing the wealth of knowledge here on CD.

[Dylan Danzeiser]

I've read a bunch about how the wheelbase has to be longer than the track width for a 4wd to effectively turn, assuming a relatively equal lateral and inline coefficient of friction. However, these talked about turning in place, not turning while moving forwards, so I was wondering if there would be much of a difference.

[Woolly]

Originally Posted by Dylan Danzeiser

I've read a bunch about how the wheelbase has to be longer than the track width for a 4wd to effectively turn, assuming a relatively equal lateral and inline coefficient of friction. However, these talked about turning in place, not turning while moving forwards, so I was wondering if there would be much of a difference.

I think you've got the inverse relationship there regarding wheelbase vs. track width there. Otherwise, turning while moving forwards or backwards while be similarly more difficult than other drive trains, but not as difficult as turning with a 4WD at a stop. Usually, this means that a 4WD will need room to swing around to pull a 180, which is a pain for the driver to plan for out on the field, and makes defense against said 4WD much easier.
Also, this means that designs that force the driver to regularly re-orient the robot, like team 1986's 2013 and 2014 robots, absolutely requires a drive train that is easy to rotate. Preferably, it should be able to rotate without the turn profile being too much larger than the robot itself, as the center of rotation should be as close to the center of the robot as possible.

[Ether]

Originally Posted by Woolly

...turning while moving forwards or backwards while be similarly more difficult than other drive trains, but not as difficult as turning with a 4WD at a stop.

^^Yes.

In fact, a robot that will not turn in-place at all will likely be able to turn given a large enough turning radius.

[Abhishek R]

Originally Posted by Monochron

He actually has it correct, really wide robots turn much easier than really long robots.

That's what he's saying. The person he quoted said wheelbase needs to be longer than the track width.

[Ether]

Originally Posted by Dylan Danzeiser

I've read a bunch about how the wheelbase has to be longer than the track width for a 4wd to effectively turn

Originally Posted by Woolly

I think you've got the inverse relationship there regarding wheelbase vs. track width there

Originally Posted by Monochron

He actually has it correct, really wide robots turn much easier than really long robots.

Trackwidth is the width of the robot (distance between front wheels). Wheelbase is the length of the robot (distance from front wheels to back wheels).