pic: drivetrain idea (comments please)

[Daniel_LaFleur]

Originally Posted by sdcantrell56

I would say it is a very safe blanket statement. I was waiting for someone to chime in with team 25 as an example. Yes they do not drop the center wheel and they do have 6 traction wheels but as far as I know, they and maybe 103 who share a similar drivetrain are the only teams that don't drop the center wheel and have had success. I would say unless you are team 25, which no one is and very few are at there level, then I would recommend dropping the center wheel. You are not sacrificing any performance and in fact are improving your ability to turn and not trip the breakers.

25, et al, are able to run 6WD skid steer without a dropped center wheel because they have an understanding of CoF and CoM. You need enough torque to overcome the friction of your wheels, and that means understanding where your CoM is, what your CoF is, and how much weight is on each wheel. Needless to say, you need to do a fair bit of math

And as far as not sacrificing anything with a lowered drivetrain ... everything is a tradeoff. Lowering the center wheel shortens your wheelbase and gives your oppenent a lever arm in which he can rotate your robot. The trick, for your opponent, is knowing when, and which side, that lever is on.

[sdcantrell56]

Another thing about 25 is that there wheels have a much lower Cof than wheels treaded with conveyor belting and they have custom patterns cut into them which most likely aids in the turning. I would say it is a safe bet to lower the center wheel unless you have a backup plan for when your robot doesn't turn. Additionally even if it does manage to turn the bucking mentioned is enough of a deterent to want to drop the center wheel. Imagine precisely aiming while the robot is bucking...not something I would want to deal with.

And as for the being easily spun with a lowered center wheel, as long as the drop is not substantial, you will not be easily turned by another robot and you will still be able to turn when you want to and push other robots.

[bmarick]

with our teams direct drive we simply did not need to drop the wheel down. but if you do drop it, i feel, that it allows for the rocking motion of your robot to be a deteriater of how well your robot will trurn when you need it. Also with a droped wheel you must make sure your whieght is truly centered or you completly loose advatage of that extra two wheels up front.

[EricH]

Originally Posted by bmarick

but if you do drop it, i feel, that it allows for the rocking motion of your robot to be a deteriater of how well your robot will trurn when you need it. Also with a droped wheel you must make sure your whieght is truly centered or you completly loose advatage of that extra two wheels up front.

Not necessarily. Look at 254, 968, 330, 60, 294 and a host of other teams that use 6WD "drop" style. They don't rock much, do they? If anything, the rock helps them.

Yes, you can put your weight dead center. But if you do, you lose most of 4 wheels, not 2. If you are balanced, only the center two are touching (if your drop is too large and you have perfect balance). When you move forward or backward, you get two more wheels. But as soon as you go the other way, you lose those two and get the other two. You keep doing this all match.

On the other hand, let's say you put your CG aft (or forward) of the center. You have four wheels in contact with the carpet and providing most of the power. You have two more that are not giving as much, but they are still contirbuting. When you rock the other way, you immediately rock back. This helps keep the robot upright (between the teams I named, I don't think more than one or two have stayed on flat the ground for more than a few seconds in years, and that was on going over sideways), as well as providing a baseline for the drivers on performance.


Oh yeah, and the rocking motion deteriorating the turn? Not hardly. The shorter wheelbase helps the turn, and with the CG at one end, you won't notice. With the CG in the middle, then you get some "fun" as first one end is on, then the other--faster turn in the middle, followed by a sudden slowdown as the end hits, repeat as necessary. Not good for fatigue in the parts.

[Daniel_LaFleur]

Originally Posted by sdcantrell56

And as for the being easily spun with a lowered center wheel, as long as the drop is not substantial, you will not be easily turned by another robot and you will still be able to turn when you want to and push other robots.

LOL. I never said easily turned, just that you'll have a shorter wheelbase and give your opponent an lever arm in which to turn you.

Again, everything is a tradeoff, and the key is to match the drivesystem to the game requirements.

[bmarick]

i meant that you have to make sure the wieght is balance because otherwise you could end up only have the four wheels touching the ground. i gess but that just makes the robot seem jerky to me because the front wheel will loose and gain tration on weather you have the robot acelerating or deacelrating.

[EricH]

Originally Posted by bmarick

i meant that you have to make sure the wieght is balance because otherwise you could end up only have the four wheels touching the ground. i gess but that just makes the robot seem jerky to me because the front wheel will loose and gain tration on weather you have the robot acelerating or deacelrating.

And there's a problem with only four wheels on the ground?

Actually, the front (or back, depending) will likely be in contact with the ground the whole time (with a small enough drop). It won't contribute as much as the rest, but it will still contribute.

Jerky? See that team list I put up in my last post? Go to the Blue Alliance and pull up some video of those teams, especially 60 and 254, who popularized the dropped-center designs.

My point was that by centering the weight precisely in the middle (which is hard to do in the first place), you lose much of the point of dropping the wheel, as the weight shifts become random and can easily throw off your drivers. By placing it strategically, you eliminate that and get a better robot.

[Cory]

Originally Posted by bmarick

i meant that you have to make sure the wieght is balance because otherwise you could end up only have the four wheels touching the ground. i gess but that just makes the robot seem jerky to me because the front wheel will loose and gain tration on weather you have the robot acelerating or deacelrating.

The entire point of dropping the center wheel is that you only have 4 wheels touching the ground at any point in time--this cuts your wheelbase length in half. When you rock, you still have 4 powered wheels in contact with the ground at any point in time. This is a good thing.

[DonRotolo]

Originally Posted by sdcantrell56

unless you are team 25, which no one is

Except team 25, of course

While several teams (25 and 103 are local examples) have excellent drivetrains with no drop, any team contemplating a six-wheel drivetrain had either better be prepared for a lot of hard development work OR have 1/8" or so of center wheel drop. Failure to do either may result in a robot that does not turn well.

The rocking effect is minimal, depending on your tread material (soft is less rock, hard is more), and we have not found it to be a serious issue, even using a stock KitBot frame.

Don

[R.C.]

Use blue nitrile tread from mcmaster, it doesn't rock as much and lasts about 2 full seasons.

[EricH]

Originally Posted by Don Rotolo

While several teams (25 and 103 are local examples) have excellent drivetrains with no drop, any team contemplating a six-wheel drivetrain had either better be prepared for a lot of hard development work OR have 1/8" or so of center wheel drop. Failure to do either may result in a robot that does not turn well.

OR put omnis at one end.

Omnis and drop together could be really interesting... I wouldn't do it. You'd get effectively a 2WD at time.