pic: Team 195 driving base 2007

[nuggetsyl]

Originally Posted by Nuttyman54

With any 4 wheel tank-drive wheelbase, long and narrow is much better at going straight, short and wide is better for turning. The problem with the short and wide is that it's easy to tip over in the forward and backwards because of the short wheelbase. The remedy is to put a middle set of wheels that is lower than the rest. This gives you essentially two sets of "short n' wide" wheelbases that you rock between. This makes you hard to tip, but also makes it much more efficient to turn, and gets rid of the characteristic "hop" that many high-traction, 4wd robots have.

another version of the 6wd solution is to keep all the wheels on the same level and replace one or both of the outboard sets with omnis (see 488's robot this year)

I would have to disagree with your logic. You do make a solid point, but by lowering the middle wheel you lose the whole point of making a 6 wheel drive robot. Think of it like tank treads but no messy tread part.

[Nuttyman54]

Originally Posted by nuggetsyl

I would have to disagree with your logic. You do make a solid point, but by lowering the middle wheel you lose the whole point of making a 6 wheel drive robot. Think of it like tank treads but no messy tread part.

I won't repeat all the arguments here, but you can find discussion about it here, and it would appear that a few of your mentors are quite familiar with 6wd systems. You'll also notice that most well-designed tank tread robots have a middle wheel that's slightly lower than the outer wheels, making it essentially a "lowered wheel" 6wd system

There is nothing here that says this is the ONLY way to do 6wd. Your team is living proof of that. I'm just trying to present the reasoning behind the lowered wheel. Many MANY teams use this design, so they can't all be wrong.

[Mike Norton]

but by lowering the middle wheel you lose the whole point of making a 6 wheel drive robot.

http://www.chiefdelphi.com/media/photos/26869

We only been doing wheel for two year so I am no expert but this system works. it is lowering the center wheel a little.

We did tank track before that and also had a center wheel

http://www.chiefdelphi.com/media/photos/18792

I think our robots has proven to have power

so what he is doing is a good way

[Lil' Lavery]

Originally Posted by nuggetsyl

Y lower the middle wheel. Makes no sence.

In order to ensure better turning, the following relationship is key.

longitudinal coefficient of friction x width of wheel base > lateral coefficient of friction (length of wheel base - 4((distance from center of area to center of mass)^2/length of wheel base))

Or simply put, there are a few ways to improve turning. Shorten the wheel base, widen the track width, reduce lateral friction while keeping longitudinal friction high, and moving the center of mass away from the center of the wheelbase.
By lowering the center wheel, you effectively create two 4WDs with a shared center wheel. Each of these has a wheelbase roughly 1/2 of that of the same drivetrain with coplanar wheels. Also, closer the center of mass of your robot is to the center wheel, further it is away from the center of area of both of the wheelbases. Additionally, the traction and width factors should be the same as if you had a coplanar 6WD, and you still achieve many of the other benefits of a 6WD.

(Thanks to Chris Hibner for this terrific white paper on the physics of turning with a 4WD).

[Arefin Bari]

Originally Posted by nuggetsyl

Y lower the middle wheel. Makes no sence.

Alright, I am going to use your team as an example. I can only assume and guess. I know you guys don't lower your middle wheels. Why? I have never been able to get a straight answer from anyone. You guys play around with your tread patterns a lot.

On the other hand, when you do a 6wd and don't lower your middle wheel, the robot is trying to turn on 6 different points. Depending on the friction you create with the wheel/tread you are using, it is very hard to turn unless you lower your middle wheel. While lowering the middle wheel, lots of team work on their CG. If the CG is low and the weight is distributed properly throughout the robot, while turning, the robot turns on 2 (middle) or 4 wheels (using only 4 different points to turn), creating less friction. At the same time, there are always 4 wheels touching the ground. I know wedgetop compresses, so if the weight is distributed properly, you get all 6 wheels touching the ground while going straight or back even if you lower your wheel. It also depends on the team how much they lower their middle wheels. I have seen teams do 1/16, 1/8, 3/16, 1/4, even 3/8 and lot of teams have been successful doing it too.

I sure would like to know how does a team turn without lowering their middle wheels. If there is anyone out there with a proper answer with reasonings behind it, please post.

Now back to topic...

As always, 195's chassis looks awesome. I am digging those wheels. Can't wait to see you guys in person. Let's not burn a victor out right before the final match this time. =)

[zander_108]

Originally Posted by Arefin Bari

Alright, I am going to use your team as an example. I can only assume and guess. I know you guys don't lower your middle wheels. Why? I have never been able to get a straight answer from anyone. You guys play around with your tread patterns a lot.

On the other hand, when you do a 6wd and don't lower your middle wheel, the robot is trying to turn on 6 different points. Depending on the friction you create with the wheel/tread you are using, it is very hard to turn unless you lower your middle wheel. While lowering the middle wheel, lots of team work on their CG. If the CG is low and the weight is distributed properly throughout the robot, while turning, the robot turns on 2 (middle) or 4 wheels (using only 4 different points to turn), creating less friction. At the same time, there are always 4 wheels touching the ground. I know wedgetop compresses, so if the weight is distributed properly, you get all 6 wheels touching the ground while going straight or back even if you lower your wheel. It also depends on the team how much they lower their middle wheels. I have seen teams do 1/16, 1/8, 3/16, 1/4, even 3/8 and lot of teams have been successful doing it too.

I sure would like to know how does a team turn without lowering their middle wheels. If there is anyone out there with a proper answer with reasonings behind it, please post.

Now back to topic...

As always, 195's chassis looks awesome. I am digging those wheels. Can't wait to see you guys in person. Let's not burn a victor out right before the final match this time. =)

I strongly agree with your argument. For your question, the Best example of a non-lowered center wheel robot that i can think of is 25 this year. And not only is it 6wd, its 8... If any members from 25 are reading... Enlighten us?

[meatmanek]

Originally Posted by Arefin Bari

I sure would like to know how does a team turn without lowering their middle wheels. If there is anyone out there with a proper answer with reasonings behind it, please post.

Team 868 did a 4 wheel, long drivetrain in 2005. How did we turn? Very slowly. But we did drive in a straight line very nicely, something that mecanum and 2WD drives aren't able to do without software help.

What I'd really like to see is an eight wheel drive, with 4 centralized drive wheels that are in contact with the ground all of the time, except when accelerating hard in one direction, or pushed from the top. The front 2 and back 2 would be raised up a bit (1/8" or so), be smaller (to reduce weight), and probably unpowered. This would have the benefits of a long drivetrain, be stable (none of the rocking back and forth I'm guessing you get with a 6WD), and have a more predictable center of turning.

[lukevanoort]

Originally Posted by Arefin Bari

I sure would like to know how does a team turn without lowering their middle wheels. If there is anyone out there with a proper answer with reasonings behind it, please post.

Psst, Arefin, read Dr. Joe's post here.