# The Drive Train Question.

Hello, CD Community!

Many of you know me, whytheheckme, as a control/electrical/programming person. Today, I come to you as a mechanical noob. :]

Can someone, please, explain to me the deal with drive trains and wheels? I’m going to give a short description of what I believe to be true, and then I’d like everyone to jump on my back and explain to me all of the things that are wrong with everything that I’m saying (as if I’d expect any differently anyway ;))

• Using 4 (all driven or 2WD) traction wheels in a non-square configuration will cause the robot to skip around while turning, due to the fact that either the front or the back wheels can’t slide on the carpet.

• Using 2 traction wheels and 2 omniwheels (all driven or 2WD) in a non-square configuration will turn freely, as the omnis can swing.

• Using 4 driven traction wheels in a square configuration will allow free turning?

• Using 6 wheel drive somehow magically works?

• Using non-square traction tank treads somehow works

My understanding goes downhill from here.

Maybe one of my more mechanically-inclined friends can help me out here.
Jacob

I wouldn’t call myself a mechanical noob, but this was very informative and provides some good examples of previous bots that have used all of the named systems, as well as some you haven’t mentioned.

http://www.firstwiki.net/index.php/Drive_train

It’s worth the read, hope it helps.

With the 6wd systems, the center wheels are usually lower than the end wheels. This means that four wheels at one end of the robot is acting as a short wheelbase 4wd robot, since the wheels at the other end are off the floor, and the wheelbase length is shorter than the track width it turns easily.

Good tank tread systems have several rollers on each side, and the middle ones are lower than the ends, so it works the same way.

Using 4 (all driven or 2WD) traction wheels in a non-square configuration will cause the robot to skip around while turning, due to the fact that either the front or the back wheels can’t slide on the carpet.
That is correct, using 4 tractions wheels in a narrow (28"x38") robot. The traction is to great for the robot to counter and thus the robot will skip as you turn it.

Using 2 traction wheels and 2 omniwheels (all driven or 2WD) in a non-square configuration will turn freely, as the omnis can swing.
If you have the omni wheels on the same side (like traction in front, omni in rear) the side with the omni wheels will freely turn. This allows the robot to turn easily, but also get pushed easily from the side.

Using 4 driven traction wheels in a square configuration will allow free turning?
This is making the robot chassis in a 38x28 format. It is the same as above, if you are using four traction wheels you will be skipping around the field, but less then the narrow setup. This allows you turn a little bit easier then the narrow 4wd traction setup.

Using 6 wheel drive somehow magically works?
Using 6 wheel drive with the center wheel lowered basically turns the robot into 4wd when turning. When turning the weight of the robot will make it pivot on the center wheel, which will raise up the other two wheels in the system. The distance between the wheels when turning goes from the full length of the frame to half, thus the reason for quick manuverable turns for 6wd robots.

**Using non-square traction tank treads somehow works **
Tank tracks provide a lot of traction, they are great for not beaing pushed from side to side. The big downfall is turning, some teams would add a bogey wheel in the middle of the track and lower it like a 6wd system. Other teams like my first year robot, we added a bogey 5th wheel that raised the front half of our tracks and let us turn easily in high gear.

Hope this helps.

Using 4 (all driven or 2WD) traction wheels in a non-square configuration will cause the robot to skip around while turning, due to the fact that either the front or the back wheels can’t slide on the carpet. True enough dependent on robot weight, drive train torque, wheel base, and wheel choice.

[li]Using 2 traction wheels and 2 omniwheels (all driven or 2WD) in a non-square configuration will turn freely, as the omnis can swing. True, the omnis will allow decent turning without side friction so the hop in the above example will be eliminated.
[/li]
[li]Using 4 driven traction wheels in a square configuration will allow free turning? ??Not sure what you mean as “traction wheels”.
[/li]
[li]Using 6 wheel drive somehow magically works? False. Those that find this drive satisfies their design criteria will have the center wheel slightly below the others. All things being equal this will artificially shorten the wheel base or when the robot is tipped so that the robot rests on just the center wheels the wheel base is reduced to zero. Again trying to eliminate side friction in the drive system.
[/li]
[li]Using non-square traction tank treads somehow works False. Tank treads do a great (perhaps superior) job of driving in two directions (FWD/REV). However, the side friction in turns is the greatest of the drives listed. Teams have lowered the center boggies (as described above on six wheels) in an attempt to reduce the wheel base, which helps. Wide arcing turns are possible, turning on a dime (zero turning radius) produces incredible currents and attempts to force the treads off the sides of the mechanism. Some teams have enclosed the tread which helps but I have seen these drives eat the treads or drive them against the sides of the mechanism in such a way that it jams the tread.
[/li]I suggest that teams prototype the drive they are considering while playing the game.

Thanks for your replies! I think I really understand it now

~Jacob