pic: High Traction Drivetrain Concept

[Osseus_Dominum]

This isn't exactly new; this wasn't an uncommon layout in 2010 and later when 8WDs started gaining traction.

Do hou have any pictures of 8 wheel drives like this? I haven't been able to find any.

Regarding your concern about lateral traction and being pushed or spun, I don't see how this would be any more at risk. An 8 colson wheel drivetrain with center drop will only ever have 4 colsons giving traction at any one time, just like this drivetrain. So why is this more susceptible to spins?

[EricH]

Originally Posted by Osseus_Dominum

Do hou have any pictures of 8 wheel drives like this? I haven't been able to find any.

Regarding your concern about lateral traction and being pushed or spun, I don't see how this would be any more at risk. An 8 colson wheel drivetrain with center drop will only ever have 4 colsons giving traction at any one time, just like this drivetrain. So why is this more susceptible to spins?

A "proper" setup for an 8-wheel with center drop will often have the outer 4 skimming the carpet. There's considerable sideways "traction" available if you're hit--more particularly if the impact rocks you "onto" the end being hit. With omnis, you have zero sideways traction available in that situation. It's not that they aren't skimming the carpet, it's that they're omnis. They're designed to allow free sideways motion.

Of course, maybe you shouldn't just trust my word for it. Weight up a proto-frame set up for 8WD (you can just have the 8 wheels on a frame) in one of your favorite configurations--you want 150 lb or some considerable fraction of that weight. Try to turn it by pushing it, hitting it, etc. Swap wheels and repeat. Quantitative data may be harder to get than qualitative, but you should be able to get an idea of how the robot will try to act.

[bstew]

Originally Posted by Osseus_Dominum

Do hou have any pictures of 8 wheel drives like this? I haven't been able to find any.

1986 used a drive like this in 2014. A video of just the drivetrain is located here. There are certainly many other examples, but this is the one that comes to my mind.

[jwfoss]

Mounting your bumpers as low as possible and limited rock is an excellent way to gain traction in a pushing match, and something 558 does as well. Where we differ is that if we are designing for maximum pushing force we believe that any wheel that touches the ground should be a traction wheel. Understand that any wheel that is in contact with the ground is providing traction, and increasing the number of contact points with the ground reduces the normal force on each wheel. Typically 558 will design an 8wd with a larger center to center distance between the middle wheels to increase scrub and make the robot both more stable at speed, and more resistant to spinning.

In basic terms, an 8wd robot with drop center will provide more pushing force than an 8wd with omnis on the outside all other things equal.

[billbo911]

By placing omni wheels in the front and rear, you are creating a drive train with what amounts to a very short wheelbase and very wide track.

This will make turning very easy, almost too easy. It may turn out to be a bit squirly to drive.

You may want to consider adding a gyro to assist with driving straight.

[philso]

It may be beneficial to consider what the normal force on each wheel would be and the behaviour of your chassis as it drives over a floor that is not perfectly flat. What happens if the four high-traction wheels in the middle have a slightly smaller diameter than the omni-wheels?

[S1LK0124]

I don't know if anybody else asked this but, what is the likelihood of it tearing up the carpet? I know that the drivetrain isn't like ours from this year in the sense that we used tank treads instead of a wheel-based drive, but our drivetrain had a lot of traction, and when combined with the high torque that we had, our treads tore up the carpet a few times, resulting in a few deactivations. We later combatted this by reducing the top speed of the robot so our deceleration wouldn't be as fast, and reducing the amount of torque. I don't know if you've already considered that, but I really like your design and I don't want it to run into trouble should you wind up using it!

[carpedav000]

Originally Posted by S1LK0124

I don't know if anybody else asked this but, what is the likelihood of it tearing up the carpet? I know that the drivetrain isn't like ours from this year in the sense that we used tank treads instead of a wheel-based drive, but our drivetrain had a lot of traction, and when combined with the high torque that we had, our treads tore up the carpet a few times, resulting in a few deactivations. We later combatted this by reducing the top speed of the robot so our deceleration wouldn't be as fast, and reducing the amount of torque. I don't know if you've already considered that, but I really like your design and I don't want it to run into trouble should you wind up using it!

I don't think this would neccessarily be an issue for this drivetrain. It isn't using roughtop/grooved/patterned wheels and the contact area on the carpet is much less than a treaded drivetrain (especially treaded drivetrains that have layouts identical to or similar to yours).

[S1LK0124]

Originally Posted by carpedav000

I don't think this would neccessarily be an issue for this drivetrain. It isn't using roughtop/grooved/patterned wheels and the contact area on the carpet is much less than a treaded drivetrain (especially treaded drivetrains that have layouts identical to or similar to yours).

Okay cool. I didn't think it would, but any question is worth asking. Especially with a train like this, it seems like a really well thought out idea, and I'd hate to see an unforeseen problem arise from this! Especially after this year, our team had a lot more unforeseen problems than in the past. We've solved these problems on the robot and in our building process, but it's still a really sad thing to see when you think you're gonna do good and something nobody expects shuts you down.

[carpedav000]

Originally Posted by S1LK0124

Okay cool. I didn't think it would, but any question is worth asking. Especially with a train like this, it seems like a really well thought out idea, and I'd hate to see an unforeseen problem arise from this! Especially after this year, our team had a lot more unforeseen problems than in the past. We've solved these problems on the robot and in our building process, but it's still a really sad thing to see when you think you're gonna do good and something nobody expects shuts you down.

Yeah I know the feeling

[jspatz1]

Originally Posted by bstew

1986 used a drive like this in 2014. A video of just the drivetrain is located here. There are certainly many other examples, but this is the one that comes to my mind.

We have used this drive arrangement multiple times.

[BrendanB]

Originally Posted by jspatz1

We have used this drive arrangement multiple times.

Care to share what has lead your team to choose this over a normal 8wd with all traction wheels or shrink it down to 6wd?

Do you keep all the wheels on the same plane or do you drop the center traction wheels slightly?

[Lil' Lavery]

Originally Posted by Osseus_Dominum

Do hou have any pictures of 8 wheel drives like this? I haven't been able to find any.

Regarding your concern about lateral traction and being pushed or spun, I don't see how this would be any more at risk. An 8 colson wheel drivetrain with center drop will only ever have 4 colsons giving traction at any one time, just like this drivetrain. So why is this more susceptible to spins?

The resistance your robot is going to be able to give is related to the amount of friction your robot has to resist that force. When it comes to lateral force, it's obvious that an omni wheel is going to present negligible friction compared to a traction wheel. Even ignoring the wheel placement for the moment (the outboard wheels will have a greater lever arm to your center of mass for resisting the applied moment), you can determine the friction from your wheels with the equation f=μN. For our purposes here, the normal force is equivalent to the weight placed on each wheel.

When the quantity of wheels in contact with the ground is increased, the normal force on each wheel is decreased (less weight on each wheel). In the case of a 8WD with 4 "drop center" wheels, you'd have the weight of the robot on 4 wheels. 100% of the robots weight would then be placed onto those 4 traction wheels. In your corner omni 8WD, the weight of the robot is distributed among 4 traction wheels and 4 omni wheels. Assuming an even distribution of weight, you'd have 50% of the weight of the robot placed on wheels with a high coefficient of friction, and 50% of the weight of the robot placed on wheels with a low coefficient of friction. As a result, the total friction your robot generates to resist that lateral force would be less than a robot with all of its weight placed upon high traction wheels.

Now, we've made quite a few assumptions to reach this point, and many things will end up being far more complicated in reality than I've presented here. For instance, resisting a spinning moment is going to be very dependent on wheel placement, drop height, and frame interactions. But I wanted to illustrate a general point. To phrase that point differently, the advantages your design has in terms of turning itself easily also serve to make it easier for other outside forces to turn. None of this is to say your design is poor, just that it will behave differently than a drop center drive. In some cases, team's have taken advantage of ultra-low resistance to turning and incorporated it into how they wanted their robot to behave.

[BrendanB]

Thanks for sharing your design! CD needs more of this.

Some quick comments. Unless you adjust the spacers on the Ballshifter third stage I don't believe there is enough room for #35 chain. Remember you want to have clearance on both sides of the sprocket for the chain.

With the 8wd if you switch your chain routes it will allow you to sneak the outer chains in a little making the shafts shorter and save a little real estate in your bellypan.

It looks really solid. Nice work!