Re: pic: 4WD Concept
 

I didn't realize these were COTS bearing blocks when I made the post, which increases my confidence in them a lot.

That said, since the bearings are hex, I think there's at least an argument to be made to be paranoid about how heavily the bearings are loaded. This year, our bearings were spaced 1.75" apart (just happened to be, we didn't design around this problem) and we had no hex bearing failures. Other teams with bearings 1" apart or less had problems with some hex bearings exploding under regular drivetrain loads. Hopefully the manufacturing problems with hex bearings are cleared up for 2015 and we don't have to think about out of spec COTS parts when designing, but if you want to be paranoid it's something to consider.

An easier solution than going with a different bearing block would be to just turn that part of the shaft round and use round bearings (less prone to failure), or better yet use a dead axle bearing block (also available from 221) since your chain is on the same side as the wheels. But this might not even be a problem next year, so...

Re: pic: 4WD Concept
 

What Wheels were you using and where was you CG located?
We had no problems whatsoever with turning.

Re: pic: 4WD Concept
 

Our team ran something similar to this in 2010 (by accident....) but it actually turned out working very well. It allowed us to pivot more around the ball that we had possession of in the front of our robot. We had one of the best acquisition units that year at FLR and we could keep control of the ball in almost any situation.

Re: pic: 4WD Concept
 

I honestly don't know why this would occur. The turning scrub of this drive-train would be noticeably less than that of a 6WD.

These bearing blocks though designed to work similarly to 221 systems blocks are in fact somewhat custom. 4451 and I CAD up these versions ourselves and then CNC milled them ourselves. They used this version on their 2014 6WD drive-train without issue. When you say the bearings were spaced (distance) apart, are you referring to the distance between the two bearings inside the block?

Re: pic: 4WD Concept
 

I was referring to the distance between the bearings, as that determines how well the shaft is supported and how loaded the outer bearing is, but it seems like that's not a very big issue, especially if you've successfully done it before.

Re: pic: 4WD Concept
 

It would depend on the CG location. Let't take a couple of extremes and then this might make sense.
1. If the CG is really close to being over the top of the omin-end, then turning can be difficult as the traction wheels act a bit like a rudder.

2. If the CG is nearly directly over the top of the traction wheels, then there will be very little lateral from the omnis which can make it difficult to drive straight.

1 can be a very bothersome configuration as you would get pushed around easy without the benefits of being really maneuverable.

2 can be difficult to drive in a straight line, but should be very very maneuverable.

There is a white paper by Chris Hibner that you can use to set up a calculator and play around with values.

Re: pic: 4WD Concept
 

Talking to 118 at IRI about their 2013 drive, they had no chain tensioners, they relied on the walls of the tube to keep from throwing chain once it stretched out. IIRC they used custom 17T sprockets like the 221 robotics systems ones, they are essentially two sprockets on one hub.

Because you are only running one chain, you could use regular 17T sprockets inside the tube and they should fit fine (This would also make it so your wheels arent cantilevered so far, which is important because with only 4 wheels, each wheel is taking a lot of weight). A chain run that long though may not be as forgiving in terms of stretching, but unless you are in a district model I don't think you will play enough matches for it to matter.

As for bearing blocks, you are probably fine without them, but if you can get something COTS like the VexPro ones I definitely would. Even if the spacing between bearings isnt an issue, what you also have to look at is the support on the outside race of the bearing. Sheet metal teams (like 971) often times beef up areas around bearings with something as simple as an extra bit of sheet riveted on just so there is a thicker face for the bearing to sit on. This may not be critical, but with only 4 wheels taking the whole weight, I would play it safe.

Re: pic: 4WD Concept
 

2 KOP traction wheels and 2 omnis
As for CG, I'm not really sure as I'm not on the Build team, but I think it was in the middle of the robot, and not very low down, but it wasn't tipsy unless you actually tried to flip it over

Re: pic: 4WD Concept
 

As much as id like to admit im magic i hate to say its actually just practice. We had the gearbox direct driven to the traction wheels which seemed to be pretty effective.

Re: pic: 4WD Concept
 

Nope. Magic. CD is agreed . :P

Re: pic: 4WD Concept
 

Well much appreciated:D but we really had no problems with turning or tbones which helped a lot in matches against heavy defense.

Re: pic: 4WD Concept
 

This times x1000. Between 2010 and 2013 a certain unnamed MAR team built some of the worst drivetrains one could conceive, but their drive team was so good (so well practiced) that it didn't matter, and they had multiple event wins as well as IRI appearances.

Re: pic: 4WD Concept
 

Well I wouldn't consider this one of the worst drive trains. Done right it can be a very competitive drive train. With practice you can out manuver a swerve and have more pushing power then a low gear on an 8 wheel or 6 wheel tank. It just takes trial and error on which wheels you want to use and the driver and the set up of the gear box

Re: pic: 4WD Concept
 

If you have multiple sizes or want to save time, you can just use a chain length calculator: https://www.rbracing-rsr.com/chainlength.html

Re: pic: 4WD Concept
 

By no means do I think this is a bad drivetrain setup, I was only trying to assert that even if it was, any negative aspects can be mitigated with practice to the point where practice is arguably the most important thing. I really like how hard it is to lock this setup in a friction pin, as well as how light weight it is.

I don't really understand how it is supposed to be able to push better than a 6 or 8 wheel tank though, considering that both robots are allowed the same motors, but one has more torque and a higher CoF with the floor.

Edit: (not that pushing is necessarily important)