I’m just curious if your only planning on powering the center 4 wheels, and if so what brought about that decision? As it is, it is well guarded against sideways pushes because it has traction in 8 locations, however if someone wanted to push from the front or back, you would only have to break traction on the center 4 wheels before they could move you at will.
the problem wouldn’t be the wheels that are powered but the weight of the robot… with 4 CIMs? it should have adequate pushability as long as theres enough mass to push with. powering all 8 wheels would just increase the turning problems that are imminent.
I’ve been thinking about a 6wd vs 8wd kit chassis drive train myself. looking at 418’s drive thie year. I was thinking for 8wd, have the middle four wheels separately driven by a cim and dewalt trans, then chained to the outer 4 wheels. We’ll see.
What I am planning on doing is driving the center 4 wheels off of the transmission and then chaining the front and back wheels to the center wheels. Also the 4 center wheels are lowered 1/8" to eliminate some of the turning issues.
This looks like a great start to a solid frame. Driving the back wheels is up to you in the case, I would try both setups. They won’t be doing much with the centered wheels being lowered also depending on what type of tread you use 1/8in offset may not be enough. I am going to recommend another thing with this design and that is use #25 chain although it needs to be lined up quite accurately it will save you alot of weight and is fine for FIRST applications. What size and materials were you looking at using for the side plates, wheels, etc?
I am planning on using 25 chain to connect the front and back wheels to the center wheels and then use 35 chain to connect the transmission to the middle wheels. The side plates are 1/4" thick and the wheels are 1.5" thick by 4" diameter. They are going to be custom machined to have a hex bore as well as the sprockets so that keys will not be necessary. I will try to post a pic of just the drive module to get a better idea. I also have posted a more updated frame that can be seen here as well as close up pictures of just the drive module.
New Framehttp://www.chiefdelphi.com/media/photos/28331?
Drive module
http://www.chiefdelphi.com/media/photos/28334
http://www.chiefdelphi.com/media/photos/28335
Thanks for all of the suggestions.
considering the weight. those transmission boxes are very heavy unless you use custom built transmission boxes like what team 254 (cheesypoofs) use which are ultra-lightweight. you’re probably looking at least about 40 pound chassis right here
The transmission plates are really not that heavy. They’re really pretty thin. The reason the cheesypoofs gearbox was so light was that they used all aluminum gears which saves a ton of weight over the steel ones. They definitely have an incredible gearbox that I got to see up close but I really don’t want to have to make a new gearbox. Also call me crazy, but I don’t feel that 40lbs is really that much for a solid 8 wheel drivetrain especially with transmissions. I would much rather have the weight in the drivetrain than up high. Also we kinda pride ourselves on never getting pushed and being able to push anyone. 
ask the designers of that gearbox (Travis Covington from 968, and others I forgot) and they admit that it was too extreme and they only did it as a senior project. Hobbing and cutting all the gears was too much work. They plan on using the gears from the AM 2-speed next year (which appears to be what you are using); The AM 2-speed is rather light for what the functionality it provides.
Yeah thats what I figured after looking at their transmission close up, its just too much work that we don’t really have time for and don’t have the resources for. I remember them saying how many hours were involved in it and it was some ridiculous amount. Basically what I am using is the Andymark transmission with the outer plate removed and the 4 bolts go directly into the sideplate of the frame. Also the frame has the holes for the bearings of the transmission machined into it. I definitely agree that the Andymarks are pretty light for what you get, and they were definitely one of the best improvements we made on our robot.