pic: 6wd Chassis Design

[Aren Siekmeier]

Originally Posted by Alex.q

But if I eliminated the axle brackets, I wouldn't be able to slide the axles to tension the chain. That was the idea behind using them as I did.

Can't you slot both sides of the box tube like you did in one side of c-channel?

Originally Posted by Alex.q

Would box tube really make it much more rigid?

Yes. At 1/8" wall it's less likely, but C-Channel will still collapse on the open side much much sooner than box tube. Think about just the profile of each of them and which would hold up better. Closed rectangle > open c.

Originally Posted by Alex.q

Do you mean a cam that pushes the chain down, or do you mean a cam that rotates to push the axle brackets farther out? Assuming you meant the second thing, then I was wondering why that keeps it tight. Couln't the force just cause the cam to rotate an pull the brackets together as you described before? (I'm not saying it doesn't work, just that I don't understand why it does)

You can do either. The principle is that of the inclined plane. The angle of the cam and the friction between it and whatever it moves are such that no force exerted back onto the cam will turn it, but you can still turn the cam. Thinking of just a block on an inclined plane, given a downward force, the static friction force <= mu*tan(theta)*mg, so if mu*tan(theta) is greater than 1, then the frictional force will always be sufficient to keep the block in place, no matter what force you exert straight down. However, if you push sideways (up the ramp), you don't get the same force of friction and can move the block farther up (or farther down) at will. The block moves when you want it to, and not at all otherwise.

[BJC]

Originally Posted by Alex.q

Do you mean a cam that pushes the chain down, or do you mean a cam that rotates to push the axle brackets farther out? Assuming you meant the second thing, then I was wondering why that keeps it tight. Couln't the force just cause the cam to rotate an pull the brackets together as you described before? (I'm not saying it doesn't work, just that I don't understand why it does)

I meant the second. The forces that are pulling the chain are linear, if your chain tensioner is also linearly tensioned then 100% of the forces on the chain will be pulling on those bolts. However, a cam uses some sort of asymetrical piece on a pivot (like a bolt) as the cam turns around the bolt the radius changes relative to a point. This means that the forces required to loosen that chain must be applyed at almost a 90degree angle to the linear forces from the chain. This means only a fraction of the forces on the chain are actually doing work to turn the cam back out. If you use a bolt as a pivot for your cam then you can tighten it with a lock nut to couneract the fractional amount of force trying to turn the cam out.

We did this last year using seemless chain with great results. Ofcourse if you were really worried about the cam comming loose you could always have another cam to keep the tenision on the first cam.

Nice start on the drivetrain. I'll say that I know that C channel seems like it will work, however there is a reason that teams who use WCD's use box tubing as opposed to C channel. Generally when muliple very good teams do the same thing multible years in a row it is because it works really well. In this case you would probably do well to join the crowd. A phrase that is repeated often on my team: "The best indicator of if a design is good is if the team who used it uses it again next year."

Good luck, Bryan

[nssheepster]

Um, I'm not a super genius at this, but my team has had problems in the past with motors overheating. Wouldn't your motors heat up really fast if they're right next to each other like that? How are you dispersing the heat? Or is it just not an issue?

[Billfred]

Originally Posted by nssheepster

Um, I'm not a super genius at this, but my team has had problems in the past with motors overheating. Wouldn't your motors heat up really fast if they're right next to each other like that? How are you dispersing the heat? Or is it just not an issue?

Running CIMs in this style is more or less standard practice; look no further than the kit's CIMple Box gearbox as evidence. Even the CIM-Sim gearbox designed for heat-a-holic Fisher-Price motors doesn't put too much daylight between the two. If you're not abusing them* (in time or in work demanded), it usually doesn't factor into the equation.

*That's not to say you wouldn't be abusing them in the context of an FRC event; many teams that go deep into eliminations find themselves resorting to ice packs or spray dusters (or I think 433 used dry ice one year) to get the temperatures down on Fisher-Price motors simply because of the tight turn-around times.

[thefro526]

Originally Posted by thefro526

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

(Ignore this if you are using a Toughbox Mini as Chris said in post #6.)

[theprgramerdude]

Originally Posted by thefro526

Your drive is going to be way too fast ( > 16.5 fps) with a cimple box direct driving a 6" wheel. You'll probably have issues with current draw as well due to under-gearing.

If they are frugal with weight and keep it at about 100 pounds w/ battery and bumpers like we did in '11, the Cimple boxes aren't a bad choice. The reduced weight makes it much easier to accelerate, and thus easier on the motors. The speed can be harder to control in tight spaces (probably not the best choice for most of LogoMotion), but it really helps when cruising down the field. Cross-field runs occur in about 4-5 seconds with it from stop, which can really save time

[Ether]

Originally Posted by theprgramerdude

If they are frugal with weight and keep it at about 100 pounds w/ battery and bumpers like we did in '11, the Cimple boxes aren't a bad choice. The reduced weight makes it much easier to accelerate, and thus easier on the motors. The speed can be harder to control in tight spaces (probably not the best choice for most of LogoMotion), but it really helps when cruising down the field. Cross-field runs occur in about 4-5 seconds with it from stop, which can really save time

Assuming you used 6" wheels (which is what the OP has), did you do an analysis to see how much faster cross-field runs from a dead stop would be* if you had more than just the CIMple box's 4.67:1 reduction?


* and less current draw, and quicker acceleration and easier control in tight spaces

[roystur44]

If you added two more c channels to the outside of the frame, lose the internal brackets and tied the channels into the front and back channels with some L brackets you have have a nice stiff frame.

[nssheepster]

Isn't it a 2-wheel design with 4 support wheels? I mean, 6 wheels, but only 2 drive wheels? Right?

[Ether]

Originally Posted by nssheepster

Isn't it a 2-wheel design with 4 support wheels? I mean, 6 wheels, but only 2 drive wheels? Right?

The front and rear wheels are also driven, via chain.

[nssheepster]

Ah, gotcha. Thanks. Senior year, fourth year on the team, but I'm stiiiiiilll new. Well, what is school for?