pic: sheet metal 6WD

[Chris is me]

Originally Posted by AdamHeard

The moment of intertia of the cross section is directly related to it's stiffness.

If you google, you'll see for a rectangle, the moment of inertia is 1/12bh^3 (base and height). Base being the side parallel to the axis the beam is being bent around.

So, a 5" tall .050" thick plate has an I = 1/12(5in)(.050in)^3 ~ 5x10^-5.

The same beam with two 1" flanges is that I plus the I of the two flanges. I = 1/12(.050)(1)^3= .004

So the beam with the two flanges has a total I of .004+.004+5x10^-5 ~.084.

.084 / (5x10^-5) ~150:1.

Even if I made a mistake in the math somewhere, this demonstrates the difference flanges make in design. I could go on for pages elaborating this, but it'd better to google and look yourself (key terms being beam, cross section, moment of inertia, etc...). You'll also then understand why I-beams are shaped the way they are.

I meant how much of a strength difference does cutting part of the flange out for the gearbox make? A poster said that it would be much stronger, and I wanted to know how much.

[AdamHeard]

Originally Posted by Chris is me

I meant how much of a strength difference does cutting part of the flange out for the gearbox make? A poster said that it would be much stronger, and I wanted to know how much.

I totally misread your question then.

Similar concepts apply, just inverse. The beam is substantially weaker due to that flange removal. Depending on how the gearbox is attached, the gearbox itself could add a lot of support to the beam where material is removed.

[548swimmer]

Originally Posted by AdamHeard

I totally misread your question then.

Similar concepts apply, just inverse. The beam is substantially weaker due to that flange removal. Depending on how the gearbox is attached, the gearbox itself could add a lot of support to the beam where material is removed.

Great explanation

The loss of a continual flange will not only decrease the second moment of area, but will also "chase" the stresses to the ends of the flange.

[Chris is me]

Originally Posted by 548swimmer

Great explanation

The loss of a continual flange will no only decrease the second moment of area, but also "chase" the stresses to the ends of the flange.

If that's the case, what's your basis for saying it'd be okay to remove the bottom flange entirely?

[548swimmer]

Originally Posted by Chris is me

If that's the case, what's your basis for saying it'd be okay to remove the bottom flange entirely?

Different design. The flange I said would be okay to remove was the bottom external flange. The top external flange used to mount bumpers will working in tandem with a properly mounted plate should, depending on material thickness, provide more than adequate structural integrity. That's why I said you could most likely remove that flange.

[Chris is me]

Originally Posted by 548swimmer

Different design. The flange I said would be okay to remove was the bottom external flange. The top external flange used to mount bumpers will working in tandem with a properly mounted plate should, depending on material thickness, provide more than adequate structural integrity. That's why I said you could most likely remove that flange.

Ah. Did you use any kind of analysis to come to this conclusion? I'm trying to learn all I can here.

[548swimmer]

Originally Posted by Chris is me

Ah. Did you use any kind of analysis to come to this conclusion? I'm trying to learn all I can here.

Not on this specific piece, though I re-designs our kicker plate (under roughly 300 lbf) to make it lighter and not deform. This was one of the things I frequently noticed.


This is with a continual .5 inch flange on the top and bottom, 100lbf, fixed constraints at the bolt holes.
http://picasaweb.google.com/11295163... 6928451693122

This with a 5 inch break in the top and bottom flanges, same load and constraints.
http://picasaweb.google.com/11295163... 6931950242546

[AdamHeard]

Another way to strengthen a frame such as this is adequate attachment between the inner and outer drive plates. This combines them into a much larger, and much stronger beam (a ~3-4" tall I-Beam).

If you understand the basic concepts, you'll quickly see that rather trivial and easy changes in design can cause appreciable increases in strength.

Kajeeven, this isn't a criticism of your design, I would definitely classify your attachment as adequate. It's just a convenient place to mention such concepts... Too much what on this forum and not enough why.

I do highly recommend adding a single baseplate that attaches to all members of the frame, or at least the full length of the front/back crossmembers and inner drive rails. This will greatly increase the rigidity of your frame. Think of it as an infinite amount of crossupports, providing strength in whatever direction is needed at the moment. Also provides a very low CG friendly electronics mount.