Rollbar
 

Does most of the strength of the roll bar come from the material thickness in the tubing or the tubing diameter?

Tubing diameter is important because having a larger diameter allows wall thickness to decrease while maintaining equal rigidity. For bending the relationship is proportional to the difference between the 4th powers of the radii. (MOI ~ R^4-r^4)

What this means is that you can have 2 equally rigid tubes that have different dimensions and weights.

For example
Let's use a 2" tube with .25wall compared to a 2" tube with .5wall

2x.25 MOI ~ 0.684
2x.5 MOI ~ 0.9375

Now let's compare the 2x.25 tube with one of the same wall thickness but double the outside diameter (4x.25)

2x.25 MOI ~ 0.684
4x.25 MOI ~ 6.62

So this hopefully shows that diameter is very important in tubing rigidity. However, a huge diameter doesn't always win due to other constraints such as material cost or size constraints. Large wall thickness is nice to have so the tubing isn't as vulnerable to local deformation. Think of a beer can being strong until you give it a small dent in the side.

I think a lot of the strength would also come from the construction of the bar, as in diagonal bars and such.

But like he said, diameter would be an important factor. There must be some reason that the scca approved hard dog bars are a thicker diameter (1.75" tubing as opposed to 1.5") than their sport bars.

well i went out and measured the maximum clearance to still fit under the top for a rollbar and when sitting in the car the rollbar sits about 1mm above my head, how do I keep the rollbar above my head and where do I mount it to? I'm going to run 2 outer braces to the floor in the corners and 1 on either side of the center console, but where else should I mount? From what i'm looking at it doesn't seem to proved any forward/backward support. I could add some horizontal bars to the shelf but i have a no-zip glass window and can't see that happening.

cut hole in rear firewall and run support bar just in front of shock tower

thats where my glass lays down

and there is not enough clearance? damn that sux, thats your best bet
i guess measure and see how far you have to come forward

i might try to copy this setup of the boss frog and use the shelf as support

http://www.teammiata.com/mall/Boss_F...ssembly-tn.jpg