You bring up a very interesting point.
It is difficult to obtain dies to bend tubing to the larger radius.
Secondly this is a complete reversal of the rules from 2004 which state the
centerline bend radius must be less than 5".
The change defies logic as well as creating a roll cage that is not as
structurally sound. I recommend posting a question to the rules committee
and seeing what they have to say. It could be just a typo, or it could be
next year's throttle cable. The link is
http://www.sae.org/students/minibajatqa.htm
It seems they are now concerned about putting kinks in the tubing by bending
it too sharply, but since there are many different tube diameters, wall
thickness's, materials and benders out there, the rules should simply state
that the tubing should not show any signs of buckling. Because of the nature
of plastic deformation, other than the stress riser effect, a tight bend is
no weaker than a large one unless the bend is so tight that buckling or a
drastic change in profile occurs.
A tight radius is more of a stress riser, but since we are building a truss,
it is best to keep the members as straight as possible so that they can
transfer the load between nodes without bending. By increasing the bend
radius each member is further from a straight line between nodes. It is now
subject to more bending stress during impact, which can lead to deformation
of the roll cage, and more stress applied to the welds at the diagonal
bracing. A tube will rarely break by itself, it is much more likely that it
will bend or the weld holding it will fracture. Many student done welds are
subject to cracking and may have questionable penetration, so the last thing
you want to do add stress to them.
Eric Hutchenreuther
Kettering University
----- Original Message -----
From: "Jon Hastie" <jhastie@coe.neu.edu>
To: <mini-baja@autox.team.net>
Sent: Thursday, September 09, 2004 5:39 PM
Subject: Bend radius
> Hey I was wondering if anybody else is having trouble with the new rule
> about bend radius.
>
> NOTE: When bent tubing is used as a roll cage member, the inside radius
> of the bend must be no less than 5", as measured by a circle gauge
> applied to the inside of the bend.
>
> This would require a 5.5" Center Line Radius bend for 1" tubing, and a
> 6" radius for anything larger.
> We were hoping to not have to build a new frame this year as it was
> built last year. As we used a 4.5" Center Line Radius, with 1-1/8"
> tube it looks like we probably will have to start over. Anyone else
> having this problem?
>
> Jon Hastie
> Northeastern University MiniBaja
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