Tube size and wall thickness....
I am in the design stages of building a chassi and plan to use 1 1/2" DOM .095 wall tubing for the main structure but am wondering what the accepted size and thickness is for the inner triangulation and such. I was thinking 1" or 1 1/4 .065 wall but don't know if I can go thinner or not? Thanks
It all depends on what you are doing... but looks way too heavy to me.
In general, .065 1" square and/or round is good for everywhere in the chassis. Highly loaded points and/or suspension mounts may need to be thicker wall. Use 4130. .049 can be used for triangulation.
The .095 DOM can be used for the cage structure/roll bar supports.
Back in the day ---
Almost everything that came from England was constructed of .065 x 1" mild steel. But it's heavy and you can get by in a lot of places with lighter material -- .058 or .049... Citation even uses .035 in some places, I believe.
Anyway, I regret that we initially used .065 chromoly for damned near everything in our FB build. If we were to do a Mark II, it would use much more thin-wall tubing. We have certainly learned that most of a tube's strength comes not from wall thickness but from its diameter.
Also, when questioning various materials actual strengths, there's nothing like doing some simple destructive tests. Hang weights on the end of a tube or composite held in a vice and see how much weight it takes before it yields. You learn so much in this tactile, no-math, made-for-dummies kinda way. You get a real "feel" for the materials -- not only about when they fail, but how they fail as well.
for what it's worth, i did the same thing. I used 1 1/2 .095 for the main and front hoop, and bracing. Everything else is 1".065 round or square, depending on where it is.
Yes, it is heavy. But I already had the dies to fit my bender in those sizes. I'll try to make up the weight by not eating for the next year or 2.
What wall thickness is generally used for the upper cockpit perimeter bar, as on a DB-1/6, Citation, etc. ?
1.125x08wt 4130
I use .049 wall for every thing that doesn't need to be bigger than that. The required roll bar structures are .080 4130 tubing. The cockpit surround is .080 1.125. If you need .058/.065 for a tube for strenght, then use 1/8 larger diameter and go for .049. In my car there are only 4 tubes other than the roll bars and braces that are larger than .049 and that is because of the requirements in the GCR. I have cars that have been raced for 15 years that have had no cracking problems.
I have done some sedan designs and I only use 16 or 14 guage on those. That is because my customers won't build lighter. 18 ga would do fine in thaos applications as well.
Thanks Rick & Steve. I'm trying to update my RF80 with a similar configuration, but is a bit difficult within the existing cockpit layout. As alluded to in that other thread, the frame of this year V/D is only 12" deep. My biggest hurdle seems to be getting the tube around the steering wheel while retaining hand clearance.
Steve, I assume the combination of your 1.125 "upper rail" and short diagonal fulfill the forward bracing requirement?
No. The forward bracing for the main roll hoop is the forward portion of the double hoop. That hoop extends downward to the top of the fuel cell and is the structure behind the upper portion of the seat. The shoulder harness fastens to the forward roll bar braces.Originally Posted by stephen wilson
The cockpit surround is a separate structure and effectively forms an x brace across the top of the cockpit opening. In the modeling of the chassis, the cockpit surround is one of the few places where I have to rely on the bending strength of a cluster joint for the strength I want, thus the .080 wall tubing.
There is or was an obscure rule that required diagonal bracing in the cockpit side to be of the same material as roll bar braces. I don't think many cars even meet this rule, if it is still effective. I have a piece of 1.0 x .083 running from the seat belt mount to the waist rail, about the drivers elbow, to meet that rule.
Years ago, Dave Weitzenhof, ran over a VD drive shaft that had not been cleared from the track after a crash. The drive shaft was kicked up by the front wing, removed the front suspension, entered the radiator opening, hit that heavy brace tube and ended up penetrating the radiator. The tube was intended to protect the driver's pelvic area in a crash. I guess it worked. Dave ended upside down off the last corner at RA.
Thanks for the clarification. Neil Robert's DB1 build site shows the large stresses placed on the cockpit surround tubing.
I have evolved my frame designs from the Z16 of the late'70s. The stress path through my frames is only similar to the DB1 as it has evolved. Both designs have followed a similar path of development. With the 1994 Citation, I was able to reduce the stress concentrations in the cockpit area that were characteristic of my earlier designs.
In the case of the 84/87 Citations, adding the tubes along the top of the cockpit doubled the chassis regidity. Adding the treatment we did in the cockpit opening further increased the overall chassis strength and reduced the stress concentrations at the dash. The DB1 has not taken the second step.
It's still there. For those whom are interested, here's the rule:
(From the FB rules)
The same rule exists in the FF/FC chassis rules ('86 on) and is the origin of the rule for FB.H.2. Chassis
A. ................
B. ................
C. The area between the upper and lower main frame tubes from the
front instrument/dash roll hoop bulkhead to the rear roll hoop bulkhead
shall be protected by...........................
1. ................
2. ...................................... The material used for the chassis braces in this area shall be at least equivalent to the roll hoop brace material.
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