Author Topic: Roll cage tubing diameter question.  (Read 6877 times)

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Offline wickedwagens

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Roll cage tubing diameter question.
« on: March 13, 2014, 01:47:16 PM »
My cage builder had a question and we wanted to make sure we have no complications down the road.  The car will run in G & H mainly, but I want the ability to run F also.  We are building the cage out of 1 5/8" and he wanted to know if every single bar has to be 1 5/8" or is it just the main hoop, bars that run to and down the A pillar, etc?  Can the door bars, the seat bar, dash bar, rear bars, etc be 1 1/2"?  Or do we have to run 1 5/8" for everything?

Also he was wondering if there is an SFI spec or other spec, that we can build to and not worry about tech?  Example:  Follow x.x spec and add the gussets, floor plates, ect from the rule book and you should be good to go.  He has read through both rule books I have and and was a little concerned about specifics on bar placement, ect.

Thanks,
Mark
1968 Volkswagen Karmann Ghia
G/CFALT  106.643 MPH  8/2016    G/CGALT  113.131 MPH  9/2016
G/CBGALT  169.462  MPH  8/2021    G/CBFALT  146.715  MPH  8/2017
G/CBGC  158.242  MPH  9/2017   H/CBGC  94.334 MPH  8/2018
H/CPRO  93.383 MPH  8/2018   H/CBFALT  101.282 MPH 9/2018
H/CBGALT  120.591 MPH 9/2018
Top Speed 170.726 MPH
36hp VW 100 mph club 9/2019 102.319 MPH

Offline dw230

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Re: Roll cage tubing diameter question.
« Reply #1 on: March 13, 2014, 02:12:14 PM »
When you contacted the tech people in section 16 of your rule book what was their answer?

DW
White Goose Bar - Where LSR is a lifestyle
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Offline Seldom Seen Slim

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Re: Roll cage tubing diameter question.
« Reply #2 on: March 13, 2014, 02:14:14 PM »
Good response, Dan.
Jon E. Wennerberg
 a/k/a Seldom Seen Slim
 Skandia, Michigan
 (that's way up north)
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Offline wickedwagens

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Re: Roll cage tubing diameter question.
« Reply #3 on: March 13, 2014, 02:29:43 PM »
Note taken,  I figured since this section of the forum was called "SCTA Rule questions" it was an ok place to ask this question and was possibly monitored by SCTA techies that might be able to answer for me.  I'll contact the appropriate people and update this thread for others with the same question.
1968 Volkswagen Karmann Ghia
G/CFALT  106.643 MPH  8/2016    G/CGALT  113.131 MPH  9/2016
G/CBGALT  169.462  MPH  8/2021    G/CBFALT  146.715  MPH  8/2017
G/CBGC  158.242  MPH  9/2017   H/CBGC  94.334 MPH  8/2018
H/CPRO  93.383 MPH  8/2018   H/CBFALT  101.282 MPH 9/2018
H/CBGALT  120.591 MPH 9/2018
Top Speed 170.726 MPH
36hp VW 100 mph club 9/2019 102.319 MPH

Offline Speed Limit 1000

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Re: Roll cage tubing diameter question.
« Reply #4 on: March 13, 2014, 03:05:22 PM »
One thing is the rule is for the minimum, some like to build stronger, it could save your life :cheers:
John Gowetski, red hat @ 221.183 MPH MSA Lakester, Bockscar #1000 60 ci normally aspirated w/N20

Offline jacksoni

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Re: Roll cage tubing diameter question.
« Reply #5 on: March 13, 2014, 04:33:29 PM »
Note taken,  I figured since this section of the forum was called "SCTA Rule questions" it was an ok place to ask this question and was possibly monitored by SCTA techies that might be able to answer for me.  I'll contact the appropriate people and update this thread for others with the same question.
Though it would be nice I think it can be said that SCTA officials, though some like Dan monitor/watch things here for their enjoyment,  will not "officially" respond so indeed, do as Dan suggested, contact them formally and directly. There is, however, a lot of good advice available (but not "official") such as that by Speed Limit 1000.

In contrast to nearly any other motorsport, weight at Bonneville is not a big deal with many competitors ballasting their cars, so why not put the bigger tubes in? Avoid the question entirely that way.  :cheers:

And it has been noted that if a competitor in tech mentions something like "NHRA or SFI or someone specifies this and that" they  may be met with comments like- "this is not that organization, meet our specs (or go elsewhere"). But couched in much more pleasant terms and without rancor of course.  :-P
Jack Iliff
 G/BGS-250.235 1987
 G/GC- 193.550 2021
  G/FAlt- 193.934 2021 (196.033 best)
 G/GMS-182.144 2019

Offline Richard 2

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Re: Roll cage tubing diameter question.
« Reply #6 on: March 13, 2014, 10:08:54 PM »
Wicked,
 When I built my roll cage I mocked it up then emailed pic's to the tech at the time. And then they made their suggestions. 
One was to use the same size tubing for bracing as the cage tubing.
Two was to add angle braces to the rear of the cage from the frame. Need to remember that the cage will be subject to hard impact 360 deg plus top and bottom, although there is nothing out on the salt to hit but the salt and signs.
Build the cage as if your child were going to be in it, advice I was given.
Richard 2
219.648 mph F/BFMR 2010 Record
4 cylinder Esslinger
Could of had a V8

Offline wickedwagens

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Re: Roll cage tubing diameter question.
« Reply #7 on: March 14, 2014, 01:34:33 AM »
I e-mailed the questions to Lee Kennedy and Steve Davies.  I'm in good hands with the cage.  My friend builds the top fuel chassis for Bill Miller of the BME racing team.  The lower frame rails behind the driver are 1 5/8", all around the driver and the rest of the frame is all 1 1/2" hence having more dies and a wider verity of bends for doing my cage.
1968 Volkswagen Karmann Ghia
G/CFALT  106.643 MPH  8/2016    G/CGALT  113.131 MPH  9/2016
G/CBGALT  169.462  MPH  8/2021    G/CBFALT  146.715  MPH  8/2017
G/CBGC  158.242  MPH  9/2017   H/CBGC  94.334 MPH  8/2018
H/CPRO  93.383 MPH  8/2018   H/CBFALT  101.282 MPH 9/2018
H/CBGALT  120.591 MPH 9/2018
Top Speed 170.726 MPH
36hp VW 100 mph club 9/2019 102.319 MPH

Offline jacksoni

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Re: Roll cage tubing diameter question.
« Reply #8 on: March 14, 2014, 08:44:51 AM »
Remember that: "the roll cage area is considered to extend from above and behind the driver's head to in front of the driver's feet and include both sides and bottom protection." From 3.B.1 bottom of page 28 2013 rule book (don't have new yet) So hoops of 1.625 are OK but 1.5" elements forward around the feet are not if you are running F/

I run G/ and entire cage is 1.75x0.134.  Don't skimp.
Jack Iliff
 G/BGS-250.235 1987
 G/GC- 193.550 2021
  G/FAlt- 193.934 2021 (196.033 best)
 G/GMS-182.144 2019

Offline redhotracing

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Re: Roll cage tubing diameter question.
« Reply #9 on: April 14, 2014, 07:52:24 AM »
Talk to Joe Timney about roll cages--- after seeing a car
slide on it's top and wear half through a 1.625" cage he
only uses 1.75" (or bigger) anymore. We went with 1.75"
.120 wall tubing for this reason.
Luke- Winston Salem, NC
Loring 2 Club- 201.252 (2010)
Ohio 2 Club- 203.712 (2013)

Offline panic

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Re: Roll cage tubing diameter question.
« Reply #10 on: April 14, 2014, 06:13:01 PM »
wear half through a 1.625" cage he only uses 1.75"

You might want to do some math on that. The difference in loaded surface area between the 2 radii isn't something you'd want to guarantee your life with.
To begin with, the initial contact is, of course, identical since it's (if memory serve, misquoting from plane geometry) "the locus of points between a curve and a flat plane, with zero area".
As load is applied, there is a really tiny difference between 1-5/8" and 1-3/4" surface area during the removal of the wall thickness (from depth zero to depth = wall thickness).
As the load passes through the lower wall into the interior, the contact surface area becomes almost identical (the wall thickness) until it reached the mid-line of each tube, at which point it is identical.

I suspect there are mechanical engineers here, but they've already given up trying to make the incoming tide go backward...

Offline manta22

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Re: Roll cage tubing diameter question.
« Reply #11 on: April 14, 2014, 06:56:20 PM »
The weight of the car makes a big difference; a 2,000lb car puts far less pressure on a roll cage tube when it's sliding along upside down that one that weighs 8,000lbs.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline redhotracing

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Re: Roll cage tubing diameter question.
« Reply #12 on: April 15, 2014, 07:58:17 AM »
As I said, only 1.75 or bigger. It didn't wear through
the entire upper cage tubing, but part of it is enough
to convince me that thicker diameter tubing is needed
if that ever happened. More upper cage structure, as
well... i.e. "headache bar" running longitudinally in the
center of the structure, sitting the driver far below the
structure and/or within a secondary cage... As PJ said,
the weight makes a big difference. You can wax poetic
on loaded surface area formulations, but the lesson is
the same- bigger tubing, thicker wall if available and
good structural design is better than building to the
Rule Book minimums.
Luke- Winston Salem, NC
Loring 2 Club- 201.252 (2010)
Ohio 2 Club- 203.712 (2013)

Offline panic

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Re: Roll cage tubing diameter question.
« Reply #13 on: April 20, 2014, 12:57:52 AM »
This is pretty significant, because simply adding a 1/3 circumferential over-lay to the outward facing surfaces of an existing tube as anti-scuff protection is pretty simple compared to a replacement, with much smaller weight penalty.
How much load it takes to buckle the cage (fold it down or inward on to the driver) is vastly greater affected by tube OD than wall thickness.

For those with incomplete plans - I hate to be Mr. Obvious, but I think some of what's been done successfully wasted tube to no effect (it was just as safe with less), and some others (especially long WB) have passed tech and leave some really weak areas.
The key?
It's something the rules (AFAIK) don't address at all: "how long is the longest unsupported span?" Anything to identify this tube, and stiffen it by triangulation (a similar tube leading from anywhere else to nearest the mid-point of the longest one) literally doubles or triples its penetration resistance with only a pound or 2 extra.
You don't have to fix everything, and the "longest tube" is certainly going to be in the first rank to collapse or trapezoid in a wreck.
For those with long memories, I once suggested an added chassis construction provision, that "... the longest unsupported span shall in all cases be less than (insert sample from safe & successful car), or in the alternative shall have this tube's OD equal or greater than (some exponential function of its length - math on request). It would affect very few cars, and catch some dangerous plans before they went forward.