adding to rollcage stiffness

[CSPTK]

I have a question about cages, and mods that actually work.

I have a Spec Miata (yeah, who doesnt!) that I'd like to stiffen a bit and I have a couple ideas.

Many people add a "Petty bar" from the main hoop near the drivers head, down to the base of the leading edge of the passenger door. Good proven design, 'nuff said. But I'd like to leave space for an occasional passenger seat.

I've also seen some cages built with a diagonal bar going from the main hoop near the drivers head, over to the top of the passenger a-pillar (along the roofline). Is this method effective in adding rigidity and tosional stiffness to the cage?

Also, I'm looking for thoughts to adding another bar going from the same point on the main hoop (near drivers head), diagonally down to the right rear, attaching near the top of the shock (tied into existing support).

These two ideas together seem, in theory to be ideal...but I'm not a cage builder....

Thoughts?

Thanks,

Tom
Finger Lakes Region

[joeg]

I refer to the first bar as an "anti-intrusion" bar. It would keep things out of the driver's compartment if you are laying on your side. Will also offer a degree of stiffness (as would trianguating any are of the cage structure).

They are actually required by some sanctioning bodies.

The traditional"petty Bar" is probably going to offer more stiffening.

As long as the second bar you describe goes to a legal mounting point(not beyond the eight you are allowed), it will help also. Once again, triangulation is the key.

Cheers.

[racer_tim]

Both of those work to add regidity. But you pay a price for the extra weight. I'd check how much weight you have to "play" with, then go from there.



------------------
Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit (Bent)
http://linerud.myvnc.com/racing/index.html

[Knestis]

I read an analysis a long time ago that suggested the roof diagonal that you describe can do a LOT for torsional rigidity.

I'm getting one as soon as the sunroof comes out - first thing in the transition from SSC to ITB.

K

[gsbaker]

Originally posted by Knestis:
I read an analysis a long time ago that suggested the roof diagonal that you describe can do a LOT for torsional rigidity.
K

Yup, this would vastly increase the torsional rigidity. Even better would be two roof diagonals that formed an X, making the rigidity symmetrical, but that's probably overkill and not worth the added weight.

(I can still hear my old structures professor lecturing on bridge design and triangulation.)

Of course, the perfect setup would be 4 Petty bars on the inside, but you could never get in the car.

------------------
Gregg Baker, P.E.
Isaac, LLC
http://www.isaacdirect.com

[kthomas]

That's weight up high. Probably do more harm than you'd ever notice benefits from. It ain't like you can adjust your remote reservior shocks to account for the extra stiffness!! Bwah Hah!!!!

Seriously, the guys up front aren't winning because of their superior cage- they're winning because they can drive. Now if you want to mod the cage to make it safer that's another discussion.

------------------
katman

[CSPTK]

Originally posted by kthomas:
Seriously, the guys up front aren't winning because of their superior cage- they're winning because they can drive.

Gee, really?! Thanks for that great leap in insight. I never would'a figured that out...work on the driver, the nut behind the wheel, yada yada yada.

Now if you want to mod the cage to make it safer that's another discussion.
[/B]

Well, consider this "another discussion" (the original one for those playing along). I never mentioned that I wanted to stiffen the cage so it would "help me win", improve my driving, etc. You misunderstood my original post, sorry if it wasnt clear. Simple explanation: I am underweight now. I can add (more) balast and still add to the cage and still make weight....I was looking for insight as to cage design, not "driver improvement"...but thanks anyway.

Tom

[whenry]

Since National decided to add 45# to the minimum weight of 1.6 SM to 2300# I was needing to add 32# myself. My research tells me that .095 tubing adds 1.44# per ft so you may not be able to add enough tubing to add the weight that you need.
I am adding several tubes that are low in the cage(one in trunk and one behind the seat) which will be about 15# and then "improve" my fire extinguisher and battery mounts to add the necessary pounds without resorting to ballast in the front passenger footwell. YMMV

[ddewhurst]

Tom, within the following attachment is a cage feast for anyones eyes. If I was in your situation & had a substantial cage I would first check out the fastest cars (Herr Coello Dewey)cages. I would rather add weight to balance the car.

www.bimmerworld.com/cages/cagepics.htm

Have Fun
David

[kthomas]

Originally posted by CSPTK:
Well, consider this "another discussion" (the original one for those playing along). I never mentioned that I wanted to stiffen the cage so it would "help me win", improve my driving, etc. You misunderstood my original post, sorry if it wasnt clear. Simple explanation: I am underweight now. I can add (more) balast and still add to the cage and still make weight....I was looking for insight as to cage design, not "driver improvement"...but thanks anyway.

Tom

Duh, my mistake. Your original post said you had a spec miata you "wanted to stiffen". I don't know why I didn't take that to be you needed to add weight. Use ballast, it will be lower than cage parts.

------------------
katman

[grjones1]

[quote]Originally posted by CSPTK:
Thoughts?

Tom,
If you don't already have them and if the Mazda accommodates them, consider adding vertical base plates to the car's frame rails up from the cage's base plates on the floor - adds safety, puts the weight low, helps rigidity, and it's legal.
GRJ

[chuck baader]

One thing to remember when adding bars..straight bars carry loads, bent bars are springs. That said, if you triangulate, do not run any bars from the center of one to the center of another. They shourld be from mounting gusset or tube intersection to gusset. As was said, one of the best ways is to take advantage of the 100in2 rule and use "L" shaped plates to mount the tubes to the floor. Then add triangulation from those points. The only problem is that the driver gets into the way of a really good cage Chuck

[badal]

Chip Herr's car has a copy of a Mitch Piper cage. He has a Petty bar, but it is mounted with huge rod ends, so it can be removed. This is one way to do it if you want room for an occasional passenger seat.

[Knestis]

Speaking of, has anyone else seen the Piper cages that don't have parallel bars from the main hoop back to the rear of the cars - only an 'X'?

Are any of those cars logbooked for SCCA club racing?

K

[Grumpy]

I have seen them and don't like them. I held a National Tech license at the time, but was locally overruled and logbooks were issued.

My contention is that at the point of the X there is ONLY ONE BAR.

Gussets at the X would help the situation. I would not sign the logbook. I would pass it to someone who thought it was legal.

This needs to have an "official" ruling.

The diagram in the GCR "suggests" the X bars as optional.

Additionally

6. Bracing:
The main roll hoop shall have two braces extending to the rear
attaching to the frame or chassis. Braces shall be attached as
near as possible to the top of the main hoop not more than six (6)
inches below the top and at an included angle of at least thirty
(30) degrees.

IT SAYS TWO BRACES.

in an X one brace does not connect the main hoop to the frame or the chassis.

Additionally With the picture of the roll cage for SS/IT it states:


IMPROVED TOURING DIAGONAL
Tube A-D of B-C is required
Note: Tubes A-F and/or B-E
are acceptable extra tubes
in addition to the
required tube.

Tubes AF and BE are the tubes that would form the X supports, and are clearly marked as acceptable EXTRA tubes.

I rest my case.

[This message has been edited by Grumpy (edited January 13, 2004).]

[Knestis]

I was thinking of you, Grumpy as I typed that. I don't remember if you were in on the conversation that I had with someone at one of the NASA HC races last year or if they just mentioned what you had said.

I think they are a really bad idea, FWIW, the cross-section of the structure being approximately the area of one tube at its narrowest point.

K

[Speed Raycer]

I built a cage with a guy (1st I'd ever done)and we did the single rear X (We had other supports running to the middle of the X from the main hoop). After personally slamming into a wall with the rear pass side, I could see the benifit of the design... but that was the only one. If it had been a rear collision... or any collision other than a direct hit on the wheel area, I believe that the cage may have been compromised.

The X is a great addition to the standard support braces, but not a replacement.

[bldn10]

This is a little off-topic but for the sake of conversation let's turn the discussion around for a moment. Safety aside, does all this extreme stiffening really make a car quicker? If so, how much? Hypothetically, take a well-built ITS car that does a 1:43 at Road Atlanta w/ a decent cage - all else being exactly equal, how much quicker could it be w/ the most elaborate triangulated super-stiffening cage? Then, assume you can adjust the suspension if necessary - quicker still? I'm asking if all the talk about stiffening cages (for speed) isn't perhaps overblown.

[chuck baader]

O.K., for the sake of discussion...without empirical testing we really don't know. However, the cages we are allowed to run in IT make tuning the suspension extremely difficult at best. What we are really doing is tuning the chassis flex with springs and shocks to make the car turn, etc. Personal experience from my Solo 1 days with a bolt in cage and now with a full weld in, springs are much softer now that the chassis is stiffer. Still we need, not only for handling but also for safety, to be able to tie the front shock towers to the cage and triangulate the front end. I'm not shure that would drop lap times, but I'm positive it would increase the integrity of the driver compartment. Chuck

[Geo]

Originally posted by bldn10:
This is a little off-topic but for the sake of conversation let's turn the discussion around for a moment. Safety aside, does all this extreme stiffening really make a car quicker? If so, how much? Hypothetically, take a well-built ITS car that does a 1:43 at Road Atlanta w/ a decent cage - all else being exactly equal, how much quicker could it be w/ the most elaborate triangulated super-stiffening cage? Then, assume you can adjust the suspension if necessary - quicker still? I'm asking if all the talk about stiffening cages (for speed) isn't perhaps overblown.

The purpose of stiffening the chassis is to make the suspension respond as it is supposed to. With a flexi-flier, you can do all you like to tune the suspension, and many times it won't do what you want/expect.

The Sentra SE-R is a great example. We are running 450/400 springs and the car is just on the understeer side of neutral. But, it had a bolt-in cage (currently being replaced and the car upgraded to EP). I also drove a road version at Summit Point Jeff Curcuit that rotated more than any SE-R I'd ever driven and that was with 450/275 springs. The rest of the suspension was pretty much the same. The difference is, the car is so flexible, the chassis has a spring rate below the 400 (essentially) that we were running.

Flexible chassis do weird things. That is why race car designers work so hard on rigidity. Does this make your car faster? Well, if you have a clue about chassis tuning, you should be more consistently faster.


------------------
George Roffe
Houston, TX
84 944 ITS car under construction
92 ITS Sentra SE-R occasionally borrowed
http://www.nissport.com