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bjhines

Struck by lightening

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Not to turn this into a bitch fest, but I think most guys are just trying to be polite in asking you why you are doing this. Trouble is you seem pretty confident that you know what you are doing and get defensive pretty quickly.

 

But you asked for criticism so here goes.

 

I don't see how randomly punching a bunch of holes in sheet metal simply because you can do it as being a well thought out and exact way to strengthen you car. Beading and flanging a flat panel will make it more rigid and less susceptible to bending, but I would think in all cases putting beads and flanges in a solid sheet will be more rigid than the same sheet punched full of holes. Strength is an entirely different argument.

 

And there has to be a limit as to how many holes is too much. At some point it will be like making a car out of chain link fence. Your rear fuel cell mounts are a case in point. I would be willing to bet that if I put my 250 pounds of glorious manhood on that piece of cross tubing and bounced up and down it would bend. Probably not the loads that piece will ever see, but why not just use a thinner wall tube in the first place?

 

The other thing to consider is corrosion. Punching subframe connectors full of holes may be great fun, but on a street machine all of that will trap water. Not a good approach IMO.

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very good points.... PnW.... thank you...

 

I did a test on a section of the 14G square stock... I used a bunch of that stuff for a ladder rack at one point... I have several 20' sticks on the shelf... I drilled a 3 foot section similarly to what you see in the horizontal sections... I set it up on 2 "I" beams and tried to bend it... With my full weight jumping up and down on it I could not make it fail... however... a moderate blow with a 5pound mallet creased it and it failed easily after that... It seems that the tubing is incredibly ridgid until it is creased in a small spot.. then it will fail easily....

 

A full section without holes was MUCH harder to crease... more like contiuous pounding and jumping was required to make it bend.. even then the bent section had considerable stregnth...

 

 

notice that I only drilled the top and bottom of the longitudinal sections... the horizontal sections are shorter and I beleive they are less stressed in an accident..

 

the tubing is only to support the fuel cell... and to replace the floor sheet and tire well... none of this was very strong to begin with... and I do not want to make the ends of the car any stiffer in an accident... think crush zones....

 

the subframe connectors have holes in the top section that will be stiched to the rust free floor panel...(original sheet metal will add redundancy on top)

I also drilled access/drain holes in 2 locations on the bottom sides... these holes will aloow me access with the welder to better stich the ends to existing structure...

 

I am trying to consider all of these points... water entrapment is a large part of considerations.... in far more areas than just the subframe connectors...

 

The plan for hole punching follows original bends and beads... the rear cargo area is 2 layers thinck in areas... the top lip is untouched.. and many folks have cut out the entire panel...

 

I also have a full cage in the car... soo I am relying on the cage to add more stiffness than would be possible with sheet metal in the first place...

 

PnW.. I would like some criticizm on my door bar placement and the validity of my rocker tip/ to TC and frame rail supports..

240Zcageplans.jpg

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I don't know that you will be gaining anything at all with your bars to the T/C mount unless it is fully triangulated, unless you did something that I have thought of and that is to put a curved bar under the engine/tranny to connect the T/c pockets together.

FWIW, the old BSR and BRE car ran that tube from the end of the rocker up thru the inner fender and connected it to end of the tube that came thru the firewall to the strut tower.

 

I'm no expert on cage building, but your door bar idea is good on paper but not be strong unless it is a true 'X" bar that connects to the bottom of both the front downleg and the main hoop, that is, if from your drawing I'm correct in assuming you want to gain room to enter the car by ending the tube from the dash area to the rocker panel.

I think a better solution would be one of two ideas, either a bar from the front downtube that connects to the main hoop but with a lower mounting point on the front downtube (about where the bottom of the dash would be, think of an X with the | leg that starts at the same level as the meeting point). Or, run a horizontal tube just above the rocker from the main hoop to the front downtube and then just put a short tube down from the kink in the front tube downward and tying into both of the door bars, and you could then tie the lower horizontal into the rockers at multiple points(leave it as it is in your diagram, but tie the bottom of the leg into a horizontal leg that runs the full lenght of the rocker between the front and main hoops and is tied into the rockers).

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BTW I highly recomend you get a copy of a read:

Chassis Engineering by Herb Adams

and

Engineer to Win by Carroll Smith (any Carroll Smith books are great)

 

Carrol Smith especially talks about techniques for safely removing weight

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What about this tool:

punch_flare.jpg

You just drill a small dia. pilot hole and then attach the tool. It makes the bigger hole and flares it in one step. I would presume the cutting action would last longer than a holesaw. Worth the more than doubling in price?

 

Also, as to the strength discussion, here's my (novice) take: there are many areas of the unibody where the sheet is merely acting as a cross piece. In other words, you could cut out the metal entirely and replace it with an X. Think tube frame. The flanging adds strength by stabilizing the material along its weak axes. Like putting a crease in a piece of paper: No crease, it flops; with crease, it can hold its own weight and more.

 

You should check out the Grape software package for doing your cage layouts.

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Let me be the first one to say I don't have a ton of racing experience behind me, and most people on this site can out fab me so it's not like i'm some great mastermind here.

 

But it seems you could save more weight by going VERY insane with hole cutting and then reinforcing the remaining sheet metal with CF (formed over the metal then rivited to the sheet seems like the most logical way to do it to me). CF isn't TOO hard to make from what I've seen (compared to what you're already doing) and we all know about it's strengths. Sure it'd be adding weight, but you could remove more weight and it should definately add some serious rigidity.

 

Correct me if I'm wrong but isn't weight only most of the battle? Isn't it also partially about weight/rigidity ratio? Shouldn't two cars with the same weight and power be identical in cornering speeds unless rigidity is vastly different?

 

In the end though this might get horribly expensive. But it's a thought. Overall I love what you've done and I think it's a great idea for the extreme track minded.

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Correct me if I'm wrong but isn't weight only most of the battle? Isn't it also partially about weight/rigidity ratio? Shouldn't two cars with the same weight and power be identical in cornering speeds unless rigidity is vastly different?

Where the weight of a vehicle is can have a big impact on how the vehicle corners and accelerates. Center of gravity and polar moment will also contribute heavily on cornering. 20 lbs above center of gravity compared to same vehicle without it can make a difference in cornering G's.

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The other thing to consider is corrosion. Punching subframe connectors full of holes may be great fun, but on a street machine all of that will trap water. Not a good approach IMO.
fortunately for bj, this is not a street machine.

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I design aircraft structure/systems for a company called Epic Aircraft and we offer an experimental kit airplane that the entire structure is carbon-fiber.

 

Although we haven't made a part that looks anything like what bjhines has done, that doesn't mean that what he has done is wrong. Weight is critical in aircraft but strength and practicality is the primary goal.

 

Every part we design that is flight critical has a computer analysis for it as well practical test data to back it up. Further, all flight critical loads are calculated for SPECIFIC cases in aircraft flight and and are designed to maximize strength for THESE cases.

 

The point that the sub-structure in the rear for the fuel cell is a big point. Design for the loads that it will see......realistically. If you want parts to be the lightest they can be, they WILL be weak in ways that they are not intended to be used. Such as jumping on a sub-frame for a fuel tank.

 

This is where I work http://www.epicaircraft.com

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The fuel cell is a $3000 bladder and steel can made by Fuel Safe... this is an ASA Vortec Pro cell.... It is literally BOMB PROOF....

 

secondly.... I am not wheel to wheel "racing".. it is a time trails car... more than likely it will not hit anything very hard in my career... I only need a hoop to run....

 

Third.... I don't want to make the rear end of the car beyong the strut towers too stiff... think Hans device... and crush zones....

 

fourth... the tubing only replaces the sheet metal floorpan and spare tiure well.... factory frame rails are intact.... it also serves as a support for the fuel cell....

 

fith.... I went out side and stood with one foot in the middle of each bar and jumped on it... I figure... what better way to load test it... I weigh more than the cell and the cell will be held by straps nearer the corners... the bars dont budge.... I tried a failure test on a section similar to the lateral pieces.... it will fail suddenly if you crease a corner over a hole...

 

sixth... notice I only drilled one plane on the longitudinal tubes....

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sixth... notice I only drilled one plane on the longitudinal tubes....

 

IMHO, you should have taken weight out of the sides, rather than top or bottom. Think I-beam for holding in the cell. Still, it sounds like it's strong enough to me.

 

Cary

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You could take it to the next step in lightening. I guy worked with built 1/4 midgets for his daughter. He was very detail oriented and experimented with wild stuff. He's the reason the rule book is so thick kind of stuff. Anyway he would take every fastener off the car and take it to his lathe. Drill 1/3 of the inner diameter out (neutral axis) and faced off the top of the head off each bolt so there was just enough to grab with a wrench. He only used allen head since the heads are smaller. Also cut each fastener to exactly the required length. Said in all many hours later it saved a couple pounds.

 

Cameron

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re reading this you guys dont know anything about structural integrity. If you crease a piece of material ( EVENLY THE WHOLE LENGTH) it increases its's strength only if you are talking about tensile strength (on the leading edge) but will destroy your integrity from side impact, since a car has LOTS of flex when you go around a hard corner, your piece of material (given the right amount of stress) will fold over and crumple. Also it's VERY hard to flare a hole in the middle of a panel, or on the parts of the body that carry structural integrity, lathough yes it does help to a degree, but not as much as it's been explained thus far. The best thing to do is to cut out holes in parts of the car that carry little if ANY structural integrity, like the floor pans, trunk pan, brackets in the dash etc, instead of doing such EXTENSIVE drilling in your side panels, and core support members (they call them supports for a reason) which is where most of your integrity in the Z is.

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I can not weigh in on the structural integrity of what has been done.

 

But I do weigh in on the concept of saving every ounce you can to make a better racing machine.

 

I raced motorcycles for years, and wieght is everything. Earlier in the post, someone mentioned 10 lbs of weight = 1 HP, it is actually 7 lbs. So the theoretical 20 lbs we are talking here is more like 3 hp than 2 hp.

 

As was stated, this car has 160 HP. If you add 3 HP to that, it is a measurable increase of over 1%. In an effort to save weight, etc, you can "add" a large portion of your horsepower in a low output scenario such as this.

 

IMHO, if you do not understand why someone would do this, then keep it to yourself. Again, not knowing how flaring and holes add or remove strength, I will not commet on it, but as to the idea of saving every precious ounce when it comes to a race vehicle, I understand and back it all the way.

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Sport Compact Car did an awesome little article on the guy who basically created the Gran Turismo series. There were three or four cars that had a ton of stuff done to them. Not your basic bolt ons, but swiss cheesing the chassis, moving suspension mounting points around, etc... The times of the cars on a circuit dropped considerably. The S2000 lopped almost 10 seconds off its time at a certain course. I never even knew that people did this stuff...and I worked for a race team (they didn't design the Porsches, just raced them).

 

By flaring the hole, you add effective depth and make it stronger.

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