How much stiffer with solid or tied in roof? Options

[sixnotfour]

Posted on Dec 15 2004, 09:40 AM
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Andy... awesome picture.  Where the hell did you get that? (we know you didn't take it because, a. it's in focus, and b. you'd probably be about 1 year old when that was taken)  

(IMG:style_emoticons/default/agree.gif) Great pic . , link? Any more , Havent seen that one before.

[TravisNeff]

What about glassing in some square or rectangular stock to the targa top (or just a stiffening U/square channel and or X brace with glass and matt alone), one on each side by the window seal, fore to aft. to give some spine to the top. The front latch is pretty stout to pull the top to the windshield frame (compared to the rear). On the rear change out the hook and pin latch mechanism for a straight down pin type latch (like the rear trunk), then you could set the pressure to preload the rear seal, or even have the pins be adjustable fore aft within a channel to preload push or pull needed to keep things square. That would hold it a bit more steady than what is in there now and could be made to be fairly light. Obviously not a super strong approach, but could use what's there with a couple tweaks for a super duty street car.

my windshield frame will wiggle easily when I grab it and pull it back and forth, I too was wanting to reinforce the A pillar area to help. If I do this, what should I use for material, some flat bar stock bent and welded, some heavy gauge sheet stock with some beads?

What about where the cowl meets the windshield base, can this use some extra support, then tie to the upright supports as I talk about above (like using 1", 1/8 flat bar stock), from windshield support to windshield support, effectively making a "U". You could bend the bar stock to get close to the windsheild base and follow the contour, wrap around the corner to the upright supports. This could be done in the front trunk compartment, or under the dash in the cabin - so it would be hidden.

[Brett W]

Guys you are missing the main problem. Not enough complete box sections in the chassis. If you can complete more boxes, increase the cross section of existing boxes, you will gain eponentially more stiffness than you would if started making all of these band-aids.

Look at the early aluminum Monocoques. They used very few tubes and many panels. You could take the factory tunnel out and put in one of a much larger cross section. You could increase the size of the factoy rocker boxes. Which are significantly stiffer than just about anything your are going to add or replace it with. Make a huge backbone through the car. This is probably the best thing you could do for the strength of your chassis.

You could build a very light weight frame of steel tubing for a seat that basically fills up the interior of the car. Make places to two seating areas and skin the whole thing in aluminum. Attach your frame to the basic 914 chassis and bond and rivet the aluminum sheeting to the frame and the chassis. This would increase your torsional stiffness incredibly. Yes it will be a massive amount of work. Would it be worth it. Absolutely.

[Mueller]

What about glassing in some square or rectangular stock to the targa top (or just a stiffening U/square channel and or X brace with glass and matt alone), one on each side by the window seal, fore to aft. to give some spine to the top. The front latch is pretty stout to pull the top to the windshield frame (compared to the rear). On the rear change out the hook and pin latch mechanism for a straight down pin type latch (like the rear trunk), then you could set the pressure to preload the rear seal, or even have the pins be adjustable fore aft within a channel to preload push or pull needed to keep things square. That would hold it a bit more steady than what is in there now and could be made to be fairly light. Obviously not a super strong approach, but could use what's there with a couple tweaks for a super duty street car.

my windshield frame will wiggle easily when I grab it and pull it back and forth, I too was wanting to reinforce the A pillar area to help. If I do this, what should I use for material, some flat bar stock bent and welded, some heavy gauge sheet stock with some beads?

What about where the cowl meets the windshield base, can this use some extra support, then tie to the upright supports as I talk about above (like using 1", 1/8 flat bar stock), from windshield support to windshield support, effectively making a "U". You could bend the bar stock to get close to the windsheild base and follow the contour, wrap around the corner to the upright supports. This could be done in the front trunk compartment, or under the dash in the cabin - so it would be hidden.

from what I have read here, it now seems to be a bad idea unless you reinforce where the windshield frame meets the cowl area (A-pillar)....

the factory roll bar is stronger than the windshield frame, if (more like when )the car flexes, the forces are going to find the weakest link and that would be the windshield frame...with enough force, the windshield could crack....I'd like to know if the factory 916s had this problem, but I highly doubt many of them have 100k+ miles on the chassis and still being daily driven (IMG:style_emoticons/default/smile.gif)

[TravisNeff]

Sorry I wasn't clear, I was meaning doing the windshield frame support and top reinforecement together, again does that sound reasonable.

Brett,

Where would you finish completing boxing of the 914 chassis, I want to visualize what or where to do this as you are stating

[Eric_Shea]

Travis, with the exception of your pin on the rear section (they bolted theirs in) it sounds like your talking about a factory GT setup. They also glassed theirs to the roof.

Gints a-pillar mod was sheet stock (10 gauge?) and followed the GT pattern. Check the photos on Jon Lowes site for more on that. It will be completely hidden once the interior trim is added. He commonly grabs his frame to get in and out of the car.

I think everyone's on (to) something here... (IMG:style_emoticons/default/biggrin.gif)

Brett seems to be talking about an all out FP type car or more.
John's car has a welded in cage that probably negates some of these issues.
Gint's car is a -6 that will have a mild-hot motor, will see some track time and won't have a cage.
Mike - ? I don't know what your plans are for that car.

We've discussed the issues of bracing the a-pillars and the roof ala a GT but really haven't addressed the question. "How much stiffer with solid or tied in roof?"

I "think" solid would be noticably stiffer (I only have a 911 to reference and old stories about the 916). Tied in like a GT would probably be noticably stiffer as well. But, as Brett and/or others have mentioned, most of your rigidity will probably come from strengthening the longitudinals. Might as well keep the targa top as it is and get an Engman kit. If you did weld on a solid roof you can still have problems if you don't address the flex that John points out (which probably means full tube, seeing as how he has a full cage and still has the flex). I raced my 911 without a roll bar or a roll cage for quite a few years and when it came time for the RS conversion the a-pillars were cracked. It's a non-sunroof coupe.

If someone were to come up with a 916 roof blank I think I'd consider it (IMG:style_emoticons/default/idea.gif)

[airsix]

Look at the early aluminum Monocoques. They used very few tubes and many panels.

Ohhh! A 962. Me likey.

(It is a 962, right?)

-Ben M.

[TravisNeff]

Just got my engman kit last week, and a welder yesterday. Just trying to figure out what things I can ruin errr um strengthen heh.

Thanks for the metal size for the uprights, I will definately do that just for the fact it's so flexy on this car.

[John]

Ok Eric,

We've discussed the issues of bracing the a-pillars and the roof ala a GT but really haven't addressed the question. "How much stiffer with solid or tied in roof?"

Back to the topic....

In my opinion.... simply tying the roof into the existing windshield frame and the Roll Bar (without the benefit of a full cage) would only marginally stiffen the chassis.

just my $0.02

[Brett W]

Eric you could use it on any type of car but it would do away with the regular up right seating and lay the driver back in a very Formula car"esque" seating position. Might be a problem for tall drivers. Look at the seating styles for some of the GTP cars, 962, McLarens, etc.

Travis
Starting at the front of the car, front trunk, big open box. (not really a big concern because it is just holding up the sheetmetal). Fuel Cell area, under windshield cowl. Open box with three panels. all connect to different pieces. None share a common joint. Both A-pillars just standing out in space. Partially bridged by the windshield cowl, sorta not really. The manufacture basically wanted the joint between the cowl and firewall to form the majority of the strength in that area. To bad it didn't work. So basically you have the strut towers connected to these beefy pieces of metal (the strut towers) with nothing on either side of any substance. So they pivot back and forth. The A-pillars flex in and out.

The tunnel is connected to one little piece of .050 sheetmetal with a couple of spot welds. That tunnel dead ends into a flat piece of material with nothing behind it but open space. The firewall wiill flex in bending and torsion. The framerails them selfs aren't all that bad. they are really stiff in bending and would be in torsion if they were connected together.

In the back the tunnel is connected another piece of .050 sheetmetal. This piece is backed with another peice of similar thickness but it is only connected in a couple of places. The firewall is folded over and spotwelded to the front engine shelf. That provides some stiffness but not a whole lot. Engmans kit helps here a little but all it does is add thickness to the one piece of sheetmetal. What it should do is build a box that comes off the firewall and is connected to the tunnel with another boxed piece. This would cause seating issues for most people.

The rear frame rails are braced by on pre-bent peice of .050 sheetmetal (the inner fender panel). So what happens when these are loaded in torsion, as the factory suspension points loads the rear suspension, The box tends to twist in and up. Basically trying to roll up towards the engine. Now a shaped panel that fits in the corners of the engine bay would do wonders for increasing the box section of that area. Adding the 2inch .043 wall tube underneath the engine lid mounts and 3in x.043 under the rear engine shelf would make noticable difference in a stock vehicle. As would bracing from the fire wall to the rear suspension mounts.

Remember most manufactures have packing issues and cost issues that come into play in deciding where stuff should be. So these issues might have gone away had the model been updated several times.

[TravisNeff]

Thanks Brett, that illustrates a lot. Was starting to dawn on me what you were talking about, with the open box cavities of the trunks (and trunk sub sections), but was not sure if you meant "box" as in the imaginary box that runs side to side between axles and fore/aft beteen the front and rear axles.

For the lower tunnel (dunno what to call it) where the steering rack goes, I guess you could make a bolt in panel with some beef (16 guage) and use a brake to make 90degree bends on the perimiter and weld them up (so it would look like a shoe box lid upside down. to give it more spine you could do rectangular stock or strap running front to back or in an X that becomes the mounting boss for the attachment bolts.

With the Engman kit, you could run some flat stock triangles (like 2-3" triangles) to gusset the outer corners to the longs/floor, inner corners to the tunnel/floor, stitch weld the tunnel to the floor, as well as the seat mount U channels, gusset the front facing corners on the floor, and the rear facing corners at the top of the tunnel to seat channel/top of the seat channel to long. Could probably even do a gusset at the rear corners at the top edge of the kit as well without any problems with the rear upholstery panel. Stitch weld the tunnel to the floor, as well as the seat adjuster channel, gusset that at the top (rather than the floor, since no one will see it, or wont get in the way). Bracing/reinforcing the seat channel to the longs/tunnel is right close to the weak spot in the cockpit, could even do flat bar stock on the floor in the side to side stamps at that crtical point, welding that to the long to tunnel and stitch along the floor. It would seem that you would need and X there to really get some strength, but that could be intrusive or very evident looking at the carpet (unless you have backing pad on your carpet that you could cut out the X so it lays smooth.

Maybe run 18 guage steel across the front gastank firewall and wrap the wheelwells to finish the box structure of the Engman kit (as well as a kickout to mount along the floor surrounding the pedal mounts. this would be double duty, one tying the front sections of the "U" to make a box of that Kit, then also reinforce the MC mount, and kickout to reinforce the pedal box mount (mine is super flexy). run spaced out stitch welds along the top and bottom of the engman kit, and stictch the tunnel to the kit. for the sake of giggles a heavier guage plate could be welded to the fore/aft ending sections of the tunnel to the firewalls (whether it is done in the gastank or engine compartment side, or inside the cabin - I dunno). and of course stitch weld the structure panels (upper to lower firewall, trunk, etc.)

The lower brace for the Suspension console is a given, I am up in the air about the top of the firewall to the shock mounts, I am trying to think of a way that those fore/aft tubes can run out of sight and out of the way. Along the upper firewall with rectangular stock doesn't bother me too much, but I would think you would need to ditch the battery location for the fore/aft tubes, or run them more inward getting in the way of working on the engine.

I have to figure out what parts to fab for the suspension console, inner scab plates, straps (like Brants' setup) the outer portion of the suspension consoles and whatever else makes sense (and someone may recomend). what about welding some sheet metal at the bottom of the suspension console to complete the box there? Could go so far as cut out the cover plate of the rear frame rail, weld a panel over the long (like the factory kit), butt weld the cover plate back on and grind down the welds so it'd be stealthy.

I will do the windshield frame reinforcement, what about changing the style of a rectangular piece, to an L, going along the windshield frame/door frame, and then inward on the inside of the cowl a touch. From Eric's photos, what about a sheet metal plate on the underside of the cowl corners on the trunk side? Just another thought.

I am going to drill out the spot welds on the front facing rear trunk panel, weld/reinforce the long to the shock mount (maybe an L piece on each side), could even (if there is room) to run a tube from long to long (maybe overkill to tie the shock towers together, but I would guess the longs could use the help. then weld the panel back over, so you wouldn't see it.

I have to weld in a front trunk floor to patch the A/C cutout, the trunk floor is very flimsy with this huge chunk gone. What about a bar to tie the front torsion bar mounts?

Like I said, just got my welder and I am finding all sorts of stuff to do, that of course will be very much overkill for what I need. But hell, it's gonna be a fun project. and I wanna throw out some ideas and have someone say, good, or your stoopid. And hopefully not have the car weight 3k when I get done, weld too much and mess up the chassis regardless of bracing while welding or whatever. Just trying to get some good ideas to ponder or try.

I am sure someone is going to ask, what are you planning on doing to your car, race, street etc. Probably HD street and some autocross, don't care about class placement right now since I will start from the ground up. All I know is I am gonna start welding, fixing the bad stuff, reinforce the problem spots, and maybe try something new along the way. mostly street I guess, it has ugpraded 150lb springs (at the moment), 22mm t-bars and koni sport shocks w/adjustable perches. No swaybar, yet - but that is coming.

[SirAndy]

Great pic . , link? Any more , Havent seen that one before.

no more pics from that particular car. no link ...
but i have my secret sources for pictures, i hope to get ahold of much more stuff soon ...


here's another interesting GT pic, don't know if that has already been posted ...

"Marathon de la Route, 1970"

Car #: 61
Drivers: Walter Simonis (D), Horst Hoier (D), Ulf Kayser (D)
Rank: DNF
Engine cc: 1991
Engine Type: B6
Group: GT

[turbo914v8]

Ok, so far I have heard a lot of great ideas regarding how to fix the flex in the 914. On one hand you could weld a bunch of steel plates here and there, seam weld, etc. On the other hand you could put a cage in and tie the 4 suspension points together etc. So much information, I am confused. Would not a cage solve most of the flex problems, combined with some plates welded in strategic places? Some one please clarify?????? (IMG:style_emoticons/default/wacko.gif)

Regards,

Turbo Paul.

[Brett W]

The reason many people shy away from the cage is streetablility. Momma doesn't want to have to crawl over the cage in the dash from the grocery store in the rain. Now if you are single, chicks dig roll cages. For the most part the cages that I have seen on street cars only meet the letter of the rules, just adding wieght with very little extra chassis stiffening. If the cages were tied to the chassis in several places that would help.

Travis, those are all great ideas, but I think you are going to end up with more weight than stiffness with a lot of those mods. If you are going from the ground up you can fix a lot of these problems by removing and relocating panels. Look at other peoples stuff first.

http://e30m3performance.com/tech_articles/...e_symposium.htm
http://sports.racer.net/images/chassis/che...tub_clecoed.jpg
http://sports.racer.net/images/chassis/che...hassis_side.jpg
http://sports.racer.net/images/chassis/che...s_aboveside.jpg
http://www.erareplicas.com/427/deslogic.htm

Look at books about airframe structure design, etc and learn about what materials have what properties. I.E. did you know that a 4"x 6" x .050 wall rectangle tube is substantially stronger in bending than a 1.5" x 3" x .250 wall tube. This is something many people miss. Always look at the tubing characterstics vs. weight. 1.75 x .065 is stiffer than a peice of 1.5 x .095 and they weigh about the same. 1.5 x .095 wieghs 1.426lbs/ft where as 1.75x .065 weighs 1.170lbs/ft

Just start doing some searching on the net and start calling the guys that do this for a living and see what they will share. Start bothering people until they share their secrets or they run you off.

[TravisNeff]

The right cage would fix most of them. Bring money, if you are going to build a cage for yourself great, if you are going to compete and want to rely on your work to save your life, that could be a tossup (and maybe not!). Getting the right sized material, the right material and figuring out the best execution can be tough. Also welding pipes together is difficult to weld with the right penetration, heat stress etc. My Dad built a sailboat and did tons and tons of aluminum welding with a MIG (and a gun with built in wire roll). Welds all look great, he got lots of practice. However his life line supports around the boat (which was tube welded to a circular base plate, every 3 or so feet) started breaking. Few broke at the weld (poor weld) and the majority of the failures were the tubes cracked above the weld where the metal got too stressed from heat. Welds looked good to me (an untrained eye), and sometimes you may not know unless you start cutting apart enough of your work to prove yourself right or wrong.

the brace from the trailing arm to the engine firewall would be separate from the cage.

You could tie in all the suspension points, then weld plates to chassis in spots, like a windshield frame tie in, rear firewall tie in, NASCAR style door braces etc. However going the super indulgent way may put you out in a fire breather class for racing. For street, it can be a bear to get in and out of and if you are not wearing a helmet those bars can do some damage on your noggin' even if it was padded up real well


So, with the right cage - problem solved, albeit a compromise on for street driving.

Or you can do a smaller scale cage - plus reinforce in strategic areas, maybe this is the best of both worlds.

Or you can stitch weld the unibody up in the trouble spots, add stiffening kits (or make your own style) and have it unobtrusive for street driving. Some works great, some long time kits don't do much, doesn't change the appearance and features (xept for flexing so much) of the car. What I am after (and everyone who has commented is probably driving in a different direction) is the tricks for a hot street setup.

This board is great, there are some very knowledgeable people and a lot of people who aren't afraid to do some digging and share their findings. There is a ton of new things as of late regarding the chassis, and this is on 30 year old cars. maybe some people knew waaay back when but the kept a lot of stuff quiet. Not here, I love it! Someone has a generalized question (roof being tied in like the GT cars, pros-cons?) we got a ton of answers, some reinforcement is good, some reinforcement is bad, options for changing the chassis around all together and a couple new ideas to ponder.

I am sure some time along there will be a pretty good guide for modifying your car, what's good, what's bad and why. Cars are getting older and weaker, suspension technology (or lets just say brute force on the chassis) is very different when the factory raced these cars when new, wide rims and sticky tires added to the mix and you can pull your car apart a lot easier than with a near stock setup.

So far, we have unboxed areas of the car, stress points and failures of general use, what the factory racers did and some speculation of what other things they had to do to counter what they originally did. The rear Longs arent welded to the rear shock towers, the whole rear of the car wiggles a bunch from the stress of the layout, the suspenion console can rust, or just rip out completely if pushed hard, the center of the car (or directly below the top of the windshield frame is the cars weak point (and a kit to help alleviate it, the factory GT kit only has a few parts that really work well and some parts were never put in the right spot, like reinforcing a cover over the frame, rather than the frame itself

[TravisNeff]

Thanks Brett, I wil check the links out. I agree that everything I have said is a lot of metal. And no, I don't know what tube types are stronger than others, don't know at which point a certain metal is strong without becoming brittle, not knowing at which point reinforcing (or I shoud say welding/heat) will have a diminished return. I am asking questions just like you said, and I aim to ask a bunch more before I get started. You wont learn much if you have someone else do it, or not participate in sounding things out.

(IMG:style_emoticons/default/agree.gif)

[Brett W]

Amen brutha'

I been buggin' the hell out of a bunch of people. I have yet to be told to go away.

[Jeroen]

This turned into another great thread!

I'm totally into chassis design and hopefully at some point in my life I will build something nice from scratch

Earlier this year I visited the Donkervoort factory here in the Netherlands (they build very high spec Super 7 type cars) and that was a real eye opener.
They build their chassis form 2x2 cm square tubing with a wall thickness of only 1mm ( 0.8x0.8" .04" wall)
A completed chassis is very strong and rigid and only weighs about 90#