To limit body flex I'm thinking about X-framing the whole under body and the rear engine firewall. Maybe in addition an interior roll-cage, later on. Also covering the whole underbody with one enormous sheet of aluminium is being considered, as it seems would be easier with a flat frame as a base.

Apart from the cost I was wondering if it would be worth the effort, I intend to attend track-days on roadrace tracks with my 86 GT (with 88 suspension and L67 waiting to go in). Also, the added weight would be a definitive negative...but would the positives outweigh the negatives is what I'm wondering about.

Below are some things I am considering, what do you guys have to add that I missed?

profile:
Would using tubular beams instead of rectangular profiles to form the X's and [] have benifits?

removability:
I was thinking using bolted down frames that can be removed easily to gain access to the gass tank for example.

basis for flat underbody:
After the whole underside is boxed and x'ed up, it is then only a small step to atatch some trimmed aluminium sheets to the frames ,as to create a totaly flat underbody for a little extra sucksion (downforce), like a real race car. would this have a noticable affect, do you think?

Weight:
Adding strength adds weight, so reinforce weakspots, overkill kills in this case.
If the flat underbody desired what would that add in the form of sheet aluminium and the webbing to support it over evary inch.

engine cooling/overhaeting:
??
exhaust routing:
??
Anybody gone this route or thinking about someting simularm feel free to chime in .
What exacly are the weak spots in the space frame construction, that can be reinforced. In other words how does it flex exactly while cornering hard with stickytires.

Wow so many questions and ideas..

Thanks
Iwan

come on guys. nobody has a view on this? I was expecting at least a couple of reactions like "are you crazy" or "you have no idea what you are talking about" and such ....

The only thing I can say is that it will probably slow you down more from the weight than it'll speed you up from the stiffining. I could be wrong, but that's what I expect to happen.
The smooth bottom plate though might help. There was another thread on the subject awhile back.

quote Originally posted by fiero-iwan: come on guys. nobody has a view on this? I was expecting at least a couple of reactions like "are you crazy" or "you have no idea what you are talking about" and such ....

OK I'm in.

A while ago I did a good bit of "reserch" in an attempt to build a ridgid Fiero targa...like an MR2, or 308 GTB. My test jig was 3 jack stands and a 10' steel tube attached to the corner with no jack stand. By adding differant bracing schemes, I was able to determine that the Fieros rigidity comes from the top structure, and not the floor.
The most serious twisting occurs at the junture between the "tunnel" and the front bulkhead.

I do like you're clean under pan...but don't see it doing any more than making.....a nice clean under pan.

You won't get downforce from a flat underpan, you will get the opposite, lift. An aircraft wing is flat on the bottom and rounded on top.

Does anyone know the flex rating for the chassis? Car makers test this on those test tracks with the uneven rolling bump things but I wouldn't know where to look for the info.

I would be worried about the extra weight from such a setup. Some well placed welded metal gussets and a few braces should add stiffness and not too much weight.

quote Originally posted by Hudini: You won't get downforce from a flat underpan, you will get the opposite, lift. An aircraft wing is flat on the bottom and rounded on top. Does anyone know the flex rating for the chassis? Car makers test this on those test tracks with the uneven rolling bump things but I wouldn't know where to look for the info. I would be worried about the extra weight from such a setup. Some well placed welded metal gussets and a few braces should add stiffness and not too much weight.

This is true. A vent from underneath to a low presure aera on top might be interesting.
The Fiero is pretty ridgid, but it becomes a wet noodle with no top. I don't know the reall numbers.
A few pounds that increase the rigidity MIGHT be worth it.

The cross bracing needs to add triangulation in the vertical plane to replace the effect when the top is cut.
Problem is that you lose ground clearance with the drop needed to gain much stiffness.
Essentially, you need to add a "truss" like frame to gain stiffness.
The effect can be gained by a brute force approach where the x frame you see mentioned
is simply strong enough to add the stiffness in the up/down dimension without too much loss of
ground clearance. Brute force means heavier material, unfortunately.
I have discussed the convertible idea a couple of times with owners and they said that I should drive
one before I commit. There is probably someone that has hit a good compromise.




Here are two frames.

[This message has been edited by TennT (edited 12-20-2006).]

Some great responses!!!
Keep them coming.
I'm searching for a thread on the flat pan, I'm sure I have seen it somewhere...

When I did my testing, I discovered that from the front bulkhead rearward, the l rigidity is excellent. The real problem is in the connection between the central “ tunnel” and the front bulkhead. All the supplemental frames shown so far add stiffness to what is already stiff.

How the tunnel ties into the front bulkhead is the key.
In my tests, I found that if the tunnel could flair outward at the front, and be completely boxed, we would have a really nice platform. My mock-up forced the peddles to the left, requiring a complete rethinking of that entire area.

I’m sure that if the Fiero had been conceived as a convertible, the engineers would have done the whole thing differently.

Years ago, I had a convertible Fiero that had an "x-frame" to reinforce the chassis; and I absolutely hated the loss of additional ground clearance that the "x-frame" uses.

If I were to ever have/create another convertible/roadster Fiero, I'd reinforce the frame the same way that Archie did for the Finale Roadster. (Pics start towards the bottom of the first page). IMHO, that seems to be the best way to reinforce the frame, as there's nothing hanging under the car.

quote Originally posted by Deabionni: Years ago, I had a convertible Fiero that had an "x-frame" to reinforce the chassis; and I absolutely hated the loss of additional ground clearance that the "x-frame" uses.

There is a good bit of room inside for reinforcement. Unfortunetly, what is really needed impacts the people space. If one can exept shareing the cockpit with some tubing, the solution is pretty simple.

I did't take any pictures of the trials I did, but after a weekend of cuting & welding, I am convinced that a boxed, and slightly reshaped central tunel is the best starting point. I could not come up with anything that was a simple add-on that did enough to consider. Exept of course, a basic triangulated cage.

apparently a flat-bottom car does provide downforce if the car is given some rake. In other words if the suspension is adjusted sothat the rear is higher from the ground than the front.

I haven't found the tread yet but did run into this site:
http://www.mulsannescorner.com/diffuser.htm

I think a rear diffuser would help also but wonder if it is necessary..

quote Originally posted by Yellow-88: I am having a little trouble picturing in my mind how these reinforcements would look. Do you think you could elaborate a little, maybe with a drawing or picture.

The most important thing is that the chassis does not twist along the longitutinal axis. Our roof does that for us.
A 4 sided form is free to twist, but once you close the ends, making it a 6 sided box...it can not twist. Our central tunnel is not boxed, so can twist. By colesing it completly, it becomes a rigid torsion box. Doing that alone helps alot, but it still needs a wider front to tie into the front bulkhead.
I did a drawing, but don't know how to attach a PDF file. (Help..??)

if I was to try and stiffen a Fiero, I would do it thru a rollcage, not a under chassis frame.

and, next, the belly pan - just making the bottom flat is not a good idea. you may end up with the reverse of what you are looking for. there are some basic rules to make it work. 1> concave - as in the middle further from the ground than the side 2> sloped up - as in, the rear is further from the ground than the front 3> front & side ground effects - keep air from getting under there by doing these 3 things - you WILL get downforce.
if you just slap some alum down there, tac it in place and be done with it - you're likely to end up with a saucer sled. an areodynamic skipping stone.

What are you trying to accomplish by "stffiening" the chassis?
If you don't take torsional stiffness measurements as you're working, then at your current level of knowledge/skill, you'll just be adding weight.

The Fiero does NOT need any additional bending stiffness.
The best thing that could be done on the underbody for torsional stiffness is to box the gas tank tunnel. C5 Corvettes box the driveshaft tunnel for this reason. Several Lotus models and the Dodge Viper also have a "backbone" chassis designed around a large central tube like structure.

quote Originally posted by Will: What are you trying to accomplish by "stffiening" the chassis? If you don't take torsional stiffness measurements as you're working, then at your current level of knowledge/skill, you'll just be adding weight. The Fiero does NOT need any additional bending stiffness. The best thing that could be done on the underbody for torsional stiffness is to box the gas tank tunnel. C5 Corvettes box the driveshaft tunnel for this reason. Several Lotus models and the Dodge Viper also have a "backbone" chassis designed around a large central tube like structure.

There is no reason to stiffen it, unless you cut the top off.
I was interested in a true targa-top, and needed to make it work.
Yes....you must test first....and know why and what your testing. I do and did.

I agree TOTALY with the central boxed bacbone design.

I read somewhere that the 88 chassis is stiffer than pre 88's (how it is stiffer I don;t know) + race cars are made as stiff as possible right? For those reasons I'm thinking about stiffening my "race"-fiero.

If I do what Yellow 88 did to test the torsional stifness before,during and after bracing etc, that would be one way to measure progress I think.

Not sure where this will end and/or if I actually ever put the theory (which is gathering here) into practice, but I find the discssion very interesting...... don't you Will? If I remember correctly you did the 88suspension retrofit to the earlier chassis aswel.

Iwan

bumpitybump

Get a iron-duke notchie take the sparetire and pass side seat out....and beat the piss out of it and have a good time coming in second and not spending 6Mo laying under the thing ,trying to figure how to spend globs of money trying to race a lead-slead...no one cares abt... youll have a good tine and youll have money to burn,,,ha.

When I made my convertible, I installed a frame like the one pictured earlier. The frame drawing and pictures are from the convertible manual. I was not happy with the flexing of the body. To solve this problem I made a corner gusset behind the seats. This went from the B pillar (door) to the rear fire wall where the rear frame is spot welded in. Again this did not solve the problem until a put a cap on it. With it boxed in with a cap, I was amazed at how much stiffer the frame was. It is not very high, but the same height as a natural bend/seam in the B pillar. I think it was about 8 - 10 " in height. The only disadvantage was I lost the last 1" of so of travel in the most rearward position of the seat. I you do not set the seat all the way back, you would never notice. I also welded in the crush zone gap on the frame that sets upon the center tunnel on each end under the console. I always said I was going to do this to my 88 coupe just to stiffen the doors, ( I live on a gravel road) but have never gotten around to it. Good luck. I pressured enough I could take some pictures, If I can dig my way back to the car..... But I have never had luck with PIP. Mark

quote Originally posted by fiero-iwan: I read somewhere that the 88 chassis is stiffer than pre 88's (how it is stiffer I don;t know) + race cars are made as stiff as possible right? For those reasons I'm thinking about stiffening my "race"-fiero. If I do what Yellow 88 did to test the torsional stifness before,during and after bracing etc, that would be one way to measure progress I think. Not sure where this will end and/or if I actually ever put the theory (which is gathering here) into practice, but I find the discssion very interesting...... don't you Will? If I remember correctly you did the 88suspension retrofit to the earlier chassis aswel. Iwan

Race cars are built as light as possible. If you go haphazardly adding pieces to the car, you'll increase the weight more than you increase the stiffness. Lightness = performance. You could mill a car out of one block of steel and it would be the stiffest chassis ever built... but it would weigh 8 tons.

Who told you the '88 chassis is stiffer?

When I was in Tempe, AZ to pick up my GT40 rebody kit back in 1990, they had a champion SCCA autocross Fiero sitting in their shop. The owner's comment I remember most was that adding a roll cage did nothing to improve frame rigidity or improve lap times. Spend your time and money on stuff that will provide results or improve appearance.

[This message has been edited by WAWUZAT (edited 12-22-2006).]

in AZ, eh? I bet I know whose car that was... It's been said several times on the Fiero Racing List that cages just add weight and should be left out unless required by class rules.

This is not to say that the Fiero can't be improved upon... just that it's fairly good already and you need to know what you're doing before you try.

[This message has been edited by Will (edited 12-23-2006).]

quote Originally posted by Mark A. Klein: When I made my convertible, I installed a frame like the one pictured earlier. The frame drawing and pictures are from the convertible manual. I was not happy with the flexing of the body. To solve this problem I made a corner gusset behind the seats. This went from the B pillar (door) to the rear fire wall where the rear frame is spot welded in. Again this did not solve the problem until a put a cap on it. With it boxed in with a cap, I was amazed at how much stiffer the frame was. It is not very high, but the same height as a natural bend/seam in the B pillar. I think it was about 8 - 10 " in height. The only disadvantage was I lost the last 1" of so of travel in the most rearward position of the seat. I you do not set the seat all the way back, you would never notice. I also welded in the crush zone gap on the frame that sets upon the center tunnel on each end under the console. I always said I was going to do this to my 88 coupe just to stiffen the doors, ( I live on a gravel road) but have never gotten around to it. Good luck. I pressured enough I could take some pictures, If I can dig my way back to the car..... But I have never had luck with PIP. Mark

Mark, if you do get some pics, just email em to me and I'll post em. For some reason i've had
pretty good luck with pip. One of my drawing progs resizes for web well.
tg

Just throwing something wierd in the mix. Ford built the Mercury Marauder and filled the front frame rails with expanding foam. Similar to Great Stuff. It decreased torsional flex br 20% without adding any weight. I did the same in my 89 Mustang race car. I only had a 6 pt cage, traction bars, and the foam filled frame. the car ran 11.21 on motor, even with 8 psi in the slicks, the car tracked straight down the track. I even let go of the steering wheel on one run. I will be doing the same on my Fiero build.

There are a lot of different grades and densities of expanding foam, from 2#/gal to 16#/gal. Personally, I don't think that filling your frame rails with 2#/gal foam with do diddly squat.

I was unaware that Ford did that with the Marauder.

[This message has been edited by Will (edited 12-23-2006).]

From this thread : https://www.fiero.nl/forum/Forum2/HTML/054448-3.html
I quote LonnieC

" I measured the torsional stiffness on my 86 coupe a couple years ago at about 248 K N-m/Rad (3200 ft-lb/deg). As a reference, the 84 C4 Corvette Targa was spec'd at 214 K N-m/Rad, the 85 Corvette convertible was 140 but a 97 C5 without the top was rated at 720 (I guess chassis engineering has made some progress over the years). Our little cars seemed to be pretty stiff for their time.
L8R
Lonnie"

Wow!!! Our little cars are NOT stiff at all by "modern standards". -(

Torsional stiffness nrs:
720 = 97 roofless Corvette
248 = 86 Fiero

Not the nrs I would have hoped to read....

For allout road racing that would mean the vette has a serious advantage over the Fiero. Even if the fiero had an LS6 swap .....Ofcourse this is not taking into acount other differences as total weights, weight distribution and many other factors that can attribute to winning or losing on a race track, I would presume. I can only hope some factors would be in the fiero's favor...

Oh well nrs are only nrs .. I love my fiero even it is as floppy as a fish

....I wonder how high an 80s or 90s porsche would score, torsional stiffness wise.

[QUOTE]Originally posted by fiero-iwan:

(I guess chassis engineering has made some progress over the years). Our little cars seemed to be pretty stiff for their time.
L8R
Lonnie"

Wow!!! Our little cars are NOT stiff at all by "modern standards". -(

Torsional stiffness nrs:
720 = 97 roofless Corvette
248 = 86 Fiero

QUOTE]

I don't know if stiffness was high on the list of requirements for the Feiro engineers. Like all things, compramise is the target. I do think they came up with a good one. I have heard that one of the main goals was to experament with new robots, and the plastic skin system. The fact that it was a 2 seat mid-engine was just to make the project more fun.

So our cars are a lot less stiff than modern day sports cars... I can sit here and cry about it, or I can look at it this way: So there is a lot of room for improvement by stiffening it up!
From all the info above it does NOT seem like a good idea to box the whole undercarrage as it would ad too much weight and eat up too much ground clearance. I do like your ideas of boxing the weakest spots for the greatest gains without too much extra weight. Still find it hard to visualize exactly where I should weld or bolt in the reinforcements for the gas tank tunnel. Especially at the front where it flares into the front bulkhead, how to go about reinforcing that area with gussets. Could somebody who is good with pictures or photoshop sketch this for us who are not so visually creative? That would help a lot.

I still like the "clean" floor pan idea as a down force mod. My plan/idea originally was to cover the whole undesride of the car with X-frames, That would serve as a convenient basis for a flat sheet of alu to be attached to. Because the x-framing is not going to happen the floorpan is in fact a totally separate project as I will need to find a way of attaching it to a "wavy" underside of the car. I have found the thread on this I was looking (below). It seems that a smooth underbode can create downforce if it incorporates some other mods: A low "air dam" type nose, functioning side skirts and a diffuser at the back. For any real effect the car would need to be as Low as possible to the ground. The underbody shape and the ground need to create a Venturi effect on the air passing underneath the car, thus speeding air up creating low pressure. That WILL suck the car to the ground.

It would need to look something like the underbody of the Enzo shown at the top of page 2 of the link below.
https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20060811-2-072729-2.html

Views or ideas guys?

Thanx Iwan

P.s.
theory of diffuser: http://www.mulsannescorner.com/diffuser.htm

I think that the stock Fiero suspension is so bad that you're wasting your time stiffening the chassis if you haven't re-engineered the suspension.

If you buy a whole lot of dial indicators and set your car up to measure torsional stiffness, you can put a dial indicator every few inches at each edge of the chassis and compile a plot of exactly where the car is twisting more. Remember to use the strut towers in the rear and not the suspension attachment points on the cradle.

I think I understand what you mean about the venturi effect, however if the air passing over the car creates a lower pressure than that under the car, you still have lift. At what speed this becomes a player, I have no idea. What type of racing are you planning? If you are talking less than 100-120mph, my guess would be "no problem" to the flat belly pan. If higher speeds are planned, maybe a working rear spoiler with front air dam would help. Seems you would need to do some testing though to make sure.

quote Originally posted by Will: I think that the stock Fiero suspension is so bad that you're wasting your time stiffening the chassis if you haven't re-engineered the suspension. .

The early cars syspension require more rework than the 88's but both can be made VERY nice .
Yes...rule one of chassis design is that it must be torsionaly rigid. A Fiero with a full top is plenty stiff enough for "normal" robust driving, but gets wicked bad if you cut the top off. You problable are wasting your time stiffening a "complete" Fiero.

quote Originally posted by Yellow-88: The early cars syspension require more rework than the 88's but both can be made VERY nice .

If by "making the early suspension nice" you mean swapping in a late suspension, yes. Otherwise, no.
My stock '88 is as easy to drive at its limits as my '87 GT with UHMW bushings, aluminum cradlke mounts, Koni shocks, good tire split, coil overs with 325# springs, etc. The early suspension just sucks.

[This message has been edited by Will (edited 12-28-2006).]

quote Originally posted by Will: If by "making the early suspension nice" you mean swapping in a late suspension, yes. Otherwise, no. My stock '88 is as easy to drive at its limits as my '87 GT with UHMW bushings, aluminum cradlke mounts, Koni shocks, good tire split, coil overs with 325# springs, etc. The early suspension just sucks.

Yes......the early cars need a lot of work. But you'rs sounds pretty ..."nice". I'll bet it would have most folks in need of an underwaer change.

Yellow 88, could you maybe email the pdf file?

You dont need to modify any part of the frame to make a flat bottom. Buy a 4'x4' sheet of 1/8" ABS and screw it to the floorpan. It's perfectly sized for the area between the engine bay and the front suspension. Two more pieces cut to fit will finish off the front and rear. Use aluminum for the rear so the exhaust doesn't melt it. Adjust your right height so you have some rake on the underbody.