quote Originally posted by Reallybig: There's no question you have to keep it alive! It's one of the great threads out there. Kinda like having a window into the mind of a mad scientist. (the ones that do cool experiments) With all this attention to the details that aren't seen, I can only imagine how amazing the finished product is going to be!

Yea...Easier to keep us updated here than support 1500 Fiero fanatics staring through your garage windows.



Bob

[This message has been edited by RCR (edited 11-26-2013).]

Thanks Sage for the bump You're right, progress has been slowed down somewhat with all that nuisance Christmas shopping (humbug!) and also by this pesky guy who keeps asking for measurements on my Stinger kit I'll have an update tomorrow though. For now I'd like to address BlackEmrald's questions about crashworthiness.

First, let me be clear that I don't mind answering questions like yours BlackEmrald... I will likely face all of these questions from the provincial inspector who must assess my car before I can register it. This keeps me honest, so here goes: Chassis modifications are almost always going to impact the crashworthiness of a car one way or the other. In the kit car and "modified" world the changes are most often negative. For example it's not rare to see people remove bumper bars and not replace them with anything at all (which is negligent in my mind if you ever carry passengers). Then there are others who make modifications like chopped roofs, convertibles, or extended frames where the impact on crashworthiness would still likely be negative but smaller if properly done. Certainly there is a level of risk tolerance you must adopt if you're going to make any of these kinds of changes.

So how much structural integrity is lost with these types of mods? That can be tough to quantify unless you're able compare the OEM's test data for things like torsional and bending stiffness before and after the changes. Unfortunately the GM test data will never be released and even if it were, none of us are likely going to equip our cars with dozens of strain gauges, run the car through a controlled load environment, and analyze the data. So the trick is to try to be smart about your modifications. For example, in the world of engineering certification, a design doesn't always need to be tested to ensure it is as safe as an alternate design. More often than not, a design is certified "by similarity" to an older design that has proven its abilities.

So what does that have to do with your questions about my car? Well, in a nutshell I am trying design modifications that when compared to the original Fiero's structure, they will be arguably stronger or stiffer by observation alone. Now, before everybody starts picking apart my every modification and throwing them back in my face, remember I said I am trying. Take for example the '88 cradle. The OEM cradle is made of a series of stamped steel pieces that are about 18 gauge (0.048") thick. The convoluted cross section was surely designed to beef up areas that needed extra strength, and less strong to save weight in other areas. By comparison, my cradle is made of 2" x 3" rectangular tube that is larger in cross section than almost every area of the OEM cradle. The tubes are made from 0.125" thick steel, and the lateral link mounts are made from 0.1875" thick steel (IIRC). The same goes for all of my suspension links, all of which are made of drawn over mandrel (DOM) seamless, thick-walled tubing and most with rod ends rated to 31,000 lbs versus the OEM stamped steel links with rubber bushings. I could go on, but I think you get the point. I believe that through observation alone, an engineer would conclude that my cradle is stronger and stiffer (and heavier!) than the OEM cradle.

If we look at the latest chassis mods where I've sectioned the upper frame rail for tire clearance, I've increased both the wall thickness and cross section of the affected area to (over) compensate for the change... and I'm not finished with this area yet. That's the subject of my next post, but in short, I've removed the vertical part of the strut tower that served as a major connection between the upper and lower frame rails, but I'll be replacing that connection with a new structure to achieve the same purpose. It will also serve as a bell crank mount for the pushrod system, something the old strut tower couldn't do. I've maintained the cross-car rigidity by keeping a large part of the strut tower top-piece that acts as a gusset between the upper frame rail and the integral cross-car brace along the trunk wall. And finally, to add even more vertical stability to the area, I plan to triangulate the upper frame rail above the bell crank with a steel C pillar dropping from the roof... something the Fiero does not have.

Your question was double ended; one regarding structural soundness, the other regarding safety. I'll try to answer the crumple zone issue with a drawing since it speaks volumes without me having to type out too much. Here is a schematic of an '88 rear end structure (albeit somewhat modified):



The drawing doesn't include the rear bumper bar, but as I'm sure you're aware, it's attached to the ends of the two lower frame rails. The red outlines show the main chassis structural components namely the upper and lower frame rails, strut tower, and strut tower brace (integral to the trunk wall). The area that's cross-hatched is the rear crumple zone on the Fiero. GM created it by weakening the upper and lower rails with the large oval cut-outs. It handily provides the necessary space to absorb a rear impact without shoving the engine into the passenger compartment. As long as this area retains the same type of features, then the crumple zone will stay intact. That said, I do plan to make some changes to the zone by recessing the rear bumper bar further inwards... but that's a post for another day!

(Edited for spelling)

[This message has been edited by Bloozberry (edited 12-13-2013).]

OK, so on to the bell crank and mounts. I apologize in advance for the verbose post, but I decided to discuss how and why I chose the various parts that will make up the all important bell crank. I decided the lower bell crank mount would be as I had already designed and shown earlier on, essentially a 2" x 3" rectangular tube rising at an angle from the lower frame rail. My plan was (and still is) to sandwich the inner race of the bell crank pivot bearings between that lower mount and one dropping from the upper rail. It followed then that I needed to know the thickness of the bell crank bearings in order to get the correct distance between the upper and lower mounts. Well, I had never gotten into very much detail regarding the bell crank design, let alone the specifics about the bearings until now.

So I did a fair bit of research into what type of bearings I wanted to use for the bell crank, only to find once again, there was little on the internet. There were a few interesting articles nonetheless and I did glean some information about the expected shock loads at a typical shock absorber. Most peg the maximum loads at somewhere between 4G - 5G, so on a 3000 lb car with a 40/60 weight distribution, the static load at a rear shock is about 900 lbs. Multiply that by 5G and you get about 4500 lbs per rear shock, not a lot, but it starts narrowing down options.

Choosing the style of bearing was next. My research bore out that I couldn't use any type of bearing that required a preload since there is going to be no practical way for me to maintain a constant preload with my mount design, so tapered roller bearings and ball bearings were out. That left either needle bearings or spherical bearings which also looked as though they would likely last the longest since the bearings would see only a small amount of rotation and cause larger roller type bearings to use only a small fraction of their rolling surfaces, which isn't good. I was leaning towards combination needle roller bearings that have needles in both axial and radial orientations like so:



With these bearings, the bell crank could essentially float on a hardened shaft bridging the upper and lower mounts with the radial rollers acting as thrust bearings. I would've needed two per bell crank, or perhaps one combination bearing and one radial bearing, but after doing a little more research into all the parts I would need (bearings, hardened shafts, hardened washers, etc) the cost per bell crank rose close to the $200 mark including shipping. That's when I started having a second look at something PFF'er Will had mentioned regarding the use of spherical bearings instead. Of course two spherical bearings would be needed in each bell crank to prevent it from tipping side to side, but these bearings are inexpensive, very strong, very compact, and yet have a large area of contact at the bearing surfaces to spread the load more evenly.

I scoured the internet for ready-made cups that could be welded to the bell crank and that the spherical bearings could press into, but came up short. I did find these from pegasusautoracing.com but also found that the cups are far too tall for the bearings that are made for them, allowing the bearing to slide inside the bore of the cup even after the snap ring is in place:



I finally decided to choose my own spherical bearings and make my own cups from DOM (drawn over mandrel) structural tubing. As shown in the drawing below, the spherical bearings I chose each have a 40,000 lb radial load capacity and 5/8" bore. That means a 5/8" diameter grade 8 through-bolt will hold it all together. The bolt shouldn't see any stresses as long as it doesn't loosen or the suspension doesn't bottom out, but if it does, I wanted to see what sort of load capacity the bolt would have in shear. The shear strength of a bolt is generally accepted as 60% of it's tensile strength, therefore a 5/8" grade 8 fine threaded bolt has a tensile strength of 150,000 lb/sq-in X 0.256 sq-in cross sectional area X 60% = 23,040 lbs. That's five times greater than the expected maximum load of 4500 lbs I estimated earlier at 5G's.



In contrast, the 1/2" grade 8 bolts holding the pushrod and the shock to the bell crank will break at 150,000 lb/sq-in X 0.1599 sq-in X 60% = 14,390 lbs, or 3.2 times the expected maximum load at 5G's. Although I have no way to measure it, I would hope the bell crank distorts before the pushrod bolt breaks, giving me fair warning that I had exceeded safe load limits. As a fail-safe measure, I'll look into bump stops that will prevent the tire from contacting the wheel house in the event a pushrod bolt breaks.

One thing you'll notice about the bell crank is that it's wider at the shock mounting ear because my Promastar DS501 shocks have 1" wide spherical bearings at either end whereas my pushrods have 5/8" wide spherical bearings, so I needed to accommodate both widths. I couldn't simply replace the bearings in the shocks with narrower ones because the housing that the bearings are installed into at either end of the shock are also too wide. Alternatively, I could have made the entire bell crank 1" wide and used spacers at the pushrod end, but I liked my current solution better.

You'll also notice that I used 5/16" thick spacers on the main pivot joint. They're needed since the bell crank will be sandwiched between two mounting plates and the spacers ensure that there is enough clearance between the upper and lower bell crank mounting plates to prevent the widest arm of the bell crank from hitting them as it swings through its arc.

Lastly, the bell crank is notched 14 mm's on the arm that connects to the shock in order for it to clear the shock's spring hat at full rebound. I'll post more on this in my next post.

[This message has been edited by Bloozberry (edited 12-18-2013).]

The design looks great Blooz and the engineering thorough... as always. Quoting a famous Nova Scotian " That's no feather duster you'll be driving". Looking forward to seeing it fabricated. I didn't notice any reference to the 40mm hole. Is it to gain access for welding the 1.75" tube? On that note...... " Friends don't let friends mix units of measure"

[This message has been edited by Yarmouth Fiero (edited 12-18-2013).]

[This message has been edited by kennn (edited 12-18-2013).]