Thanks Bob! As for those high-tech jounce limiters ccfiero, those look pretty cool... I'll have to look into whether I can afford them, and whether I have the space to incorporate them in the engine bay... maybe on a separate arm attached to the bell crank.

I've been busy planning the shock bell crank mounts for some time now... playing with different ideas. One thing I realized quite early on was that the crank pivot has to be secured from the top and the bottom to ensure the rigidity and safety of the system. That means having one mount come up from the lower frame rail (as I've already shown), but also dropping one down from the upper frame rail too. The portion of the upper frame rail that's usually hidden inside the strut tower is a great location for this upper crank mount since it's surrounded by the heavy gauge steel used in make-up of the strut tower and it's an area that's well braced. Vertically, the fore and aft walls of the strut tower are formed of heavy, curved steel "channels" that interconnect the upper and lower frame rails together, and laterally, there is the cross-car strut tower brace that's integral to the trunk wall.

In my conversion to the SLA set up, I maintained the integral cross-car brace in the trunk (so far), and the rear curved "channel" of the strut tower that connects the upper and lower frame rails. I did cut away the forward strut tower channel to make room for my upper links, but I plan to have my bell crank mounts bridge the gap and restore the structure lost to make way for the arms. The rusty condition of the inboard wall of the upper rail in that area led me to be a little concerned about the integrity of the entire area though, since much of it is covered in OEM seam sealer.



To be on the safe side, I decided to take the time to remove all of the sealer on both rails to get a feel for how deep the corrosion might have gone between these interconnecting structures:



I was happy to find that the rust was limited to the one area I've already addressed. After priming, I now have peace of mind to continue my plan to install the upper bell crank mount to this general area without fear that it will simply break apart from underlying rust.



The next thing that had been eating away at my subconciousness was whether I should make the necessary clearance to the underside of the upper frame rail to allow the tire to move into it's full design range of jounce, namely 3". If you recall, in my previous post I cycled the suspension through 2.25" of jounce before the tire made contact with the underside of the frame rail:



After spending a fair bit of time coming up with what I believe to be a solid plan to reinforce the area and provide a suitable mounting surface for an upper bell crank mount, I decided to notch the upper frame rail:



Here's what it looks like from the outside:



And from inside the engine bay:



With the notch removed, the wheel can take advantage of the full range of suspension travel that I had envisioned from the start. If you recall, my earlier drawings also forecasted the need for this notch. It was just a matter of deciding whether I would allow the full range of travel or not. By notching the rail, I keep the door open:



So, with the inboard and bottom walls of the frame rail removed completely, and the outside wall reduced to about half of it's original height, my next step was to regain the structural rigidity I'd lost. My solution was to use a 2" x 3" x 1/8" wall rectangular tube to bridge the space between the strut tower brace (integral with the trunk and aft strut tower wall) and the forward strut tower wall, like so:



I still had to clearance the new stiffener for the tire:



But from this view you can see how much wider the frame rail will be than before, and how there are now three vertical walls that will make up the rail:



The next post will show how I intend to close off the rail and the stiffener, and how the bell crank mounts will support the forward end of the stiffener, as well as bridge the gap between the upper and lower frame rails.

Nice image placement...



Bob

[This message has been edited by RCR (edited 11-22-2013).]

I see that you notched the outer edge of your 2" x 3" HSS structure for your wheel / tire clearance but will there be a wheel tub in there someday to?

There will, but as I learned with my 308 kit, the wheel tub doesn't need to be anything more than a thin sheet of flat fiberglass flexed into a half-cylindrical shape and bonded into place. It doesn't need to extend the full depth of the wheel house either, just as long as it meets up with the sheet metal that comprises the rest of the wheel house. What I'm trying to say is that the fiberglass tub doesn't need to have all the complex shapes to cover everything... only enough to cover the areas that are open into the engine bay and body flanks. Once bonded in place, the entire wheel house can be sprayed with a rubberized rocker guard to minimize the transfer of sound into the cabin from stones and such, and to hide whatever seams may show when the wheel is off the car.

Is that what you were getting at? I do plan to close up the steel stiffener and upper frame rail with sheet metal, if that's your concern.

[This message has been edited by Bloozberry (edited 11-22-2013).]

I was just wondering if you would be using a wheel tub even though your suspension will be quite different than my set up.

Also, save the notched out piece in case I have to trace it onto my chassis

No probs YF... piece saved.

Here's a quick couple shots showing how I'm closing off the upper frame rail and the stiffener. First, I trimmed the crescent-shaped portion I cut out of the rectangular tubing, and then recessed it and tacked it back onto the stiffener:



Then I made a cardboard template to mock up the shape of the floor that will bridge the stiffener to the frame rail and close off the area. Once I was happy with the fit, I traced the shape onto some 1/8" thick steel plate, cut it out, and rolled it to the right curvature:



Here's what the rail's floor piece looks like mocked up. One of the things I want to do is weld the floor piece to all three vertical walls (the original outside wall, plus the two walls of the stiffener) for added rigidity. The way I plan to weld the middle wall to the floor is by drilling some small holes in the floor in-line and underneath it, that way I can rosette-weld the two together despite very limited access once the floor is in place. I've also got to prime and paint the inside surfaces before it gets welded up too.



Here's a direct side-on view:

Just when I start "loosing" some interest in my "shade tree" roadster project, I get caught up on this thread and get enthused again. Thanks again for sharing your work Blooz - I always find "answers" to whatever it is I am struggling with on my project - in your threads. I got lost looking through all the suspension geometry drawings but now I can see it in "production" form - Truly great work! I'm headed out to the cold garage right now to get the heater going for a late night work session.

Pat

...and just when I start to wonder whether keeping this thread alive is worth all the extra effort, feedback like yours spurs me on! Thanks Katatak for your kind words... now I'm off to catch up on your thread...

quote Originally posted by Bloozberry: ...and just when I start to wonder whether keeping this thread alive is worth all the extra effort, feedback like yours spurs me on! Thanks Katatak for your kind words... now I'm off to catch up on your thread...

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!

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).]

I actually laughed out loud when I read your posts Reallybig and RCR. Thanks for the encouragement and the laughs.

This is just a quick update to show what the finished upper frame rail notch and reinforcement looks like. One other mod which isn't shown is that I welded shut the circular OEM hole at the apex of the rail notch:



If you look carefully you'll see a row of dimples along the curved portion which are the traces of the rosette welds I made to secure the hidden wall of the 2X3 to the lower skin. They weren't necessary since the skin is welded along the length of both of its sides, but tying it into the hidden wall was easy so I did it.

Here's the view from inside the engine bay:



I've got a few details here and there to tie up such as closing off the little triangular area at the bottom right of the photo above, but otherwise I'm well into duplicating the notch on the passenger side now. I've only got to get some more 1/8" plate to make the lower skin and then weld it all together. In a couple days I'll be into the bell crank mounts and the bell crank.

Having withdrawal symptoms here........

We understand you're busy...and Christmas is coming...and it's getting colder by the day...ok...guess we'll just wait till you feel sufficent progress has been made, and you have time, for an update.

In the meantime...


HAGO!

I know that your frame modifications are well enginereed and thought out, but have you considered the crashworthyness of your modifications?

God forbod you wreck the car in one way or another, will those frame rails hold up the way the factory intened them to? Did you cut through any crumple zones, spot welds, Taylor rolled edges or blanks? (I'm asking because I don't know).

I just want the car to be as safe and structurally sound as possible.

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).]

quote Originally posted by Bloozberry: Of course two spherical bearings would be needed in each bell crank to prevent it from tipping side to side,

Why?

Don't spherical bearings ordinarily have some amount of misalignment capability? If so, two sandwiched together would eliminate misalignment since they would cancel one another. Perhaps you are considering clamping the bearing tight enough to not allow movement, therefore no misalignment? Or, perhaps two bearings accommodate the width of the bell crank necessary for other connections to the shocks and push rod. Why am I answering anyway? Ha ha. We'll see what Blooze says.

Ken

Edit

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

quote Originally posted by Yarmouth Fiero: I didn't notice any reference to the 40mm hole. Is it to gain access for welding the 1.75" tube?

It's there simply to lighten up the crank since it will be plenty heavy as it is. Besides, the load paths don't pass through the center of the crank so the material isn't needed... and it looks cool.

quote Originally posted by Yarmouth Fiero: On that note...... " Friends don't let friends mix units of measure"

LOL. Yeah... I was going to mention that. That's what you get when you mix metric and SAE parts together... like who would know that 30.16 mm is 1-3/16" if I had converted it?

quote Originally posted by Will: Why [two bearings per crank]?

Recall that there are spherical rod ends at both ends of the shock and both ends of the pushrod. If I were to have a single spherical rod end in the bell crank as well, the system would be akin to pushing on one of those toy articulated snakes. I surmised that as the forces on the pushrod become slightly out of plane when the suspension cycles through it's range, those forces will tip the bell crank out of plane as well if there is only one spherical bearing at the crank fulcrum.

Using two spherical bearings will limit the bell crank movement to a single plane, and the out of plane forces will be resisted by the spherical nature of the bearings. But Id like to hear your views.

I was asking why you need to resist side loads.

How much out of plane loading will you see with a single axis pivot? The load analysis you've presented doesn't address that.

Also, with a multi-DOF pivot, how far out of the original plane will the arm move throughout it's range of motion?

I had calculated and posted earlier on that the pushrods would see 3.8 degrees and 2.5 degrees of misalignment at 76 mms of jounce and rebound respectively. Granted, it's not much but since my bell crank mount design sandwiches the bell crank between two plates that leave 0.25" clearance on either side, I would rather resist the loads than accommodate them. I didn't include the side load analysis given the negligible forces involved compared to the axial load capacity of the spherical bearings.

(Edited because "negligent" isn't the same as "negligible")

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

quote Originally posted by RCR: Yea...Easier to keep us updated here than support 1500 Fiero fanatics staring through your garage windows. Bob

Now that's funny!

You in there Blooz?

quote Originally posted by Bloozberry: I had calculated and posted earlier on that the pushrods would see 3.8 degrees and 2.5 degrees of misalignment at 76 mms of jounce and rebound respectively. Granted, it's not much but since my bell crank mount design sandwiches the bell crank between two plates that leave 0.25" clearance on either side, I would rather resist the loads than accommodate them. I didn't include the side load analysis given the negligible forces involved compared to the axial load capacity of the spherical bearings. (Edited because "negligent" isn't the same as "negligible")

Ok. I was thinking "Why bother reacting the side loads if you can allow it to self-align".
I was also thinking more about the torque the side loads would put on your support struts.

Katatak, you crack me up.

It's spring time! OK, well more like shock-and-spring time. Until now, all of my previous drawings were based only on generic shocks and springs meant to reassure me that the general concept would work. Getting into the nitty-gritty of the bell crank design forced me to take out my pencil and paper (mouse, really) and have a detailed look at two separate issues that hadn't previously been studied:

1. whether the spring and shock would interfere with the bell crank at any point in the suspension's travel; and

2. where my adjustable height springs needed to be positioned to give me the suspension travel I wanted without the shocks bottoming out, nor the springs binding up in full compression, nor becoming loose at full extension.

It's amazing how much of this stuff is already figured out for you when you're only throwing together a bunch of parts that someone else has already proven (or more likely not!). Coming up from scratch has been an eye-opener. I'll start with number 2 since I build upon the results to answer number 1. To get started, I ordered the Promastar DS501 shocks because they had the longest stroke available to accommodate my suspension's design travel. Fully extended they're 17" eye-to-eye, and fully compressed they are 11.625" giving a maximum stroke of 5.375", which imposes a new, lower maximum travel envelope on my suspension, which was able to accommodate up to 6". (I could change the lengths of the bell crank arms to give me a different input to output ratio so that I could still use the full 6" suspension travel while only generating 5.375" of shock travel, but I don't think it's necessary). Also, according to the literature that came with the shocks, the Promastar's are designed to be operated at an optimal static length (length at ride height) between 13.25" - 14.5".

At first I thought it made logical sense to chose the halfway point between the shock's fully extended and fully compressed lengths as the length at ride height. That would have assured equal amounts of jounce and rebound at 2.69" each (5.375" divided by 2). But after thinking about it a bit, I decided that allowing more jounce at the expense of some rebound travel was a better compromise since in a roll, the wheel in jounce has the majority of the weight on it and therefore the control. In other words, it would matter less if the wheel in rebound (inside corner) ran out of travel while cornering, than if the wheel in jounce (outside corner) did. So I chose to offset the ride height length of the shock to the maximum recommended 14.5". This will give me 2.875" of jounce travel and 2.5" of rebound travel.

Enough math for you yet? Get some popcorn because it's not over yet. The thing is, I can arbitrarily choose the length of the shock absorber at ride height because I haven't cast the the location for the hard mounted end of the shock absorber in stone yet. But when I do, I also have to make certain that the spring I chose isn't so long that it will bind up (ie the coils come into contact with each other) before the shock reaches its fully compressed state, AND make sure it isn't so short that it flops around loosely when the shock is fully extended.

To satisfy these two conditions now that I've chosen a 14.5" static shock length, I first have to calculate how long the spring will be when the car is at static ride height. That's easier than it may sound since I estimate the car will weigh about 3000 lbs, so the 40%/60% weight distribution will result in about 900 lbs at each rear spring, give or take. Since I had already bought my 12" x 350lbs/in springs with my Held conversion kit (now gathering dust), I could figure out how much they would compress by simply taking 900 lbs divided by 350lbs/in to give 2.57" of compression. Subtract 2.57" from 12" and I get 9.43" as the length of the spring when supporting the weight of the car.

So now that I know how long the shock absorber has to be (14.5") and how long the spring will be at ride height (9.43"), I can draw the coil-over at ride height, as well as what each are doing at full shock absorber compression and extension:



Starting with the top image, you can see how the threaded adjuster ring on the shock body has lots of leeway in either direction to lengthen or shorten the spring in case I've over or under estimated the weight of the car. The middle drawing shows the coil-over at full shock absorber compression and shows that the spring hasn't bound up yet since there are still a couple millimeters between each coil. That solves that tricky condition. Lastly, the bottom drawing shows that when the shock is at full extension, the spring is only just reaching its maximum length. In fact the spring is actually 12.125" in length, so even at full shock extension, the spring won't be flopping around on the shock. That solves the other condition. (It may be important to note that the adjustment ring is at the same location on the shock body in all three drawings.)

In the first paragraph, I mentioned that another concern in the design process was to be sure that the coil-over would not contact the bell crank at any point in the suspension's travel. Well, I can now use the information from the above discussion to draw out the bell crank and coil-over geometry at any point in the travel range to be sure. Perhaps the most important discovery in doing this step was that I found that I wouldn't be able to orient the shock absorbers with the adjustment ring and knob towards the rear, where they would be more easily accessible. The trouble with placing that end closest to the bell crank was that an overly large cut-out would have had to have been made in the bell crank to clear the shock when the suspension was at full rebound (fully extended). By simply turning the shock around 180 degrees, I was able to minimize the cut out to 14 mms as shown in the bell crank drawing a couple posts ago. To resolve the conflict in my head about having to put the knob at the other end, I told myself that it would be harder for kids to reach when the deck lid was open at future car shows. Again, here are the scale drawings showing how the coil-over and bell crank behave at ride height, full jounce, and full rebound respectively:



Finally, I'm armed with enough details draw how the bell crank and shock mounts will be positioned in the chassis (by night), and start fabricating some parts (by day). Now if only that pesky Christmas holiday weren't in the way!

quote Originally posted by Bloozberry: (I could change the lengths of the bell crank arms to give me a different input to output ratio so that I could still use the full 6" suspension travel while only generating 5.375" of shock travel, but I don't think it's necessary).

I think you should change your bell crank ratio.

If the shock has less travel than your suspension, then the shock is the travel limiter and that is BAD. Crashing the shock is going to be quite detrimental to it, whereas having the tire hit the shell you welded in really won't hurt much. (I'd still want to hit a bump stop before that happened, though). Personally, I'd set it up with maybe 1/4" of damper travel left when the suspension is at its mechanical limits (tire into the shell)

Blooze,

If you don't already make your living at technical writing, you certainly have the talent. And, you could use this treatise as part of your resumé.

It appears that as the shock compresses that the change in mechanical advantage makes for a somewhat variable rate at the spring, which seems like a good thing if I'm correct.

Ken

edit: I also think that Will has a good point with regard to bump stops. Don't your shocks come with a bump stop at the shaft end? The Held struts that I am fitting apparently employ a Bilstein insert that has an integral stop within the housing (not visible).
------------------
'88 Formula V6
'88 GT TPI V8

[This message has been edited by kennn (edited 12-20-2013).]

quote Originally posted by kennn: It appears that as the shock compresses that the change in mechanical advantage makes for a somewhat variable rate at the spring, which seems like a good thing if I'm correct.

The loss of MA on the driving end compensates for the loss of MA on the driven end.

quote Originally posted by Will: I think you should change your bell crank ratio. If the shock has less travel than your suspension, then the shock is the travel limiter and that is BAD. I'd set it up with maybe 1/4" of damper travel left when the suspension is at its mechanical limits (tire into the shell)

quote Originally posted by kennn: I also think that Will has a good point with regard to bump stops.

After a second sober thought, I realize that you guys are right. Peer review is one of the great things about this forum. I'll redesign the bell crank as you suggested Will, and that will give me the space to add a bumper on the shaft Kennn, BTW, I don't believe the Promastars have any such integral/internal bumper... they make a definite thunk when they reach max and min limits. Back to the drawing board!

Hey Blooze,

This may be a type of bump stop that is located on the shaft of the shock. I didn't confer with QA1.

<http://www.qa1.net/qa1_motorsports/catalog/product/gallery/id/7021/image/77/>

Ken

edit: Or this one <http://www.qa1.net/qa1_motorsports/catalog/product/gallery/image/1201/id/7021/>

[This message has been edited by kennn (edited 12-21-2013).]

nevermind, I was wrong.

Joe

[This message has been edited by motoracer838 (edited 12-21-2013).]

OK so this post might look like déja-vu but it's kind of like one of those games where you have to spot the differences. Following Will's advice, I've revised the ratio of the bell crank from 1:1 to 1:0.88 by reducing the length of the arm that the coil over attaches to by 13mm (from 107 mm to 94 mm) so now it looks like this:



By keeping the same 14.5" shock length at ride height as before, the simple change in the bell crank allows the wheel to travel the full 3" of jounce while compressing the coil over by only 2.625". This means that when the wheel reaches the end of its upward travel the shock absorber will still have 0.25" of travel left (ie it won't bottom out and destroy the shock). Conversely, in rebound when the wheel travels to its rebound bump stop 2.5" downward, the coil over will extend by only 2.2" leaving a buffer zone of another 0.25" at the opposite end of the shock's travel.

Here are the new drawings of the coil over at ride height and at the two extremes. Notice how the new configuration increases the length of the spring at full jounce over the previous design thereby reducing the likelihood of the coils binding. A similar improvement also occurs in full rebound where the spring remains compressed by a little more than 0.4" making sure it won't flop around even in the worst scenario.



Shortening the shock absorber arm on the bell crank had the potential to worsen the interference between the spring hat and the crank especially at full rebound so the last thing to check were the clearances with the new bell crank. Again, everything checked out fine:



Two other benefits of having modified the bell crank are that it lowers the coil over in the engine bay making a bit more room under the deck lid for future adjustments, and with it being lower, the sight lines to them from the open deck lid improve as well. These things are way too good looking to be hidden!

The bell crank dimensions finally worked out, I could now complete the design of the upper mount for the bell crank, as well as the forward stationary mount for the coil-over shock. Again, the best way to describe what I propose is to simply post the drawing and point out a few of the most important features, so here is the rear view:



Of the three different views I'll be posting, the rear view is the most revealing. The upper and lower crank mounts (A & B) are the most clear in this one... they're just capped off 2" x 3" x 1/8" wall rectangular tubes. The upper mount drops down from the new upper frame rail stiffener (C) that I added to the chassis a few posts ago. There are a few things to note about this area so I've broken them down the following sub-paragraphs:

1. you can clearly see how the original cross section of the upper frame rail has been modified. I've left a barely visible outline of the part that was removed (D) just to show what it used to look like. I've also drawn an outline of the wheel and tire at full jounce (76 mm) to show why it was necessary to notch the bottom of the rail;

2. the stiffener (C) will serve as the main anchor point for the upper bell crank mount (A). The top of the mount will also be welded to the underside of what's left of the old strut tower. That portion of the strut tower is made up of two heavy gauge steel plates welded together. This area will get a further boost in strength when I eventually drop a steel C-pillar down from the roof to the top front corner of the strut tower. The previous owner had already done something similar with some round tubing however it isn't properly anchored to the roof;

3. the rear view also shows how the bell crank (E) and coil over (F) clear the mounts side to side and from the bottom. I had to square off the top corner of the lower crank mount (B) since the aft end of the coil-over angles downward by about 20 mm in full rebound and would otherwise have contacted the top of the mount;

4. to tighten the bell crank bearing bolt, the bell crank mounts will have an angled access hole (G) machined in one wall large enough to get a socket wrench on the nut and bolt;

5. the angle of the bell crank and pushrod wasn't just arbitrarily chosen. Notice how a line (H) drawn though the center of the crank down through the lower end of the push rod lines up perfectly with the outboard lateral link pivot. Having these parts line up this way ensures a 1:1 ratio between the wheel movement and the pushrod movement... i.e. 1" upward movement of the wheel translates into 1" upward movement of the pushrod (at least initially at ride height). This ratio can be whatever you want it to be, but the longer the movement of the pushrod, the more easily fine adjustments can be made to damp the spring. (Of course I reduced the ratio of the motion actually transmitted to the shock with the non-equal length arms on the bell crank); and

6. lastly, if you recall from earlier drawings, I stole an idea from Yarmouth Fiero by adding an upper cross car brace (I) under the rear window. The plan is to use that brace to mount the stationary end of the shocks, the engine torque struts, and the deck lid hinges. The change I made to this area from earlier drawings is that I now plan to run the drop down supports (J) vertically to the lower frame rail rather than angling them outwards at the top to meet up where the cross car brace meets the upper frame rail. Making them vertical not only makes them easier to weld and cut, but also very conveniently makes them line up directly with the stationary coil-over mount, giving the area better strength and rigidity to react the spring's forces.

Tired of reading? You should try typing this stuff out. OK, on to the side view to see what gems might be hiding there:



In this view you get a better idea how the vertical supports for the cross car brace (fuchsia) are anchored to the lower frame rail, and despite being mostly hidden by the upper frame rail, you can also see how the stationary coil-over mounts project backwards to meet up with the shock.

You can also see how the coil-over is at the same height as the upper frame rail (well-below the deck lid). This keeps the forces exerted on the cross car brace in the same plane (in the side view) as the main structures that react them. It just means that the cross car brace will be under a simple bending force rather than a compound bending and torsional force.

Two last tidbits from this view: if you look closely, you'll see the large hole in the upper bell crank mount to provide access to the crank bearing bolt with a socket wrench I mentioned earlier and; for those who haven't fallen asleep yet, you'll notice I've pencilled in an additional pushrod off the crank that leads to a sway bar (brown). I got the idea for a rear mounted bar off the bell crank when I saw this particular photo online:



For now it's just a concept since I may or may not use a sway bar, but interestingly enough, it appears the stock '88 rear sway bar would fit in that space like a well made glove.

Lastly, the top view is always the most obscured view since everything is layered very much vertically. The view does give a bit of insight into how some of the pieces fit though:



Here, the stationary coil-over mounts are much more visible, as is the shape of the left-over strut tower (light green). I've mentioned it before, but I left the top piece of the strut tower in a triangular shape to act as a gusset between the upper frame rail and the old strut tower brace that's integral with the trunk wall. If I do indeed use a sway bar set up like I've pictured here, I might need to replace the old strut tower brace with a different design so as not to interfere with the movement of the sway bar arms.

One final thing to note about the potential use of a sway bar connected to the bell crank is that it would place some more complicated loads on the bell crank bearings due to the slight offset of having the sway bar pushrod mounted to the side of the crank. Some more research would be needed if I decided to go this way, but I'm not going to study it at this point.

So there you have it. This pretty much completes the design phase of the rear SLA suspension. Please don't be shy if you have any questions, concerns, or I didn't explain something clearly.

Looks really good Blooz. It's certainly going to be an interesting feature when you look in the engine bay. Is there any room to add gussets to the lower end of part B and the lower frame rail?

What is the red car in the picture you used? Looks like their bell crank has a much large ratio.... maybe 1.5 : 1.

Merry Christmas to you and your much better half from the Oakleys.

There is a fair bit of room for gussets if needed, though given the stoutness of the lower mount and the direction of the forces involved, I think it'll be OK.

The bell crank on the red car in the photo doesn't make a lot of sense to me too. The shock length looks too short and the spring coils too close for there to be much jounce travel, and the bell crank ratio certainly doesn't make things better. It's a race car though so I doubt it sees anything but smooth asphalt. I didn't pay much attention to the actual article but if I recall correctly the car was one of those American university Formula SAE cars. As I recall, the article was written by an FSAE technical judge who wanted to point out some of the areas where past teams had lost points. In this example he was pointing out how the bell crank pushrod and shock were held to the crank in single shear.

Best of the season to you and your family too.

With the holiday-hoopla winding down, I found some time to finish modifying the passenger rear upper frame rail with the tire notch and reinforcement. I also managed to get a few parts in the mail for the bell crank from Summit Racing. These are the SG105 stainless spacers made by QA1 that are 5/16" thick and have a 5/8" bore for the crank fulcrum bolt:



Santa also delivered the crank bearings too. These are the FK S10 spherical bearings that I plan to use (two per crank). Notice they have a predrilled lubrication hole and groove. I'll have to update the crank hub design with zerk fittings:



I also found some time today to start fabricating the parts for the first bell crank. The pieces below are the main body of one crank and are cut out of 3/16" thick steel plate. The smaller pieces will be overlapped onto the larger pieces when welded, making the spacing between them 1.0" while the spacing between the main body halves will be 5/8". The drawing a couple posts up shows how they're overlapped. I haven't drilled out the lightening hole in the center yet because I want to wait until the parts are welded to the crank hub to minimize warpage.



Next step is getting more 3/16" plate to make the other crank, and popping by the machine shop to see about getting some DOM tubing machined to accept the bearings and a snap ring. Hopefully that won't cost too much.

While I am waiting for an opportunity to use a friend's lathe for the bell crank bearing housings, I thought I'd get around to something I've been meaning to do for a while: create an index for the thread. For convenience, I've edited my very first post on page 1 with a guide to navigating the past 22 pages.

I should get around to some real progress by tomorrow.

Still watching this build! Has to be the most well thought out and methodical builds I've ever seen.

May I ask where you bought those spherical bearings?

I'm interested in doing a similar project, and this thread has been a great resource. One of the main things that may prevent me from pursuing a project like this is liability. Has anyone done any in depth research about what kind of liability a builder may have if they eventually sell the project?

This project has the benefit of being reviewed/approved by a PE, but there isn't a requirement, that I am aware of, in Indiana.