PS. Can you or one of your readers give me approximate dimensions for a stock '88 Fiero rear sway bar (eye to eye, arm length, and diameter)? I haven't been able to find one or reference for this information. I'd like to see what rate was stock and decide if that will work for me. That will give me a starting point if I need to make my own.
45" Eye to Eye .863" diameter (but the bar has a few offsets in the center to clear the stock muffler) 12 3/8" from eye to pivot at the bushing.
I would measure an 88 rear bar for you, but that's the only 88 rear suspension piece not sitting ok my garage floor right now, but R Runners pieces are top notch, and have about any range you need. I do have a low opinion of heavy rear bars in an open-diff RWD car though and there tendancy to pull up on the inside wheel.
Thanks for the link. Looks like good stuff but probably more than I need for my application. I agree with you about heavy rear bars. A little bit can help (maybe) but you have to be careful not to go too far. That's the reason I want to figure out what the original size (er, rate) is and work from there.
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I agree that expecting 6 degrees of role is an unreleastic expectation. But as a design engineer by trade, I feel to not take the time to analyze a system at all conditions, even the most extreme, is inviting critical errors into the system. One thing I would also see as a benifit to Blooze's analysis' would be the effect of bushing deflection, as it can be very significant. But is see no fault to design something to 10 tenths, perfection cannot be reached, but it can be approached, and the reward is knowing nothing was left on the table. If the suspension is capable of 3 inches of jounce and bump it can, somehow, achieve roll at those 3 inches.
I really hope that I'm not being misunderstood. I'm not faulting or criticizing the work that's been done at all or even the direction that it seems to be headed. What I AM saying is that by MY way of thinking there is a place where one must accept the original design parameters and constraints for the project and design and build accordingly---OR, change the design parameters. I think Blooze is approaching that place. Point is that if a 10 tenths design was part of the original design plan, then I dare say none of us would be using a 20 something year old mass produced passenger car platform as a starting point. There is just so much that can be done with these parts. How one tackles these problems is a personal decision and we all approach things a little differently.
[This message has been edited by m.mcc (edited 12-31-2012).]
Thanks for taking the time to put together a well thought out set of comments there Mike. I have to admit as I started reading the build up to your comments, I was getting worried you found something that would send me back to the drawing board, literally. If I understand correctly though, you believe I'm on the right track, but designing for conditions that are outside the likely conditions the car will ever see. Because of that, you believe that a whole sale redesign of the rear suspension is overkill. You're perhaps right, and it's good that I am challenged to explain my position because it forces me to take a step back and rationalize what I'm doing and why.
The "why" part of it has morphed several times as I've gained more knowledge about suspension systems and about potential registration problems in my province. I started out thinking that I wanted primarily to give the car the right stance, and as a secondary objective improve the handling if possible. The handling issue quickly turned into a more important issue than the aesthetics when I learned of pending legislation prohibiting changes in suspensions unless proven to be at least as good as OEM. I initially assumed the impact of various changes I planned would certainly result in better handling, but was enlightened with the simulations provided by the Lotus Suspension Analysis software. That showed me that with the Chapman strut design, the angle of the lower lateral link at ride height is extremely important. But that was as odds with getting a reasonable wheel to fender gap given the IFG body I have. I briefly evaluated several options including Fieroguru's lateral link lowering hardware which lowers the attaching point at the knuckle, as well as several other knuckle designs, but decided to explore the SLA concept, thinking if it didn't pan out I could always return to the first two options. I'm still not 100% convinced it will fit with my engine and transmission choice, but the concept seems to have enough advantages that I believe it is worth developing until I achieve my Eureka Moment or I come across an insurmountable roadblock.
With my motivation perhaps a little clearer for you, I'll attempt to address your comments:
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Originally posted by m.mcc: You have invited input and constructive criticism and I hope my comments will be received as such.
Absolutely. I thank you for your interest and concerns... the entire sum of my knowledge in suspension systems is what you see in this thread, and I have learned it as I progressed.
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Originally posted by m.mcc: ...regarding the rear suspension, did you include the various Chapman strut upper mounting datum point movements during chassis roll in your analysis?
I only provided the coordinates for a pivoting point upper strut mount. Your concerns may be valid but I'm somewhat skeptical that the upper end of the strut which is mounted in a polyurethane bushing of approx 40 mm dia will allow enough of a change in strut position other than its angle to be significant, especially given the roughly 600 mm length it acts upon (upper strut mount to lower lateral link mount). The analysis does calculate changes in strut angle, just not movement of the upper pivot point in the lateral, longitudinal, and vertical axes relative to the frame.
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Originally posted by m.mcc: I would be concerning myself with the suspensions behavior up to about 3 degrees and that’s about it. I would have little to no concern beyond that.
I agree with you regarding the likely roll behaviour to remain no more than about 3 degrees. In Road & Track's Sep 83 issue, the '84 Fiero with P215/60R/14 tires and (approx) 250 in/lb springs had a purported roll rate of 3.5 degrees per lateral g. As you mentioned, my tire and spring choice (so far) will lessen that roll rate even further. I analyzed a maximum of 6 degrees of roll for several reasons: because the software allowed it; I wanted to design an envelope that would accommodate more than 3.5 degrees of roll for a margin of safety; and because of my growing interest in suspension behaviour. I intend to limit the overall roll angle to less than 6 degrees through the use of the springs and bars that suit me, but the precise amount will likely be determined by how much work I can avoid in modifying the upper frame rail for clearance!
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Originally posted by m.mcc: With this in mind, most (but not all) of your scenarios fell within what I would consider to be generally acceptable limits within that roll range. Again, maybe not absolutely perfect for track use and not as desirable as say the GT40, but again the car is being built from a production car platform for road use.
I understand the above comment is in regards to the lowered Chapman strut design, not the SLA exercise. Again, my motivation for a better design than stock is in the pending legislation changes I mentioned above. I will undoubtedly need to rely on the simulation results to appease the inspector who must certify the roadworthiness before I will be able to register the car. To that end, the inspector may be as lenient as you and find the data from the lowered Chapman strut design within an "acceptable" range. But that would involve speculation and hope on my part since his precise criteria are unknown, potentially ambiguous, and likely subjective. One thing is certain, the lowered Chapman strut design shows a degradation in performance over stock, which doesn't meet the draft requirements.
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Originally posted by m.mcc: Personally, in this application I would be more than fine with a camber curve of one degree (or even slightly more) per degree of body roll. The .7 to 1 of the GT40 is great but I don’t think you’d know the difference in this application. If you can get to .7 to 1 without a lot of hassle, fine. If not, get as close as you can without major modifications and call it good.
I understand this comment to apply to the SLA design since this is where Fierowannabe brought it up. If so, then modifying the camber curve was dirt simple at this stage... about an hour's work to redefine the location of the control arm and draw it out. The impact on future frame modifications to implement this change was insignificant so I went ahead with Fierowannabe's suggestion.
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Originally posted by m.mcc: One last comment on camber. I do think you are correct in fixing the rear lower control link angle so it is level at static ride height.
I believe you were referring to the Chapman strut design with this comment. If so, then I agree. If you were referring to the SLA design, then I believe I've shown that this isn't necessary, although I haven't completed the assessment with the Lotus simulation yet. I'm still working on packaging issues for the upper control arm given my engine and transmission choice.
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Originally posted by m.mcc: It’s an established fact that strut suspensions produce massive roll center movement. It’s also an established fact that there are a lot of very, very, good performance road cars that use struts on one or both ends and these cars don’t suffer from erratic handling. My friend’s WRX is a great example and the little Lotus that Colin Chapman first put these on was no slouch either.
All very good points, but the trouble is the Fiero doesn't handle very well with the Chapman strut. Why some cars do and others don't is a mystery to me as well but if we want to forge an understanding, we must start somewhere. None of the OEM's publish their suspension data to allow the average joe to compare apples to apples analytically. To scratch the surface, all that us peons can do is reverse engineer the stock setup and run whatever suspension simulation software we can afford and make some educated assumptions. I know there are limitations to what I'm doing here (not the least of which is neglecting bushing compliance) but it's a starting point considering nobody else has gone down this road in the Fiero world (as far as I could tell, and if they have, they're not sharing the information). None of the companies offering modified suspension parts for the Fiero (and most other cars for that matter) offer anything but unsubstantiated claims regarding the improvements of their designs. No one should arbitrarily accept those claims especially if they come under the "for off road use only" banner. That just won't cut it when I try to certify and register my car for road use.
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Originally posted by m.mcc: I’m of the opinion that in your application, or mine for that matter, that the roll center migration is worthy of note, and perhaps should be addressed within practical limits, but not enough to cause dramatic redesigns given the original project design criteria. If we were going F1 or Indycar racing it would be a different deal. Again, this is my opinion but I do also respect yours.
Now that I've started going down the SLA road, I'm finding the challenge to understand and design a different suspension as rewarding as I hope the end result will be.
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Originally posted by m.mcc: I agree with FieroWannaBe in that I don’t think you need to be overly concerned with it. Just design into whatever you end up with for rear suspension about the same as original factory and call it good. If you felt that you needed a little leeway just in case of axle tramp you could design in multiple forward trailing link mounting points (or provide for them). And leave the front alone.
That's what I intend to do with the SLA design. I've kept the trailing link at the same angle as stock. I haven't designed the front mount for the trailing link yet, but I'll certainly take your advice about providing additional mounting points when I do.
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Originally posted by m.mcc: In summation, the areas that I personally watch closely in these types of projects are tire choice, camber curves that I may have messed up by lowering, brake bias, scrub radius if I’ve changed wheel offset, and especially roll steer. I try to keep the CG as low as I can (without tearing the car apart), the track widths wide, and let the tires do the rest.
Great summary so I measured my progress to date against your criteria and believe that I haven't strayed far from your own ideals:
1. Tires were chosen with a rear weight bias in mind (and of course looks!); 2. Camber curves have been studied and now improved with the SLA design following lowering; 3. Brake bias will remain the same as stock by using the same MC and calipers though larger rotors; 4. Scrub radius has been improved over stock; 5. CofG has been lowered; 6. Track width has been widened 6"; and 7. Roll steer is being addressed currently.
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Originally posted by m.mcc: PS. Can you or one of your readers give me approximate dimensions for a stock '88 Fiero rear sway bar (eye to eye, arm length, and diameter)? I haven't been able to find one or reference for this information. I'd like to see what rate was stock and decide if that will work for me. That will give me a starting point if I need to make my own.
I'll PM you the info later this afternoon.
Edit: I see Fieroguru beat me to it.
[This message has been edited by Bloozberry (edited 12-31-2012).]
OK. The design criteria changed (morphed) in mid project. The journey is becoming as important as the destination. Understood completely. Happens to me all the time
We don't agree that the '88 doesn't handle very well but that's a subjective thing. Earlier models were junk and I do think the skid pad numbers for the '88 were much better than those you cited for an early production car.
I'm not sure that we are communicating well regarding the movement of the upper Chapman strut mounting point during chassis roll but it's a moot point with your change to a multi link configuration.
"The next thing to do before designing the mounts for the shocks and bell cranks is to locate the upper control arms in the side and top views."
For the next couple weeks I kept busy avoiding that and doing mindless stuff like creating engine drawings instead. I was trying to avoid confirming this terrible feeling that I wasn't going to have enough room for the upper links. When I finally added the engine and transmission to the drawings, things started looking brighter.
Fro sure the aft upper link is still going to present some mounting challenges where it connects to the frame rail, but at least I have the space to deal with the necessary frame modifications without them interfering with the pulleys on the engine, nor with the transmission. One arm down, one to go.
Locating the forward upper link and determining its length came next. The purpose of the forward control link in this design is two-fold. The first is to triangulate the upper part of the knuckle so it moves only up and down. The second is to generate the same amount of toe at the top of the knuckle as the lower lateral links do, in roll.
After dusting off my scientific calculator and doing some serious head-scratching, I believe I've located where and how long the upper forward link needs to be, and it looks very promising from a mounting perspective. Here's what both upper links look like in the top view on the driver's side (the upper links are in purple) This drawing shows the room around the F40 transmission:
And here's what they look like in the top view on the passenger side, showing the clearance with the Northstar engine:
Finally, here's what they look like in the side view:
Before wasting any time figuring out frame mounts for the links and the shocks, the next step is getting the coordinates of all the major pivot points off to Zac to see how this system really behaves. Hopefully the beads missing from my abacus weren't too important.
Very nice Blooz. Will the Lotus Suspension Analyzer detect if there is any binding in the links as they travel together. I'm curious how all the links will move in unison considering the various lengths and attachement points in 3D space. Its almost like two different 4 bar links that share some common attachemnt points yet each with links at different lengths and angles.
Yes it does by giving strange flat spots when you graph the data. But based on all the digital stick models I've created so far (not shown), I'm pretty confident that the minor binding action that takes place in the trailing link, and to a much lesser extent in the lower lateral links, is easily accommodated by compliance in the poly bushings in those locations. That same binding is present in the stock configuration on those arms. The uppers will have spherical rod ends and from what I've been able to determine, they'll have enough range of motion to cycle through the full bump travel without being at odds with each other.
Beautiful drawings and thoughtful considerations. Looking in side view I notice that pivot axis of the upper control arm link attachment to the chassis appears to slope down to the rear, which I think may be contrary to anti-squat requirements, Am I off here?
I am in keen anticipation to see your mounting solution for the crank pivot.
Thanks for your willingness to take us through this process with you on your journey of discovery and creation.
Thanks for your kind words Ken. With respect to your observation about the pivot axis of the upper control links, I did not consider the impact that angling the upper link pivot axis in the side view would have on anti-squat. I was focused on getting the right toe change and finding a mounting location that would fall on the frame, so thanks for pointing that out because it's worth considering.
In the pure stock Chapman strut configuration, the trailing link angle is what defines the amount of anti-squat, which I retained. The addition of the upper control links probably affects that as you mentioned, though I'm not sure at the moment how to calculate it. Perhaps someone else may have an idea, otherwise perhaps the Lotus simulation software will show it.
Edit to add: The bellcrank pivot is going to take some brainstorming. I don't have the solution figured out yet, but I suspect the forward wall of the strut tower will play a part in locating the mount.
[This message has been edited by Bloozberry (edited 01-15-2013).]
Well, I think I've finally reached the Eureka Moment I've been working towards, but not before first having to throw out the design from my last post (again). I'll take a few moments to explain why my previous configuration wasn't going to work.
For starters, after a lot of research I became painfully aware that the 18" X 9.5 ET38 rear wheels that suited the length of the 3" extended HT lower links, were made of "unobtainium". Sure, I could have had the wheels custom made for no less than about $1000 each, delivered to my door, but then any company that could make the rear wheels to my specs could not make matching 17" front wheels to my specs as well. It seems very few companies make a custom 17" wheel anymore, and those that do, either don't offer designs that I find attractive, or offer nothing higher than a 44mm offset for a 7" wide wheel. I need a minimum 48mm offset for the fronts.
I decided that if I had to make a compromise regardless whether I bought expensive custom wheels or inexpensive off-the-shelf wheels, then it would be foolish to waste money. During the roughly two-years of scouring the internet for off-the-shelf wheels, I only ever found one manufacturer that offered the style I wanted in staggered sizes in the right dimensions, except that the rear rims would have to be 9" rather than the 9.5" wide as I had designed for. Luckily the 265 section tires I bought for the rear are approved by Goodyear for mounting on a 9" rim.
The 9" rear wheels unfortunately meant that I had to redraw the rear suspension to accommodate the available wheel offset otherwise they would have stuck too far in. At first I thought it would be a breeze to simply move all the links outboard as necessary, but then I didn't like how the new upper links would have had to be embedded in lower frame rail pockets (like the '84-'87 cars do with the lower control arms in the cradle), weakening it. There were other reasons too that I won't get into for the sake of brevity.
With the knowledge I've gained from doing this exercise more times than I cared to count, I started what I dubbed the "final redesign". As Archie once put it: "At some point you have to take the pencil away from the engineer." My wife was threatening to do just that. This time before even putting pencil to paper I chose a static rear roll center to give a roll axis of 2.7 degrees sloping down to the front; I wanted the 0.7:1 camber to roll ratio, minimal roll center movement, no toe gain, and I wanted none of the link mounts inside frame rail pockets, nor to protrude into the engine bay. With that, I hit the drawing board.
It took some coaxing and some teasing and some more head-scratching but I finally came up with a no compromise solution. The key to solving the problem was to ditch the 3" extended HT lower links that were artificially constraining the design. That freed up the ability to move the lower link mounts outboard substantially, until they cleared the overhanging F40 transmission, and then upwards to re-establish roughly horizontal, shorter, lower links. A few digital sketches were needed to locate the upper links for the right camber curve and roll center height/movement, and all that remained was to await the dynamic results from Zac88GT. I got those this morning, plotted them, and couldn't be happier. I'll post the results tomorrow showing how this last design performs compared to most of the others I've been through in order to give anybody that cares an opportunity to catch anything I may have missed. Stay tuned!
OK, so here is the final set of graphs showing how the various suspension modifications stack up against each other compared to stock. They are the same as ones I've posted previously with the exception that I've added the reddish-brown curves which represent the Short/Long Arm (SLA) suspension dynamics of my latest design. The key to realizing the significance here is that despite an overall 2" drop in the rear, I've managed to improve the suspension's theoretical performance, and maintain a 5" ground clearance. I'll explain the significance of each reddish-brown-colored curve briefly:
Camber vs Roll:
Here's one area where the SLA design shines over the stock Chapman strut. Where the rate of camber change decreases under jounce with the Chapman strut, the SLA can be tuned to do whatever you want. The little pictogram below illustrates what happens to the angle of the tires relative to the road in a 6 degree roll.
The SLA design keeps the outside tire (the one with all the weight on it) nearly perpendicular to the road surface throughout the entire range, whereas the stock '88 Fiero geometry tilts the tread patch away from the road by about 3.7 degrees in a 6 degree roll. The lesser the angle between road and the outside tire, the greater the potential cornering power.
Roll Center Migration vs Roll:
Here again having upper control arms or links that change angles with respect to the chassis allows a significant ability to control the roll center movement over the fixed angle of the Chapman strut design. As before, I've drawn a scale representation of the above graph showing where the roll center migrates to on the '88 Fiero.
With an SLA design, you can eliminate roll center movement entirely, though I needed to make a small concession in this area to keep the control arm mounts outside the engine bay and outside the frame rail.
Roll Moment vs Bump:
By raising the roll center and controlling it's movement, yet another beneficial trait appears, namely a smaller and more stable roll moment.
Recall that the greater the difference in height between the center of gravity and the roll center, the greater the leverage that forces acting on the center of gravity have on rolling and pitching the car. The above graph shows nearly a 40% reduction in the roll moment at most points of the SLA suspension travel versus stock.
Camber vs Bump:
A side effect of greater camber gain in roll, is also greater camber gain in bump.
Here the graph shows that as the suspension compresses or extends under jounce and rebound, the camber changes more rapidly than stock for a given amount of travel. Under straight line acceleration, this results in a greater loss of traction for the same reasons it increases traction in a turn, so the key here will be to limit squatting under acceleration.
Anti-Squat:
My new SLA geometry shows that for any given amount of jounce or rebound, the anti-squat remains nearly constant, though it is less than the stock configuration at most points.
Since the angle of the trailing link is what governs the amount of anti-squat, I plan to mount the forward end of the trailing link to a bracket with multiple holes that drops down from the lower frame rail, rather than to a fixed point on the engine cradle. It should be easy to make the trailing link angle, and thereby the anti-squat, tuneable.
Toe vs Bump:
Lastly, I struggled somewhat to understand GM's use of a shorter forward lower lateral link in the stock suspension only to have it produce a tenth of a degree of toe-in, and only under the most extreme cases of suspension travel.
In order for me to achieve the same amount of toe-in with the SLA design, I would have had to make the forward links a whopping 0.5mm (0.020") shorter than the aft links, top and bottom. It just isn't practical to work in that scale, so I hypothesized that other factors must have been at play with the Chapman strut which would have caused the rear to toe out unless the forward lateral links were made shorter. (Since toe-out in the rear causes oversteer in a turn, it has to be kept in check.) I don't believe this is the case with the SLA design since the data shows that with equal length fore and aft lateral links, the toe basically mimics the stock Fiero suspension in jounce, where it matters most. Of course bushing deflections aren't taken into account here.
It has happened to me more times than I can remember, when a design was going well and pretty far along, only to be completely destroyed due to an off the shelf part being non-existant.
Thanks guys for the comments. I am discontinuing my involvement with PFF from this point forward. For those interested in following my F355 build, you can look me up at the Canadian kit car website Madmechanics.com under the same user name. See you over there.
I am member over there too (same username). You will give me a reason to check it more often.
Thanks guys for the comments. I am discontinuing my involvement with PFF from this point forward. For those interested in following my F355 build, you can look me up at the Canadian kit car website Madmechanics.com under the same user name. See you over there.
Please reconsider leaving PFF, you are a well respected member with lots to contributate.
Please reconsider leaving PFF, you are a well respected member with lots to contributate.
X2! Please don't pack up and leave. Blooz you are in a very small handful of people here that are as helpful as they are knowledgable...the people like me need the people like you!
[This message has been edited by 85sliverGT (edited 02-09-2013).]
Thanks for your support guys. The most recent changes to the forum have given me reason to consider staying on. Kudos go to Cliff for making a simple, but much needed change regarding how the forum is moderated... I hope it lasts. I'll update my thread in a couple days.
Blooz, getting back to the topic at hand, once upon a time, I used to consider myself very ocd and anal retentive, you've convinced me I'm no such thing, j/k , I doubt anybody has put near as much effort into Fiero suspension as you, now "put down the pencil" and build the thing.
Thanks Joe... (at least I think it was a compliment)
I've finally completed the schematics of what I plan to build for the rear suspension, well, at least as far as I'm going to develop them in electronic format. They aren't 100% complete because I've reached a point of diminishing returns (and I'm starting to get cabin fever). Before I describe the important aspects of the design, I want to mention that I've strayed from my original goal of making this an easily retrofittable modification. I apologize to anyone who might have been hoping to duplicate my design, but I ultimately had to choose between either a relatively simple modification that would not work very well, or a more involved project that would give good performance. Since I am primarily building this for myself and had no plans to market any kits, I chose the latter. Now, onto the final configuration...
As I mentioned in one of my previous posts, I had a breakthrough in the design process when I finally ditched the idea of keeping the extended Held/HT Motorsports lateral links I had bought earlier. Their length meant that any new upper link mounts would have needed to protrude into the engine bay if I wanted decent suspension performance. With that constraint removed, I shortened the lower lateral links considerably in my drawings until the inboards mounts cleared the overhanging F40 transmission on the LH side. That then allowed me to raise their mounting points as well. The rest of the geometry was a piece of cake from that point. With the geometry set in stone the first thing I needed to do was design a new cradle to tie the new lower link mounts to a framework that included the front and rear cradle mounts:
THE CRADLE
Since my OEM cradle was rusted out anyways, I started a clean sheet design by placing everything I had (engine, transmission, raised cradle mounting points, and new lower lateral link locations) on a 3-view drawing, then drew a set of four 2" X 3" X 1/8" beams to attach all the known points together. I chose this size tubing to maintain a cross sectional area that was at least as large as the OEM for greater strength. While the design of the cradle side rails is basically complete, the shapes and locations of the cradle cross members aren't. They're just "fill-ins" until I have the time to design mounts for the Northstar, the F40, and the exhaust layout. Here is the top view with the new cradle in blue compared to the OEM cradle outlined in red:
From this view you can see that I moved the forward lower lateral link mounts outboard by 96 mm and the aft link mounts 123 mm further outboard, per side, than stock. Two neat things followed: the first was that both link mounts now lined up perfectly with the front cradle mounting point, eliminating any need to joggle the cradle side rails inward like the OEM cradle (I believe this will make the cradle stiffer); the second was that it added some wiggle room at either end of the engine/transmission. One side effect however was that with the new side rails being further apart, the rear cradle mounts would have to be offset inwards to meet up with the lower frame rail of the car. At first this concerned me knowing that I would have to create offset rear cradle mounts until I realized that the OEM mounts are offset by an equal amount outwards. That meant they could be swapped sides and all would be well again. I also shortened them to raise the cradle 25 mm. Here's the rear view showing what I mean... again, the blue is new, red is OEM:
I'll evaluate the need for gusseting the rear mounts better once I get into the actual fabrication of the cradle.
Finally, here's the RH side view showing the profile of the cradle side rails.
The important features to note are that:
1. the cradle mounting points have been lowered 25 mm from stock, which will result in the cradle being raised up into the chassis;
2. the lower lateral link mounts have been raised an additional 70 mm from the stock location as well as having been moved outboard;
3. the cradle bottom extends 10 mm below the oil pan allowing a skid plate or other oil pan protection to be installed; and
4. the rear cradle kicks up at the rear allowing for such features as exhaust system clearances, a marginally better ramp angle, and/or the installation of towing hooks.
Up next, the whole schemozzle installed in the chassis along with the new upper links.
(edited to update drawings to "as built" configuration)
[This message has been edited by Bloozberry (edited 07-26-2013).]
Here's the side view of the final configuration (well at least until I start actually building it!). You always see a better way of doing things when you're actually sitting in an empty engine bay:
Starting from the bottom left of the image, the important things to take from this view are the following:
1. the purple piece that extends and meets the front cradle mount is a 4" square tube that was welded into place by the previous owner as a means to extend the cradle mount back for the 3" frame stretch. If I had my druthers, I would have just made the new cradle longer but this 4 X 4 is very securely welded in place and I have no intention of trying to get it out, so I'm working with it;
2. the forward end of the trailing link will now be captured on both sides with mounting ears (dark green) that drop down from the lower frame rail. This is different from OEM where the trailing link is pinned to the side rails of the cradle. The trailing link is what transmits the accelerative and decelerative forces to the chassis so with the wider tires and greater torque of the V8, I wanted the trailing link captured on both sides. Notice how I plan to add a few extra mounting holes above the stock angle to allow easy changes to anti-squat;
3. the upper and lower lateral links are in light purple. From the side view it's obvious the lower links are nearly horizontal (though it's even more evident from the rear view). The new upper links are attached to the knuckle with a pair of custom brackets (in blue) in-line with the old upper strut mounting hole. The aft bracket is just a flat plate of steel and is really only necessary to locate the lower end of the shock pushrod. The forward bracket is more complicated since it serves to locate the lower end of the shock pushrod AND provide the correct angle for the upper forward link. I wanted the forward upper link to triangulate the top of the knuckle for better knuckle control under various loads. The angles are too large for hemispherical joints to make up the difference by themselves so the bracket is needed to keep the joints within their design limits.
4. the upper links are attached to the top surface of the lower frame rail with custom brackets (dark green). These brackets are clearer in the rear view.
5. above the knuckle you'll see the pushrod bell crank that's attached to a cutaway beam (in red). I haven't completed the bell crank mount yet because I haven't decided exactly what will work best. One idea is to use a beam as shown welding the aft end to the old strut tower wall, and the forward end to the new pink transverse beam.
6. The forward shock mount is going to be anchored onto a custom-shaped 3" X 3-1/2" transverse beam (in pink) that will also act as decklid hinge mount, and Northstar torque strut mount. Tearing a page from (PFF member) Yarmouth Fiero's book of design, I'll remove the lower half of the stock transverse beam under the rear window to raise the new beam high enough to solidly anchor it to the two OEM upper frame rails. The stock upper rails are only attached to the sheet metal of the B-pillar so this should add some rigidity. The new transverse beam will also be tied into the lower frame rail with at least one vertical 3" X 1" rectangular tube per side.
Here's the rear view:
Some of the more important points in this view are:
1. the angles and lengths of the upper and lower links. Starting with the lowers, the forward ones were shortened 14 mm and the aft ones 43 mm over stock, despite the overall track width increase. The added track width is now accomplished through the widening of the cradle. Those paying attention to details will also notice that the overall track width increase was dropped from 6" to 5". As for the upper links, the aft one is shown in this view while the forward one is shown in the drawing below. Their inner pivot points are in line with each other both transversally and vertically.
2. the static angle of the axle is 7.4 degrees which is not bad at all considering tripot joints need a minimum of a degree or so to maintain proper lubrication. At 76 mm jounce the axle is at 18.4 degrees and at full rebound -3.9 degrees, which are within a typical CV joint's maximum operating angle of 24 degrees, though I doubt I will allow that much travel. As for compression and extension, the axle length changes by 20 mm from full jounce to full rebound, easily within the safe operating range of the tripot. Now I can start planning the source for my axles.
3. the bell crank mount beam is clearer in this view as is the firewall transverse beam and it's link to the lower frame rail. I'm pretty confident I will be able to come up with a satisfactory bell crank mounting system. I've superimposed the wheel and tire at 76 mm jounce (in light grey) to show how the bell crank mount must be on the inboard side of the bell crank, otherwise it would likely interfere with wheel travel. If all else fails, I have left enough room above the knuckle to replace the pushrod with a coil over shock absorber (not a strut), mounting the upper end to a custom bracket at the top of the strut tower. On the other hand, if the pushrod system does work out, the strut towers will sectioned and greatly reduced in size.
Here's the rear view of how the upper forward lateral link mounts to the lower frame rail:
Finally, here's the top view.
This wraps things up showing the final details of the custom bracketry for the upper links,. Again, notice how they don't impede into the engine bay making the loaded cradle installation much easier than if they had.
As always, I'm open to constructive advice so don't be shy to point something out if you think there may be a better way of doing things. Keep in mind that the locations of the links can not be changed, though the way they are mounted can be. The entire pushrod shock system is not subject to the same constraints, so if you see an alternate approach to orienting, mounting, or strengthening an area, don't be shy to discuss it.
Great job on the drawings Blooz. I know how much work it is to get those drawings so accurate. I have been measuring and drawing.... and remeasuring and redrawing all winter and I haven't progressed half as far as you. I think your design is well thoughtout and I, like many others are waiting to see your execution of the fabrication.
I see that you are going to run your transverse frame to the outer edges of the upper frame rails which will put it inside the B pillars; do you see yourself opening the pillar up to secure the end of the frame? I may have better access as I'm cutting my B pillar off at that point.
Also, thank you for adding the cross section of the 355 body as I have been just going on a wing and a prayer ( and a few photos from your shop) that my rocker reinforcements would be well inside the final body shape in the vacinity of the firewall and wheel well.
Thanks Bob and Graham... I too am looking forward to gettin' goin'.
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Originally posted by Yarmouth Fiero: I see that you are going to run your transverse frame to the outer edges of the upper frame rails which will put it inside the B pillars; do you see yourself opening the pillar up to secure the end of the frame?
My job was made easier when the previous owner cut away the B-pillar as well, then welded in a tubular brace running from the rear edge of the roof down to the upper frame rail in line with the strut tower. Here's a picture of what it looks like now, only the hole where the B-pillar was is covered with bondo. I'm sure there are surprises under there...
Damn! Gorgeous drawings. Which software are you employing? Even while asking this I am reminding myself that the true contribution made is the measuring you have done to arrive at the drawings. That is the part that I have found challenging in my own endeavors as I attempt to achieve as much accuracy as possible.
I too am modifying my subframe, but not to the extent that you are. Like you, however, I am raising my lower inboard links 1" to somewhat coordinate with my 1 1/2" raised spindles up front. A thing that caught my attention about the relative locations of the lower links is that the forward link is shorter as you have indicated. I wondered why, so I took the links through their range of motion and found that the result was a small amount (1/32-1/16) of toe-in that resulted in jounce. It seems to me to make sense with a rear-biased weight wanting to easily come around that the engineers may have wanted to either compensate for bushing deformation or merely wanted to make sure there was some toe-in and not toe-out, which would seemingly contribute to some dicey handling. The result for me, then, is to retain the offset just to be conservative. But then, my timidity may be a result of my in-the-body memory of the time I drove my brother's 356 Porsche in to a turn at a pretty quick pace and through some miracle was able not to wreck it as the back end wanted to be in control of the whole process.
edit to add: Sorry, Blooze. I just read in a prior post where you have considered this and feel that it is resolved with the more rigid SLA design.
Ken
------------------ '88 Formula V6 '88 GT TPI V8
[This message has been edited by kennn (edited 02-17-2013).]
Thanks Kennn. You certainly got the part about the true contribution right! I have yet to tally up the time I've spent just running back and forth taking and re-taking measurements, then still finding something that didn't add up. I do keep track of the time I spend in a spreadsheet though, so it will be interesting when I find the time to plot out the hours for different tasks.
As for the drawings, I wanted to keep the source files accessible to the largest possible audience so I use the simple vector-based drawing tool available in MS Office. In hindsight, I should've used a more sophisticated CAD program, but again when I make the source files available, just about anyone will be able to manipulate them as they are now.
Thanks for your support guys. The most recent changes to the forum have given me reason to consider staying on. Kudos go to Cliff for making a simple, but much needed change regarding how the forum is moderated... I hope it lasts. I'll update my thread in a couple days.
I let a few run me off for a couple of years and went to the Fiero kit car forum during that time. Decided to come back after a name change to the forum and not having a kit car anymore. I decided to not let the Hatters get to me. So I understand why you wanted to but really glad you are here to stay.
In regards to your upper forward lateral link pocket mount. I'm not sure how you plan to weld it in, but what I have found with that type of pocket is that it's a lot easier to make the full pocket. Then cut your opening front and back so that you can just slide in your pocket, weld it on both sides of the frame rail and have a solid full weld with no open holes for water to get in. Also use a 10 degree or so angle on the bottom of the pocket so water and gunk doesn't collect there.
I made a quick sketch it's not to scale or detailed but it should help you understand my text
Access for welding. Frame rail gets closed up. You have access to the bolt (also weld a nut on the back end since you won't have access to that after).
Before cutting your angles into the pocket. Cut your opening in the frame rail, slide in your pocket. That way you can scribe both sides with your exact contours. In order to get the nut to pass though the opening cut a slit, peal back the sheet metal (that way to close you just have to bend back the sheet and lay a short weld bead).
I'm not trying to tell you how to fabricate, I'm just giving you options and ideas.
Nice to see that you decided to raise the inner lower link mounts too.
[This message has been edited by aaron88 (edited 02-26-2013).]
Thanks for the input Aaron. I think you may have misinterpreted my drawing though. I can see how you might have mistaken my upper inboard mount as being inside the frame rail, but if you look closely, it's actually sitting on top of the rail. If you look in your own engine bay, you'll see that the top surface of the lower rail is fully exposed inside the compartment. Since my new mount will be welded on top of the rail, I'll have full access to the pivot bolt for the link inside the engine bay. The hole in the sheet metal where the link protrudes through is actually just the metal wheel house liner. Hope that helps clear things up.
Got it! It was late when I read through your latest posts, I guess my brain wasn't really working well.
You've got me re-thinking my SLA design though. I've got my current design done (which is a strut design) and I'm about to make up the fixtures for the lower control arms, but I'm looking at my design and I still don't like that I only gain 1.8 degrees of negative camber per inch of travel. I'd rather have 3.4. My SLA design mounted the upper control arm rearward not forward like yours. I just don't seem to want to go though the extra expenses, but I'm starting to re-think it now.
my current design
You said you needed to pass an inspection for safety. So are your upper link rod ends DOT approved for street (I know there are some but they are expensive)? Or is there something I missed when it comes to upper mounts?
One cool thing about my SLA design was that it was swapable. Rubber bushings, poly and rod ends were all interchangeable. So if I was going to do a track weekend I cloud just swap in my rod ends(...then do an alignment).
Always amazed at your detail Blooze! It's always a treat to get caught up on this thread. I too am very glad you decided to stick around. Thanks for sharing all your hard work!
Thanks Katatak, glad to see you're back in the game again.
Aaron, your drawing looks great. I'm not sure what year knuckles you're using but it looks like an '84-'87 given the lower ball joint, but then I don't see a tie rod. Are you using your CAD program to simulate how much camber gain you're getting?
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Originally posted by aaron88: I still don't like that I only gain 1.8 degrees of negative camber per inch of travel. I'd rather have 3.4.
That is the fundamental limitation with a strut design... not only is camber gain limited, but the rate of camber gain decreases rapidly as the wheel moves up in jounce. This is one of several reasons I moved away from the strut design. If you have '88 knuckles, the modifications to get rid of the strut aren't really that complicated once you determine the new coordinates for your upper and lower control arm pivots. You certainly don't need a pushrod shock absorber to complicate your design since there is room vertically to replace the strut with a shock absorber in place of the pushrod. The key is to decouple the rigid mount of the shock to the top of the knuckle by making it a pivot since a rigid mount is what limits the camber change. If you have '84-'87 knuckles, I'm sure there is a way to do the same, I just haven't studied the particulars of that set up.
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Originally posted by aaron88: You said you needed to pass an inspection for safety. So are your upper link rod ends DOT approved for street (I know there are some but they are expensive)?
These are the rod ends I'm considering:
QA1 HMR8-10 (5/8R" X 1/2") RH thread QA1 HML8-10 (5/8L" X 1/2") LH thread
The QA1 website details all of the properties needed to help make a decision which one is right for your application.
If I understand correctly though, you believe I'm on the right track, but designing for conditions that are outside the likely conditions the car will ever see. Because of that, you believe that a whole sale redesign of the rear suspension is overkill. You're perhaps right, and it's good that I am challenged to explain my position because it forces me to take a step back and rationalize what I'm doing and why.
When I finally added the engine and transmission to the drawings, things started looking brighter.1.JPG)
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I'll take a few moments to explain why my previous configuration wasn't going to work.
