[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:

quote 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.

quote 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.

quote 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!

quote 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.

quote 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.

quote 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.

quote 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.

quote 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.

quote 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.

quote 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.

quote 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).]

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'88 Formula V6
'88 GT TPI V8

[This message has been edited by Bloozberry (edited 01-15-2013).]