Thanks for that timely reminder to sift through your old pics Don... it opened my eyes to another possibility that I had not yet considered. For those that weren't aware, 355Fiero (Don) bought an IFG kit a few years before me and painfully reworked every single panel in a major way. To say he "tweaked" them would be woefully understated. He sent me nearly 200 photos of how he did it, which have at once inspired and discouraged me. Inspired, because now I know this kit can be made into something great, but discouraged because of all the work that lies ahead! So, thanks Don. (If you want to see what I'm talking about, you'll have to prod him to start a thread showing off how he spent several years turning a pig into a purse.

So to get back on track with rear suspension mods, I decided to take an iterative approach to modifying the baseline drawing in my last post, starting first with cradle and link modifications, then on to new wheels, followed by a spring drop, and finally a fender reshaping. I needed to find a method of keeping track of each successive change I made so I could backtrack in case an earlier modification conflicted with a later one. So I decided to highlight changes in each successive stage in red. Only new changes from one stage of modification to the next are highlighted, not the cumulative changes from the baseline drawing. If that doesn't make sense, it soon will as I describe the next set of drawings.

Stage 1 - Cradle Raising & HT Links.

From my earlier kinematics analysis, I knew that I would achieve most of the needed lowering with a spring drop, and needed either to raise the inboard lateral link mounts relative to the cradle, or to raise the entire cradle up into the chassis to counteract the adverse effects the spring drop has on the lateral and trailing link angles. So with measuring tape in hand, I quickly realized that if I took the raised inboard lateral link mount option, interference with the F40 transmission would limit the amount to about 25 mm, with a 10 mm clearance for power train movement. I abandoned that option when it appeared I could raise the entire cradle a full two inches instead.

Once I had drawn out that scenario and confirmed that a 1" diameter axle wouldn't contact the underside of the lower frame rail at full jounce, I was quite happy with the result. That is, until I took a few height measurements of the newly raised Northstar engine relative to the stationary decklid. Raising the cradle two inches would've put the highest point of the engine 5 mm above the underside of the decklid, not including any room for powertrain movement. That wasn't going to work.

In the end, I worked my way from the top down rather than from the bottom up. I started by choosing a 20mm clearance between the underside of the decklid and the top of the engine, which then automatically determined the relative location of the transmission, which subsequently set the maximum height of the inboard forward lateral link mount allowing for 10 mm of powertrain movement, and automatically set the oil pan level in relation to the cradle bottom. This also established the axle location. With all these interrelated vertical dimensions resolved, that only left room for a 25 mm (1 inch) raising of the cradle. In this side view drawing all of the areas of concern are in red: the transmission to lateral link area, the shortened rear cradle mounts, the lowered front cradle mounts, and the F355 decklid height. Notice that the forward cradle cross member is also in red since it interferes with the Northstar engine, and though it appears to interfere with the transmission it does not:



To keep things a bit less cluttered in the above drawing I left out the strut, so here is the same drawing from the rear view, less the engine, but with the strut detail added:



This view shows that I've added the longer lateral and trailing links, lowered the chassis onto the cradle, added the coil-over strut (which needed to be shortened to accommodate the lowered chassis on the cradle), trimmed the weld flange on the upper frame rail, and changed the angle of the axle due to the location of the transmission. I've also drawn the axle using the G6 inner tripot joint with the Fiero outer CV joint. These are all changes to the baseline rear view drawing posted earlier so they are in red, as well as their effects on various measurements. One thing not clearly shown is that the engine/transmission assembly was centered between the frame rails leaving 35 mm of clearance at either end as measured at the crank centerline. Here's a summary of all the impacts of stage 1 cradle raising and HT links vs stock suspension and cradle:

Strut travel in jounce: reduced 13 mm
Strut angle: 1.25 degrees less vertical than stock
Strut to lateral link angle: 1.25 degrees less than stock
Trailing link angle: unchanged
Lateral link angle: unchanged
Axle angle: 1.6 degrees angled further downward
Engine to F355 decklid clearance: 23mm more than stock 2.8L engine
Oil pan to cradle bottom clearance: 10 mm
Transmission to cradle vertical clearance: 10 mm
Half-track: increased 76 mm
Wheel to F355 fender vertical clearance: (edit) reduced by 25 mm
Wheel to F355 fender horizontal offset: reduced 76 mm from stock
CofG height: dropped from 495.3 to 476 mm in combination with the front suspension changes made earlier (lowered 39 mm)

I won't explain the significance of these impacts since this is only an interim level modification and therefore most will change by the time I get through the next 3 stages. Next up is ditching the stock wheels and tires to see how that changes things.

(Edit to change CofG height in drawings and text to account for front and rear suspension drops)

[This message has been edited by Bloozberry (edited 08-18-2012).]

quote Originally posted by Yarmouth Fiero: ...it appears a little diabolical as I attempt to see where you are going with this.......... lower the front cradle pivot point, raise the cradle, lower the rear cradle mount but still show 6.25" of ground clearance......... diabolical indeed.

It might be easier to wrap your head around it if you consider the cradle was held stationary and the chassis was dropped 25 mm by lopping off the top part of the cradle. The ground clearance to the bottom of the cradle remains the same, but the chassis height was lowered in relation to the cradle.

quote Originally posted by Yarmouth Fiero: I haven't looked at one page for so long since I found my dad's Penthouse when I was 10.

Only a fellow engineer would admit that.

[This message has been edited by Bloozberry (edited 06-28-2012).]

[This message has been edited by Bloozberry (edited 06-28-2012).]

HI all

Blooze, or anyone else, how much can you widen the Fiero? Like how wide can you make the front tire, say a 17 inch tall rim, how wide can you make that fit and work before having to go with a wider suspension?

Thanx,

Air-eek

edit: 17 inch tall of course

[This message has been edited by pavo_roddy (edited 06-28-2012).]

Stage 2 - Wheels and Tires

The next order of business was seeing how much difference new wheels and tires would make to the clearance problems. There were six variables to consider : tire diameter, lateral tire placement (depth), tire width, wheel diameter, wheel width, and wheel offset (now I know why there are so many like-new wheels for sale on eBay)

Starting with the tire diameter, I wanted to get a reasonable size to fill the wheel well, but also not be so tall that it would change the rake of the car, causing a need to lower it even more on the springs than otherwise needed. Most people also consider staying with the stock overall tire diameter so as not to throw off the calibration of their speedometers. That was a non-issue for me since I plan to use a GPS calibrated aftermarket speedo. After playing around with the drawings, I decided a tire in the 25" to 25.5" range would be a good compromise between filling the well and not raising the rear too much.

Next I needed to choose the lateral tire placement. I don't like tires that stick out beyond the fenders and besides, I needed to be sure the wheels would tuck in enough under jounce to avoid hitting the fiberglass fender and making a mess. So the goal was to get the new tire to stick out about 11 - 12 mm beyond the lip of the fender with zero wheel camber. That way once the static camber was added, the top of the tire would be flush with the top of the fender opening, as viewed from the top. The math involved is a simple formula using the tire height multiplied by the tangent of the static camber in degrees, ie:

Amount top of tire tucks in = tire height x tan (static camber)
= 643 mm x tan (1.0 degree)
= 643 mm x 0.01745
= 11.2 mm

Two down, four variables to go. Choosing the width of the tire wasn't just a matter of seeing what the maximum width possible was. Instead, to optimize performance (at least theoretically) tire widths should be proportional to the weight distribution to ensure equal tire loading at all four corners. Since the car's weight distribution is approximately 46% front 54% rear, and my front tires are 215's, a 265 mm rear tire gives 45% tire patch up front and 55% out back. Close enough, and it also happens to be the same width as the real F355 too (well... for the back anyways)!

Choosing the wheel diameter was again more than a simple matter of preference. 17 inch wheels with 265mm tires would not clear the top of the knuckle, so 18 inch wheels were the minimum diameter needed. 19 inchers just made too much of a difference with the fronts, which are 17's. So 18 it is. From that decision, the sidewall height automatically fell into my hands since I had already decided the overall tire diameter and width. A 265/35/18 measures 643 mm (or 25.25").

The required rim width was the next thing that needed defining, but the choice of 265 tires narrowed the options. I visited several sites which recommended a minimum of 9.0" and a maximum of 12.0" wide for a 265/35/18. The Goodyear tire site even listed several additional dimensions of the installed tire on a 9.5" wheel, so that's what I chose.

The last variable to choose was the wheel offset, but that was pretty easy since I had already determined how much the tire needed to stick out beyond the fender (before adding camber), that automatically meant the 9.5" wheel would need a 38 mm offset given the amount of sidewall bulge defined in the Goodyear tables.

That's all the data I needed to know to be able to draw the wheel and tire assembly, which I then added to the latest suspension drawing, and here's the result:



The only things drawn in red are things that have changed since the Stage 1 drawing from my last post. The most noteworthy are that the overall ground clearance of the cradle (not the floor pan, Don) was increased by 20 mm, yet the fender gap only closed up 7 mm up top. The reason for the apparent dichotomy is that the new tires have much stiffer sidewalls than the original 215/60/15 tires so the tires are rounder, lifting the chassis up from the bottom more than it closes the gap at the top. Who'da thunk it?

Other notes:

-The rotor was redrawn to reflect the 12" Corvette brakes, which added to the impact the new tire and wheel had on the half track width.

- The CofG was raised about 14 mm due to the taller tires raising the entire rear half of the chassis;

- The vertical clearance of the tire to the fender still remains at an unsightly 130mm (5.1")

Next: Stage 3- Spring Drop.

(Edit: changed CofG in drawing and text to account for combined effects of front and rear suspension drops)

[This message has been edited by Bloozberry (edited 08-18-2012).]