Picking the "ideal" fender gap is obviously subjective, but there are more things to consider than just the looks. I like the fender gap on Don's car in his photos above, so I would consider that pretty close to "ideal looking", but then Don mentioned that to get his car looking that way he finds himself with less than the desired ground clearance. On the other hand, had he raised the cradle up into the car like I'm proposing, the tire to upper frame rail clearance may have become an issue. So the answer is inevitably a compromise between looks, available bump travel, kinematic performance, and ground clearance. Each builder will have his own unique set of priorities.

Changing topics for a minute here, in case you're the type that notices these things, I've changed the title of the Stage 3A modification from 30 MM Spring Drop & 24 MM Fender Mod, to: 149MM Cradle Ground Clearance & 24 MM Lowered Fender Mod. I realized that once I added the modified lower adapter, the length of the spring drop becomes meaningless, so I combined the effects of the spring and adapter into a single more easily understood cradle to ground clearance measurement.

Stage 3B - 125MM Cradle Ground Clearance & 24 MM Lowered Fender Mod

Similar to Stage 3A, this modification uses the modified lower strut adapter to regain jounce travel and tilt the bottom of the strut further outboard. The major change here is that the spring was shortened even more than stage 3A to achieve a tire to fender clearance of about 2" (51mm with zero camber).



Doing so caused several side effects though:

a. it increased the angle of the axle to 7 degrees. Old-schoolers might find this excessive but then the axle isn't terminated with universal joints, but rather constant velocity joints. Automotive CV joints actually require at least 2 degrees of angle to maintain relative motion between the internal components of the tripot joint and provide proper lubrication. Furthermore, typical automotive CV joints can operate at up to about 26 degrees without detrimental effects;

b. the angle of the lateral links increases to about 7.7 degrees. This wields a strut to lateral link angle of approximately 74.2 degrees which is slightly worse than the stock angle of 73.5 degrees. The result is that the camber curve will be slightly less progressive than stock, which is a move in the wrong direction. Regardless, this is considerably better than simply having lowered the stock suspension by 2" as most people do, which would have resulted in an even worse 80.5 degree separation between the strut and the lateral link;

c. the strut angle drops a further 1.1 degrees away from vertical over Step 3A and a full 7 degrees more than stock. This should help limit the lateral movement of the roll center, but will necessitate an upper strut wedge spacer to keep the upper strut bushing within a reasonable amount of deflection;

d. the maximum strut travel in jounce is reduced by 5 mm from stock (93mm) to 88mm. Given the higher rate 350 lb/in springs, this amount of jounce should be adequate to prevent bottoming out on the struts; and

e. the cradle ground clearance is reduced from 6.25" in the stock configuration to 4.9". Unless the floor pan is modified slightly, the floor pan's clearance will be 1" less or 3.9". This means that if the struts are allowed to bottom out, the floor pan will be approximately 1.5" above the ground;

f. the car's center of gravity is reduced by approximately 14 mm over stage 3A and by 22.5 mm over stock.

There are several other side effects of this modification, though these are also impacted slightly by the addition of 1 degree of negative camber to the rear wheel as shown in the above drawing. When the effects of the camber are added to those of the spring drop:

g. the half track increases by 1.6mm. This is a combined effect of the narrowing that results from angling the lateral links upwards, and a widening from the negative camber which pushes out the tire contact patch;

h. the fender gap decreases by 2mm more than the simple 24mm chassis drop due to the tilting of the tire top in negative camber;

i. the outside top edge of the tire gets pulled in so that it is in line with the outside edge of the fender at ride height. This is an area that I will focus on again later when I get the kinematic suspension results back from the Lotus Suspension Analyzer program. Ideally, as the rear suspension compresses beyond the 49mm vertical clearance to the fender, the dynamic camber curve should tuck the tire top in enough to clear the fender horizontally and allow the maximum strut travel (88mm) without the tire contacting the fiberglass. If the software shows that there is insufficient tuck-in, then slightly higher offset wheels or additional fender modifications would be needed; and finally

j. the upper frame rail is modified to gain the additional clearance as shown. Without this modification, the rotating tire would contact the underside of the frame rail limiting wheel travel in jounce to 50mm.

To get an idea what a 2" fender gap will look like, I'll post a few pictures in my next post. Again, this modification step represents the largest suspension angles I'd likely be willing to accept to achieve a good looking fender gap, whereas step 3A illustrates the maximum acceptable fender gap with the least angular impact to the suspension. Neither option explores the possibility of lowering the fiberglass fender more than 24mm, but this is certainly an option as well. My final decision on how to proceed will be made after I show the Lotus Suspension Analyzer data.

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

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

Hey Blooze, those tires are very nice. Can't wait to see them installed on rims
and on the car permenantly. I suppose that will be 2 years from now??

Keep it going Blooze...

------------------
fierogt28

88 GT, Loaded, 5-speed.
88 GT, 5-speed. All original.

Thanks for the comments there fierogt28! Your guestimate for a completion date is probably pretty accurate. I'm hoping to make great strides this fall and winter as I've cleared my schedule for "me-time".

Well, it's been a while since I last posted an update here... so long in fact that something had to be done to get my thread off the second page of the Construction Zone. The good news is I have more to report than just a simple shameless bump. The bad news is that it looks like I'll be going back to the drawing board.

A few weeks ago, I sent a spreadsheet full of suspension coordinates representing stages 3A and 3B off to Zac88GT. He's been kind enough to run all my data through his Lotus Suspension Analyzer program and send me the results. I plotted the information within a day or so of hearing back from him, but I've been hesitant to post the results because they weren't what I expected, and I had no plan B (more like plan F at this point!).

I won't bother posting all the graphs again since I doubt I will use either Stage 3A or 3B modifications, however I'll summarize the results here. As I began plotting the data, the camber, toe, and anti-squat versus bump were encouraging... both new suspension configurations showed notable improvements over stock performance. The roll couple (Roll Center to CG Vertical Distance) vs bump was pretty much on par with stock performance for stage 3A, but was significantly worse than stock for stage 3B. The camber vs roll angle plots for both were marginally worse than stock as well.

The show stopper for both configurations was the roll center vs chassis roll plots. I'll post these diagrams because a picture is worth a thousand words. In this first plot, the blue line represents the stock movement of the rear roll center as the chassis rolls up to 6 degrees in either direction. The pink and red lines depict options discussed earlier in this thread. The orange and green lines show how much the roll center migrates with stage 3A and 3B modifications respectively:





Clearly roll center movement in both vertical and lateral planes is significantly worse than stock, which is bad enough to begin with. To help visualize the impact of the roll center movement, I superimposed the plots from the stock suspension and stages 3A and 3B over top of a scale drawing of the stage 3B rear suspension below. The squares represent 10 cms (almost 4"). Bear in mind that I stopped connecting the dots for all three plots at 3.5 degrees of roll since the scale of the entire drawing would have been unreadable if I had continued the orange line out to 6 degrees. I did leave the data points on the other two plots to give an idea how they behaved beyond 3.5 degrees, in 1/2 degree increments.



So where to go from here? I know I've said the primary goal was to get the car looking good, with performance a secondary objective, but this is just too much of a deviation to accept without at least studying other alternatives. I've been inspired by Datsum1973 (a PFF member) to look into a short/long arm (SLA) style rear suspension:



(from this thread: www.fiero.nl/forum/Forum2/HTML/117227-9.html ).

I've spent several hours now measuring the rear chassis, studying my previous front and rear drawings, and generally assessing the feasibility of his design. My first impressions are that it looks do-able, so I'll spend the next while brainstorming and posting my thoughts on how his design might be adapted to suit my car, then hopefully Zac will still be around to provide the ever-important number-crunching.

[This message has been edited by Bloozberry (edited 08-28-2012).]