I have a question regarding your strut offset brackets. They appeared in your last entry but I can't see where you mentioned the offset you chose. You indicated a 6" track wide increase so can I assume that meant a 3" strut offset?
Yes... the correct offset adapters are part of the HT track widening package.
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Originally posted by Yarmouth Fiero: Also, the 1.25 deg reduction in strut angle is due to the 25mm cradle pivot point move?
No... the slight change in strut angle occurs because the HT strut adapters seem to be made (either by design or fluke) to give the maximum range of negative camber possible. If you go back to the stock suspension drawing, you'll see that the bolt in the slotted hole at the bottom of the strut is centered in the slot when the camber is zero. In my last drawing with the adpater the bolt hole is all the way to one side of the slot. For me to be able to obtrain the same 16.5 degrees on the strut, the slotted hole would have needed to be slotted even more. As I center the bolt in the slotted hole, the camber goes more negative... which is a good thing because it allows up to about two degrees of neg camber before running out of adjustability. That's better than the other way around.
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Originally posted by Yarmouth Fiero: Will this require a tapered shim at the top of the strut?
Not at this low angle, since the upper strut bushing can handle the slight variance. If you go back several pages where I show the stock suspension metrics using drawings, you'll see that in the stock config the strut moves through an angular change of around 3.5 degrees from full jounce to full rebound. It's the job of the bushing to handle that change. In my next stage of modifications though, you'll see that I do end up using a tapered shim at the top as you suggest, but only because I purposely tilt the strut over even more (as I've hinted at several pages earlier) as a means to control the lateral movement of the roll center somewhat more than stock.
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Originally posted by Yarmouth Fiero: I see the strut offset brackets for sale on the AMS site with various offsets available, but they still talk about a kit to relocate the top of the strut in the strut tower. Why would this be?
I'm not 100% sure what site you're referring to, but it may be to convert pre-88's with an '88 cradle since the strut tops are located closer together in the '88's. If that doesn't explain it, then perhaps you should attach a website link so I can have a better look.
Thanks doublec4! But given the drastic measures needed for the rear of this thing, the jury is still out whether the rear suspension is going to be any better than stock.
Stage 3A - 149 MM Cradle Ground Clearance & 24 MM Fender Mod
Well, with 130 mm gap still left between the top of the tire and the fender, it's apparent that a combination of spring drop and fender mods are going to be needed to get a decent looking car out of this. Rather than make two separate stages, one with a spring drop and the other with a fender mod, I decided to analyze two different spring drops in separate stages combined with fiberglass fender mods in both. The first is this one where I lower the car 30 mm and reshape the rear fender by lowering the edge 1" as Don suggested. This combination results in a 5.9" (149mm) ground clearance with the cradle, and decreases the fender gap to 76 mm (3.0") which is still a bit too much for my liking, but it is worth looking at in depth to weigh the pros and cons of appearance vs performance between this stage and the next.
So here is the drawing for now, the Lotus Suspension Analyzer results will follow after the schematics of both stage 3A and 3B, as long as Zac88GT is still out there. :crosses fingers:
In a rather roundabout way, I achieved the 30 mm drop in the chassis by modifing lower strut adapters. This new design drops the entire chassis by 50mm but when combined with a lengthening of the coilovers by 30 mm, the resultant chassis drop is 30mm. You'd think this would result in a 20mm chassis drop, but the angle of the strut comes into play.
One of the more significant impacts resulting from the redesigned lower strut adapter is the regaining of available jounce travel before the strut bottoms out. During the earlier first stage when the cradle was into the chassis, that used up some of the jounce travel at that time. A further shortening of the springs in this stage without the modified lower strut adapters would have reduced the spring travel in jounce to an unacceptable 50 mm (2"). I took advantage of the space between the top of the CV joint boot and the bottom of the strut to extend the strut bottom downwards. I haven't made the adapter yet, so it's only a concept at this stage but as you can see the jounce travel increases significantly to 110 mm. This idea came to me when 355Fiero (Don) thought at one point that I had installed the HT adapters upside down, which would have resulted in a similar looking adapter (though they wouldn't fit correctly that way). That's what's so great about this forum... so many different views coming together in one place.
This new adapter also has the benefit of offsetting the lower end of the strut further outboard, which moves the strut angle further away from vertical. That isn't always a good thing, but on the Fiero, it's a means to reduce the lateral movement of the roll center (that's one of the reasons the '88 has less distance between the strut tops than the earlier cars). Also remember from an earlier post that as the angle between the strut and the lateral links approach 90 degrees during jounce, the camber gain goes from negative to positive, which is a bad thing. Angling the strut by tipping the bottom outboard (or the top inboard) helps keep the relative angle between the strut and the lateral links under 90 degrees. In this case, the new adapter combined with the somewhat neutralizing effect of the angled lateral links results in a strut to lateral link angle of 71.9 degrees which is slightly better than the stock 73.5 degrees.
To accommodate this new strut angle, I've added a 5.9 degree wedge shaped spacer between the top of the strut and the underside of the strut tower. The upper bushing normally takes into account the angular change in the strut through it's bump travel (about 3.5 degrees total) but I consider 6+ degrees enough to warrant the spacer.
The overall 30mm chassis drop causes the axle and lateral links to angle upwards 3.6 and 4.3 degrees respectively, so they're redrawn in red.
I kept the angle of the trailing link constant by raising the forward trailing link mount on the cradle 30 mm's and so that's why the link isn't redrawn in red but the cradle is.
There are other things that happen as you shorten the springs: the half-track width narrows very slightly (1.1mm) and the CofG lowers as well.
Lastly, this drawing depicts a new profile for the rear fender to close the gap above the wheel. As mentioned before, at this stage it's worth actually having some tires and setting the body at the new ride height to see whether a 3" fender gap is way too much or borderline acceptable since the drawings can be misleading. I knew what tires I wanted, so I ordered them, and they're now in. I spent a couple hours playing with ride heights and the camera, so I've decided that while the 3" gap is still too large for my liking, I will analyze the performance data to see how much I lose by stepping up to stage 3B.
(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).]
When looking at the fender height above the wheel, have a good look at the rear bumper and how the front edge of the rear bumper really raises up compared to the rear section. When lining the straight body line up front to rear as on oem, you will see that the front edge of the rear bumper will have to be lowered by about 1.5"-2" IIRC. Given you are at 3" with a 1" drop in the wheel well, you can probably get another 1/2" or more by leveling out the rear bumper to line up down the body.
You will find you need to adjust the rear bumper down, fender well down, body line in front of fender well and on rear clip down, door cut and just about 3/4" at the rear edge and about 1/2" at the front edge down and then rotate the front bumper rear edge down about 1/2" as well. This will line up the body line and lower your rear wheel well down to a better location to the rear wheel.
I will see if I can find my old pics of the body cut up and pasted back together before fiberglassing everything back together. And then try to figure out the pff upload tool again...
Keep up the posts as these are great to read.
Cheers Don
Edit: Finish my sentences and spelling.... and I took your 3/4 side view and added lines to show how the body ends up being cut and pasted back together to make it acceptable. Red line is where the body line should be from end to end and the black lines are where I cut to reposition the panels to get the straight body line. Made a world of difference to the look of the car.
I put the photo on Photobucket as a start to see if it works. (works....)
[This message has been edited by 355Fiero (edited 07-04-2012).]
Don - Very dramatic cut line on Blooz's photo. Hard to believe there was a plug made at one time and may still be out there waiting for some poor soul to splash resin and glass on it.
I started with the same body and after lots of cutting and pasting all the panels, cutting off and making separate rockers, and rebuilding door panels and the rear bumper, I had a set of panels I was happy enough with. Not as good as some other panels out there but much better than they started with. I am confident Blooz will take as much care and thought into sorting out his panels as he has on the suspension so we are in for a treat of a transformation.....
My, almost, finished product to show it can be done if you are willing to put in the time. Lots of time........
Don, I know exactly what you mean and what you went through since I still have all the photos you sent to me including the cut and pasted panel shots on the car you referred to. I've poured over them many times now. I understand the belt line is supposed to be horizontal and not change angle from one panel to the next between the rear bumper, door, and front end. In fact I've had several rows of masking tape stretched down the side of my car for months now, just like the line you drew in the photo above to remind me what lays ahead. One masking tape line is horizontal to the ground, another one bridges the two fender arches, and a third follows the crooked belt line. It's almost comical to see the zig-zagging and divergent lines. It's such a hodge-podge of pieces that I can't help but think IFG had three people working in three different locations form each of the moulds without ever having talked between themselves. I also have a 36" laminated poster on the wall above the car of an actual F355 from a dead-on side view, to remind me what it's supposed to look like. Now that you've reminded me not to overlook the impact of changes that need to be made to the body regardless of anything else I do, I know that the car can be made to look good. I don't worry about that anymore... I have your pictures to prove it
Perhaps to some it seems like I'm still grasping at straws for a solution, but I've posted the three (soon to be four) modification stages not because I'm still scrambling to find an answer, but to show how I arrived at the answer most suitable to me. In particular, I left the spring drop and fender mods for last since these two mods are more flexible in the amount and direction I take with them. More importantly, I don't need to make a decision on these last two attributes to get on with fabricating a new cradle, which is at the top of the priority list. I can take my time analyzing the impact of several different amounts of spring drop at this stage because I'm convinced that I've exhausted all other avenues for lowering (or giving the appearance of lowering) the car with the least negative impact on suspension performance. In fact, I won't likely make a decision regarding the exact amount of spring drop nor fender mods until I have the rear cradle fabricated, the rear suspension components installed, and weight on the wheels. Though it may take longer, I prefer this method over fabricating components through trial and error.
This body is, actually, a splash from IFG, not an IFG directly. I found that the front area had been in an accident and another fellow fixed the front bumper and then splashed the panels from there. The front bumper is a bit more level from IFG. I think this is a good example of the results of a splash of a splash of a splash etc.... Like a photocopier, things degrade as you copy a copy.... also, people just not paying attention to the details when copying.....
I looks forward to seeing how you make a new rear cradle.
Yarmouth Fiero;
The door hings stays in the same location as stock Fiero. The front fender rear edges are shaped the way the are to allow for a fully opening door panel without interference. You need to make the rear edge fairly thin though to clear the door panel when it first opens. I can send you some pics to your email of what I did with the doors an rockers to show you the changes. If you go with an AD355 body, you won't need to do these changes as they are already in the panels.
I snapped a few pictures the other day to give a better appreciation of approximately what stage 3A would look like. Remember, this isn't the look I want, it's just a better visual of what the drawings above are saying: basically a three inch fender gap is too much. First, here are the newly acquired rear meats... Goodyear Eagle F1 Assymetric2's:
And although this next picture doesn't actually have the fender reprofiling I've lowered the chassis on the wheel to show what 76 mm (3") looks like. From the side:
Edit to add:
And from the rear quarter view:
[This message has been edited by Bloozberry (edited 07-08-2012).]
Nice rubber you have chosen. When its all done, what is considered the " ideal look" with regard to wheel and fender clearance? Is a uniform fender gap around the tire desireable? Also, a page back you showed a +/- 76mm suspension travel in your rear suspension analysis. When its all done, what is " normal travel" for the suspension on a car like this? I've been tweeking my chassis additions and I've angled the rear support quite far back to maximize the strength, clear the gas fill plumbing but still give ample room for wheel travel. But once you have selected your ride height, spring rates and such, what do you expect will be normal wheel travel ( assuming I'm driving on typical Nova Scotia roads like you )
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).]
Here are the pictures of the 2" fender gap I promised. Remember, this is just me playing with height of the chassis to illustrate a point, none of the fiberglass fender mods have been done to achieve this look yet. I find that the fender gap in the direct side view always looks like crap without the wheel well liners like in this first pic:
The next two photos give a better impression of what the 2" fender gap should look like:
And the whole car (ignore the masking tape and the front fender gap)
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:
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).]
I am wondering how much of an improvment FieroGuru's link relocation/camber curve kit will make to the roll center migration. Its worth a try, isnt it?
Fieroguru got Zac88GT to run the numbers on his relocation kit, so I'll let him post the data if he wants. It's dawned on me that the trouble with the Fiero rear suspension is that the fixed angle of the strut significantly limits roll center control. An upper control arm provides much greater freedom to counter the effects the lateral links have on roll center location.
I am wondering how much of an improvment FieroGuru's link relocation/camber curve kit will make to the roll center migration. Its worth a try, isnt it?
Zac ran the numbers and posted the graphs half way down on this page for a car at stock ride height. The overall benefit depending on ride height of the car. https://www.fiero.nl/forum/F...2/HTML/117227-7.html
As you work to lower the chassis a significant amount, the 88 rear suspension (and front suspension for that matter) really starts to work against you unless you start with lowering uprights/knuckles to accomplish the vast majority of the lowering.
Blooze, glad to see you back working on this swap!
Since you are widening the overall track width, have you considered making the rails of the cradle wider? This would lower the overall length of the lateral links and get you further away from interference issues with the transmission and possibly allow the inboard lateral links to be raised further. However, it would be very cool to see you come up with a dual a-arm setup as well!
Blooze, glad to see you back working on this swap!
Since you are widening the overall track width, have you considered making the rails of the cradle wider? This would lower the overall length of the lateral links and get you further away from interference issues with the transmission and possibly allow the inboard lateral links to be raised further. However, it would be very cool to see you come up with a dual a-arm setup as well!
On a lowered fiero, the longer links help to lessen the camber gain on compression once the links have passed the horizontal position (but also negatively affect camber loss before they pass the horizontal) since the linkage end travels a larger radius arc. They also lessen the veritcal displacement of the roll center for a given amount wheel travel due to a smaller angular displacment (although the horizontal displacement is greater). Which is why I think combining your relocation brackets can help soften the negative effects of Bloozberry's first proposed solution, but probably not enough to justify it.
Obviously, provided the right coordinates a double wishbone (or a 5-link type like most modern production cars and Datsun1973's solution) a can provide the best selection of geometry kenimatics, and force vectoring.
For Fieroguru: Earlier on, I did look at making the cradle wider, and shortening and moving the inboard lateral link mounts outboard and upwards, although only superficially. I quickly realized that the F40 overhangs the forward inboard lateral link mount by way too much to ever make enough room for it vertically. The other problem with shortening the lateral links is that it would only exacerbate the angle of the links with respect to the ground, worsening the roll center movement.
One of the things I did to get a warm fuzzy feeling about whether an SLA set up like Datsun1973's might fit, was to overlay the scale drawings of my front control arms onto the rear suspension. My logic here was that since the front geometry works so well, if it could be replicated in the rea, then the rear roll center could potentially be tamed equally well. Here's my first crude attempt to see how the front geometry (in red) would look if it were moved to the rear:
I simply cut and pasted the two front control arms in the exact same orientation they are to each other from my final front suspension configuration, onto the rear Stage 3B rear modification drawing. Even though I don't intend to replace the rear lower lateral links with a control arm, for this exercise, used the front lower control arm simply to locate where the upper rear control would lie. I aligned the front lower control so that the lower ball joint was in line vertically with the hole in the rear knuckle for the lateral links and the inboard pivot was at the same height as the lateral link mounts. The upper control arm then just fell into place.
My first observations are that:
1. the geometry can't be exactly the same as the front since the lateral links are shorter than the lower control arm, and the upper ball joint does not fall conveniently where Datsun1973 has located his upper joint on the knuckle;
2. since the lower lateral links will be shorter, the upper control arm will likely need to be proportionally shorter as well, which will make it easier to package within the available space;
3. the lower frame rail appears to be well located to support the upper control arm pivot, or at least a new structure to support the pivot;
4. with a pushrod set up and shock orientation like Datsun1973's, the strut towers become redundant and could be replaced by a simpler and less intrusive structure tying the upper and lower frame rails together and making more room for the Northstar rear valve cover (not to mention the dead give away the strut towers provide to the actual lineage of the car when the decklid is open.);
The next thing I want to do is draw the rear lower frame rail to see how the front suspension geometry fits in the side and top views... more soon.
Edit: Updated drawing
[This message has been edited by Bloozberry (edited 09-09-2012).]
Since you will have to relocate the lateral link attachment at the upright, why not center the upper ball joint to the center of the upright casting. Then you can drill a hole through it, machine the sides flat and use a rod end or poly bushing on each side. Or shift it inboard 1" so you can have a bracket attach at the stock strut attachment, then come down to the proper area with a cross shaft right inboard of the casting.
Looks like the lateral link attachment would be about 70mm lower (if I can count right on this small net book screen), and I am pretty sure that would fit in your wheels, but the trailing link could be an issue. You might want to consider shortening it and attaching it directly to the front lateral link (but probably want to up-size the tube.
Blooz, could you repost these pictures with your analysis in my suspension redesign thread? - This is very interesting. I was always hoping an SLA would work in the rear and use the lower frame rail.
For Fieroguru: You may have a good point, but I'm not sure yet that I'll have to relocate the lateral link attachment at the upright like you suggest. Even if both control arms would need to be angled upwards at ride height as my initial sketch shows, it shouldn't matter much if I can duplicate the control of roll center movement achieved in the front suspension. What I mean is that in the front, no matter what the control arm angles are at, the roll center stays roughly stationary. The challenge will be to define the coordinates of the control arms that not only control the roll center though, but also the camber and toe, given different packaging constraints than the front.
Fierogt28: Good eye. I do have new tie rod ends to install, they're just not painted yet. The lower ball joints are new, but the uppers aren't. They're actually not worn out, but they do detract from the rest of the new parts dont' they? Now that you've pointed that out to the whole world, I feel like I have to get new ones... thanks alot.
exoticse: Thanks for the compliments... it takes an experienced eye to see the potential in a kit car at this stage.
Austrian Import: I would take you up on your offer since the more brainstorming there is, the more likely the best solution will come to the surface. As you likely know though, posting pictures is easy. What takes time is explaining them... more than I can afford at this very early stage. Perhaps once I'm out of the preliminary concept stage I'll have more time. Feel free to download my images from here and repost the ones you think will generate discussion over in your thread though. I always check out new posts in there... afterall, Datsun1973's contribution comes from your thread.
Edit to add: thanks too 355Spider!
Now back to the latest update: It's taken a considerable amount of effort to map out that rear lower frame rail, especially since not all of it is easily accessible for measuring. As usual, I head out to the shop, take a whole slew of measurements and make a hundred little sketches, come back to the office and try to make everything fit. The computer is amazing at simultaneously showing how accurate something can be and at how terrible I am at measuring things. It's no wonder you hear that saying "Measure twice, cut once" so often among people who work with their hands. If I'm lucky, I'll only have a few discrepancies to iron out... wasn't the case this time around. So as one PFF'er states in his signature, "you get what you pay for", consider how much you paid for these drawings before building a new Fiero using these as blueprints!
For now though, just to show that I AM working at it, here are the top and side views of the stretched frame and raised cradle. For the eagle-eyed among you who are curious why I indexed the longitudinal axis in both drawings where I did, well, it's a long story but it boils down to using the rear trunk wall as the index point since it was the closest repeatable measuring point for all other points on the frame rail... and it was the easiest point of reference to use while sitting in the empty engine bay.
I'll superimpose the front suspension geometry over top of them in my next post:
[This message has been edited by Bloozberry (edited 09-07-2012).]
Thanks 85LAMB... you're very kind. Between myself and Yarmouth Fiero (another PFF'er building an F355) we have some neat stuff brewing in the pot for the bodies.
Thanks 85LAMB... you're very kind. Between myself and Yarmouth Fiero (another PFF'er building an F355) we have some neat stuff brewing in the pot for the bodies.
Does Yarmouth have a build thread here? We need more kit car build threads.
As promised, here are the two remaining views of the rear frame rail and cradle with the control arms from the front suspension superimposed on them. Again, this is just a crude way to get an idea how well or ill-suited the stock lower frame rail is oriented to locate some mounts for an upper control arm and a pushrod shock. The rear view was posted earlier, so here is the top view with the lower control arm being blue and the top one red:
Once again, I'm not planning to replace the '88 lateral links with a lower control arm. The only reason I left the lower control arm on the diagram was to show the relative positions of the lateral link mounts on the cradle vs where they would need to be to retain an identical geometry with the front. The lower control arm's function is replaced by the two lateral links which are roughly 35 mm shorter than the aft leg of the lower control arm and roughly 65 mm shorter than the forward leg. This view shows clearly that the front geometry cannot be incorporated lock, stock, and barrel onto the rear, but that doesn't mean it's not possible to achieve a similar performance.
Since the lateral links are shorter than the lower control arm, the result would be decreased camber gain in jounce vs the front suspension, if the upper control arm were identical. But this drawing (and the rear view one posted earlier) show that the upper control arm would need to be shortened to bring the ball joint in line with the top of the knuckle. The impact of that change would be to increase the camber gain in jounce... perhaps enough to compensate for the effect of the shorter lateral links.
Here is the side view:
Of course the upper control arm is angled for anti dive when anti-squat is what's needed, so ignore the front to back slope of the upper control arm. The other thing that jumps to mind in this drawing is the amount of caster, indicated by the misalignment of upper and lower ball joints vertically. Caster is needed in the front suspension to make steering more stable, but not needed in the rear so the upper ball joint location could be moved towards the front of the car if it were useful to do so. The only other notable point I can think of about this drawing is that it shows how much room there is rearward for a fore-and-aft oriented coilover similar to Datsun1973's design.
Overall then, the layout and location of the stock rear frame rail appears to be amenable to an SLA-like suspension, only in htis case it would be more like a four(?) link design (is the shock pushrod considered a link?). Certainly worth further development in my opinion, so I'll continue refining these drawings by looking into a suitable upper control arm pivot design or two, and a pushrod shock system. Questions or constructive comments are always welcome.
(Edit: corrected side view drawing to illustrate correct ground clearance)
[This message has been edited by Bloozberry (edited 09-16-2012).]
From what ive gathered around different sources (Milliken, Smith, Adams, Haney) a anti-roll bar on the rear of a vehicle equiped with an open-defferential is more prone to lossing tractive effort on the inside tire in a turn, and be more akin to oversteering, I havent had the chance to read Olley's book yet, but from the OP in the linked thread, and my quick look at a few more High performance racing setups, the inclination fo the roll axis is to angle it upwards to the rear. I wonder what the effects of the stock 88's is.
I've spent the last couple weeks working on and off trying to refine the location of a new upper control arm. I must admit it's taken much more time than I expected mostly because I needed to leave it alone on many occasions to let things sink in, and because I don't have simulation software. I had to try many iterations using a digital stick model approach to narrow down the best coordinates for the new arm. I wanted to make things as easy for myself as possible in the fabrication stage too so I began by setting some design limitations. Though it didn't necessarily make things easy at this stage, I decided to:
a. leave the locations of the lateral and the trailing links intact (following the 6" track width increase);
b. use the upper or lower strut mounting holes on the knuckle to locate the outer pivot point of the new upper control arm;
c. use the 25 mm raised cradle; and
d. leave a 125 mm ground clearance to the bottom of the center of the cradle.
Starting with the rear view, my goal was to locate the coordinates of the inboard pivot point for the new upper control arm, such that the roll center movement and camber gain were improved over the stock configuration. In my head it seemed simple enough to locate a single pivot point, but practically speaking it was anything but.
Initially I just picked a random point near the lower frame rail for the inboard upper control arm pivot, then I rolled the body over 6 degrees and noted what happened to the roll center and the camber of both tires. To keep the impact of jounce out of the equation, I made sure that when I rolled the body I kept the ground clearance as measured at the centerline of the cradle at 125 mm. This may not prove to be what happens in real life but I needed to hold this constant to prevent too many variables from wreaking havoc on my simplistic methodology.
I then changed the location of the inboard pivot point, and repeated the experiment until I got a feel for how the camber and roll center were impacted by various vertical and lateral displacements of the pivot point. I was amazed at how sensitive the camber gain was with only very minor pivot point changes, but I was eventually able to find a location near the bottom outside corner of the lower frame rail that seems to be a good compromise between both roll center movement and camber gain. Whether this pans out when these coordinates get plugged into the Lotus Suspension Analyzer software or not is yet to be seen, but as I said, it's a good start.
Using the best coordinates I found for the uppper control arm, this first drawing shows the static location of the rear roll center being about 92 mm above the ground. If you recall, the static roll center of the modified front suspension is lower at 72.4 mm. The result is a roll axis that is raised slightly at the rear. This is a good characteristic and is what FieroWannaBe was referring to earlier as the roll axis inclination. I won't get into details at this point but suffice to say that this new design is on the right track.
(I apologize in advance for the poor resolution of these drawings, but unfortunately it's a draw back to using PIP.)
This next drawing shows how the same suspension behaves when the body is rolled 6 degrees to the left, as it would be in a high G right hand turn. The most important things to note are that the camber change is almost perfect (excluding the effects of bushing deflection). The outside (LH) wheel counteracts the body roll by tilting the same amount in the opposite direction, which keeps the tire contact patch flat with respect to the ground, giving maximum traction. The inside (RH) wheel comes within 1 degree of countering the body roll angle, which is a significant improvement over stock.
The other important thing to note is that the roll center moved considerably less in this design than my earlier stage 1 or 2 modifications. Here, in a 6 degree roll, the roll center migrates only about 7 mm down and 320 mm outboard as compared to nearly 470 mm down and 2670 mm outboard in my stage 2 design using Chapman struts. Again, I still have to verify that my drawings are correct using the Lotus Software. To do that though, I'll need work on the top and side view drawings to get the additional coordinates in the x-plane.
(Edited to rename both drawings)
[This message has been edited by Bloozberry (edited 10-20-2012).]
This may be of small interest. At a recent SAE chapter meeting, an engineer from the Ford GT project gave a presentation detailing the design process. He had mentioned the design goal was to mimick the Ferrari F360. Using a kinematic test of the ferrari, they designed a suspension to preplicate it, mostly. (http://www.morsemeasurements.com/store.asp?pid=33665&catid=20028) The Ford was finished with a rear camber curve of 0.7 Degrees of camber change per degree of body roll (or something to that effect, IIRC) and 0.5 for the front (this matched the ferrari); and a ride frequency of 1.7Hz in the rear, 1.5Hz in the front, I think this was slighty higher than the ferrari. Those are the numbers I seem to remeber, they may be off, but it was less than unity.
I would be concerned about the foward tractive capability with an overly agressive camber change.
Originally posted by FieroWannaBe: The Ford was finished with a rear camber curve of 0.7 Degrees of camber change per degree of body roll (or something to that effect, IIRC) and 0.5 for the front (this matched the ferrari); and a ride frequency of 1.7Hz in the rear, 1.5Hz in the front, I think this was slighty higher than the ferrari. ...they may be off, but it was less than unity.
Interesting stuff.
quote
Originally posted by FieroWannaBe: I would be concerned about the foward tractive capability with an overly agressive camber change.
Originally posted by Bloozberry: Can you elaborate a little on this?
The reason the Ford GT and Ferrari F360 have less than unity camber change characteristics is becuase in straight line acceleration and deceleration, the subsequent suspension deflection will cause the tire contact patch to deform and shrink from wheel camber change. This is countered by the anti geometries, however it is impracticle to run 100% anti geometries to eliminate the issue, 100% anti sqaut can cause brake hop on rough surfaces and in transient conditions, because it becomes pro-lift under braking. Too much anti-dive in the front can cause bump-steer and caster change on uneven surfaces. The suspension can become too stiff for uneven conditions with anti-geometry under accelerations as well since forces are attempting to seperate the body mass from the wheel assembly. A lot of this can be compromised and iterated forever. But settling on a best of all worlds and leaving room for adjustment might be the easiest solution. Tuning the roll-stiffness and rebound damping can also help to control oversteer/unsersteer relationship, and probably more effective than roll center/camber change placements by themselves.
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. 
