I'm looking at doing a 4.0 swap into my '84. What does this have to do with suspension you ask? Let me tell you my idea. The '84-87' suspension has some limitations (namely bumpsteer), small brakes, and small bearings. All of these can be solved or fixed one way or another. Now, with this 4.0, I have the complete cradle and suspension from the '97 Aurora. What drawbacks would there be using the Aurora front suspension on the rear of the Fiero? Would the "built-in" safety features of a front wheel drive setup (under steer vs over steer) be a detriment in a rear wheel drive setup? Can this be fixed or avoided? The Aurora gives me the ability to keep the axles the same, gives me bigger vented brakes, large bearings, aprox 6" wider stance (wide body coming), sway bar, etc...

Thanx for any input.

You need to get a book called "How To Make Your Car Handle" by Fred Puhn, it's still in print last I looked. It has all the math you'll need to see if this will work. My first impression is that the reason the '84-87 Fiero suspension isn't that good is because they borrowed the rear from a front drive car and had to make compromises in the front to deal with the resulting rear geometry problems. I don't think that you'll be able to avoid that same set of problems using the front Aurora suspension in the rear of the Fiero.

James

Thanx Jazman...But if the only issue that remianed was the bump steer, at least that can be reduced. Will this set-up create more problems? My concern is the way the front mount is on the a-arm. I'm not particularly happy with the way it's connected. Here's a picture from Ebay to show what I mean:


That front mount is vertical, not hoizontal and parallel to the back.

Bob

[This message has been edited by RCR (edited 12-30-2006).]

bump for 2007

I saw this last week when I was at the bone yard. I couldn't believe that GM would put that on a car and charge $$ for it. While you have the cradle out, move the arm through the full range of travel and see if the rubber mount restricts the motion. Also, for a performance car you would not want to use the rubber mounts and if that vertical mount was poly, I would think that would really be restrictive.

Along the same lines, I'm looking at converting the back (as well as the front) to the 115 mm bolt pattern to improve options for wheels and brakes and gain some bearing. That actually looks fairly easy with early '90s fwd bearings and spindles (Bonneville, 6000 etc.) Next on my list is to improve the suspension in general (I have an '87) and try to lighten the sprung mass. The front is another story but I have the same goals.

So to answer your question - I would have to think long and hard if that is the best solution for an upgrade. On the other hand you can always change it later.

The L-shaped control arms are actually VERY GOOD for combining handling precision and good ride quality. The forward pivot (at the corner of the L) is a hard pivot. The rear pivot (at the long tip of the L) is a soft bushing. When you corner the car hard, the lateral force goes through the hard ball joint and hard forward pivot giving you precise control of the vehicle. When you hit a bump, the control arm pivots around the forward pivot and compresses the bushing at the rear, absorbing the impact.

BMW has been using that setup for twenty years or more at the fronts of its cars.

The second potential problem I see is differences in the kingpin angle between the two cars. Despite having similar track widths, the 97+ W-body front suspension has the strut tops further apart than the Fiero does. Putting this car's suspension into a Fiero would require pulling the struts closer together at the top (increasing kingpin inclination) and probably resulting in uncorrectable excessive negative camber.

Thanx Will...Great info. I've seen some spacers used to move the strut back off the upright for wide-track setups. If the angle becomes too great, I thought I might be able to use something like that, or move the strut tops out. You've given me a lot more confidence in this setup.

Bob

If you're cutting metal, you can certainly use camber plates.

I'd go with deep dish wheels to match a wide body rather than try to widen the suspension. Zero offset wheels have the least wheel bearing load while sitting or driving straight. As the offset goes positive (wheel moving outboard relative to hub) cornering loads on the wheel bearing go DOWN until they hit a minimum when the offset divided by tire radius equals the cornering G.

IOW, for an overall diameter of 24 inches and 1G of cornering load, wheel bearing load will be least with a +12 inch offset... the wheel bearing won't even be inside the wheel.

According to WCF, stock rear geometry can take a 17x9-45 wheel. This has 6 1/4" of backspacing and 2 3/4" of front spacing. If you are widening by 3" per side, you won't even get to zero offset if you go with 17x12 wheels with the same backspacing. That's with the stock geometry and track width on the suspension and LESS overall wheel bearing load than if you had widened the suspension. You can also run 315/35-17, or even 335/35-17 tires with that setup.

[This message has been edited by Will (edited 01-02-2007).]

Will,
So given the oppurtunity to pick one over the other, it sounds like you're recommending staying with the Fiero suspension, and correcting/minimizing the bump-steer issue. The bearing issue can be solved by switching to the HD 6000 upright/hub.

If I choose to go that route, what axle combination would you recommend using the 4T80E, since I could no longer use the Aurora axles?


thanx a million for your input...

Bob

Will you be trying to optimize the supension at all, for RWD, or are you going to go the path of least resistance and just slap stuff together so it just rolls down the road?
If you pick up a GOOD book on suspension design you'll find out REAL FAST that there are a lot of subtle differences in the geometry of the front vs. the rear.

Yes, the Fiero rear suffers from a lot of problems due to the fact that it began life in the front of a FWD car.

If you're building a wide body, you should have the fab skillz to correct most of these.

I don't know what axles work with a 4T80E because I've never put one together.

[This message has been edited by Will (edited 01-03-2007).]

"Yes, the Fiero rear suffers from a lot of problems due to the fact that it began life in the front of a FWD car"
Gee, ya think?
I was refering to tha Aurora suspension installation at any rate....

quote Will you be trying to optimize the supension at all, for RWD, or are you going to go the path of least resistance and just slap stuff together so it just rolls down the road? If you pick up a GOOD book on suspension design you'll find out REAL FAST that there are a lot of subtle differences in the geometry of the front vs. the rear.

Well, it's not a matter of what I plan on doing, it's a matter of what you recommend, as I'm asking for advise from suspension experts. I do have one book on suspensions, by Herb Adams, but it doesn't cover much on strut-type independent suspensions, other than saying they are not as good as double wish-bone suspensions for race applications.

Now, what say you???

Bob

The biggest problem is bump steer, which you can fix with a Held kit.

The next problem is that anti-dive at the front end = pro-squat at the back end. Fixing this would require moving the rear pivot of the control arm down.

Since the rear pivots are further inboard than the front pivots in order to compensate for the castor change induced by the anti-dive, you'll have to move the rear pivots outboard as well.

quote Originally posted by RCR: Well, it's not a matter of what I plan on doing, it's a matter of what you recommend, as I'm asking for advise from suspension experts. I do have one book on suspensions, by Herb Adams, but it doesn't cover much on strut-type independent suspensions, other than saying they are not as good as double wish-bone suspensions for race applications. Now, what say you??? Bob

If this is about ideas....and you want to talk about serious IRS setups....I'm in.

In some ways an IRS is way eieser to do than a front. A double a-arm is not terribly difficult to design and build for the rear because it will never, and must never steer. Either the early or late cradle can be used as a platform to carry the a-arms, and hub carriers that don't need to steer can be made from existing front hub carriers.

The IRS that I designed and partialy built uses a modified early cradle, fabricated a-arms, and modified 88 hub carriers. The geometry matches the 88 front exactly.

yellow88: Pictures man....

Bob

What do I recommend-- is that if you are determined to get a different suspension setup on the back of your Fiero--is to edumacate yourself with a couple of quality books on suspension design. I have that book by Herb Adams-- and it will not do the job for you at all.
The reason that I got a little snitty in my first post, *and I apologise for that*, is that this thread was going to hell in a hand basket from a design standpoint. You were concentrating on a small detail (that didn't matter a whole helluva lot) at a time when you should be looking at the general layout of the design. In a good design, you should spend a HUGE chunk of time just contemplating and planning the overall design.
At least, at the aiplane company I work at, that's what *I* do.
So, as an example.
Basically, you have to find out what the CG height, and it's fore and aft position in the car, put a load on the CG (a good guess works) and find out what loads it puts on the unsprung mass (body) and the resulting loads on the tires, and design a suspension to accomodate that.
Further, the rear suspension has to relate it's roll center, height wise, with the front suspension as well as parallel the mass centroid axis that runs fore and aft in the car (relates to the overturning roll moment and steering transition effects). How much anti-squat to put in? If I move the roll center up, what spring rate will I need now?...
And the list goes on and on and on.....
Now you can make some good guesses about what some of this stuff like mass centroid axis , CG height, etc. is but you have to know that it exists it in the first place.
Will put in a good example -- the front suspension anti-dive and FWD power anti-lift geometry turns into pro squat and braking pro-lift when used on the back. You can use the Aurora front end as delivered and compensate for it's weirdnesses to a certain extent with stiffer springs in the back, but then the car will probably oversteer, and have generally screwy handling coupled with a bad ride.
I'll be willing to bet SOME people on this forum couldn't tell a good handling car from a pack of gum. So they don't worry about throwing some greasy mongrel engine and tranny in the back and taking it to the drag strip. The fact that it goes down the road without falling apart is good enough for them.
But my opinion is that if you are going to go through a lot of work to change the whole suspension, you should at least take the time to figure out what direction things like suspension arms should point.
I would say that if you don't want to immerse yourself in serious suspension design books (at $90 a pop) for at least a couple of months, then you should consider keeping the the standard rear suspension. Just renovate it to where it works like new, then work from there.
There is no shame in that.
I bought a old Honda CRX, wanted to put the engine in the middle. Didn't look like a good idea at all after reading some books so I sold it. Bought a 1988 Fiero intending to put a V8 in it after redoing the suspension. It's fast, with a turbo'd V6 and it corners well, but the handling is NOT world class, and it NEVER be world class.
So I bought a Lotus Esprit. Ahhh, that scratches the itch.
Sorry for the long post, but like the man said, I didn't have the time to make it shorter.

quote Originally posted by gixxer: ..... so I sold it. Bought a 1988 Fiero intending to put a V8 in it after redoing the suspension. It's fast, with a turbo'd V6 and it corners well, but the handling is NOT world class, and it NEVER be world class. So I bought a Lotus Esprit. Ahhh, that scratches the itch. Sorry for the long post, but like the man said, I didn't have the time to make it shorter.

The geometry of the 88 front end is world class...in my opinion. Being sold to the "general comsumer" it has to be somewhat complient. When fitted with hard bushings, and adjustable spring shock rates, and bump steer is.....word class.

The 88 rear....though somewhat beter than the early cars, suffers from the fact that it is a strut design. Its unequal lengh laterial links, and trailing arms don't work well with complient bushings, but can be improved with hard control points, ONLY if one takes the time bump steer it.

Yellow is set up with solid front bushings, and is bumpsteer corected. In the rear, I changed to equal lenght laterial links, with solid control points. I used equal lenght because they are much easier to bump steer. Both cradles are rubber mounted to isolate the chassis noise assosiated with solid bushings. At this point, spring and shock rates are stock 88. Alignment is set to zero chamber front ant rear, zero toe front and rear, with 6 deg of front caster. Roll is controled with stock 88 bars. Yellow feels nothing like any other Fiero I'v'e ever driven.

Yellow is not a Lotus.....but it is as pedictable and confident as the legendary cars built by Mr. Chapman. Yellow is proof that a Fiero is worth some carefull "adjustments".

Gixxer: No offense taken and long posts are usually good posts. I appreciate your info. I bought the Herb Adams book because I wanted to understand some of the terminology that was being tossed around in several other suspension posts. It quickly gets over the head of the casual reader without some background. I do have another text book, called Race Car Vehicle Dynamics, that I've browsed through, but haven't read.

Yellow88: thanx for your input, too.

I guess I should re-state some of me goals (and concerns) in why I'm thinking of this.
- biggest concern and I'll be perfectly honest here. budget. No real money to throw at this with 4 other mouths to feed/cloth/and provide shelter and one income right now
- Safety...see above
- Long term goal: Wide (custom) body
- better brakes
- better/bigger bearings
- 4.0 is going in, how can I ease the axle situation and keep it affordable
- 4.0 fits the Aurora cradle better, but either need to be modified to fit
- one day, I would like to buy/design a true tubular upper/lower a-arm suspension and cradle, but that's in the future

The car will be a "nice" day driver with little chance of seeing a race track and even smaller chance of seeing a quater mile. But...I want it to handle well for spirited driving and for the off chance of seeing a Fiero track day at one of the local road courses. I want to be able to show it at some of the local Fiero shows (25'th anniversary, etc).

So, I find myself in a pickle barrel with this swap. How can I use the Aurora setup (and improve it if I can/have to) or do I stay stock and improve upon that. Any extra tech info is a bonus for me (and the forum as a whole).


I also had a thought. With the steering arms in the back, I believe this causes bad over-steer when located in the rear. Can the uprights be swapped right to left to move the arms to the front to help reverse this? Can this be done on the stock Fiero suspension?


Thanx, and I hope we can still add more info for the tech data base.

Bob

quote Originally posted by 87GTZ34: ..... Next on my list is to improve the suspension in general (I have an '87) and try to lighten the sprung mass. The front is another story but I have the same goals...... .

We are one the same page about unsprung mass. (I'm assuming you mean the stuff between the springs and the road.)

There is a lot of iron in Fiero syspention. Unfortunetly.....there aren't too many components that one can just bolt in.
Willwood calipers and fabricated aluminum control arms are problably the esiest., and many after market wheels are lighter than stock. But It's the hub carriiers and hubs that would be nice in aluminum.

Access to a database of curently used automotive components would be nice....

quote Originally posted by RCR: I guess I should re-state some of me goals (and concerns) in why I'm thinking of this. - biggest concern and I'll be perfectly honest here. budget. No real money to throw at this with 4 other mouths to feed/cloth/and provide shelter and one income right now

What....a budget...?? I got a feeling you're not unique in that.

I think.....that building cars should not be about how much mony one puts into it. Perhaps because I've never had that option, Ive learned to do stuff myself. I can't afford fabrication, but I can afford to learn welding. The expanded electric bill for practise welds somehow is easir to justify than one to a fabriction shop.

Good design is almost alway based on good comprremise. Don't be assamed of reality
My most exotic designs may not get built, but the exersize has identified good compremise.

Chassis dynamics sounds wicked complicated, but once you understand the basics you'll find that it is not.
Once you have that understanding, you'll find that almost any chassis can be much nicer with simple, inexpensive modificatins. So often....people look right past the most basic stuff and go right for the big dollar, nice looking stuff, only to end up with no real improvmet at all.

quote Originally posted by RCR: I guess I should re-state some of me goals (and concerns) in why I'm thinking of this. - biggest concern and I'll be perfectly honest here. Budget

This narrows it way down (same as me, lol). Stick with the stock cradle.

Then as you save some cash: fix the bump steer, add a sway bar, struts of your choice (hard, soft, whatever you like), bigger brakes, etc. Bring the car up to your standards over time and have fun. IMHO.

quote Originally posted by RCR: The car will be a "nice" day driver with little chance of seeing a race track and even smaller chance of seeing a quater mile. But...I want it to handle well for spirited driving and for the off chance of seeing a Fiero track day at one of the local road courses. I want to be able to show it at some of the local Fiero shows (25'th anniversary, etc). Bob

This is a very reaistic goal.
A slightly modified Fiero can easily be what you've stated here.

New balljoints, shocks, and poly bushings. Tires and wheels that match the original dia. and offset keeps the geometry as it was intended. On the 86, a rear anti roll bar is nessasary.

Now....absolutly. ....by far the most important......most noticable.....is alignment. A lot has been said about settings, specs, good and bad shops...bla...bla..

It is my VERY firm opinion that you must do your own if you want to be completly satisfied with the results.
It simply is'nt realistic to expect a shop to meet the needs of someone who is using hard bushings on a mid engine 2 seat sports car. They honestly can't spend the time nessasary to fine tune to the level you will need. With hard bushings the factory secs most likley will not be the feel you like, even if the tiny tolerances needed by hard bushing are met. With hard bushings you have allmost NO tolerance, and you will need a good bit of testing to be sure of what changes do what.

Once again your stated goal is very realistic.

quote Originally posted by gixxer: Basically, you have to find out what the CG height, and it's fore and aft position in the car, put a load on the CG (a good guess works) and find out what loads it puts on the unsprung mass (body) and the resulting loads on the tires, and design a suspension to accomodate that. Further, the rear suspension has to relate it's roll center, height wise, with the front suspension as well as parallel the mass centroid axis that runs fore and aft in the car (relates to the overturning roll moment and steering transition effects). How much anti-squat to put in? If I move the roll center up, what spring rate will I need now?... but the handling is NOT world class, and it NEVER be world class.

Isn't the point of hot-rodding to MAKE things world class?

quote Originally posted by Yellow-88: The geometry of the 88 front end is world class...in my opinion. Being sold to the "general comsumer" it has to be somewhat complient. When fitted with hard bushings, and adjustable spring shock rates, and bump steer is.....word class. The 88 rear....though somewhat beter than the early cars, suffers from the fact that it is a strut design. Its unequal lengh laterial links, and trailing arms don't work well with complient bushings, but can be improved with hard control points, ONLY if one takes the time bump steer it. Yellow is set up with solid front bushings, and is bumpsteer corected. In the rear, I changed to equal lenght laterial links, with solid control points. I used equal lenght because they are much easier to bump steer. Both cradles are rubber mounted to isolate the chassis noise assosiated with solid bushings. At this point, spring and shock rates are stock 88. Alignment is set to zero chamber front ant rear, zero toe front and rear, with 6 deg of front caster. Roll is controled with stock 88 bars. Yellow feels nothing like any other Fiero I'v'e ever driven.

I bet it still oversteers at the limit and has a noticeable lift throttle reaction.
Have you taken pyrometer measurements to make sure that zero camber is what the car really wants? 6 degrees seems like a lot of caster... That would compromise contact patch feel too much for me.

I disagree on your assessment of the '88 lateral links. They are set up so that at stock height they give slight good bump steer... toe in on compression, toe out on extension. The problem comes with lowering the car. This does nothing but screw up the rear geometry. I thought you said you'd duplicated the front geometry in the rear?
I'd leave the rubber bushings in the trailing arm but go to rod ends in the lateral links. That will give handling precision without killing ride quality.
Why on earth did you rubber mount the cradles? That undoes everything you just did with the hard bushings.
.
.
.
Good handling is nothing more and nothing less than allowing the car to utilize its contact patches in the best way possible.
This starts by giving the car the same contact pressure front and rear by using tires in proportion to weight distribution. This will put the front and rear tires at the same starting points on their grip curves.
The next step is to make sure that the CHANGES in contact pressure are the same front and rear. This is trickier. As gixxer alluded, you need to match the inclination of the roll axis with the centroid axis. This insures that contact pressures change in the same way front/rear as the car rolls. In a Fiero, this is mostly accomplished by raising the rear roll center. Duplicating the front geometry in the rear is not a good solution because it puts the rear roll center too low. There's nothing wrong with the '88 rear that raising the inner lateral link pivots doesn't fix. It's not that it's a strut system... it's that it's slightly copromised for powertrain clearance.

As far as books go, I like "Tires, Suspension and Handling" by John Dixon. It's VERY high level material with very few pictures and lots of equations, though.

[This message has been edited by Will (edited 01-07-2007).]

quote Originally posted by Will: I bet it still oversteers at the limit and has a noticeable lift throttle reaction. Have you taken pyrometer measurements to make sure that zero camber is what the car really wants? 6 degrees seems like a lot of caster... That would compromise contact patch feel too much for me. I disagree on your assessment of the '88 lateral links. They are set up so that at stock height they give slight good bump steer... toe in on compression, toe out on extension. The problem comes with lowering the car. This does nothing but screw up the rear geometry. I thought you said you'd duplicated the front geometry in the rear? I'd leave the rubber bushings in the trailing arm but go to rod ends in the lateral links. That will give handling precision without killing ride quality. Why on earth did you rubber mount the cradles? That undoes everything you just did with the hard bushings.

It oversteers only when I want it to. Near the limit, I can steer with the throttle if I want to. Is that what you mean by "reation..??
Equal lenght links allow much easier adjustment of roll steer. I don't like toe in on compression. Yellow is adjustable to whatever you like.
My trailing arms are still rubber, I agree with you there, and that lowering screws things up.
I like a lot of caster, maybe because I like zero toe.
I use "tire waer test", because I don't have a pyromiter. 40,000 miles with perfect waer is problably what it likes.
I rubber mounted the cradels to isolate chassis noise. Their not real soft, just enough to damp the noise a bit. Torsioal ridgidity is the same as when they were solid. I did measure it.