One of the better autocross drivers did an analysis of what are the important elements needed in an autocross run. Here's the breakdown of a 59.18 second run at a national event: http://www.rhoadescamaro.com/build/?page_id=481
For that matter, looking at his next car.
That looks like a really cool resource... I'll have to check it out in a couple of weeks when I can sleep...
I understand that discussion is an important part of development, but I really want to see pictures of a fully modern suspension on a Fiero. There are a lot of supercars out there from which we can draw. Personally, I like the setup on the Ford GT. Anyway, I'm no engineer, so I guess I'll shut up now and let some more brilliant minds come up with a solution. I know I've been impatient, but I really want to see the end product.
I do not agree that the geometry should be used to limit body roll. This deprives the driver of the ability to feel the chassis load up.
I couldn't agree more. My point is that the McPherson strut design is limited by its inability to keep the contact patch intact during jounce and rebound. In order to keep the patch on the ground requires more initial negative camber than I'm willing run on the street. The picture I posted was to show the camber on the loaded rear wheel just before it lost traction. This is with 1.5* negative camber initial and all poly links. It's the best I think my streetable Fiero can do with this suspension design. Yes, I could use a pyrometer to optimize the camber for maximum traction going around a cone at 20 mph, but I wouldn't like driving it the other 99.9% of the time. Hence my interest in an SLA setup.
~Neil
[This message has been edited by sspeedstreet (edited 07-31-2011).]
My point is that the McPherson strut design is limited by its inability to keep the contact patch intact during jounce and rebound.
BMW and Porsche do just fine with struts. As does Honda since they introduced the RSX. There's nothing inherently wrong with struts, as long as they can be packaged appropriately. They fail, for example, in a car like the GM W-body in which the strut tops are pushed further apart than they are on the Fiero in order to package a larger powertrain. This obviously strictly limits the capacity of the geometry for camber gain.
After that, try raising the inner pivots of the lateral links. That will raise roll center, move dynamic contract patch loading closer to equal and give the suspension more camber gain.
And run a staggered tire fitment that matches the car's weight distribution.
[This message has been edited by Will (edited 07-31-2011).]
it's not as good as say, Milliken, but Haney does better than others like Adams, or Gillespie, at focusing at the root of the suspension problem, analyzing the history of current solutions that are being used, and has insight from many in-practice professionals. Camber gain, roll center, and wheel rates are all control solutions, but its more important to understand why. Haney here does a good job of using the ground-up engineered approach, starting with what a tire is , how it behaves, and using this behavior, You wont design a Formula Car after reading it, but its not a Textbook, but a book meant to illustrate the point of why tires are the most important factor in a Vehicle's dynamic behavior. I cant claim I'm a technical expert, Ive read a few different authors on the subject, talked about it with several people in the field, they all really try to say the same thing. This book explains it best that I have read. My personal experience is limited to my collegiate design, and what Ive tried to absorb from others.
Establishing a desired tractive force at each corner, under specific operating conditions is the goal, and the suspension allows drivers to do so by controlling specific load, and contanct patch at each wheel.
If someone really would want to go ground up with a fiero, they really need a copy of Milliken's Race Car Vehicle Dynamics, but as for optimizing the FIero towards a specific behavior, Haney is a great resource to begin an analysis.
------------------ 1986GT 4-speed DOHC and 1987GT Auto
BMW and Porsche do just fine with struts. As does Honda since they introduced the RSX. There's nothing inherently wrong with struts, as long as they can be packaged appropriately. They fail, for example, in a car like the GM W-body in which the strut tops are pushed further apart than they are on the Fiero in order to package a larger powertrain. This obviously strictly limits the capacity of the geometry for camber gain.
You're killing me here, Will. What does defending struts as used on other vehicles have to do with the shortcomings of the 1988 Fiero design?
I read that some time ago. It did a good job of convincing me that rod ends were too much maintenance for me on my street driven Fiero. Worked for you? Glad to hear it. I ended up making my own Delrin-poly hybrid bushings. There's less than 1/4" of poly for compliance. Very smooth, very quiet and much firmer than stock. Works for me.
quote
After that, try raising the inner pivots of the lateral links. That will raise roll center, move dynamic contract patch loading closer to equal and give the suspension more camber gain.
Again, looked at that hard, but this modification is limited by its total lack of adjustability.
quote
And run a staggered tire fitment that matches the car's weight distribution.
You're killing me here, Will. What does defending struts as used on other vehicles have to do with the shortcomings of the 1988 Fiero design?
Again, looked at that hard, but this modification is limited by its total lack of adjustability.
The simple act of being a strut suspension doesn't automatically condemn a design to mediocrity. Just like SLA, struts are subject to the packaging constraints of the implementation. In moving from the early rear suspension to the '88 rear suspension, the Fiero engineers actually did pretty much all the right things.
Not sure what you mean by "total lack of adjustability".
[This message has been edited by Will (edited 07-31-2011).]
Raising the inner pivots wouldn't result in any less adjustability than the suspension had stock. Since it was already significantly and meaningfully adjustable for camber and toe, I'm not sure what your point is.
[This message has been edited by Will (edited 08-01-2011).]
I have no interest in dedicating my car to track use only . Wide sticky tires, stiffer springs and higher rate sway bars only mask an inherently poor suspension design. Reducing body roll by design would be my goal. Then the spring rate in the rear could be reduced to make the car stick better in wet or uneven road surfaces i.e., The Real World.
From what I've read so far, body roll, by itself isn't that bad, as long as the suspension geometry compensates for it. That's why SLA's have unequal A-arms, so that the tire contact patch remains firmly on the ground, even if the body of the car leans.
If you can come up with struts that have better geometry than GM's design 30 years ago, then it's one solution to the problem. We can come up with multiple designs, SLA's, struts, multi link, trailing arm, etc. Let's see which one works best. The only one I draw a hard line on is live axles. In the words of Top Gear, that design belongs on lorry's. Actually, let me ad swing arm to that as well.
Awhile back I made some measurements of the front and rear suspension pivot points on my '88 for a suspension program that I have. The front roll center was a little bit HIGHER than the rear roll center. (WTF!) By eyeballing where the masses were in the car, it suggested to me that the roll center axis was out of whack with the mass axis of the car. In other words, this means the the rear overturning rolling moment was much higher than the front and explained the need for the rather large stock anti-roll bar in the back of the Fiero. Not that this is entirely bad, as it might help the car pass whatever dynamic steering quality tests that the car had to go thru to pass GM steering and stability tests (maybe). This would explain the feeling that the back end of the car would flop over a bit, even with Koni shocks, which was a real party killer to me because I wanted to put a big 'ol Chevy V8 in it. A big V8 would have made the car scary at the limit, or when recovering from a slide. The only solution I could see was to put a LOT lighter engine in the back--or lower the existing package-- and raise the rear roll center. (I would have used this opportunity to remove more weight from the front to keep the weight distribution the same, as that is just fine.) However, raising the rear roll center puts more lateral load onto the outside rear tire in a corner and makes the car fundamentally more prone to oversteer. Reducing or eliminating the the rear bar to compensate for the raised rear roll center aggravates the lack of rear suspension travel and would make the rear end more prone to hitting the bumpstops. What a mess! At this point, the EASIEST thing to do, because I am lazy, was to put all-season tires on it and and buy a different car for serious driving.
Some of the design problems can't be solved with improved geometry, such as CG height. I want to lower the car to lower the CG, and create issues with camber changes-- although some of that can be fixed with changed the mounting points of the lower control arms.
I took my C4 vette to the last autocross, since the Fiero is broken, and I had forgotten how much fun that car is. You can throw the car around the course with confidence and the results are predictable. The C4 CG is 15"-- while, according to the drawings by Bloozberry, the Fiero CG is 19.4". The 80's MR2 CG is about the same-- 19". That is going to be tough to change.
The Corvette has issues, such as chassis flex and is heavy for it's power by today's standards.
If you wanted to improve the 88 rear suspension setup, just raise the whole cradle 1" while lowering the engine relative to the cradle about the same amount. It would give you better camber gain and anti-squat and since the engine/transmission elevation remains about the same, the CG should be largely unchanged.
A 2nd cradle sleeve could be welded to the cradle, or the front tabs on the chassis drilled for the 2nd set of holes. The rear of the cradle would need to be sectioned, but the rear uprights are vertical so that makes if pretty simple. To lower the drivetrain you can just section the 3 mounts.
If you wanted to improve the 88 rear suspension setup, just raise the whole cradle 1" while lowering the engine relative to the cradle about the same amount. It would give you better camber gain and anti-squat and since the engine/transmission elevation remains about the same, the CG should be largely unchanged.
A 2nd cradle sleeve could be welded to the cradle, or the front tabs on the chassis drilled for the 2nd set of holes. The rear of the cradle would need to be sectioned, but the rear uprights are vertical so that makes if pretty simple. To lower the drivetrain you can just section the 3 mounts.
Except that you can't really do that because the left inner CV joint is essentially right on top of the left inner toe link pivot.
You have to move the toe link inner pivots forward in order to move them up. If you do that, you might as well leave the cradle where it is and weld on the new toe and lateral link mounts.
[This message has been edited by Will (edited 08-04-2011).]
Except that you can't really do that because the left inner CV joint is essentially right on top of the left inner toe link pivot.
You have to move the toe link inner pivots forward in order to move them up. If you do that, you might as well leave the cradle where it is and weld on the new toe and lateral link mounts.
The last stock 88 2.8 drivetrain I measured had the crankshaft center-line 8 13/16" from the bottom of the cradle with 172K miles on it with worn out factory mounts. The stock height with some good condition mounts is around 9 to 9 1/4". On pretty much every swap I do the drivetrain is lowered on the cradle to get the crankshaft centerline in the 8 to 8 3/8" range... I have done this for SBC, 4.3, 4.9 and 3800SC swaps (all on 88 cradles) with manual transmissions and never have had any interference issues between the transmission/tripod and the lateral link mount boxes.
So there is indeed room to lower the stock drivertain on an 88 cradle to allow the rear cradle to be raised.
the two ball joints seem to mimic an unequal A arm setup .i really think this is ingenious .no way i could ever duplicate this setup , and i find it hard to believe cash strapped gm developed this all by them selves . the 911 is struts all around and it out handles pretty much anything , do they have a similar setup ?
[This message has been edited by wftb (edited 10-29-2011).]
Here's the effect of raising the inboard mounting points of the lower control arms. I think. I think the previous owner raised them. Right now the cradle ride height is 4" on 15" slicks with a 22.9" diameter.
This is probably lowered beyond optimum.
The simplest way to lower the car is use shorter tires.
Looks like they were raised about an inch. It does help the camber gain in your lowered car. Have you tried any other ride heights since you got the car?
------------------ yellow 88 GT, not stock white 88 notchie, 4 banger
I've lowered the car about 2". The issue at the front with the car this low is bottoming the shocks. I think a simple solution will be to cut the upper mount and add a 2" extension.
I understand this thread is about an ideal suspension or at least as idealized as possible.
I'm mostly interested in tweaking the existing design to it's maximum potential. The previous owner was quite fast with the car as it is. I'd like to make it quicker, if possible.
Most of you are looking for a good handling street car.
I'm looking for the best handling track car (although the setup for autocross and road racing are not necessarily the same). Finding the balance is the key.
I do intend on building a street rod Fiero down the road using either a 4.9 or 3.8.
What I hope to gain out of this is an understanding of the inherent handling flaws that are prompting the desire to redesign the suspension. What doesn't it do that it could do with a better design?
What are the specific design goals?
I will probably re-read the thread when I have some extra time. Much of my question may already be answered.
I'm in the track car category myself. I've had the same front issue. Ended up doing about the same thing, but went to taller spindles at the same time. On the rear, doing about the same but designed a new knuckle to lower the links and use a bigger bearing. I can run 315's now.
------------------ yellow 88 GT, not stock white 88 notchie, 4 banger
Have you looked at how much clearance there is vs. the relocation that would be required to achieve a worthwhie effect?
Nope. Mostly because chassis ride height, spring rates, swaybars, tire side wall deflection, etc. all come into play when you look at the current state and compare it to the modified state. For favorable camber curves on compression and extension, the inboard lateral link pivots need to be higher than the outboard ones at ride height. How much higher they should be really depends on the amount of suspension travel, body roll and tire defection your setup will have.
Originally posted by bse53: The issue at the front with the car this low is bottoming the shocks. I think a simple solution will be to cut the upper mount and add a 2" extension.
Another option is to use longer bolts and spacers between the lower shock mounting shaft and the bottom side of the lower a-arms.
Nope. Mostly because chassis ride height, spring rates, swaybars, tire side wall deflection, etc. all come into play when you look at the current state and compare it to the modified state. For favorable camber curves on compression and extension, the inboard lateral link pivots need to be higher than the outboard ones at ride height. How much higher they should be really depends on the amount of suspension travel, body roll and tire defection your setup will have.
The springs, bars and dampers depend on the suspension and tires, not the other way around. The ideal suspension has one degree of camber gain for one degree of body roll. Assessing where modified geometry is relative to the ideal is not a hand waving argument citing complexity in dependent processes.
The big gain for the Fiero is really the centroid axis inclination vs. roll axis inclination. When the rear roll center is raised, the roll axis inclination more closely matches the centroid axis inclination. This means that the contact pressure on the outboard tires in a corner grows at the same rate per lateral g front and rear. If the static contact pressure is matched front and rear and the suspension acts to grow the outboard contact pressures in the same way front and rear, then the car will remain neutral.
That assumes similar camber performance front/rear, springs/bars/dampers chosen intelligently, etc.
The springs, bars and dampers depend on the suspension and tires, not the other way around. The ideal suspension has one degree of camber gain for one degree of body roll. Assessing where modified geometry is relative to the ideal is not a hand waving argument citing complexity in dependent processes.
The big gain for the Fiero is really the centroid axis inclination vs. roll axis inclination. When the rear roll center is raised, the roll axis inclination more closely matches the centroid axis inclination. This means that the contact pressure on the outboard tires in a corner grows at the same rate per lateral g front and rear. If the static contact pressure is matched front and rear and the suspension acts to grow the outboard contact pressures in the same way front and rear, then the car will remain neutral.
That assumes similar camber performance front/rear, springs/bars/dampers chosen intelligently, etc.
I think I would like to lower the CG and move it closer to the roll center axis. The CG is high on the car because the car was never designed as a sports car, but a sporty car. If the Fiero CG is 19", compare that to the Porsche 911, it's CG is 15". My question is how can I do that. Lowering the cradle and raising the inboard mounting points of the lower control arm/toe arm is a start. I can now get as much static camber as I want.
I think I can balance the car with springs and dampers. Bars not so much. I want to control dive because the front suspension has such limited travel.
I've considered lowering the engine/trans also. I have a spare cradle I was modifying with different engine/trans mounts (and perhaps SLA upper mounts) anyway, so this would be the time to do it. I've also looked at weight at the top of the engine compartment. I've moved the battery to the front and the wing is the next to go. I've got the materials for a carbon fiber lightweight deck lid but haven't had the time to start on the mold. All that should make a difference in the rear CG.
Unfortunately, there's the 3.4 DOHC cams up there. Ever pick up a cam carrier off one of those? It's ridiculous. Wonder if I could get them gun drilled?
~Neil
BTW, it's going to be hard to match a Porsche rear CG when it has a flat 6 motor in it.
[This message has been edited by sspeedstreet (edited 08-06-2011).]
Porsches have horizontally opposed engines and a lot of them have dry sump lubrication systems, which allow the engine to really sit low in the chassis.
The Corvette mentioned above may have a lower CG than the Fiero, but it's also 600# heavier. If you bolt 600# of lead ballast into the floorboards of a Fiero, the CG will be lower, but you really don't want to do that. There really isn't much room to drop the Fiero CG without doing something extreme like going to a dry sump sysem and lowering the engine while rotating the transmission.
As mentioned above, there's only so much room to raise the inner pivot of the left toe link before it runs into the left inner CV joint. There's more room to raise it if you move it forward about an inch. I would go for raising the pivots more than lowering the cradle. If you raise the pivots an inch and lower the cradle an inch, you haven't done anything to improve your geometry. If you've lowered the car with shorter springs than stock, you've already compromised the geometry relative to stock, which wasn't great to begin with.
What I hope to gain out of this is an understanding of the inherent handling flaws that are prompting the desire to redesign the suspension. What doesn't it do that it could do with a better design?
What are the specific design goals?
I will probably re-read the thread when I have some extra time. Much of my question may already be answered.
Brian
That was the goal of this thread, unfortunately it has since been taken off to too many tangents.
Porsches have horizontally opposed engines and a lot of them have dry sump lubrication systems, which allow the engine to really sit low in the chassis.
The Corvette mentioned above may have a lower CG than the Fiero, but it's also 600# heavier. If you bolt 600# of lead ballast into the floorboards of a Fiero, the CG will be lower, but you really don't want to do that. There really isn't much room to drop the Fiero CG without doing something extreme like going to a dry sump sysem and lowering the engine while rotating the transmission.
As mentioned above, there's only so much room to raise the inner pivot of the left toe link before it runs into the left inner CV joint. There's more room to raise it if you move it forward about an inch. I would go for raising the pivots more than lowering the cradle. If you raise the pivots an inch and lower the cradle an inch, you haven't done anything to improve your geometry. If you've lowered the car with shorter springs than stock, you've already compromised the geometry relative to stock, which wasn't great to begin with.
I was merely pointing out some of the obstacles in making the Fiero a true sports car. Assume the C4 Corvette is 3300 lbs and CG is 15" and track is 59". Lateral weight transfer at 1 g is 838 lbs. Assume Fiero is 2700 lbs and CG is 19" and track is 59". Lateral weight transfer at 1 g is 869 lbs.
Take 100 lbs off the car and you've equalized the weight transfer (assuming the 100 lbs doesn't come low on the car and actually raise the CG in the process).
Getting the CG lower will make a big difference. Any suspension design should include this consideration. Keeping unsprung weight to a minimum should be a consideration. Any design shouldn't add weight.
All modern sports cars use aluminum extensively in the suspension elements. Corvette uses magnesium to help balance.
Bumpsteer is not as big an issue, even in a pre-87. This design by Held looks effective.
Another important element that I believe is a weak link are shocks. The Fiero has a limited choice of dampers.
This thread is starting to read like we should just install a flat boxer motor with a dry sump to keep the CG low and all problems will be fixed.
So nobody things the geometry of the suspension could be improved with the engine sitting where it does now?
The assumption I made as to CG height was based on a drawing on this thread. That was based on a ride height of 6.2". Raising the inboard mounting points of the lower control arm/toe rods allows the rear to be lowered a couple of inches without too much affect on geometry. Add the suggestion of lowering the engine in the cradle and you've lowered the engine and CG even more.
If a person is swapping engines, what the engine does to CG should be a consideration for a track car, IMO. I could be wrong. The Fiero I'm messing with has the 4.9l and is producing as much power as it probably can without adding boost. It's relatively light in comparison to the 2.8, but even there aluminum block/cast heads puts the weight up high. The all aluminum LS engines might be a better choice, or what CCfiero350 is doing with the 2.2 ecotec might be the wave of the future for lightweight power.
I'm just suggesting that weight is part of the equation of making a car handle.
Weight absolutely is. Lightness is the best thing you can add to a car to make it handle. A tire with less weight on it can generate more grip. In addition, if you go from an iron 2.8 to an aluminum 4 cylinder, you can run wider tires in front.
Assuming you're running the widest tires you can in the rear and a proportional tire in front, then you'd end up with something like 225 front/275 rear or 205 front/255 rear with 45/55 weight distribution.
If you can bump that just a couple of percent to 47.5/52.5, then you can widen the front tires from 0.8x the rear to 0.9x the rear. That means 245 front/275 rear and 235 front/255 rear.
Less weight AND you get to put more rubber on the road.
[This message has been edited by Will (edited 08-10-2011).]
Aereodynamic downforce is our friend. Unfortunately most autocross class rules don’t allow areo and the amount of area needed to apply any significant amount of downforce at autocross speeds is pretty large.
Downforce is free weight, since it doesn’t affect lateral weight transfer, which is what we were talking about.
There are three ways to reduce lateral weight transfer- lower the CG, reduce weight or increase track width. So for our Fiero we should optimize each of these and then improve the suspension geometry to keep our tires at maximum contact.
We can lower the CG a couple of inches fairly easily, we can lose a few hundred pounds by taking off all the comfort amenities and we can increase track width with wide wheels—since the design of the car forces the wheels outside the car as we increase the width.
On my car, the maximum backspace I can fit is about 5.5”, so a 10” wheel is going to stick outside the stock fenders slightly. Simple solution—widebody. A 10” wheels also allows the 275 size tire. I’m referring to 15” wheels.
So, assume a 60” track and 1g of cornering. Our 2700 lb car at 19” CG will generate 855 lbs of lateral weight transfer. Lighten the car to 2500 lbs and we’ve reduced the WT to 791 lbs.
But let’s lower the CG 2”. Now our 2700 lb car will generate 765 lbs of WT. Lighten the car to 2500 lbs and we’ve lower the WT to 708 lbs.
Pretty obvious we want as light and low a car as possible. If we could lower the car to 0” CG, there would be no weight transfer and our cornering ability would be the limits of the tire. Remember as we transfer weight, we’re adding weight to the outside tires. If the Force at which tires slide is the coefficient (expressed in g’s) times weight and that Force is a function of the tire, as we lower the weight on the tire we increase the g’s we can generate, which translates to speed around a corner up to the stiction of the tire.
Assume a 100' radius corner. At 1g we'll be traveling 38.6 mph. Increase that to 1.25 g and we've increased our speed to just over 43 mph.
Here’s a very good resource on handling by Brian Beckman:
