What coil springs to use on a fox?

[TurboShortBus]

I ran spherical upper bushings on the housing side at a couple of events two years ago (unfortunately one of those was the first NASA Nationals) and it was the biggest setup mistake I have made yet with my car. The bind was unbelievable with and without the panhard bar installed. It was as if I had put a 2-inch rear sway bar on the car. The binding was so bad it tore the rubber bushings out of the chassis side of the uppers and by the end of the championship race, the mounting bolts were flopping around in the bushing shells and the rearend was slamming back and forth under braking and acceleration. I think you could make the spherical bearings work if you only ran one upper arm, but the car would react asymmetrically on right and left turns.

 

You mentioned bind "with and without the panhard bar installed." Now, during that time, how long were you running without the panhard bar at all? That is more than likely the reason why you destroyed the stock rubber bushings (that, and the fact that your 1986 rubber bushings weren't exactly fresh a handful of years ago). The more heim joints you add to the control arms, the more you lose lateral stability (without a panhard, that is; but, you're still relatively fine due to the angle of the uppers). Running spherical upper bushings on the axle end meant that those poor upper chassis end bushings were now 100% responsible for locating the axle, rather than 50%. Throw in 15-odd years of dry rot, and you have a recipe for failure. I would not blame this on binding due to the suspension geometry. Besides, the range of travel of the rear axle (with stiffer springs, etc.) is much smaller than the car would see on the street in stock form.

 

Several years ago, when I first noticed that my 60-foot times weren't doing so hot in my street/strip car (it was a low-12 to mid-11 daily driver), I started inspecting the rear suspension and found that the stock upper bushings had been taking quite a beating and were torn (I also autocrossed the car back then, and generally beat the snot out of it every chance I got). Having trashed upper bushings meant that the chassis was free to do its own thing laterally (in relation to the axle), and since I had installed stiffer polyurethane bushings in the lower arms a year or two before, this allowed a small hairline crack to develop in the lower torque box. Losing lateral stability meant that the lower arms were moving side to side much further than usual, and the stiff poly lower bushings helped the large "levers" (lower control arms) try to "open up" the torque boxes, and a crack formed at the corner of one of the torque boxes, right where you would expect it to start.

 

Mark

[TurboShortBus]

As explained to me...

 

you have two lengths of control arms, long lowers and short uppers. As the axle wants to move up and down, the arc described by these two arms are different. This difference will cause bind in the suspension if all the links were solid (e.g. rod ends). I believe that is why Ford put the large oval rubber bushings in the forward end of the factory equipment lower arms. The soft rubber bushings allowed the arms to translate fore/aft during bump and jounce.

 

Since you have solid arms in the lower, there is no way that lower arms will deflect, so they describe 100% of the axle’s motion. Therefore, all the compliance must be taken from the upper control arm bushings. Unfortunately the upper arms are unable to translate fore/aft enough to provide this compliance

 

The only way for the upper arms to not bind were if the pinion was allowed to freely rotate. However, the engine torque is resisting the pinion rotation (actually creating pinion rotation in the opposite direction as the engine torque is wanting to drive the pinion up and the compression of the suspension is wanting to drive the pinion down.

 

This suspension bind is the reason you are able to run such a soft rear spring but still have a “loose†feeling in the car. Basically you are getting the standard Mustang snap oversteer.

 

I am not in agreement with some of this. While I'm not exactly a suspension engineer (just a dumb architect), I have spent more than my fair share of time under Fox chassis cars since my first one in 1990.

 

First of all, not all Mustangs came with those oval lower bushings. Round bushings came in the SVO models, and we pulled a set of round-bushing lowers out of a 1990-ish 4-bang-Stang that my buddy was parting out. I never took the rear suspension out of my old 1987 cop car LX 5.0, so I can't say for sure what the police package cars had.

 

Now, let's go back in time about 15-20 years and see who remembers this example. Mustangs that use those old-school Southside lower control arms are asking for trouble. They used completely solid bushings (note that my use of the word SOLID does not mean HEIM). Southsides were attached with the typical bolts, but the bolts passed through a very thin plastic/nylon sleeve, which passed through a steel tube that was welded to the arm. This "design" (or lack thereof) meant that there was no compliance, flex, or give. It's bad enough that the lower control arms are not perfectly parallel to each other, but once you throw in some lateral movement, all the Southsides did was tear the torque boxes apart. Granted, they were mostly intended for drag racing, but guys who ran them on street cars were busy welding their torque boxes back together a few years later. This is why heim joints or rubber/poly bushings (and not solid bushings) are still needed on the lower arms.

 

I agree that the short upper arms and the long lower arms move in two different arcs, which causes the the pinion angle to change through the axle's range of motion. However, how large is that range of motion during a typical race?

 

Now, rather than worrying about having 2 different arcs due to short and long control arms and their affect on pinion angle, I'm more concerned with the angles of the arms as viewed from the top. Keep in mind that the upper arms are at such extreme angles specifically for lateral stability. The upper arms are not moving in the same plane as the axle (really, nothing moves together back there lol); they are basically moving 45* to the axle travel. Now yes, with rubber or polyurethane bushings, the suspension will bind. Bind becomes worse with the poly bushings, since they are stiffer than the stock rubber. Rubber is used to allow a certain amount of flex or pivoting at the control arm ends. However, with the use of heim joints (which I do not consider to be solid; to me, solid would be a 1-piece metal sleeve), the arms are free to rotate or pivot however they need to at the connection points. They are just doing the same thing that the rubber bushings do, but without being soft.

 

Mark

[snakebit]

Okay, I'm gonna jump in here and ask....where should I start with setup for my 04 GT. I'm planning on running CMC here in NASA SE. What springs 900/200? Koni DA's? What front sway bar? Any rear bar? Panhard? Stock rear control arm bushings? Are Steeda offset a-arm bushings (front) legal? Are lowering ball joints legal? If not can the stock ones be modified? What about the rear quad shocks?

 

Thanks,

John

[TurboShortBus]

John,

 

It looks like we're building identical cars. Sonic Blue 2004 GT here.

 

Based on Tony G's recommendations, I just installed 900 lb x 12" length x 5" OD front springs and 250 lb x 11" length x 5.5" OD rear springs last weekend. I'll double-check the boxes to make sure. Unfortunately, the car was only lowered by 3/4", and could stand to go down another 3/4" to 1" (I believe that Tony suggested maintaining 23.5" from the bottom wheel lip to the fenderwell lip, and that's with 16" wheels). I'm only running a lower spring isolator up front, with none at the top. Out back, I reused the stock upper and lower isolators, so yanking all of them will give me another 1/4" up front and probably 1/2" out back at the very most.

 

I installed Tokico G-Spec shocks and struts because I got them for $475/set, new in box, delivered. I'll probably wear them out in HPDE before I'm eligible for CMC.

 

If you go with a more solid rear control arm setup (heims, etc.), then the quad shocks can go away. They are there mostly to dampen standing-start wheelhop with stock arms and bushings, but I would imagine that they might help a bit during hard downshifts as well.

 

Are you planning to run at all in Florida? There are no CMC cars that I know of down here.

 

Mark

 

(edit: I fubarred the rear spring length the first time)

Edited April 4, 2008 by Guest

[snakebit]

Mark,

 

Thanks for the info. I have a set of Tokico D-spec's for the front, nothing yet for the rear.

 

Florida is a bit far for me but not out of the question. I'd love to run Sebring! My wife (currently separated) is from Orlando. She has family there still and we had planned to come done this summer, I was going to do a Chin event. On hold for now.

 

Here's a pic of the car in it's current state. It's a theft recovery with only 3100 miles. I have a 2000 GT donor as well.

 

How do you attach pics in this forum?

[TurboShortBus]

I have yet to run Sebring. All of my track time has been at Homestead-Miami Speedway. The second half of NASA's season down here is almost exclusively at Sebring. I'll eventually jump in with Chin or SAFE for more track time than NASA gives.

 

I'm right next to Orlando, in Winter Park.

 

As for attachments, I'm not smart enough to figure it out, either. lol

 

Mark

[King Matt1548534716]

What Boudy posted is exactly how I understand it as well. If I had to guess, that sounds like Jack Hidely from MM, but regardless of who wrote it, it does describe what is happening in a Mustang rear suspension. In my case, the sphericals on one end definitely expedited the destruction of the rubber on the other, but it still remains that you can't have non-compliant material at all four suspension points because the arms must effectively change length as the rear axle rotates due to the angle of the uppers. If you had a parallel four-link with a panhard, it would be a different story.

[TurboShortBus]

Fortunately, heim joints will let the control arm connection points rotate around as necessary in order to prevent binding. Through the axle's range of motion, the control arms will not always be perpendicular to the bolts that anchor them. That is why the factory uses compliant rubber bushings; as a compromise to allow this rotation and reduce NVH at the same time. Poly bushings are stiffer and will put up a fight, which causes the binding. Heim joints are free to rotate in nearly any direction (within reason), although NVH goes through the roof.

 

Tell you what...get under the back of your car and install heim joint upper control arms. Put the chassis on jack stands and run the axle up and down with a floor jack. You tell me where the binding is happening. If it's binding with these heim joints, then it won't be long until the factory spot welds are torn from the unibody, correct? You should be able to see all sorts of sheet metal flexing, right? Nope. I've had this setup on my car, and it worked fine. This May will mark 13 years of assbeatings for this car, both on the street and on the strip.

 

Here's another example. If you have ever installed upper control arms with fresh rubber or poly bushings, it was probably a bitch to get the control arm bolts through the sleeves and completely installed without raising or lowering the axle to its "sweet spot" so that the holes and everything lined up, along with prying the end of the upper arm one way or another. This is because the arms are not always perpendicular with the bolts. This is the binding that everybody talks about. Now, with heim joints on both ends of the uppers, you don't have to go through so much BS. All you have to do is swivel the heim joint one way or another, and whammo, the bolt goes right through. The only variable here is the length of the control arm itself, but that's an issue no matter what you're using for bushings. But, my point is that the heim joints eliminate the binding that occurs with rubber and poly bushings.

 

Mark

[King Matt1548534716]

Read through this thread: http://www.corner-carvers.com/forums/showthread.php?t=9237&highlight=four+link+mustang+spherical+bind

 

The upshot is that while the sphericals in all locations will not bind as long as the range of motion of the rear suspension travel does not exceed the misalignment angle of the bearings, it introduces highly unfavorable pinion angle changes and roll steer conditions. But if you want to try it, please get back to us with a race report.

[bsim]

I about as far from a Mustang guy as you can get, but after reading through all this it seems that the "bind" being referred to isn't a bushing bind at all (once replaced by spherical/hiems).

 

Once the bushings are replaced, the "bind" is when the axle wants to rotate (due to the unequal length arms) which changes pinion angle. All this twisting movement will also be either compounded or fought against depending whether you're accelerating or decelerating.

 

Sounds nasty on a racecar.

[King Matt1548534716]

Correct. The fact that the sphericals don't add wheel rate like a rubber or urethane bushing does not make them a desirable solution to the fundamental geometry problem.

[TurboShortBus]

Correct. The fact that the sphericals don't add wheel rate like a rubber or urethane bushing does not make them a desirable solution to the fundamental geometry problem.

 

So, besides a "for sale" sign, what is the solution for a CMC Mustang? I'm all ears (and nose, if you've ever met me).

 

Mark

[mitchntx1548534714]

Sounds nasty on a racecar.

 

In terms of what a 4th gen has to work with, maybe.

 

But what I don't get is if it's predictable, and heims should remove much of the unpredictability out of the equation, a reasonable chassis tuner should be able work around this issue and even take advantage of it.

 

People use bump stops on the rear suspension of a 4th gen to get the car to rotate in a turn. Can't get any more instantaneous bind than hitting the suspension travel bump stops and the effective spring rate goes infinite.

[TurboShortBus]

Mustang guys are pretty much SOL when it comes to pinion angle anyway, since we're not allowed to change the length of our rear control arms. There's no opportunity to attempt to set the angle at something that might work reasonably well most of the time, so we're stuck with the factory settings (before the car is lowered, that is). It isn't desirable if the pinion is pointing up in relationship to the driveshaft while at rest (since it will only get worse under acceleration).

 

I'll have to get under the back of my 2004 GT HPDE car and see what the pinion angle currently is. I know that my 1994 GT is set at 2* nose down (sphericals on axle ends, 3-piece poly bushings on chassis end; best street compromise).

 

Mark

[King Matt1548534716]

Given the rollsteer and pinion changes it would be predictably very scary and hard to drive, especially at the limit. But to answer Turbo's question, there really is no "solution" to the Mustang's rear suspension problems given the CMC rules, but there are workable compromises involving good rear lower controls and tuned spring rates, shocks, rollbars, etc. A panhard might be part of that equation too, but it has advantages and disadvantages, all part of the compromise. Some popular combinations have been discussed in this very thread.

[TurboShortBus]

True, all we can really do is compromise, since there is no "perfect car" for us (unless Scuderia Ferrari is signing our paychecks). A long torque arm, rigidly mounted to the axle, with shorter lower control arms is also a geometric fubar (to a degree), due to the different arcs and the axle's inability to rotate to compensate (although this keeps the pinion angle more consistent).

 

But, even a perfectly set-up car with a ball-less driver (the latter exists at my house) is no match for another car with a rear suspension bind-fest and a driver with huevos. lol

 

Mark

[Boudy1548534717]

Well Mark, mine's not for sale yet. I'm in process of making some changes on my car for testing at the next event. The reason these guys say that you can't run stiffer springs is that stiffer makes the current problem worse, you can't just swap springs and go see what happens. I'm implementing a combination of changes that should allow for stiffer springs and a lower ride height without agrivatating other issues. Rolling over like a sail boat is surely not the fastest way around but it's been the compromise we've grown to live with.

 

rb

[TurboShortBus]

Rolling over like a sail boat is surely not the fastest way around but it's been the compromise we've grown to live with.

 

Right...and, as long as everybody's sailboat is rolling over the same amount, it should be pretty fair, right? May the best rolling sailboat win...lol

 

Mark

[TurboShortBus]

rb

 

Oh hell, it just hit me...this is Robert at Argent Lab, isn't it? I spoke to you a few months ago regarding Ultra-Shield seats and lame HPDE rules. You know my brother, Scott, in the Factory Five Cobra, and his buddy, Pat, in the AI Mustang. What's up?

 

Mark

[mitchntx1548534714]

a reasonable chassis tuner should be able work around this issue

 

there are workable compromises

 

 

 

Like in any of the platforms, each has it's Achilles heel. Something else I don't understand is that if it's so bad, why are there Mustangs still running in this series?

 

What I find laughable is that the platform a particular racer owns is absolutely the worst ... every single time. It's nothing more than a transparent attempt at laying ground work for "politickin'" come rules massaging day.

 

Well, after trying my best at doing just that, let me share something with you. It ain't happenin' ... we're all just killin' electrons here.

[King Matt1548534716]

Yeah, there are workable compromises, but putting solid bearings in all four locations isn't one of them, unless you plan to run springs so stiff that there is no rear suspension movement. Let's see you tune around that! The compromises that work with a stock Mustang 4-link involve balancing stiffness and compliance with sufficient articulation to make the thing drivable within the limitations of the poor rear geometry.

 

As far as platforms go, one things that is pretty hard to refute is that given the fact that the SN95 and Fox Mustangs share the same engine, weight, and chassis, the difference in track width is a significant factor.

[CMC#11]

How about we come up with a list of 5 reasons why a GM is better than a Mustang?

 

Also 5 reasons why a Mustang is better than a GM, and one of the reasons can't be superior driver skill.

[TurboShortBus]

and one of the reasons can't be superior driver skill.

 

Yeah, driver skill is overrated...lol

 

Mark

[Boudy1548534717]

Like in any of the platforms, each has it's Achilles heel. Something else I don't understand is that if it's so bad, why are there Mustangs still running in this series?

 

What I find laughable is that the platform a particular racer owns is absolutely the worst ... every single time. It's nothing more than a transparent attempt at laying ground work for "politickin'" come rules massaging day.

 

Yeah sure, every platform has an Achilles heel but this one has an Achilles leg. I've maintained all along that these Fox cars can be made faster with time and effort put into the developement. Not much has been done, that I've seen or read to get this platform to it's fullest potential. The only documentation I've ever seen proves that the geometry stinks, period. Why are they still around? You drove your's even when you though it sucked didn't you? Not a good arguement.

 

What I find laughable is when guys believe one thing and post the opposite. Sorry, but politickin' ain't my game cause I stink at it. You know me well enough to know that I'll take it on the chin for the next guy any day. But the facts are what they are. You are wider, you have coil overs, you have a torque arm, you have access to springs and tuning gear made for your car, you have a panhard bar that makes the car better not worse, horsepower to spare, I have more... Now I have heard how each of these items is actually a GM's disadvantage but that's where the real politikin' kicks in.

 

All I asked for last year was a weight concession to compensate for the lack of track width that the Fox fenders restrick it to. Too much? I think it's more than fair as track width can easily be tested in relation to laptimes as can weight. Pretty black and white or do you think your track width is hurting your laptimes?

 

As for the rest, I can't honestly ask for anything else because I don't feel the car is fully developed. I've got 5-6 really sharp guys helping me make changes and I'll get this thing to its full potential soon. However, I do believe that even then, it will still fall short of keeping up to you guys under the current rules. You believe it too!

 

Look for me testing my latest revelations:

 

Edited April 4, 2008 by Guest

[Boudy1548534717]

rb

 

Oh hell, it just hit me...this is Robert at Argent Lab, isn't it? I spoke to you a few months ago regarding Ultra-Shield seats and lame HPDE rules. You know my brother, Scott, in the Factory Five Cobra, and his buddy, Pat, in the AI Mustang. What's up?

 

Mark

 

Yep, same jerkoff buttlick you spoke to. Don't get me started on that brother of yours...

 

rb