NEW MUSTANG REAR SUSPENSION 1

[BillyShope]

What with the emphasis on dragstrip applications, does anyone know why Ford didn't offset the upper link to the right on the new Mustang rear suspension (ala the Jaguar C-Type)?

 

[Joest]

Billy,
My best guess is that it was not done because of packaging. Placed in the middle, the link can use the driveshaft tunnel. With an offset configuration, it might conflict with the rear passenger seat or be too short to maintain good geometry.

 

[BillyShope]

Yes, that would be my guess, too (though they ended up with a very short link, anyway). I'm just hoping that someone who was involved will respond to tell me that they were at least aware of the Jaguar design and considered it.

Or, they could have cut costs even further by using quarter elliptics, clamped at both ends (forcing the spring into the secondary bending mode), and a single upper link, clamped at the rear and pivoting at the front. They'd have ended up with no Panhard and only one bushing. (It's fun to be a Monday morning quarterback.)

 

[carnage1]

they probibly would have been even beter off using the coil spring watts link arrangement that they have been using on the police cars for 6 or 7 years. (do you think they would mind if I "borrowed" one for my custom truck?)

 

[GregLocock]

http://www.mach1performance.org/news/images/2005fordmustangrearaxle.jpg

My /guesses/ are

A) An offset link would introduce an asymmetry in the rear suspension steering characteristic under braking and acceleration

B) They didn't think of it

C) Packaging (as Joest suggested)

D) They didn't care

E) They don't read eng-tips

F) Faint hope - they tried it and didn't think the result was worth the aggravation

G) They need as much wheelspin as they can get from the V6 to impress the customers.



Cheers

Greg Locock

 

[BillyShope]

Greg, your "G" comment reminded me of a funny situation which existed with the early Fordomatic transmissions. The '54 V8's would often emit a tire "chirp" as the Fordomatic upshifted to second. The kids thought this was great, of course. What they didn't realize was that the chirp was because of a shifting problem with the transmission. In other words, for an instant, the transmission was engaging both first and second gears.

As for the braking comment, everyone brings this up and, if it wasn't for the success of the C-Type, I'd be inclined to accept it. I guess it worked because most of the braking is occurring at the front and, also, the asymmetry at the rear wouldn't really be noticed until you reach the limits of adhesion. And, after all, you don't want those rears locking up prematurely, anyway.

 

[NormPeterson]

Perhaps the intent was to mimic the Trans-Am road racers instead of the quarter-milers. After all, the best side view angles for the lowers (and by implication, rollsteer) that go along with the offset 3rd link for best dragstrip performance probably aren't all that friendly for the lowest-common-denominator buyer in normal road driving. Symmetry is a much easier compromise to make.

For some time now, there has been discussion on a couple of other fora dealing with at least one aftermarket 3-link retrofit kit specifically for Mustangs, a similar Shelby Cobra replicar company option, and a number of one-off individual efforts. Not to mention some similar products under development for some of the earlier GM RWD cars.

Even small braking asymmetries might bring with them some ABS consequences. Or stability control system issues, if that was envisioned. In all fairness, the replicar product is slightly offset, but that car does not anticipate the installation of either ABS or stability control.

It may have been decided that taking the 3rd link loads off the pumpkin was preferable to forcing the right side axle tube to pumpkin attachment to handle the full torque reaction. That's what the previous 4-link did, hence the changes to the axle assembly would have been limited to the details of providing a single longitudinally-oriented ear instead of two angled ones.

Or maybe the rear suspension re-design budget was small enough that everybody decided to just keep it simple.

Norm

 

[BillyShope]

Thanks for your comments, Norm. Of course, until someone posts who was involved, we're all just guessing, but, as working engineers (at least, I used to work), we know that your last sentence is probably the best guess yet.

You imply that rear suspension asymmetry is common among dragracers, but, apart from some static preloading, this is certainly not the case. The Ramchargers (a group of Chrysler engineers) attempted to utilize the Jaguar design back in the early sixties and there was a handful of imitators, but, unless something's happening recently of which I'm not aware, that's about it.

But, perhaps something is happening, for I was certainly not aware of these aftermarket kits to which you refer. I've been pushing this sort of thing...among dragracers...for about half a century now and it appears that I might actually see some activity before I die. They love their 4links so very much! Didn't think it would ever happen. In fact, I've recently started pushing adjustment of the 4links to place the IC above the no squat/no rise line on the right and below it on the left. Achieves the same effect as the asymmetric 3link and it allows them to keep their precious 4link.

Incidentally, it looks like someone else took high school latin. I'm going to make a search for these "fora" to which you refer. (I've been ribbed aplenty when I refer to turn "apices" and football "stadia.")

 

[NormPeterson]

You imply that rear suspension asymmetry is common among dragracers, but, apart from some static preloading, this is certainly not the case.

I didn't mean to imply that it was common anywhere just yet. Certainly not at the dragstrip (which at my somewhat casual glance appears to be mostly 4-links and ladder bars) What I was suggesting was that the details of the geometry were following road race preferences rather than incorporating tuning specifically oriented toward the drag strip in the interest of less demanding overall road manners.

Sites that I know of include corner-carvers.com and corral.net for the Mustang, Factory Five, and early Camaro fitments. Factory Five probably has its own site. Pro-touring (currently found at

http://66.70.234.198/forum/index.php?

) and montecarloss.com discuss the Camaro and some other GM applications. Note that there is some overlap of project discussions and people involved across the boards.

Maybe I had a study hall in a room where Latin was taught during other periods. Unintentional learning via osmosis?

Norm

 

[GregLocock]

In production they could eliminate straight line brake steer even with an offset third link by tuning the left and right hand lower arm bushes independently. They could also do the same in acceleration by using different snubbers on each side. It's not rocket science.





Cheers

Greg Locock

 

[BillyShope]

Greg, you might be missing something on these asymmetric setups. Suppose you had 3 trailing links, 2 below and 1 offset above and, further, suppose that they are perfectly oriented to provide driveshaft torque cancelation and no squat or rise (so that the angles won't be changed as the car launches). While the forward thrust forces, at the rear tire patches, are equal, the loads in the lower links are not. So, though the links are asymmetrical when projected onto the XY plane, the resultant link force lies in the XZ plane and there is no adverse steering effect. (I can provide some numbers, if you're interested.)

In braking, however, this ideal situation does not exist.

 

[SusTestEng]

At the end of the day..... it's still a Ford Mustang. Everytime we buy one as a competitor, nobody wants to drive it more than 30 minutes. I still don't see how Ford can sell so many of theese cars, when they are not very good at anything... besides making tires bald.

Anyway, here is a pretty good picture of the layout

http://www.rsportscars.com/foto/05/mustanggt05_detail.jpg

 

[MoreWing]

Good picture STE.

I think you actually kinda hit the nail on the head. They're real good at making tires bald. I've no real doubt that the trailing arm layout was a compromise between 'better that the awful thing we've been selling for the last 25 years' and $$$.

At best a Mustang is about 90% of a really good, fast car. And that's only after a lot of work. Regardless, when I was a bit younger I had a hell of a good time in many of them.

I guess you get what you pay for.

 

[SusTestEng]

last weeks episode of Top Gear featured the new Mustang GT. They drove it down the longest straight highway they could find in England, as to not have to worry about making any turns. They made an analogy of the rear suspension that is pretty true: "It's like walking around with your shoelaces tied together."

 

[BillyShope]

Funny how you can get so many different opinions from these magazine "testers." Richard Fleury, also from "Top Gear," seemed to think the Mustang handling was superior:

"The way the Mustang carves confidently into corners, making freeway on and off-ramps something to look forward to."

Liked that "shoelaces tied together" comment, though my favorite is still Tom McCahill's "It handled like a cannon ball falling down a chimney." Boy, I wish I could talk purty like that!

 

[GregLocock]

And there's always Colin Chapman "Any suspension can be made to work, if you don't let it move"

I think he was thinking about Corvettes and Mustangs at the time!



Cheers

Greg Locock

 

[dyna3d]

I don't know much about what they ended with, but I can tell you they started with a 4-link rear with a lateral toe-link, a lateral camber link, a lower lateral spring link, and a trailing blade (compliant in torsion) with a huge front bushing. This arrangement is recently common in front-wheel drive cars and front-wheel drive dominant cars that offer a "real-time" 4-wheel drive (really a rear-wheel assist), like the Jag. X-type and about 10 or more small SUV's. The blade simply couldn't survive addition of the high torque acceleration loads that the rear-drive V8 would create. Anyway, the live axle is probably much cheaper and, since the GM F-bodies are gone, the Mustang has no natural competitors so they don't need to win customers with improved features. They will sell them all from looks alone for the first few years until everybody that wants one has one and then they will have to figure out how to update a retro style without rehashing 40 years of less desirable designs.

 

[GregLocock]

"The blade simply couldn't survive addition of the high torque acceleration loads that the rear-drive V8 would create"

Um, no.

Check out Ford Australia's Typhoon. 4 litre turbo charged with a blade type IRS. 550 Nm of right pedal fun. The killer load for blades is not simply engine torque.

I think your last two sentences are correct.



Cheers

Greg Locock

 

[dyna3d]

Greg, you are right, but I didn't think the specifics were necessary. The killer event was a WOT run up a bumpy incline (I seem to recall it might have even included pulling a trailer- but I might have dreamed that part). The combination of high longitudinal loads combined with rapid articulation (resulting in torsion in the blade as the 4-link is overconstrained and can only operate with the torsional compliance of the blade and the conical compliance of the trailing blade bushing) made the blade a risky design. I am not familiar with the Typhooon, but most blade IRS's that have rear drive have a torque limiting feature in the rear differential (gerotor pump, viscous coupling, etc.) and rely primarily on their front axle for movtive power and the rear axle for assist when the front wheels slip. I would be surprised if that Typhoon didn't have some reinforcement on the blade to stiffen it torsionally near the knuckle. Can you shed any light on its design?

 

[GregLocock]

"Can you shed any light on its design?" Not really. I like my job.

I think my main point is that there are other features of the suspension (ie tune and geometry) that cause blade failures, you just need enough grunt to set the problem off, but any additional torque just goes into relatively harmless wheelspin.



Cheers

Greg Locock