S550 Suspension School - Integral Link IRS

[Grimace427]

I am trying my darnest to keep up with this debate about virtual pivot point, upper and lower, outer and inner, front ICs and RCs, and even psychedelic distorting control arms, but I am totaling missing out on understanding what all this terminology means regarding track performance.

I get the fact that the new design is lighter than a traditional A-Arm, and is potentially more adjustable, but what is it likely to deliver in terms of steering control, under/over-steer, steering feel, turn-in, etc?

Did BMW achieve some improvement in racing performance as a result of this design? If so, does anyone know how this steering architecture contributed to the improvement?

Feeling like a dunce. :shrug:

As far as over/understeer that is reliant on the entire chassis setup. What the double ball joint front suspension does directly is give the Ford engineers more flexability as far as agressive alignment tuning, brakes and wheel packages, and hopefully an improvement in front steering feel. The setup is lighter for a fact, it also might be easier to upgrade as far as bushings go(compare to BMW upgrades).

Many of the German manufacturers have been using the setup for decades and they certainly contribute to refined handling. I work on Mercedes and I can say they make the heavier cars feel a bit lighter on their feet, but we are only talking about a small piece of the puzzle. The only thing that matters is how potential buyers feel about the suspension/chassis package as a whole after they've driven the car.

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[Norm Peterson]

. . . what is it likely to deliver in terms of steering control, under/over-steer, steering feel, turn-in, etc?

Looks like it makes it easier to get a small scrub radius - the lateral distance between where the steering axis intersects the ground and the tread center - without requiring wheels with huge positive offset (think tire/wheel to strut clearance). Scrub radius goes to rough road behavior and steering kickback, with a bigger scrub radius being worse than a small one.

I also think I'm seeing a reduction in mechanical trail for the outside tire in a turn. This gives better warning as you approach the limits of lateral tire grip.

(Race Car Vehicle Dynamics)


Norm

 

[thePill]

@Norm

I misunderstood the initial instruction.

 

[Norm Peterson]

@Norm

I misunderstood the initial instruction.

Not sure what you're getting at here. Maybe it's too early, or maybe the coffee hasn't kicked in yet.


Norm

 

[thePill]

Not sure what you're getting at here. Maybe it's too early, or maybe the coffee hasn't kicked in yet.


Norm

I got to look at some BMW technologies in 2011. This is where I first heard the term. The explaintion I got was likely misinterpreted... He spoke English well but not properly.

The trouble learning chassis and suspension in Germany and Japan... Japan is nearly impossible.

 

[Grimace427]

All the German engineers I got to meet over the years with Mercedes spoke perfect English. I could say they spoke better English than some of my coworkers!


However I can easily understand how confusion can occur when reading technical descriptions and designs in German, their words get so long you really have to sit there and sound them out! Last time I took some factory training we were learning the 722.9 7-speed transmission and they had a cutaway labeled. Holy crap I stood there laughing at some of the words.

 

[Norm Peterson]

Even when the spoken English is pretty good, accents and occasional reversions to first language sentence structure can get in the way of instant comprehension. It takes a little while to pick up on when to do a little on-the-fly mental rewording or "translating". With continuous exposure, you almost don't notice what you're doing.

The particular engineering department in the specific company I spent most of my career at had people on staff from all over the globe, and on a few occasions we worked directly with outside consultants whose spoken English was very good but still clearly a second language. Been out of that job long enough now to know I've lost a lot of this particular communication skill.


Norm

 

[let me ride]

I found this great writeup on the underside of the S550.





Here you can see what appears to be an all aluminum 8.8" center section for the IRS rear axle. Lots of control arms going willy nilly, but the geometry might actually work this time. This IRS replaces the tried and true solid axle that has been part of the Mustang line-up for 50 years. Sure, there was an IRS in the Cobra Mustang from 1999-2004, but it was fraught with geometry troubles and was more of a "parts bin" add-on than a proper IRS design. The S550's system looks like the real deal. Its no Corvette IRS (no room for a rear transaxle if you want back seats) but its the next best thing and much welcomed in a ~$26K (base) Mustang.

The diff housing likely still has the 8.8" ring and pinion inside, which has been well proven in the 1986-2014 V8 Mustangs. This means that there will be dozens of optional rear gear selections available (from Ford and other sources) from day 1, as well as lots of aftermarket limited slip differential options. Ford seems to be on a "Torsen T-2" diff kick of late - in the higher performance GT500, Boss302 and Track Pack Mustangs - but I suspect we might still have a clutch plate "Traction Lok" diff in the lower end models; We shall see. I'm still likely going to upgrade to a better aftermarket diff (Torsen T-2R or OS Giken) in our 2015 Mustang, and the gear ratio might not stay within the standard range of three choices that include "3.31, 3.55 or 3.73", if the gearing/speed numbers don't work out. The Getrag MT-82 is staying in the S550 and that 6-speed manual doesn't play well (3rd gear is needed by 55 mph with the 3.73s) with the lower numerical ratios that come optional in the '11-14 cars, from what we've seen.



This close-up of the right rear lower control arm shows eccentric bolts that would be part of the rear camber and/or toe adjustment. This is a big plus for the IRS over a solid axle, as a fixed axle has fixed toe (zero) and camber (zero), which hurts overall cornering grip and corner exit traction. Sure, a solid axle "loses no geometry" in dynamic use, but it is far from perfect. And yes, that is black spray paint on the rear exhaust section. Probably done to cover up some sin in fabrication, the production exhaust will likely be a dull grey as usual. And the routing of the exhaust... as tortured as you would expect, so look forward to some exhaust flow gains to be had from a custom or aftermarket "axle back" exhaust on the S550s.


Does this solid axle Mustang look like it is going into positive camber on the outside rear tire, in this image?

We've seen what appears to be a loss of camber on the rear solid axle of our S197s under load, so dialing in a smidgen of static rear negative camber will be one of our first mods on the S550. Look at the image above of our 2011 Mustang exiting hard at NOLA's Turn16 corner. Does it look like that rear wheel is showing zero camber, or a tick positive? With an IRS we could dial in something like -1.5° or even more negative camber, which should make for more grip (a modern radial tire likes a little negative camber, even when loaded), and that rear toe-in trick is something we've also used on BMWs for a decade to get them to turn in faster.

Neil Roberts described this phenomenon best in his book ThinkFAST, on page 133, in the section called "Turn Radius Effects on Toe Settings":

Have you ever wondered why every competitive alignment includes front tow out and rear toe in? It is a simple consequence of simple geometry...

I don't want to plagiarize his whole book but the next 2 pages explain why this works, with diagrams. If you don't own this book and are a racer, get it and read it. Just know that adjustable rear toe and camber is a very good thing and IRS brings this, and much more. A 100+ pounds less unsprung mass is also a huge benefit of IRS over a solid axle! Not only will it ride significantly better than the solid axle 3-link it replaces, it should handle better as well.



In these pics I was trying to show how much unused room there was in the back of this chassis for something like alternate exhaust routing, or a diff cooler mounting, or a big ass rear diffuser. The shot of the rear spring is to show the fact that this layout does indeed seem to follow the image below, which I posted previously. Yes, these images I took are crap - and they are the good ones. :/



I couldn't see jack squat under the front end, as the car was low and the circular platform prevented me from looking up under the front bumper. But from what little I could see, the S550 at PRI appears to have the same front suspension shown in the image above, which was in the press releases two weeks ago. I would have paid money to be able to open that hood and inspect the strut towers (we make camber plates, remember??) but I think we will have the room we need to get the front negative camber adjustment this car will surely benefit from in performance driving or track use.

That dual ball joint suspension is popular on German cars (BMW, Merc, etc) and the benefits should apply to the S550 as well. Separating the tension and compression forces in the 2-piece lower control arm is also a German thing... and heck, it was kind of how the 1964.5 model Mustang did things up front, in the most basic of ways. The big (apparently) 4 piston front calipers looked beefy but they were definitely not a Brembo design I was familiar with. Did Ford make their own, or source this from another vendor? If it was a Brembo you know it would be printed and legible as such from 30 feet away. Strange, but if it works...

My biggest take away from this brief look at the pre-production 2015 Mustang is: this thing is not a smaller car as some are saying. It is still plenty big, especially the rear width, and therefore I don't see how it could possibly be "200-300 pounds lighter" than the outgoing car, if you compare the same drive trains from old to new. We shall see, and I hope I am wrong about this.

 

[Norm Peterson]

Be a little careful just what you assume about stick axle alignment specs, or that suspension geometry being the only cause of camber change.

From later in the same thread (post #65).

Does this solid axle Mustang look like it is going into positive camber on the outside rear tire, in this image?

I'm absolutely certain that you're getting a little positive rear camber . . . not due to suspension geometry of course, but due to lateral load transfer and tire vertical stiffness effects. This effect is worth something like 0.5°/g - 1.0°/g, and since this happens outside the suspension it will happen in an IRS car as well. It even contributes to sprung mass roll, which explains in part why the "apparent roll" in pictures ends up being slightly greater than what you might calculate from roll stiffnesses alone. It's probably part of the reason people sometimes modify stick axles for a bit of negative camber . . . or don't get an axle "fixed" to zero camber when little negative cambers are found.

 

[WestRace]

There was also talk about the strut tower height being lower as well but I'm still not sure if that is true. I guess if they are celebrating about the lower hood, it would suggest everthing else stayed the same... But then again, how can that be on an All-New Platform?

Edit: When I say the tower height is lower, I mean the tower mount is lower. Closer to the datum plane.

There is a picture of the S197 and S550 opened hood and it seems like the strut mount point is indeed lower vs the s197.

 

[Jarstang]

 

[DivineStrike]

I know old thread but it's still on the first page and this is the first legitimate update to this. Link with several pictures and I'm posting the best pic

There's also a good picture in the Production line thread

http://www.lethalperformance.com/in...ontent/2015-ford-mustang-gt-first-look-at-irs

 

[TothiusB]

As far as over/understeer that is reliant on the entire chassis setup. What the double ball joint front suspension does directly is give the Ford engineers more flexability as far as agressive alignment tuning, brakes and wheel packages, and hopefully an improvement in front steering feel. The setup is lighter for a fact, it also might be easier to upgrade as far as bushings go(compare to BMW upgrades).

Many of the German manufacturers have been using the setup for decades and they certainly contribute to refined handling. I work on Mercedes and I can say they make the heavier cars feel a bit lighter on their feet, but we are only talking about a small piece of the puzzle. The only thing that matters is how potential buyers feel about the suspension/chassis package as a whole after they've driven the car.

Mercedes like always, invented this twin lower ball joint design that should bind and not steer at all. Most mechanics I have word with have no idea? They get pissed and take the day off if any German car comes in. I don't get it (I understand it's about time and time is money) but to a point! You will never hear about it or find info on it on the web? Secret? This is what it does on an Alignment rack and I cheat (No bar or weights or special head mounts for Mercedes rims) You have to drive it and readjust, like getting the wheel straight at times. When you go into a corner it gives more support to load wheel and scrub radius is improved because wheel is not pivoting around and axis point and at the same time the caster and camber have a new relationship, where the wheel plants. Yes, caster moves. That left - Left wheel is turning left and weight shift to it and eccentrics of two pivot points will make the wheel position forward and camber in (In is top in and caster is always top forward or back for pos or neg) Now the wheel must always be fixed and always has, until now and everyone misses it. Now you see it on Tesla's, Camaro's and Mustangs for example. When you see a BMW front wheel move back 3 or 4 inches when braking, that's not this! That's bad bushings in lower firewall, sell that job fast! Mercedes has invented most everything today Half of ford is patent releases from Mercedes! Sorry, wordie but should be helpful and who's going to contest it (I wish someone would!) Just kidding. Good luck.

 

[moffetts]

Did you intentionally find the oldest post and then feel compelled to reply to it?

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