Tipping Sensation During Hard Cornering (roll center?), SP080 springs, PP swaybars and Struts

[TheLion]

Alright gents, I've been slowly pushing the car harder on some beautiful empty country back roads (no drive ways, clear views, two sweeping turns etc.) and I pushed it harder than I have before recently. I've noticed some characteristics that I'm not fond of.

Now there's obviously a bunch of things going on here and multiple aspects of geometry, spring rates, damping rates etc., so pleas do forgive me if I'm incorrect as I'm still learning the fundamentals of suspension geometry.

During hard cornering the car gives me a sensation that it's going to roll over which I'm NOT a fan of. My suspension modifications are very conservative at this point and as some of you know I'm planning a quite extensive over haul based on R&D testing BmacIL and few others have undergone and proven out, but I plan to do it piece wise so I can work through the changes and see how they impact the car's handling (well that and it's much easier on my wallet).

I have a 2016 PP GT 6M. I do have a Power Pack 2, cat back exhaust and oil catch can and am running Mobil 1 Advanced Synthetic, so the 5.0 runs pretty strong and consistent compared to when it was stock, keep that in mind as it's making a substantial amount more mid-range power that it didn't stock.

I am running BMR's SP080 springs for the time being, stock PP sway bars, camber / caster plates, CB05 cradle lock out and FP outer toe link spherical bearings. So the back end and front end are nice and tight.

1. This has been on my to fix list for a while, Tramlining with my +33mm offset lightweight wheels from RTR. Does tramline a very small amount with the stock PP wheels I'm currently running (so I can burn up the P Zeros), but it's less noticeable because of the smaller scrub on the stock offset.

2. A tipping sensation during hard cornering that I don't ever recall experiencing at the stock ride height with stock springs.

If my memory serves correctly, doesn't a lowered roll center (from lowering springs) increase jacking forces and body roll and is that why I get this "tipping sensation" during cornering? Normally lowering springs have a slightly higher spring rate to compensate to some degree, however the SP080's have nearly identical spring rates to stock, hence there's nothing to counteract the increased leverage on the suspension due to the lowered roll center.

It feels as if the inside tires just want to lift up and the car wants to roll over. But I have yet to hear of a GT rolling over during cornering, the CG is low enough that I would expect to break traction well before that point, especially on crappy stock P Zero's and stock PP wheels.

I'm seriously thinking about getting Steeda's roll center and bump steer correction kit for the following reasons (assuming I'm on the right path here):

1. If I am understanding how a lowered roll center affects the leverage on the suspension, lowering the car while keeping the same spring rates, damping rates and bars will increase body roll which also results in increased jacking forces that "lift" the car during cornering (pushing up on the roll center) and that's the "tippy" sensation I feel.

2. Bump steer obviously is an issue that gets exacerbated more by uneven surfaces, especially street use and causes car to tram-line due to excessive toe angle changes as the suspension moves through it's arc of motion. This will be exacerbated by a increased scrub radius which effectively amplifies the leverage on the wheel.

No I have NOT had an alignment yet, however I have a pretty good idea of approximately where the geometry is at and I don't believe an alignment is going to resolve the above issues unless some one can convince me other wise. So I'm holding off on a performance alignment until I decided weather or not to correct the roll center and bump steer, but I'd like have a good handle on these quirks I have introduced by lowering the car.

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[Brian V]

I want you to change out those tyres pronto and reevaluate your testing . Those P Zero s are affecting the drive .. I too was getting the tramlining and such with worn tyres in front . Had an alignment and toe was off and I still tramlined . Changed the wheels and tyres to 20 inch set with Nitto NT555 . No tramlining . Get an alignment also ... oh yeah I am installing those springs after I am able to purchase a few more bits .

 

[Norm Peterson]

2. A tipping sensation during hard cornering that I don't ever recall experiencing at the stock ride height with stock springs.

1. If I am understanding how a lowered roll center affects the leverage on the suspension, lowering the car while keeping the same spring rates, damping rates and bars will increase body roll which also results in increased jacking forces that "lift" the car during cornering (pushing up on the roll center) and that's the "tippy" sensation I feel.

You're kind of on the right track here except that it's not jacking forces that are increased. It's the increased roll itself. It sounds like there's two sources of increased roll going on here - some from the lowering with essentially same-stiffness springs and I think you're getting some because you're probably cornering harder now.

Maybe there's a third effect going on here, where the amount of roll is enough greater than what you've typically experienced in all the rest of your driving, that it simply feels like it's too much. I'm having to do a little guessing here, partly because when I'm driving I just don't pay much attention to the amount of roll that's happening.

Jacking is related to roll centers and front view suspension geometry but is a distinctly different effect. Actually, this effect would have been reduced by your lowering.


Norm

 

[TheLion]

Here's what I've found about roll center:

1. Lowering roll center too much causes more dynamic weight transfer due to increased roll couple, making the car more "twitchy" at the limits with increased body roll. It effectively reduces your bar and spring rates with regards to how they affect body roll. It also makes the car more sluggish to steering inputs. I've noticed both of these traits since lowering the car.

2. You can compensate to some degree with springs and bars, however you can only do this so much or you loose the suspensions ability to do what it does and keep the tires on the ground at high speeds, tire float = bad. Also for a street car you might be required to use spring rates stiff enough to start making the car uncomfortable to ride in = not fun.

Given that I've lowered the car with springs that have virtually the same rate as the factory PP springs and I haven't touched the bars, the car is going to roll more than it would have stock. But it does have a lower CG so there is less weight transfer, the down side is the other effects seem to out weigh the benefits of the lower CG and correcting the roll center provides the benefit of reduced weight transfer without the negative side effects.

My goal is to tweak the suspension little by little and I believe the first step I should really take before jumping into stiffer spring and struts which is my ultimate plan, is to restore the designed roll center of the car.

Both the current spring setup (SP080 and PP Struts) and the one I plan on going to (SP083 and FP Track Struts) lower the front by about 1" or slightly more, so there is benefits to running corrected front end geometry. I do know for a fact that the GT350 runs different front hub knuckles than the regular GT, partly for lightening (aluminum vs cast iron) but also to correct the geometry for the 0.7" lower ride height.

Obviously the Ford chassis engineers felt that dropping the ride height by about 3/4" of an inch called for corrected geometry, so it would only make sense to do it here as well and thankfully we have Steeda who stepped up to the plate and is the only ones I know of currently offering extended ball stud links in their correction kit (also fixes bump steer geometry).

So at this point I really think that's the next logical step. It all works out as I haven't yet had an alignment, so I can do it all at once. Obviously I'll have to do a second alignment when I do the SP083 struts / FP Track Dampers as the ride height is just a hair different, but it will be minor tweaks as it's still pretty close.

Thanks for the info Norm, I think your right, I'm sensing the combination of increased body roll due to an uncorrected roll center, same spring rates and higher corner speeds. And I certainly need to add a little more camber up front because I can see wear on the outer tire all the way to the edge of the shoulder, so it may have been rolling the tire a bit, since I have camber / caster plates that's not an issue.

Just for reference MotoIQ has a great set of articles that corroborate this and explain geometry reasonably well:

the-ultimate-guide-to-suspension-and-handling-its-all-in-the-geometry-part-one-the-roll-center

I'd rather go into tuning with stiffer bars, springs and damping rates with a corrected geometry and work from there. Sure, you can get away with a lowered roll center to some degree, but it's not the most ideal and there's a reason the factory roll center was set where it was.

 

[AmericanV8Guy]

Although springs do play a large role in the overall handling of any car, there are also many other components as well. For instance, I'm sure that the "tipping" sensation you are experiencing can be gotten rid of or at least reduced by adding stronger/thicker front and rear sway bars. Even though it's the PP, there is PLENTY of room to improve. Yes, the PP is miles ahead of your base mustang, but there is also a lot of room for improvement as well. I also have a PP, but have done full suspension to it (shocks/struts, springs, sway bars, vertical links, rear toe links, IRS Subframe support, K member support), and my car feels like an absolute track beast now. I can EASILY take corners at twice the speed that I used to be able to do in my OEM PP GT. Keep in mind, these are also on the stock PP wheels, with 285 Michelin SS tires to replace the old OEM Pirellis I had on. My only issue now is that I'm strongly considering a harness bar/harness install, as I have a hard time staying planted in my seat.
I am unfamiliar with the BMR springs, so I'm not sure what their ride quality/performance abilities are like, but I have the Steeda Ultralite Linear Springs (much better than progressive springs) paired with Ford Performance Shocks and Struts, and they exceed my expectations. Granted, I installed all of my suspension parts at once, so it's kind of hard to tell what exactly is helping with what, but I'm pretty sure that you will definitely benefit from getting some thicker (and adjustable) front and rear sway bars.

 

[Bluemustang]

You need to get some SP083s IMO. I had the same springs you mentioned, along with much stiffer shocks and sway bars and I experienced a similar sensation. And I was running the bars at the middle setting back then.

My car is really flat in the corners on back country roads, even at high speeds. I have the bars as soft as they can go.

Stiffer spring over stiffer sway bars for me.

To add to that, some Ford Racing dampers would be a great complement and I think that setup would get you there.

 

[Norm Peterson]

I'd rather go into tuning with stiffer bars, springs and damping rates with a corrected geometry and work from there. Sure, you can get away with a lowered roll center to some degree, but it's not the most ideal and there's a reason the factory roll center was set where it was.

That's a plan that makes sense. Fix the foundation before building anything more on top of it. Though any significant amount of roll center correction will end up involving bumpsteer correction as well.

Strut suspensions are kind of evil with respect to how far the geo roll center drops for any given amount of lowering - a good enough ratio for discussion purpose is double the drop or a bit more. Most other independent suspensions drop the geo-RC closer to the same as the amount of lowering. Only a few suspension types - the S197's 3-link/PHB stick-axle being one - drop the RC less than the amount lowered.


I think your MotoIQ link was missing a few letters, as took me quite a while to find the article (I was finally able to chase it down starting with a search on 'MotoIQ' and stumbling around in their menu). Hopefully this will work for anybody following along.
https://motoiq.com/the-ultimate-gui...all-in-the-geometry-part-one-the-roll-center/

Incidentally, what the author calls the "Mike axis" is a crude approximation of the mass centroid axis (sprung mass only). Which I'm not sure means a whole lot until you start getting interested in what the actual chassis torsional stiffness value is (compared to the suspension roll resistances), how it varies along the wheelbase, how much the chassis is twisting, and where. But that's beyond where Mike went with it, and well beyond anything we're doing here.


Norm

 

[Dr. Norts]

I'm seriously thinking about getting Steeda's roll center and bump steer correction kit for the following reasons

This.

Take a picture of your toe links / tension links. You ideally want them parallel to each other / the ground like this. I have the steeda roll center / Bumpsteer kit.

The picture gives an illusion they aren't level with the ground but they are. It's hard to get under there to snap a pic at a good angle.

 

[Coyote Red]

What I don't see in your sig may help you along with better struts & shocks is BMR CB006, 4 pt. brace. It's helped my pony and I will do Bilstein struts & shocks soon to help my lowered pony. Hey Norm, salute.

 

[Roadway 5.0]

A car can feel unpredictable and loose with toe-out on all four wheels, which is exactly what you have after lowering the car and forgoing an alignment. That, and as you mentioned, lowering the car without adding spring rate could certainly present an odd feeling from behind the wheel.

You’ve said the answers to the test for the way forward in really dialing-in your suspension. I look forward to hearing how things go for you once complete

 

[TheLion]

Thanks for the input everyone. I'm dead set on the @BmacIL setup using SP083's, FP Track dampers and GT350 bars. I'll try just a 34 mm GT350 front bar with that setup first and if I need more rear bar I can always add the 24 mm rear bar as they are dirt cheap.

But the reality is with my current setup I can TELL that the roll center is NOT correct. Hence why the car feels a bit "tippy" once I push it hard, turn in is a bit sluggish and the car pushes even more going into a corner than it did before, at least with PP staggered setup, the 4 square RTR's with PSAS3+'s compensated for the push somewhat. I'm guessing that most guys buying the SP080's aren't pushing the car nearly as hard as I am or they would be complaining about the same issues, either that or they are pairing them with stiffer bars and other mods that to some degree compensate.

I'm actually glad I'm doing this incrementally, that way I can see how each item affects the car and weather the change has had a desired or undesired impact on handling. The Engineering side of me can't logically move forward however when I know the foundation isn't quite optimized yet and I'd rather build a platform up from a solid foundation (geometry wise) as oppose to trying to over compensate with more bar.

The idea is to run a little more spring and little less bar to keep the suspension independent which makes the car more settled on bumpy roads. Correcting the roll center has a similar effect to adding more bar up front but without reducing the independence of motion of each wheel, which I see is advantageous to a good track / street car.

Sure, absolutely none of this matters without good tires and the P Zero's are not by any means a great performance tire, but more of a basic "sport touring tire", they do ride nice, at least I feel like they do compared to the MPAS3+'s, but sadly they have noticeably less lateral grip, are extremely vague compared to even those high performance all seasons from Michelin. PS4S's are coming in the spring. I'm debating on weather I want to go snow tires on my PP wheels or good all seasons like the Geforce Comp 2 AS's this fall for my "winter tire", but I digress.

I've already ordered the Steeda Roll Center & Bump Steer correction kit. That kit will work with SP080's I already have on the car and the SP083's I plan on upgrading to next yearish. After that I think I'm in good shape a balanced street / HPDE car. I honestly lost interest in auto x after my back road "track sessions". Soooo much fun! So I'm slowly setting the car up for HPDE as that's what the S550 chassis excells at when it's properly set up. I'm not knocking auto x for those of you that compete, it's just not my cup of tea.

The car isn't bad by any means. Once I get the roll center corrected, the lower CG will then actually be of benefit, right now I feel that any gains due to lower CG are likely off-set by the losses in handling due to increased roll over stock, more sluggish steering inputs and greater push on corner entry. Even with throttle steering I need to use more wheel to hold my line, so the car is pushing more on exit as well. That needs to be corrected first as the car is going to stay at a lower than stock ride height now and in the future.

While not dirt cheap, it's not uber expensive to correct the front roll center either. $335 in parts, I already have all the tools etc. and still haven't gotten an alignment so it works out in the end. I'll report back in a couple weeks once I get the parts in and have an alignment done and I'll go from there.

 

[Norm Peterson]

If you're not against the idea of running shorter tires than OE, you can get some CG lowering (all of it, including the unsprung masses) without giving up as much as you would with the same amount of sprung mass lowering accomplished via spring changes. Maybe something like 285/35-19's???


Norm

 

[BmacIL]

If you're not against the idea of running shorter tires than OE, you can get some CG lowering (all of it, including the unsprung masses) without giving up as much as you would with the same amount of sprung mass lowering accomplished via spring changes. Maybe something like 285/35-19's???


Norm

This. 285/35 in most street tires is a great fit on a 19x9.5. In tires like the RE71R, which you wouldn't get anyway, 275/35R19 is a better fit, and is what I run on my track set of wheels.

I agree with your plan, @TheLion.

 

[Brian V]

I think that there is an equation that is being overlooked here .

With progressive springs and bumpier than all get out the ride quality is far better while DD ing every time 1 has to drive to A to B twice a day . Lineers just keep bouncing trust me I drive 294 and it plain sucks on these PP springs and shocks ..
Figuring out that 1 needs to correct Roll center is an ideal plan and 1 should give careful thought to while lowering to go along with correcting the alignment ..

So give me the ideal alignment Specs . For lowering an inch in the front and a half inch in the back plus the correct valving shocks and struts with BMR SPO80 Springs ......

 

[Dr. Norts]

BMR street recommendation

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