Chassis stiffness

[MerlintheMad]

I posted here, because I deliberately want to attract the comments/wisdom of those who know. I do not know. I only read and drive my car.

Here in the promo book, in the Nurburgring chapter (the best written, most engaging bit of text that Kia has produced for my eyes and heart), it says: "Indeed, huge thought goes into negotiating the white-knuckle Flugplatz section with its short uphill straight, quickly descending into two fast right-hand kinks. It steals your breath, widens your eyes and tightens your grip on the wheel. The Stinger's stiff chassis lets the electric-assist steering work its magic, though. Turning the thick, grippy wheel helps deliver the right on-center feel, the right weighting, and just the right amount of information about what the front wheels are doing."

This is the opening argument for why I say that the Stinger is a stiff car, i.e. necessarily a feature of fine communication with the front end, what the steering is doing and tells you it is doing, etc. This has been my experience, limited though it is thus far.

I am not going to drop any names here, or appeal to anyone on the forum. If you read this, you'll recognize instantly which "camp" you belong to, and probably where I got my information from.

I've been told very recently that the Stinger's chassis is very flexible compared to other cars, performance and non performance. And in a posted review, the writer has this to say: "A last concern: chassis stiffness. From the lack of precise feedback from the front end when completely loaded in a turn, to a disconcerting creaking over every raised entrance to a gas station that put a load on one corner, this Stinger might benefit from some bracing. It's a big, heavy, long-wheelbase car with a giant hole for the liftgate, to be fair — none of that helps with stiffness. Even with that caveat, there's more flex than you'd expect, and this undercuts its otherwise convincing aura of quality and competence." 2018 Kia Stinger GT Quick Spin Review | Running the diaper-to-racetrack gamut Which on the face of it is diametrically opposed to the Nurburgring assertion. How this guy gets "imprecise feedback from the front end" and the Nurburgring driver gets "just the right amount of information about what the front wheels are doing", is beyond explanation by me.

I've read on here that the Stinger has "good bones" for being a track car: and that Pobst very recently sang its praises at Laguna Seca, pretty much saying the same thing; that the car when driven past "75%" hunkers down and stops swaying, etc. This is countered by the same reviewer cited above, who says: "The takeaway from this week of extremes — child-rearing mundanity and track shenanigans — is that the Stinger GT is most successful in between. There's space for lots of luggage, the handling at around three-quarters of the car's limits is lovely ..." This is countered by one of our own car guys, with a lot of track experience, who has written this past week that the car really doesn't start to behave like a track car until it is pushed hard. So which is it?

"Track bones"? Or diaper bag carrier that happens to accelerate well?

Does the chassis flex and feedback steering response inadequately? Or is the Nurburgring description accurate? It can't be both, or can it? Do individual cars have better or less stellar "electronically controlled suspension/steering"? Is the reported mushiness or precision a matter of driver taste/talent? Is the reported "creaking" as the car goes over uneven entrances or negotiates steep driveways, etc., a function of a "flexible chassis", or is the noise generated by something else? Could the electronically controlled suspension/steering send back noises on some cars?

Is the reported noise from doors/sunroof at speed in any way related to the assertion that the chassis lacks stiffness?

And finally, I get to report my own observations, again. Elsewhere I've mentioned that the opportunity was offered, and instantly seized upon, to go fast. I took the car up to 155 MPH. In a straight line, but only horizontally. Vertically, the car was up and down with the undulations in the road: which at the speed limit and considerably above, were not noticeable; but over 140 MPH turned into humps and then at the top end, thumps. At the end of each "up" the car planted back down immediately, remaining smooth, firm and stable. The steering was impressively confidence-building, feeling completely controllable. A flexible chassis would not have allowed this, would it? The expected result of high speed would be a tendency toward instability and the steering would not remain the same at 150+ as it feels at c. 70. That's what my logic tells me, anyway.

Is there a way to resolve this dichotomy? I think that Albert Biermann made sure the Stinger's chassis is stiff. And others have an opposite opinion, or even certainty, that the flex is excessive, especially compared to other cars.

 

[Helo58]

This doesn't cover everything, but considering that these reviews rely on qualitative (qualities) rather than quantitative (quantities) descriptions, it depends on who the reviewer is. Someone used to driving an 80s Oldsmobile may think the Stinger is rigid, someone driving an Ariel Atom may not. It could also depend on the track where the car is driven. Manufacturers are notorious for taking their cars where they will "shine" when reviewed. Short of measuring the car for torsional stiffness and getting a value to compare with other cars tested in a similar fashion, it is mostly subjective.

On a side note, I just took a 11 hour road trip in my Stinger GT and it was the MOST comfortable trip I have ever taken in a car. Fast and comfortable is exactly what I purchased the car for. It does that perfectly despite how it performs on a track!

 

[robz32]

Is the reported noise from doors/sunroof at speed in any way related to the assertion that the chassis lacks stiffness?

And finally, I get to report my own observations, again. Elsewhere I've mentioned that the opportunity was offered, and instantly seized upon, to go fast. I took the car up to 155 MPH. In a straight line, but only horizontally. Vertically, the car was up and down with the undulations in the road: which at the speed limit and considerably above, were not noticeable; but over 140 MPH turned into humps and then at the top end, thumps. At the end of each "up" the car planted back down immediately, remaining smooth, firm and stable. The steering was impressively confidence-building, feeling completely controllable. A flexible chassis would not have allowed this, would it? The expected result of high speed would be a tendency toward instability and the steering would not remain the same at 150+ as it feels at c. 70. That's what my logic tells me, anyway.

Is there a way to resolve this dichotomy? I think that Albert Biermann made sure the Stinger's chassis is stiff. And others have an opposite opinion, or even certainty, that the flex is excessive, especially compared to other cars.

Yes, the reported issues with sunroofs and doors are related to chasis flex. The issue is easily replicated when entering a gas station, a home's driveway or a parking lot with an incline.

Going at a high rate of speed and feeling the car planted has more to do with suspension setup and aero dynamics of the vehicle than chasis stiffness.

The stinger will benefit form more bracing that addresses vertical flexing.

 

[eflyguy]

tldr, but I will say this - chassis "stiffness" isn't some absolute parameter that needs to be attained. I spent a day throwing my daughters unbelievable fabulous Challenger around the tightest road I know - the Dragon in TN - and it handled just exactly as I knew it would, but when reviewing some of the pictures captured by multiple photographers, I was very surprised to see visible evidence of flex. Later I was reading an article by someone that had recently added struts/braces to stiffen up their already wonderfully stiff vehicle, and had to go back and make major suspension changes to compensate.

I know a lot about motorcycle suspension and although it's different, it's similar in many ways, and there is so much more to understand than many vehicle owner/operators realize. Chasing one particular parameter - in this case, "stiffness" - will almost certainly lead to a vehicle capable of doing only one thing well, vs. a well rounded, balanced vehicle capable of being enjoyed in many different driving situations.

 

[MerlintheMad]

The stinger will benefit form more bracing that addresses vertical flexing.

Ah, vertical flexing of the body panels? That isn't the same thing as horizontal flexing in the sense of the chassis flexing one wheel off the ground on a steep twisting driveway, or entry to a parking lot. And vertical flexing would definitely mess with the fit of window seals and the like. When added stiffening of the chassis results in more noise, hasn't the designed balance of chassis and body resulted in a detrimental outcome? At least as far as comfort is concerned. But the handling might be improved. Trying to get back less noise requires more "fixes", or addons, and then you are chasing cause and effect.

 

[MerlintheMad]

Chasing one particular parameter - in this case, "stiffness" - will almost certainly lead to a vehicle capable of doing only one thing well, vs. a well rounded, balanced vehicle capable of being enjoyed in many different driving situations.

This is very insightful. The Stinger gets criticized for not doing both (performance and luxury) better. But most drivers are pleasantly surprised that both goals in the design do so well.

 

[Tonkabob]

Ah, vertical flexing of the body panels? That isn't the same thing as horizontal flexing in the sense of the chassis flexing one wheel off the ground on a steep twisting driveway, or entry to a parking lot. And vertical flexing would definitely mess with the fit of window seals and the like. When added stiffening of the chassis results in more noise, hasn't the designed balance of chassis and body resulted in a detrimental outcome? At least as far as comfort is concerned. But the handling might be improved. Trying to get back less noise requires more "fixes", or addons, and then you are chasing cause and effect.

I think all flex is verticle which often becomes rotational. Only have 4 tires on the ground that can be lifted or lowered. Horizontal flex would require crashing into something.

 

[Ozstung]

All I'll say is the car is set up as a true GT vehicle and stays true to those principals. I have broken some of those rules lowering springs and aftermarket exhaust.

 

[MerlintheMad]

I think all flex is verticle which often becomes rotational. Only have 4 tires on the ground that can be lifted or lowered. Horizontal flex would require crashing into something.

Okay, I see what you're saying. I was thrown off by the use of "vertical".

But what I was visualizing was actual chassis flex (lower) separate from the flexing the body panels do (upper). Is there a distinction? Or is the body attached to the chassis proper a "whole" when addressing flex/stiffness? I guess what I am asking is if the chassis design for stiffness was a design unto itself, and the body design added to it (or visa versa)?

When it is suggested that stiffness could be added and that the Stinger would benefit, is it being suggested that redesign is to the chassis or body or both? And being more stiff, would this actually decrease extraneous noises or tend to increase the problem?

A stiffer chassis would potentially benefit race car applications but ruin the economy employed in the Stinger as-is to provide a comfortable, quiet "luxurious" feeling ride. To achieve both better handling through increased stiffness, and retain a quiet ride, would require such upgrades as to push the car's price into the same range as the competition, rendering the Stinger "stingless".

 

[robz32]

Here is a great article that will explain the benefits of increasing chasis rigidity.

Chassis 201: Maximizing Torsional Rigidity

No, it will not compromise the "comfortable, quite, luxurious" ride as those attributes are given by how the suspension is setup on the vehicle. On the contrary it will help the suspension work much more efficient.

 

[MerlintheMad]

Here is a great article that will explain the benefits of increasing chasis rigidity.

Chassis 201: Maximizing Torsional Rigidity

No, it will not compromise the "comfortable, quite, luxurious" ride as those attributes are given by how the suspension is setup on the vehicle. On the contrary it will help the suspension work much more efficient.

And right up front, this:

"In reality, increasing the torsional rigidity of a vehicle improves ride comfort quality by allowing the suspension to work more efficiently."

Echoing exactly what you said.

So, the title of this thread should be "Chassis torsional rigidity".

What does "Nm" stand for? Newton meter (this comparison can also be rendered in joules, but I am even less of a math guy than I am a car guy! ), or One Newton of twisting force applied per meter of length will start to achieve one degree of twisting angle to the moment arm, in this case the chassis with both rear wheels still held down horizontally; and out at the forward end of the chassis, the flex is one degree of twist off the horizontal. The '66 Mustang only requires 5K Newtons to twist one degree; the Bugatti requires 60K Newtons to twist one degree. Where does the Stinger fall? And how do other like priced cars compare?

Obviously, when @CaliSting calls for a shortened, two-door version of the Stinger, he is actually asking that a completely different car be designed that only resembles a Stinger! But it would definitely be stiffer for similar cost. It would not, however, be in any sense of the term a "gran turismo". Only a race car.

 

[ZyroXZ2]

To be blunt: a car's behavior almost always relies on the driver's ability. This is why that saying, "90% driver, 10% car" exists. Sure, there's really no way a Honda Civic is going to "beat" the Stinger GT on Laguna Seca, but what it DOES mean is that a skilled driver will get more out of either of those cars than is normally deemed "possible".

So to answer people who don't quite understand "track physics" (aka pushing a car to its limit), there is a lot that has to do with one's handling of the vehicle and not the vehicle itself. For example, many people report the squirrely rear-end when cornering hard. I can replicate that... by turning in wrong and not maintaining cornering speed. If I take a turn hard, turn in correctly, and apply the correct amount of gas to maintain my speed, there is absolutely nothing squirrely happening in the rear at all and it flies right through the corner.

All of this discussion depends highly on the driver. The better the driver, the better the results, PERIOD. It doesn't matter what car they're driving from a Geo Metro to a Lamborghini Centenario: neither car will "control poorly" in a turn because the driver knows exactly how to handle it no matter how poorly or how well it's made. The Centenario will win every time, but the Geo Metro won't be "not stiff enough" to a genuinely skilled racecar driver. The driver will know what speeds, when to turn in, how much gas to apply, etc to maintain the vehicle's composure.

You don't see the F1 drivers on Top Gear doing the Reasonably Priced Car complaining about how the car lacks chassis stiffness compared to their F1 cars, now, do you?

 

[MerlintheMad]

@ZyroXZ2 That wasn't meant to be a thread killer was it? I really like your input. It's how I think about driving cars fast. My personal experience with high performance is this ONE. All my previous vehicles have been utility to the max. That doesn't mean that I didn't push them hard from time to time.

I noticed what a change of tires could do. My Voyager turned into a completely different corning machine when I got the right sized tires (previous owner had smaller diameter tires than OEM on for some "reason"), with enough grip. The oversteer was tremendous and caught me by surprise the first time I pushed into a corner: it felt like the steering wheel went "over the top" and almost clicked into the cornering position. I watched for that the next time and it always did it. The faster the cornering speed the more pronounced the transition was. It was the tread pattern, apparently: very aggressive when the wheels attacked a corner at a certain angle. I got used to it and even used that to enter a corner with more deliberate speed. It almost felt like I had "power boost steering", for lack of a real term. I'd have the van leaned over on donuts entering the freeway, striving for forty plus MPH, the tires making the "cornering noise" (never pushed beyond that, because of lean and also I didn't know what would happen if the tires started to howl; I'm chicken that way).

Now I corner much quicker (duh) and am slowly working up to where the tires make noise that sounds like they might break loose; don't want to experience that! Chassis "torsional rigidity" never even entered my mind, until conversations started by some drivers criticizing the Stinger's flexibility, going diametrically against what the "scripture of my conversion" (the promo book) had said in the Nurburgring chapter.

 

[ZyroXZ2]

@ZyroXZ2 That wasn't meant to be a thread killer was it? I really like your input. It's how I think about driving cars fast. My personal experience with high performance is this ONE. All my previous vehicles have been utility to the max. That doesn't mean that I didn't push them hard from time to time.

I noticed what a change of tires could do. My Voyager turned into a completely different corning machine when I got the right sized tires (previous owner had smaller diameter tires than OEM on for some "reason"), with enough grip. The oversteer was tremendous and caught me by surprise the first time I pushed into a corner: it felt like the steering wheel went "over the top" and almost clicked into the cornering position. I watched for that the next time and it always did it. The faster the cornering speed the more pronounced the transition was. It was the tread pattern, apparently: very aggressive when the wheels attacked a corner at a certain angle. I got used to it and even used that to enter a corner with more deliberate speed. It almost felt like I had "power boost steering", for lack of a real term. I'd have the van leaned over on donuts entering the freeway, striving for forty plus MPH, the tires making the "cornering noise" (never pushed beyond that, because of lean and also I didn't know what would happen if the tires started to howl; I'm chicken that way).

Now I corner much quicker (duh) and am slowly working up to where the tires make noise that sounds like they might break loose; don't want to experience that! Chassis "torsional rigidity" never even entered my mind, until conversations started by some drivers criticizing the Stinger's flexibility, going diametrically against what the "scripture of my conversion" (the promo book) had said in the Nurburgring chapter.

You might be surprised how much of a car is really under your control. Sure, there is still 10% of the car in play (tires, design, etc. etc.), but believe me when I tell you I've watched slower cars beat faster cars simply because of a person's "feel" of the car on the road. Turning hard is one thing, cornering is another. Don't worry so much about the numbers (aka tires, chassis stiffness, grip, etc), worry more about how you can make the best of it and you might just find yourself blowing by people worrying about the numbers, lol

 

[CaliSting]

I have posted M4 chassis and Stinger. Frankly Stinger looks in some ways better engineered. That big beefy boxed cross in middle of car makes torsional rigidity class leading. In addition to beefy design they use tons of structural adhesives and high percentage of high strength steel.

Most Showroom stock cars flex a bit on track when on sticky tires. Heck my old 914 was flexi-flyer until I seam welded chassis, welded in reinforcements and caged the car. I seam welded, reinforced and caged my 370Z ....talking stiff! And my caged Cayman is like a solid piece of billet. But man when I compare Kia to my Cayman the Stinger looks over engineered!

Can pretty much assure you not many are going to experience chassis flex with street tires and stock suspension. I haven’t felt any and have driven crap out of this thing in AX. Won’t really be able to determine whether car has significant chassis flex until you get sticky tires and stiffer suspension under it. I have pushed car hard on flat cement and really couldn’t detect chassis flex. But it certainly doesn’t have gross flex that impacts handling significantly. Frankly I’d expect front suspension arm flex before we would see Stinger chassis moving around. Suspension arms are kind of “pee pee” compared to German and even Japanese cars like my past Z cars.

And I disagree that a slightly shorter chassis makes a completely different car. Plenty of reviewers in G70 say car feels like a stinger just 10% sharper . Plenty of sedans have more enthusiast focused coupes. Don’t see silly arguments like this in Infiniti or Honda 2 door coupes verses sedans on same platform. Just fact coupe is more enthusiast focused.

 

[eflyguy]

The amount of flex in the chassis is included into the overall suspension system component selection, and it's not just figured out with maths. That's the point of real-world / track testing in vehicle development, to take the data from engineering and compare it to the desired outcome in actual driving scenarios, where the flex also plays a role. The performance of the suspension components is pretty well defined but the outcome when bolted to an imperfect chassis, with both static and dynamic loads varying with acceleration in every direction, is still not able to be accurately determined. It's why manufacturers put thousands of hours into actual driving tests, and even top-level racing teams need practice and test sessions to understand how the vehicle will perform.

So, again, "stiffness" will certainly allow the components of the suspension system to perform closer to their design parameters, but that doesn't immediately translate into "better" handling or driving experience. They may, in fact, need to be replaced or re-tuned to account for the additional stiffness that has potentially worsened one or another subjective part of that overall experience.

On a race track, comfort and compliance are far behind raw grip under extreme loads for the sake of minimum lap times.

Despite observing panel gap shifts under load in my daughters Challenger - in photos I saw several days later - I have no intention of "stiffening" her chassis, as I felt nothing requiring any changes, the vehicle handled perfectly under those very severe and unusual conditions, which it has rarely needed to deal with before.

I can't wait to take my Stinger up there and put it through the same tests, but even though I have less than an hour behind the wheel of different examples on local roads, I'd be very surprised if I felt it needed to be "stiffened" to make it a better all-around vehicle. If I was planning to track it every weekend, on perfect asphalt, and was therefore willing to endure the horrific ride quality during the daily commute, then so be it. But then I probably wouldn't buy the Stinger in the first place. And, I work from home, so....

 

[xot1]

The M3/M4 has a lot of reinforcement added to the chassis here and there.

 

[MerlintheMad]

@CaliSting and @eflyguy, you two guys made my day. Thanks for the comments and personal observations. This is what I was hoping to hear: gearheads talking from a lot of experience. It's one thing to talk about how much the driver determines how well the car does. And it is germane to the topic of chassis "torsional rigidity" to point out that making a chassis more rigid is justified by lower lap times, but not to pursue higher performance at the expense of a good ride as a daily driver. I especially like the observation that heavy testing and practice are the only ways to determine if a car is working out. Start with a really good driver and then throw cars and changes at him (her?).

This all started by some assertions that the Stinger needs to be stiffer to reduce extraneous noise, implying that Kia somehow cheaped out where they shouldn't have. I'm even further convinced, after these few responses on this thread, that there is no basis for such assertions; that stiffening, chassis and suspension, is more likely to take a well designed car and Ef it up, creating noise where there was none before, and/or worsening noise that was already there.

 

[Waynerm002]

A lot of good, useful information here. The thing I don't like is how the car's rear end can get upset over bump in the road and how disconcerting it feels in corners, on the street. I don't track my cars, they are my daily drivers but I like to have some fun at times and there are some places that the car rear end bounces more than I want it to.

 

[MerlintheMad]

A lot of good, useful information here. The thing I don't like is how the car's rear end can get upset over bump in the road and how disconcerting it feels in corners, on the street. I don't track my cars, they are my daily drivers but I like to have some fun at times and there are some places that the car rear end bounces more than I want it to.

"Bounces"? Not my AWD in Sport mode. If the road pushes the car up, it comes right back down instantly. There is no bouncy, bouncy, bounce. If there were, I'd have felt it at well over a hundred forty going over those "humps" on Hwy 95 in Oregon. I had stability and traction control back "ON" for that experience. Maybe that made the difference: electronically controlled suspension seemed like a good idea for my first foray into the realm of Speed.