Chassis Stiffening, Suspension Ideas, Handling Improvements

[wytbishop]

Just a few unanotated, unsupported thoughts on the subject of chassis stiffness.

The issue as I see it is with energy absorption. Transmissibility is a concept taught early in Engineering. A perfectly rigid body transmits 100% of the forces which act upon it. A perfectly non-rigid body, of such a thing existed, would absorb 100% of the force acting on it. The question to be answered is "What are the affects of energy absorption in the chassis on the handling and quality of ride?"

Taking run of the mill car with a pretty weak chassis and only moderate cornering ability like say a stock 323 or Escort and looking at what happens while it's cornering you would see that although the chassis and the suspension are reacting separately, they are reacting to the forces being applied to the car simultaniously. The springs absorb almost all of the vertical forces, but because the other suspension components are fairly rigid they transmit the majority of the lateral acceleration of the vehicle into the chassis. Some is absorbed by the bushings....don't get me started...but most goes right into the chassis. The chassis then immediately begins to absorb a good deal of that energy. Because the chassis is weak it begins to flex at relatively low levels of cornering force. Because the chassis is flexing very early in the process, the driver feels the car sort of melt into the corner. It's comfortable, but not precise.

So if you take a car with a lot of flex in the chassis and make it significantly more rigid, the chassis will resist that flexing until the forces get much stronger, so either much later in the suspension travel or only in extremely hard turning situations. So as the energy of the car's lateral acceleration is transmitted through the suspension it is also transmitted through the chassis to a much greater degree. This allows the tuner to make a much better handling car because the suspension is doing most of the energy absorption (in the lateral plane) but it makes the ride quality worse because the energy is being transmitted to the driver. Far more precise, but at the point when chassis flexure does occur, the car is experiencing much greater force and so the energy which is transmitted to the driver is significant.

From a pure handling point of view, look at a high level racing Go-cart. Its only suspension is it's tires and until you get one to the very edge of its traction envelope it is the best handling thing on four wheels on the planet...as long as you're on a smooth track with no bumps or imperfections to absorb. That is because when you initiate a corner in a vehicle with effectively zero suspension travel and a stiff chassis it immediately begins to generate reaction forces. Almost no energy is absorbed so when the earth starts to push on it, it immediately starts to push back. A car doesn't do that. When the earth starts to push on your car the suspension compresses, the chassis flexes until the whole thing is loaded up and THEN it starts to push back.

You could turn the MX-3, or any other car for that matter, into a virtual go-cart and it would handle better than anything you've ever driven on a flat smooth road...but I wouldn't want to drive it to work.

I'm tired.

[_-Night-Shade-_]

Some is absorbed by the bushings....don't get me started...

[onlytrueromeo]

Ya, I have read that article, 300.

Thanks for input guys!

[Norm Peterson]

Just for starters, as I've only started going through this topic. There's too much to try to address in only a single post anyway.

With respect to calling in research done by others without it passing a sanity check or posting its stated limitations, well, I won't go so far as to call it "salesmanship", but it does come up short as far as useful application of any such reference is concerned. Not intended as flame, BTW. Just my position as a (still) practicing engineer.

With respect to the paper itself, something is clearly amiss in that ABAQUS analysis. No way does a STB of any design reduce the relative rotational deformation between the car's A and C pillars as the sketches show. The paper's author already admits having difficulty with the program and goes on to write a lot of the unexpected difference (order of magnitude, no less) off with only a few words. His final conclusion . . .

The conclusion is that analyses like these are very time, computer, and man intensive.

that while being obviously correct after scanning through the paper, is completely off-topic and irrelevant to the structural issue at hand. "Bit off more than he could chew" is the phrase that comes to mind.

Be VERY careful how you use anything from that paper. I don't want to review it in any greater detail for fear of what I might find there. Not yet, anyway.


Norm

[Norm Peterson]

A couple of further thoughts.

NVH is a dynamic condition, which requires more than a = F/M to describe (a=F/M + some function of α=T/I, actually, since most bumps involve only 1 or 2 wheels). Think combinations of heave, pitch, and roll, and that's just quasi-statically.

Anyway, humans are sensitive to vibrations, and it is possible to add stiffness such that bothersome structural vibration modes are chased off to frequencies and/or maximum displacement locations that annoy the driver to lesser amounts. The details by which this is accomplished may or may not increase general torsional or flexural rigidity by enough to matter.

Energy imparted to the suspension certainly does get dissipated within the suspension, mostly in the form of heat developed in the dampers. Lots of it. I doubt that I'd want to grab a competition vehicle's shock body barehanded immediately following its hot laps. Milliken gives numbers like 15% - 25% critical damping for shocks primarily intended where ride quality is the main concern and 45% or so for best cornering. Still higher values are commonly used to affect transitional handling.

Structural damping within the chassis is much, much lower. It is common in my industry to use values between 2% and 5% of critical damping for welded tubular structures and 7% for bolted ones. My point here is that structural damping isn't going to make the chassis look much stiffer dynamically than its static stiffness, as the suspension sees it.

You can only build in so much chassis stiffness before considerations such as added weight and cost or passenger / cargo / repair access make you stop. But you do need a certain amount of stiffness, otherwise tuning the suspension to tweak the handling becomes difficult. Roll stiffness distribution depends on the chassis to carry torque that wants to be resisted at one end down to the other. This is a 3 springs in series sort of thing, and if the spring in the middle resembles cooked spaghetti it won't matter what you do to the other two, the LLTD wouldn't shift and the handling balance would stay right where it was. Note that as suspension roll stiffness goes up, so does the need for greater chassis torsional stiffness. This is the handling side of the coin. For ride, you still need enough torsional stiffness to drag a bit of LLTD from one end to the other without having to go crazy with springs and sta-bars, as well as a certain amount of beam stiffness just to make the car commercially acceptable (you do want to be able to open and shut the doors without too much difficulty if one corner is on a little hill or in a little hole, don't you?).

Don't overlook seat design either, as that's the third spring between the road and the driver's butt (with its own frequencies).


Norm

[onlytrueromeo]

A couple of further thoughts.

NVH is a dynamic condition, which requires more than a = F/M to describe (a=F/M + some function of α=T/I, actually, since most bumps involve only 1 or 2 wheels). Think combinations of heave, pitch, and roll, and that's just quasi-statically.

Anyway, humans are sensitive to vibrations, and it is possible to add stiffness such that bothersome structural vibration modes are chased off to frequencies and/or maximum displacement locations that annoy the driver to lesser amounts. The details by which this is accomplished may or may not increase general torsional or flexural rigidity by enough to matter.

Energy imparted to the suspension certainly does get dissipated within the suspension, mostly in the form of heat developed in the dampers. Lots of it. I doubt that I'd want to grab a competition vehicle's shock body barehanded immediately following its hot laps. Milliken gives numbers like 15% - 25% critical damping for shocks primarily intended where ride quality is the main concern and 45% or so for best cornering. Still higher values are commonly used to affect transitional handling.

Structural damping within the chassis is much, much lower. It is common in my industry to use values between 2% and 5% of critical damping for welded tubular structures and 7% for bolted ones. My point here is that structural damping isn't going to make the chassis look much stiffer dynamically than its static stiffness, as the suspension sees it.

You can only build in so much chassis stiffness before considerations such as added weight and cost or passenger / cargo / repair access make you stop. But you do need a certain amount of stiffness, otherwise tuning the suspension to tweak the handling becomes difficult. Roll stiffness distribution depends on the chassis to carry torque that wants to be resisted at one end down to the other. This is a 3 springs in series sort of thing, and if the spring in the middle resembles cooked spaghetti it won't matter what you do to the other two, the LLTD wouldn't shift and the handling balance would stay right where it was. Note that as suspension roll stiffness goes up, so does the need for greater chassis torsional stiffness. This is the handling side of the coin. For ride, you still need enough torsional stiffness to drag a bit of LLTD from one end to the other without having to go crazy with springs and sta-bars, as well as a certain amount of beam stiffness just to make the car commercially acceptable (you do want to be able to open and shut the doors without too much difficulty if one corner is on a little hill or in a little hole, don't you?).

Don't overlook seat design either, as that's the third spring between the road and the driver's butt (with its own frequencies).


Norm

Yes.

You say things more elegantly than me...I tend to be long winded

Haven't done too much reading into this topic lately, been very busy wrapping up the semester at school and tryin to find a job for when I graduate in a month

If you're wondering what I plan do use the car for (I have this in other threads somewhere) it may help to understand where I'm going with all this.

I want to build a DD with much improved handling and comfort, but to no extreme in either case. I believe I can reduce NVH through the chassis stiffening topics discussed, and improve handling at the same time. I will be increasing the spring rate/damping from stock, but I haven't gotten that far yet to determine what range I want and how much I want to spend. Koni yellows/reds is most likely my upper limit, w/ ground controls or similar custom spring rate. I have not done enough reading to determine if I want linear/progressive spring rate, or what style "bumpstop" I want. (I know its no longer a bumpstop, but I don't remember the technical term for it)

I also plan to add additional noise reduction/damping materials so I can keep my stereo in the car, and get rid of any and all rattles on the plastic pieces, and add closed cell foam to block road noise.

I think many of the creaks/groans I hear on the car area due to things being old. I have replaced everything except my bushings, and while they are the cause of some of the noises, most are from the flexing of interior panels, trunk latch/alignment and the actual metal of the car. I expect that the 150,000miles has weakened some of the welded joints on the body, and things are flexing more now than they were originally.

[Norm Peterson]

You say things more elegantly than me...I tend to be long winded

Been at it a bit longer (this coming Tuesday the 10th, check the "Congratulations" block at the bottom of the list of forums page).

Hopefully, you'll soon get to find out that answers in industry aren't quite as clear-cut as they are in the academic world. You'll want to look downstream of the immediate conclusions a bit. If you don't, it gets way too easy to grow a set of blinders (a little thoroughbred horse racing reference). Reread and edit your posts as necessary (I've edited this one several times).

And don't be too picky about where or from whom you might learn something, but at least give it a chance while making whatever it is pass a thoughtful sanity check. I expect as much for myself from having joined here.


It doesn't sound like you're asking for the moon here, but on the other hand I wouldn't expect to calculate things like this to within 15%. There's likely that much error just in the structural modelling assumptions that you make, and assuming that the chassis actually rotates in roll about the suspensions' geometric "roll centers" is also erroneous.

Even if you did get a "final answer" to within 15% of actual by the time you get to details like spring rates and bar stiffnesses, testing would probably bring about changes anyway. If you'd like to see in a little more detail what vehicle dynamics gets into mathematically, try to track down a copy of Walter Bergman's paper "The Basic Nature of Vehicle Understeer-Oversteer". It's a rather old paper (early 1960's and not available online as far as I can determine), and it's not without a couple of faults. But the section with the equations should at least provide some idea of what-all is involved.

Subscribe to Mark Ortiz's "Chassis Newsletter" (it's free, just e-mail him a request, PM me for the address). His newsletters appear as columns/articles in "Racecar Engineering" (around $10/issue at Barnes & Noble).


Norm

[_-Night-Shade-_]

Been at it a bit longer (this coming Tuesday the 10th, check the "Congratulations" block at the bottom of the list of forums page)

Happy Birthday old timer

[mitmaks]

I certainly would be interested in subframe connectors for my mx-3, it's been on a "parts to get" list for a while

[jaydog5678]

Just my .02. I've been an Mx-3 owner for a very long time and during that time, I have come to realize certain handling problems with our cars. I have done everything to make a FWD Mx-3 handle as good as it's going to get, well almost.. I've done chassis stiffening, rear sway bar, bushing upgrades, spring and adjustable struts, roll cage, race tires, 300hp and all that doesn't make me the fastest car around the racetrack. I am a member of SCCA and NASA and have been to several diff. road courses around the southeast U.S. Here are several problems with our suspension and overall design. We have Macpherson struts at all 4 corners. The car is front engine f. wheel drive. A less than desirable center of gravity. A less than desirable weight distribution. Under braking. From my point of view, a fellow enthusiast and racer, the car could be better but only through a radical chassis and engine re-configuration. Don't get me wrong, I've had plenty of fun toying with the Mx-3 over the years and beating out many higher priced cars around the race track. The bottom line is either buy another street/track car or re-configure the Mx-3 to mid engine, rear wheel drive.

[weaver]

Hi All, I am a first time poster, I have read this topic with great interest, and very interesting and intelligent views.
I read it with great details before taking the decision to perform chassis foaming myself.

Short intro of myself and why I am posting here. I am from a part of the world that you guys probably don't know where it is (Malaysia). I am driving a Toyota Vios (in some places it is called Yaris Sedan), for those of you who has no idea what car it is, it is a small compact and budget economical sedan. It is a very light 1020 kg dry weight car.
I wish to post here to share my experience and hope I am able to help anyone of you guys out there in making a decision, I am willing to share experience because I find reasonable experiences always lacking in internet (no offense), so I try to do my best here and hope I offer a good knowledge here based on my experience.
My English is not good, but I try my best to explain so that you guys know what I am talking about.

Warning - Don't take everything I said as 100% correct, because I am not the most knowledgeable person, I share input based on my own level best understanding and logical thinking.



OK time to cut the long BS. .

I have to agree with the comment of Inodoro Pereyra that chassis foaming does compromise the ride comfort.
My car Vios originally is a car with very soft absorber setting, with very comfortable ride in most situation, be it going over speed bumps, or going over small wave of road undulations, uneven roads,...bridge joints,...ripples. Of course in its original form, high speed ride is not so good because of very soft settings.

What happened to the ride quality after I did the chassis foaming?

1. Going over bridge joints or ripples, it is harsher as if I had put in much higher air pressure on tires
2. Going over uneven road level connections - from lower to higher level (with smooth tarred connection)
- the ride is much firmer,...you just felt that impact harder without being harsh, probably due to soft absorber setting.
Before chassis foaming, the car beautifully absorbs the changes.
3. Going over uneven road level connections - from higher level to lower level (with smooth tarred connection)
- You can feel the car squats less when it hits the lower level. Before foaming, it squats more,...but squatting more also makes it more "cushiony" feeling. Again,...the foaming effect makes it firmer without bordering on harshness.
4. Going over road with uneven small bumps,...whereby there it has bumps randomly all over the road,..left side with small bump,...but right side of road no bumps...etc...
- After foaming, I felt the car is more prone to "rocking" from left to right. Before foaming,...the car manage to smooth out these unevenness much better.
5. Going over speed bumps - before foaming, going up the bump,...it absorbs more of the impact,...going down the bump, it squats more and even out the force. After foaming much firmer.
6. Going over ripples or ripple lines - after foaming, you can feel the vibration much more, alittle bit like the whole chassis resonates.
7. Highway / freeway with very good quality road conditions - Not much difference before and after foaming here.

My own conclusion is (for a car with the same parameters as my car) -
1. The ride quality is harsher after foaming.
2. You just do not realise how much your car chassis flex, doing the foaming makes you realise it.
3. For cars like mine (Vios / Yaris sedan), chassis flex is part of the "absorber" work in absorbing the energy before reaching passenger
4. I agree with both the theories here -
a - chassis foaming / stiffening makes the absorber works more because it puts more force to the absorber when there is less flex
b - chassis foaming/ stiffening will make the ride quality harsher because stiff chassis is not able to flex and absorb some of the impact energy.
Therefore, even though flexible chassis does not make the absorber work so much, but its flexible chassis does absorb a huge amount of energy impact,..smoothering out the ride. Thus resulting in better ride quality.
Chassis flexing does not really make an uncomfortable ride because we are sitting in the middle of the car,...the flex happens at both the front and rear end of the car.

I am using this two examples -
Stiff chassis - put a thick plywood / plank on 4 springs. The springs need to work more because the rigid plywood forces it too. But whatever impact from the wheel, undulations and so on is felt alot on the plywood.
Flexible chassis - imagine putting a mattress (or something like that) on 4 springs, the spring does not work so much and the mattress flexes more when there is energy and impact from the spring.
But if you sit in the middle of the mattress, it does not vibrate so much, not so harsh or firm with all the movement.

c. Why is it that making the original chassis stiffer deteriorates the rides, while the manufacturers are always claiming to build stiffer chassis with better rides and handling?
My guess is that,...when for example Toyota tests the suspension and absorber setup for the Vios sedan,...it is done based on this type of body / chassis with such amount of flex. So if we are to change the rigidity of this original chassis, then the suspension / absorber is not suitable for it anymore.
My light weight Vios may need a softer absorber to retain the same level of comfort with the current chassis rigidity.

So for other more expensive cars with every increasing chassis stiffness, my guess is that when they are testing the absorber and suspension setting,...it is done with that chassis stiffness,..and adjusted accordingly.


Hope my long winded input helps,...

[Josh]

Holly thread revival Batman...