Tom400CFI

What is your position here; that B/c some "boring cars" use a leaf spring, it's no good? Is THAT your version of "engineering and logic"??


Hey Dave, that's O.K. I'm used to it. Lots of people resort to insults, sarcasm and drama when they are confronted w/facts and solid evidence. You asked me;
"what is the other car at any price point that uses it any price point?"
So I did. I gave you 4 legitimate examples w/o any effort at all. You come back w/a horsesh!t response.

I say someone needs to run a test. GM may be wrong about their claims, but I doubt it. If all that we had to go on was a "marketing blurb" about the springs...yeah, I'd be skeptical. But the chief engineer of the car wrote in his book (after retiring from GM) about this effect from the spring. He's an engineer. Is THAT "engineering and logic has left the room", too?

.

MatthewMiller

Tom is not wrong on that. The torque from the transmission output shaft is sent to a differential that is bolted to the frame, and that differential is not rigidly attached to the tires*. So the transmission torque is reacted into the frame of the car, period. You are just wrong about this. As long as the two rear tires have equal traction and equivalent trailing arm and toe geometry, the car will not "torque over," it will accelerate straight ahead, and it will break the tires loose at the same time (even with an open diff).

OTOH, a solid axle car sends it transmission output to an axle that is not solidly mounted to the frame, but instead is effectively solidly mounted to the road. Since the frame is only connected to the diff by compliant springs, the engine and frame and the axle can't spin around the driveshaft (because pavement), then the engine tries to spin the whole frame around the drive axle. The only thing that keeps the car's body from spinning in endless barrel rolls is that the springs eventually resist the torque enough to arrest the roll and/or the tires run out of traction. If you took the springs off altogether and hung the car from a crane by the spring attachment points with the brakes applied to keep the tires from turning (or the ring gear welded to the pinion so it wouldn't turn), the body would endlessly spin around the driveshaft axis. But...what if the axle were solidly mounted to the frame with no suspension at all. Then what would happen if you hung it from the crane again? Thing about that, and then consider that an IRS is effectively the same thing because its diff is also mounted solidly to the frame without a meaningful suspension.*

*Save for some rubber bushings, that are not consequential to this matter and can be eliminated for more race-oriented use anyway.

Yeah. Good luck with that. Because if you drive sensibly then nobody will ever cut you off or pull out in front of you or otherwise cause you to have to take emergent action with brakes and/or steering wheel to avoid an accident. I'm sure it will work out fine...

Tom400CFI

^Worded much more concisely than my post. Well said.


My memory was a bit off; there was no reference to the Camaro, but this is straight from the horses mouth; the Chief Engineer of the car. One might claim that this is marketing rubbish. Let's consider the source; a book published in '02, by the engineer of the car. The guy is writing about a car no longer produced by the company that he no longer works for. In addition, Dave has no problem disparaging many of GM's practices throughout the book....including the Targa top (which BTW, didn't come from "styling"). I'm just not seeing where "marketing" would come into play here. Guys, I think the McLellan was telling the truth.

MatthewMiller

Yeah, I think I'm going to end up standing corrected here. I did some thinking and found an interview with a guy from Pfadt. Ironically, I think he got it exactly backwards, but there is a way the transverse leaf could resist roll by reducing suspension independence. It could not happen if the spring were clamped solidly across the entire 18" center section. But because it has freedom to flex in between the two mounts, I can see how moving one end of the spring down might move the middle portion up, which could then move the other end downward somewhat. That would provide some measure of roll resistance exactly the way a sway bar does. Whether that is a significant portion of the roll resistance is open to question. Also, if this extra variable is deemed a problem, then the easy solution would be to fabricate something to clamp the spring all the way across the 18" center.

It's still worth modeling, but I suspect that will demonstrate that the effect is real. To find out its significance would require a real Corvette with a scale and a calibrated way to weight one end down a measured amount. The question would be: if you move one end of the spring down by 1" (and therefore put a known amount of force on the spring based on spring rate), then how much force does the other end exert onto the scale? Now I'm curious. Also curious is how the effect changes by altering the spacing of the mounts, and by altering the dimensions of the spring in the middle section (e.g. tapering the thickness or the width up or down).

ddahlgren

I read about the targa top deal on a Corvette forum though not sure which one and the quote was credited to Gordon Killibrew(sp?) and have experienced the difference in mine with top on and top off so they lost the rabbit somewhere in chassis design. I personally suspect the large open area under the floor pan with nothing in shear. Adding the roof closes the box and adds a shear plane. I think if they had bonded a rigid floor pan to the frame including the front and rear bulkheads it would have made a large difference but no doubt out of the budget.

From my experience with everything from NASCAR modifieds and road race cars high roll stiffness comes with a price tag of needing a smooth track surface with little compliance for any sort of bump or void. Watkins Glen is a good example as last time I was there to work on a car 90% of the track was very nice but the hill after turn one was a challenge. as pretty rough. I have only found 2 times to limit suspension travel the first a ground effects car that requires strict clearances and the second one with poorly designed pick-up points with bad camber curves and bump / roll steer. Attached is a link to what claims to be wheel rates for various years a sub models of C4 production and sure looks like they were all over the place. I am sure if I were to race a C4 or any other car I would start with chassis stiffness and once fixed move on to suspension geometry and pick-up points. My opinions free and do with them what you will...

http://www.netmotive.net/articles/hib/c4/sustunch.pdf

blackozvet

Im thinking that if GM had the maths on how much difference leafs make to anti-roll they would have been telling everyone for years to defend their decision, but we still rely on urban myths to keep it rolling. They probably didnt have the time or money during c4 development to test the theory. So unless someone has empirical data proof, the jury is still out on this one ?

So as far as the engineers saying that the front leaf stiffened the front end (and required less roll bar) as long as it was clamped 18" apart, Im thinking that clamping the spring between the 2 chassis rails is introducing a chassis stiffener and thats how it works, as opposed to being a defacto sway bar ? Maybe needs a minimum of 18" to be stable under roll and weight transfer ?
The reason Im thinking this is as a result of making a front camber bar and the result you get from tying those 2 front chassis rails together. If you stiffen a chassis you need less roll bar.

MatthewMiller

The way the spring is clamped by two straps, it has multiple degrees of freedom to move. It could not provide meaningful chassis stiffness because of that.

Like I said, I think it would be trivial to test this idea, and I'm 100% sure that GM did test it. What their exact findings were is another story.

ddahlgren

There is always the 'tick tock factor LOL. Lost a years production just use what you have factor.

blackozvet

Am I misunderstanding your description, the middle of the spring cannot move where it is bolted to the chassis ?

Tom400CFI

You are absolutely right about all of that. If they'd had more time, more money...it could have been a C5...that was still FUN to drive.
Read on, if you'd like...about the structure, and how the roof plays into that;










In the space between the two mounting points, the spring is free to move...and it does move in that space.

Tom400CFI

I was agreeing w/all of this, 100%. Was the effect meaningful? Or not really. McLellan said that they could "achieve a major portion of the roll stiffness" from the spring...who knows what "major" means, but to me, it means over 50%.

I did a little more looking and found this, very specific data about both the springs contribution to roll stiffness and the ability to tune that by changing mounting point locations. Read on....

MatthewMiller

Yep, this all makes sense to me now. The is realizing that the spring has freedom to flex in between the clamps. That's the part I was missing. It's an important detail! I am glad I jumped into this thread.

BTW, that book by McLellan looks fascinating. I need to get it. Is it Corvette from the Inside?

383vett

Back to,the op's original question, I drove my C4 for years on the street with no front sway bar and drag shocks. It was a street/strip car back then and the handling was actually very good thanks to the Z51 springs. I didn't enter any autocrosses, but it was very safe on the street. I had no issues.

ddahlgren

Just how do you go about corner weighing the magic leaf spring? Endlessly playing with shims under the mounts all day?

MatthewMiller

You adjust the lengths of the bolts connecting the ends of the rear spring the uprights.

ddahlgren

And in the front?

The front having a higher wheel rate will change things much quicker without ending up with a silly ride height as well.

Aardwolf

I did some minor corner balancing on mine a few years back. I didn't think it was that bad to do. Me in the car with a full tank:

Wheel Stander

Thanks for getting us back on point. I never intended for my inquiry to result in a prolonged debate on the physics of vehicle suspension configurations. Oh well, I guess people sometimes just get carried away.

MatthewMiller

What ride height change? You lengthen the distance on one end of the rear spring and shorten the distance on the other. The overall ride height stays the same - you are just changing preload. On the Xtreme front spring from VBP like I run, you can do the same on the front. Obviously on OE or OE-style springs you don't have that option. You can mess around with shims on the pads if you really want, but the rear bolts are a whole lot easier and accomplish the same thing.

If you really read what was posted, the crux of it is very pertinent to your topic. Even if you disconnect the front sway bar, you will still have a very significant amount of anti-roll effect just from the front spring by itself. So you aren't really accomplishing much. Even more, the reasons that people disconnect swaybars on solid-axle cars for drag racing may not even apply to a car with IRS. It's kind of hard to discuss a question about suspension dynamics ("Should I disconnect the front sway bar or not?") without discussing suspension dynamics.

Tom400CFI

Yes, you're right; Corvette from the Inside...it is a fantastic book. The later pages I posted were from "The Newest Corvette from A-Z52", by Michael Lamm

Both books that should be bought w/any C4 purchase, IMO. "Corvette form the Inside" is "better" because you get so many details from "behind the scenes" From Dave himself. Invaluable, IMO. In "The Newest Corvette" is "better" because there is more technical detail about the C4 itself, such as the frame structure, etc. More of that book is dedicated to technical info, where as Dave's book gets more into politics, motivations...etc. The way Dave words things make implications that allow you to fill in blanks about certain things. Good stuff.



What Mathew Miller said....and this goes back to what I said, way back in post #24; people who track/race their car and are making continual adjustments for tracks, conditions, etc....those people benefit from the more readily adjustable, coil over suspension. For the rest of us, that is not a benefit b/c we're not making spring adjustment/rate changes every weekend...or several times a weekend. Probably 98% of Corvette owners never touch their springs, yet enjoy the benefits of the composite leaf spring (likely w/o even knowing it). Probably ~ 1.9% of Corvette owners mess w/the springs (and suspension) initially or at some point, then run the car for the rest of their ownership, yet enjoy the benefits of the leaf spring. And...I'm guessing that ~.1% of Corvette owners track/race the **** out of their cars, use coil overs for their readily adjustable features and those people enjoy the benefits of the coil over design for it's adjustability/tuneability at the track.

.