dizwiz24

Curious why our cars have transverse leaf springs?

weight? Cost?

to me, it doesnt seem ideal or truly independant -as whats happening on one side will still get ‘telegraphed’ to the other - despite the isolators.

we all know that there is a benefit to coil overs ...

my old 04 bmw 330i felt so much more ‘planted’ on one of ohio’s typical filled in pothole patchwork roads vs my wide-tired vette that shakes me up all over the road.

on a smooth racetrack, sure my 275/315 tire shod vette is going to outhandle that bmw.

so back to the fundamental question:
why transverse?

Tom400CFI

This could get ugly...but has been covered many a-time. The transverse leaf springs advantages are:
1. weight -less then 1/2 the weight of a stock coil spring (a single spring -you need two)
2. life -in testing, it out lasted a steel spring
3. packaging
4. some 'sway bar effect", depending on how it's mounted
5. very easily adjustable (depending on how it's mounted)

Disadvantages:
1. it's not easily adjustable (replaceable) for RATE. Requires a spring replacement which is more expensive than a coil.

"Truly independent". I laugh every time I read this. People "want" (think they want) "true independent suspension"....as if the thing is a crawler and we seek massive articulation But then what do we install? Bigger/stiffer sway bars! What do sway bars do? They tie the L&R suspension together, limiting "independence"! So....

Your BMW felt more planted b/c it had better shock/valving/damping. Not b/c of the type of spring. I've tracked a friends' M3, and it felt "more planted" (I'd say more dampened) than my C6 did. Better dampers.


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Tom400CFI

Right from the engineer's (Brian Decker, C4 Powertrain and chassis design) mouth:
"The reason we went to fiberglass is because it has the right properties to do the job: the most efficient storage of energy per unit of mass. It also packages very well in the car."

Tom400CFI

Here is some reading on the matter...

FAUEE

Well, Tom covered it pretty well.

Basically, it is in fact better as a performance option. I suspect that they could get really fancy and figure out a way to do progressive rate springs like they do for coils of they really wanted to.

The real question is, does GM own IP around it that keeps other manufacturers from doing it, or is a cost thing?

383vett

The transverse spring has been used in Corvettes from 84 to present day. The C7s provide world class performance at a much lower cost than other ultra expensive cars. It has been a winning concept.

Tom400CFI

Thanks!

You could totally make a progressive rate transverse leaf; you could even make it adjustable with the right design. But basically, if you made another "upper mount" -a mount of rubber pad/bumper above the spring, outboard of the current mount. Position it at the elevation where you want the rate increase in the travel, and far enough out to give the rate increase that you want; the farther out, the greater the rate increase. You could get really elaborate with this, using an elliptical interface that runs horizontally outward from the outside of the upper mount, and curves upward as it goes out, with such a curvature as to provide an infinitely progressive increase -the curve created the rate increase rate...that you want.

IDK why others haven't used it. Cost, is my guess. It does packages super nice in an SLA suspension and it is super light weight. IDK why more/all manufacturers don't use the design.


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Meanmyz

Great reading, Tom. What magazine or book is that?

Tom400CFI

That was from "The Newest Corvette. From A through Z51" by Michael Lamm. Killer book for any C4 owner.

There is also several pages w/a side bar specifically about the mono leaf spring, in Dave McLellan's "Corvette from the Inside".

Meanmyz

Thanks Tom! I actually have both of those. I agree, both are excellent! The Michael Lamm book is one of the best ever that I have seen on the C4. I don't think I have read the whole thing, or I have forgot...it has been a while. I think your post is going to motivate me to do some more reading.

Road & Track had a magazine on the new 1984 Corvette that I remember as being a treasure trove too. It had a gold '84 on the front.

pacoW

Just to give the Top Gear old brit hosts something to complain about.

MatthewMiller

You're really asking two different questions:

1. What are the pros and cons of a transverse leaf spring vs coil spring; and
2. What are the pros and cons of a double-A-arm front and five-link rear suspension (C4) vs a McPherson strut front and whateverthehellitis rear suspension (BMW E46).

Rather than re-type all the stuff I've written about the springs before, I'll point you to an article I put together for our local NCCC club: Transverse Leaf Springs: A Corvette Controversy. As I've posted before, a spring is a spring is a spring. They all do the same thing: store the energy that is input at the free end, and then release it back at the same free end. The "form factor" of the spring - flat, coil, torsion - doesn't change that. You typically use the spring that does the job with the best packaging, lowest mass, and best cost/availability. Sometimes that's a flat spring, sometimes a coil, sometimes a torsion spring.

Regarding the independence, a transverse leaf (flat) spring can be set up to be truly, fully independent if the center clamps are close together (like on the rear of a C4 and/or there is no unclamped space between the clamping points. It can also be set up such that movement on one end creates some movement in the same direction on the opposite end (anti-roll), by spacing the clamps pretty far apart, such as on the front of a C4 and both the front and rear of a C5/6. In this sense, that arrangement isn't fully "independent." Any suspension with a swaybar (aka "anti-roll bar") is not independent either, for the same reason: movement of the suspension on one side creates movement in the same direction on the other side. So almost no car has fully independent suspension in that sense, except for a few off-road vehicles (some allow the decoupling of the swaybars for rough terrain). But that's not really what is meant by the term "independent suspension." That term really means that one side of the suspension is not rigidly connected to the other. A solid axle or De Dion suspension are examples of non-independent suspensions, where there is rigid connection from one side to the other. The C4 and E46 are both examples of fully independent suspensions, even though both come with swaybars front and rear that create an elastic connection from left side to right side. The end result of any anti-roll spring property (whether from a swaybar or a widely clamped monoleaf) is just that the wheel rate of the car in roll is higher than in pitch or heave. That's it. We shouldn't make it more complicated than that.

Question #2, about the actual suspension geometry and type, has giant books written about it. So I won't go into it too much here. Suffice to say that up front a double-A-arm setup like the C4's is generally superior to a strut like the E46's, although of course the specific geometry built into it can hobble a double-A-arm design or a strut design. For example, the double-A-arm design of the C5/6 is generally better than that of either C4 geometry (84-87 or 88-96). At the rear, a five-link like the C4's is probably inferior to the double-A-arm design that debuted in the C5, but geometry matters here, too. The C4's rear geometry was somewhat constrained by making the halfshafts serve double-duty as the upper control arms: that required some "baked in" locations for the upper arm geometry. It still works really well, however. That 85-link/squishy-bushing/super-alignment-changing contraption the Germans built for the E46? I have no friggin' idea. My guess is that it's really good for a street car that needs to be comfy and compliant over public roads, but that it has compromises when you try to use it in a more race-focused application. You will never, ever, see a clean-sheet race car designed with anything like that suspension, but then that wasn't the E46's mission.

Tom400CFI

Totally! And all the other ignoramuses who bash something that they don't "get".

The irony of this thread is that Dizwiz is a big proponent of "light weight". To the point that he uses a stock radiator on a supercharged car to save a couple pounds compared to an aftermarket radiator. For a guy that into weight savings Diz ought to be a transverse mono leaf spring cheerleader!

auburn2

Your BMW may have felt more planted to you, but I believe it is also uses a Macpherson strut setup in front vice a double wishbone. It is an inferior suspension design compared to a C4, despite being much newer.

The C4 suspension is as truely independent as any suspension on any car, the springs don't telegraph anything from one side to the other any more than the control arms or roll bars do and significantly less than the tie rods.

You can tailor the spring rates a bit easier with coil springs (with progressive rate springs), but other than that there are not any inherent advantages in the design.