Sorry I'm unable to visit the W113 Pagoda SL Forum more often. As a purveyor of springs for Mercedes-Benz any discussion of suspensions is a welcome one! I became involved in supplying springs because M-B took me for granted by superseded an order I placed for M-B 280SL sport springs years ago; they sent standard springs without telling me. I didn't discover it until my suspension was dissembled. They just said "Sorry, M-B doesn't make the parts you ordered any more so our numbers reverted back to standard springs." Ugh. I had the springs I wanted custom-made and fifteen years later I have delivered many hundreds of custom springs, all for Mercedes-Benz, for all models from 1926 through 1980. Some have even been sold to Mercedes-Benz.
The availability of progressive rate springs is relatively recent.
I've accumulated much first hand experience during these years but do come to you "WSR" (without slide rule). I don't have the engineering background of some of you may prefer. I can draw on years of first-hand racing the infamous 300SL Gullwing and more recently a 300SL Roadster, including four La Carrera PanAmericanas (2,000 mile six day races through Mexico). My knowledge of suspensions comes from hand-on experimentation with dozens of setups including a personal 280SL five-speed (since sold but still loved). With this experience I've read your messages of this past week and believe following thoughts and facts should be added to the discussion in no particular order (you may want to download this and read it later; it's long!):
1) One writer observed that Mercedes-Benz engineers [I'd add their suppliers such as Sachs and Continental] put so much thought into each of their products that it is presumptuous for us to think we can second guess their decisions. Hmm, those same engineers would be the first to admit that every decision they made was a compromise. In cars sold to the public, suspension choices are many. Additionally, a two passenger car's wheelbase is short and its overall weight less compared to sedans. This alone magnify the effect of variable payload, not to mention where in the car the added weight occurs. Add diverse driver expectations (and abilities) and any standard SL is a watered down version of various potentials. In 1985 I had the honor of interviewing Hans Werner Aufrecht, one of the founders of AMG. He confidently stated that no matter what Mercedes-Benz offers for sale, he could improve on it. AMG has become an icon doing just that because Mercedes must take a far greater range of clientele into account than AMG. We, as discerning owners are pursuing the same thing as Herr. Eng. Aufrecht.
2) All three primary German shock absorber companies, Bilstein, Koni, and Sachs, employ the philosophy that it is the spring's job to handle the weight and downward (spring compression) strokes of a car and the shock absorber's job to mute rebound. Consequently the specifications even for adjustable German shock absorbers show 80% (plus or minus 10%) of their absorption on the rebounds stroke and 20% on the compression stroke. American shock absorbers are generally 50/50. plus or minus 10%. This may account for the difference reported among you re shock absorbers accentuating minor road variations at slow speeds, i.e. a Mercedes vs. an American car or Suburban.
3) Several e-mails pictured cars going out of control due to sudden changes in spring stiffness or suspension design. Of course all cars will go beyond driver control from overloading, poorly placed loads, too much speed, loss of road adhesion, tripping over obstacles, just to mention a few. Mercedes-Benz has been a leader curtailing these nemesis with "involuntary systems" such as anti-dive geometry in the 1980s, and ESP and ASD in the 1990s to shut down or correct for driver-naivety and their car's resulting trajectories.
Several of these hazards affect oversteer/understeer. Car companies declare "payload" maximums based of the strength limits of their parts but also to put a limit on oversteer from too much weight at the rear. This is not to say oversteer is always "bad." For decades race drivers preferred cars with oversteer. When front wheel drive cars became prevalent intentional understeer became necessary. High performance driving schools like Bob Bondurant's, prefer cars with close to neutral oversteer/understeer so drivers can shift the car's weight to whichever corner needs the most traction -- more over the steering wheels (via light braking) during the first half of corners and more weight over the driving wheels (rear for Mercedes) through acceleration after the apex of a corner (accomplished through so-called "heel and toe" footwork). In short, there is no one-fits-all answer, unless it is Bondurant's.
4) On August 15 Longtooth states as fact that the 230SL had significantly greater tendency that the 250SL for sudden oversteer.
FACT: I don't know which automotive journalist or author started this notion that there is a difference in the suspension amongst the W113 SLs but it is not true. Gernold Nisius set me straight on when he was proof reading my book THE SL EXPERIENCE before publication. There is no change whatsoever in the basic suspension components of the 230, 250 or 280 SL. I did previously know the springs retain the same standard deflection rates per 100 kilo. on all three models. Different springs are specified for preserving ground clearance of later cars delivered with A/C, automatic transmission, and power steering, but for no other reason. Major changes occurred on non-SLs during these years, but the only change to the W113 was a reduction in the number of grease fittings, and those changes were not specifically on wheel-to-body suspension parts (they were on the drive shaft and steering box). To come to the defense of the 230SL (are the rest of you 230SL owners sleeping?) it was that first model in which Rudolf Uhlenhaut is credited with lapping the Montreux race track within .2 seconds of a V12 Ferrari 250GT Berlinetta driven by Ferrari designer Michael Parkes (1963). It is also the 230SL which British automotive journalist L.J.K. Setright always favors as more agile than it's successors, the 250 and 280SL, due also to the three main bearing engine. I don't agree; I think the three models are close enough that superior engine and suspension tuning will determine the victor of any contest between them. Personally, I think all three models as sold to the public had atrociously mushy rides. Sport springs and a 50% stiffer compensating spring are listed in the W113 Workshop Manual and Technical Data books as options, but Americans never seemed to discover them. I guess too many W113s were gifts to wives and Stuttgart catered to it, at least in the USA. We know an increasing number were delivered with automatic transmissions and A/C every year. Now we come along and expect a little more of the car's "sportive" potential.
5) Longtooth's description of a car's lift during cornering is a risk for all automobiles however the 300SL Roadster and subsequent W113 SLs do reduce the phenomenon rather well by their low, single pivot differential. The very low pivot point successfully keeps the angle of the outside wheel (on a corner) negative (tipped in at the top) -- the most desirable angle due to tire roll. Separate parts between the body and suspension address lateral spring motion. The wide track is also helping. In 1987 I used a G-Force meter for tests on my 280SL and one of Carol Shelby's famous 289 Cobras over the same four mile loop of country roads. The 280SL recorded higher G-forces (just over 1 G) on all the sharper corners and nearly matched the Cobra's elapsed times despite 100 less horsepower. Both cars accepted controlled-drifts at will with negligible body lean, and the 280SL never rose on it's pivot axle. An open secret here (that all Mercedes owners should know) is never lifting the accelerator once it is applied in a corner. Lifting gives up the benefit of low pivot and negative angle of the rear wheels. With steady to increasing power, cornering is conspicuously superior to the fixed-axle Ferrari or Cobra. I'm sure all of this can be reduced to a mathematical model. M-B engineers undoubtedly did concoct the concept on paper first since they had to justify building a prototype low-swinging rear end before they could verify if it worked. When they got all done they (or the marketing whizkids) realized their typical clientele was more interested in a soft ride. ;-(
6) At the front wheels, suspension components intentionally tip the top of wheels inward of a curve as the wheels turn (more pronounced in the 1970s and till 1986), in anticipation of tire roll. Mercedes-Benz pooled their design with tire technology of the time which favored sidewalls strong enough to handle roll and tread going up the edge of each tire. Even with soft springs the W113 SL does what it is supposed to do ... deliver negative camber on the outside of corners. It's just that the "busy-ness" of the soft ride distract the driver from believing it!
[SIDE BAR: #6 is why flatter-bottomed tires with more abrupt edges, available since the early 1980s, are not well-suited to pre-1986s Mercedes-Benz. Flat bottom tires resist the roll a W113 is designed to "expect," and the flat bottoms very easily cause unwanted tracking diversions on crowned rural roads. The S-Class and SL Class did not fully adopt to the newest tires until 1986.]
7) When I read Jeff's "phase one installation report" on Sunday, trying out his new rear side springs at highway speed, I thought of a chart that I printed in my book and again in my SL Market Letter last month:
......................INCREASES......INCREASES
ADJUSTMENTS....OVERSTEER......UNDERSTEER
Ft. Air Pressure.....Higher..........Lower
Rr. Air Pressure.....Lower..........Higher
Front Camber.........More -.........More +
Rear Camber..........More +.........More -
Front Springs........Softer..........Firmer
Rear Springs.........Firmer..........Softer
Ft. Swaybar.........Thinner........Thicker
Rear Swaybar........Thicker........Thinner
Weight...............Rearward.......Forward
Front Tire Width.....Larger........Smaller
Rear Tire Width.....Smaller.........Larger
On any vehicle when stiffer springs are added to the rear only, it increasing potential for oversteer, all else being equal. This is why I always recommend changing all for corner springs. Introducing new sport or progressive rate springs to both ends at the same time avoids second guessing M-B's original front to back balance intentions. I'm not opposed to changing this relationship with thought but that is a separate decision unrelated to the present mission of less nose dive in braking and less body lean in corners, without a harsh boulevard ride. So Jeff and others, beware of more oversteer than you had before, until all springs are changed.
For Longtooth's question about how a non-tapered bar (coiled or otherwise) could still be "progressive" we may have to look to a "real" engineer for that answer, however I can tell you that my "progressive" springs are custom pitched to new angles at several intervals along the coil before the hardening process so the active coils differ in their angle to the descending weight where the loops are furthest apart. When placed in a spring tester the weight needed to squeeze the spring (deflection rate) increases progressively with every inch it is squeezed. Actually it becomes more complicated than that as the intended installed height has to be factored in first to deliver a prescribed curb height. Only after that is achieved can the desired beginning to ending deflection rates be programed (15 different calculations are involved for each custom order).
I hope this long winded dissertation is of some help. Please forgive any stupid typos they seem to be my foré and it's after midnight here!
Mit Vollgas (pedal to the metal)!
.....
JohnJohn R. Olson, Editor - SL MARKET LETTER
Snail Mail: 2020 Girard Av. So. Mpls, MN 55405
Phone: (612)-377-0155 - FAX (612)-377-0157
Website: slmarket.com - office@slmarket.com
1959 300SL Ro - 1977 450SEL 6.9 Wagon
