Mercedes Benz 300SL Suspension Tuning for the Road and Vintage Racing

Some of you know that in a previous century I vintage raced a 300SL Gullwing and later a Roadster. I was always hungry for clues about Mercedes racing modifications. There is a brief section in the back of earliest 300SL Workshop Manuals listing adjustments the factory recommended for maximum engine performance. The subsequent 300SL Workshop Manual printed in 1958 omitted this section but included part numbers and specifications for sport springs and firmer shock absorbers. When I tried to order a set the sport springs they were no longer available so I had them made in the USA. After two unsuccessful attempts I found a company with the patience to get the rear spring “pig tails” right and duplicate the sport deflection rates. They are not harsh, but definitely reduce body pitch during hard or emergency braking and turning.

Later in the 1980s I had the pleasure of befriending the late Paul O’Shea, the star of the 1955-1957 O’Shea-Tilp Racing Team. Paul accrued the most wins and/or SCCA points all three years. Paul was even selected as Sports Illustrated’s first Athlete of the Year in a big NYC ceremony in late 1955.

Paul told me the 300SLs provided for him were initially prepared from Hoffman’s USA inventory. 1956 and 1957 cars were purpose-built by Stuttgart for George Tilp of Montclair, NJ., and Paul O’Shea. The two 1957 Roadsters were lighterthan-stock by 500 lbs. putting them in SCCA’s largest Class that included 4.9 liter V12 Ferraris.

In addition to sport springs and shock absorbers Paul said the flat metal “bottoming straps” under the rear axles of the Gullwing Coupés were shortened an inch and more (to the preferences of team drivers) to restrict positive camber on corners and reduce unsprung weight. In 1988 retired factory race driver Hermann Lang told me that he recalled 300SL Coupés with adjustable chains in place of rear axle straps for quicker adjustments. These two changes, springs and straps, noticeable reduced body lean and nose-dive in hard braking. I sold the first set of 300SL sport springs to another owner in 1987 and have retained a record of all subsequent purchasers.

Another common mistake in those days was ignoring the Gullwing specification for 3º to 3.5º negative camber at the rear wheels, at the curb with driver aboard. When set at zero they go into positive camber too easily when cornering. I remember observing only 5 of 25 Gullwings at a late 1980s GWG Convention set up right. I wonder how Saratoga Springs will compare 20 years later.

After ten years savoring GW # 5500771 I switched to a Roadster (not willing the wait until I might own both concurrently). It continually amazes me how different and similar these cars are at the same time. So many identical parts yet so different to drive. Unidentical twins! Suspension wise, except for different front spring and anti-sway bar stiffness, the suspension are identical. The big revision was at the rear; its lower, single pivot differential made axle straps and double (inner and outer) rear springs obsolete. Sudden oversteer while cornering also disappeared. While skilled drivers obviously learned how to go very fast in Coupés, Roadsters became almost like a Buick… downright soft. They go into negative camber nicely in corners, as they should, but the ride is softer than many people expect for the car’s heritage.

ANTI-SWAY BARS: Front anti-sway bars grew in size twice. Gullwings began life with 19 mm. bars. The first Roadsters were 22 mm. They grew to 25mm. in approximately 1961. During the 1980s the late Joe Thompkin, another serious vintage racer driving an alloy Gullwing no less, used and sold several dozen 25 mm. anti-sway bars to club members. The Gull Wing Group Store now sells a 25mm. bar complete with 10 Mercedes Benz rubber bushings. The same shape bar fits both Coupés and Roadsters. If this interests you put a micrometer to your antisway bar (a little behind your front bumper) to be sure your bar isn’t already 25 mm! I do recommend them as documented M-B “tuning” that curtails body sway and oversteer when cornering (see chart). It isn’t harsh and only engages during cornering.

Even though Mercedes-Benz withdrew from racing the two factory prepared O’Shea-Tilp Racing Team roadsters have remained an inspiration to owners. It is certain these cars were significantly modified specifically for O’Shea’s 1957 season. Erich Waxenberger, a retired M-B engineer reporting to Rudolf Uhlenhaut, told me in 2006 that the two cars supplied to O’Shea were built in different departments for considerably different costs but delivered essentially identical performance. Two full time works mechanics were sent with the cars to care for them. Paul told me new ideas were tried continually between 1957 races, often at his or Mr. Tilp’s suggestion. Venting ducts were added through the front of front fenders for brakes. American valve springs were tried. Suspension springs were also changed. A few times O’Shea drove one of the cars to a race, competed, and drove on to the next race with no trailer.

SPORT SPRINGS: In 1991 the first of what later became three SL Market Letter subscribers, offered to pick up the bill in exchange for navigating and co-driving my car in Mexico’s six day, 2,000 mile Carrera Pan-Americana. I couldn’t resist. I began testing alternatives to tighten up my Roadster’s standard suspension beyond the 25 mm anti-sway bar I’d bought from Joe. I’ve found GWs & Roadsters are quite responsive to suspension tuning. Adapting to differing speeds, roads and “payloads” is always a compromise, short of driver-controlled suspensions (as we have in today’s senior Mercedes-Benz). Over a dozen factors go into any car’s “spring recipe”: the length (number of loops and overall diameter), thickness and type of steel rod that is heat-treated to achieve a specific curb height and deflection-rate. When you read the allowed payload of any car (often found on a door pillar and in owner’s manuals) it reflects the limits of that car’s standard suspension. A civil goal is for SL springs to travel a little short of one inch per 100 kilo (220 lbs.) of compression caused by bumps or change in payload. One inch is common, Two inches is soft. Adding a passenger, a full tank of gas and luggage quickly reaches load limits. Substituting (or adding) curvy roads and sportive driving makes the case for firmer springs conclusive. For overall weight-management and handling of 300SLs I’ve come to favor sport springs in front and progressive rate springs in the rear.

PROGRESSIVE RATE SPRINGS: The first progressive rate springs appeared on Studebakers in 1961. Mercedes-Benz began using them in the 1980s. Simply stated, they have more and closer loops on one half than the other half. The half with the most loops is softer and compresses first, thus being “progressive.” Cost is the same to make, and they particularly compliment the 300SL’s swinging rear axles. While not authentic I’ve yet to hear of any 300SL being faulted in a Concours for having progressive springs. In fact, I’ll plead Gullwings deserve a “free pass” for safety reasons. Here’s why:

COUPÉS: You will recognize the Workshop Manual line-drawing on the next page of the Gullwing’s rear springs. They are a primitive effort at progressive-rate springing that Mercedes initiated back in the 1930s. The idea seems fine but sudden engagement of the inner spring is not fine.

As shown on the box chart, “textbook” ways to reduce oversteer are softening rear springs and/or tire pressure, stiffening the front springs and/or tire pressure, or a thicker antisway bar. Now think, what happens if you are drifting nicely in a corner when the rear tires hit road unevenness that engages the inner spring: the rear suspension gets suddenly firmer, increasing oversteer … exactly what we don’t want. What did Mercedes do to improve handling of Roadsters? The standard deflection rate was changed from 21.8 mm to 39.3 mm per 100 kilo (twice as soft) and the inner spring was eliminated! A HUGE change, essentially confirming what we’ve always known. To be kind the inner-spring is counterproductive. What to do about it? One option is simply remove the inner spring.

Granting that I sell springs, I wouldn’t otherwise have studied the issue this closely. I’m convinced that for Gullwings a valid safety upgrade is replacing the four (inner and outer) rear springs with two progressive-rate springs calibrated to remain softer than the front spring(s) regardless how much they are compressed. If the front is concurrently changed to a “sport” rating (to reduce nose-dive) it also serves the objective of always keeping the front firmer than the rear. You will sense the difference immediately; it is not a harsh change and definitely reduces the risks of sudden oversteer. A side benefit is eliminating the “broken axle” look of Gullwings when at rest or on display. Progress-rate rear springs are installed at or near zero degrees of camber, just as Roadsters. Being progressive, they go into negative camber more easily in medium speed corners.The reason Gullwings must start out at 3+º negative camber is to compensate for being too stiff and the axle’s high(er) fulcrum point!

ROADSTERS: Both the Gullwing and Roadster suspensions, front and rear, were initially designed for the much larger, slower 300 and 300S bodies. Small wonder they were not perfect for the 300SL. The Roadster’s single, low-pivot differential (debuting on the 300S) missed being included in Gullwing production engineering commitments by only six months in 1954. It debuted in the 1955 300Sc and 300SL in 1957. The low, single pivot concept in lighter scale, served M-B cars for the next 18 years in every series up to and including the 280SE 3.5 Convertibles, the 6.3 and 108/108 Body into 1962.

Equalizer Springs: A 1960s debate among M-B engineers is evidenced by widely varying equalizing spring deflection rates. When introduced to the 300SL Roadster the deflection rate was mellow 2.1 inches (57 mm) per 100 kilo. Conversely, the 1960s 220 SEb was assigned an equalizer spring with only .4 inch (10.1 mm) of deflection per 100 kilo., almost eliminating it’s function. When the Pagoda SLs arrived with identical wheel base and weight to the 300SL Roadster, its equalizing spring rate per 100 kilo was 1.2 inches (29.4 mm). While distance between the axles’ fulcrum point and the axle’s springs would change the spring rate, there appears to be little agreement for the role of these equalizer springs. I‘ve tested 1) 57mm (original), 2) 29mm., 3) progressive rate equalizer spring (26mm to 17 mm) on my own car. I prefer the latter two. 56mm is too soft and busy.

Front Springs: Original front Roadster springs are the same stiffness as Gullwing: 19.3 mm., but not the same part number due to different body weights. Gullwing sport springs are 14% firmer, easily welcome on Roadsters.

Rear Springs: Originals rears springs have a 39.3mm. deflection rate. A progressive rate starting at 26mm and progressing to 18 mm. preserves a comfortable ride without the supersoft ride that is so common in Roadsters today.

Shock Absorbers: Most important is to understanding the job of shock absorbers. It is to mute the rebound action of springs. Shock absorbers do not “carry” the car, that’s the job of springs. Other than that just be sure they work. I have no brand preference in shock absorbers.

Get custom Sport and Progressive Rate Springs for your Mercedes-Benz from the SL Market Letter by visiting our Custom Springs Page.