I've recalc'ed the spring rates once again, this time based on detailed spring data from the MB Technical Data manual, as opposed to pictures in the Service Manual and rough measurements made while coils are installed in the car.

Calculation inputs are from the Technical Data manual. Calculation formula is the standard formula available on any spring rate / and most spring mfg'ing co's web sites (formula shown below). The calculated spring ratesw are 1% - 2% less than the reported (MB Tech. Data manual) spring rates.

The design values are shown for front and rear springs respectively for the 250SL/280SL versions

Active Coils = 9.75 & 5.5 compressed to Curb Wt

Wire Diam = 0.614" & 0.622"

Coil OD is 4.94" & 5.35"

Mean Coil Diam. = 4.33" & 4.72"

Parenthetically, Free Ht of springs are 15.26" & 11.40"

Compressed Ht of springs are 9.96" & 8.43" at 672 & 550 lbs load/spring respectively.*

*Note that the loads at curb weights with and without the hard-top do not match the above stated compressed spring loads for the compressed spring heights given. The sum of the front and rear spring loads at the compressed heights given above are 18.3% less than the curb wts given for the roadster without the hard-top, and 21% less than the roadster with hard-top installed. Therefore, the additional spring compression will occur over and above the compressed heights given above for the standard 250SL/280SL at curb weights with and without the hardtop installed.

At Curb Wt, without hardtop the spring loads are 792 lbs & 704 lbs/spring (front & rear respectively). Therefore, an additional compression at curb weight, without hard-top installed, of 0.475" & 0.424" are expected. This would put the compressed spring heights at curb weight

without the hard-top at 9.5" and 8.0" (front & rear respectively).

The hardtop weighs 45 kg = 99 lbs (230SL/250SL/280SL) based on subtractions of the car's weights given with and without the hardtop in the Technical Data manual. The distribution of curb weights to front and rear axels with the hardtop installed are not provided. The distribution

without the hardtop is 53%/47% for the 250SL/280SL, & 53.7%/46.3% for the 230SL.

Assuming the commonly used modulus for coil spring steel,

Calculated (see formula below) Spring rates as installed are:

Front:

**252.7** lbs/inch (255.7 MB Reported).... Calculated is 1.2% less than Reported... equivalent to 0.12 less active coils.

Rear:

**363.0 ** lbs/inch (370.8 MB Reported).... Calculated is 2.1% less than Reported ... 0.12 less active coils.

There are several coil spring calculators on-line. I used

http://www.auto-chat.net/help/math22.shtml The formula is (applies only for

**vertical** springs... any off-vertical mounting angle requires an adjustment which is significant):

Spring Rate (lbs/inch) = TM x WD^4 / 8 x NC x CD^3

Where:

TM is torsional modulus of spring material (11.25e^6 psi **);

WD = Wire Diameter (inches)

NC = Number of free coils

CD = Mean Diameter of Coil Spring = OD - WD

** Modulus shown is commonly used in all coil spring rate calculators... I have no knowledge of whether or to what degree this modulus applies to the MB springs or any other for that matter, but with the calculated values within 1% - 2% of the reported values there's no reason to believe the modulus is any different than the commonly used modulus.

Assume the estimated spring rates are approx. correct, then the questions as they relates to any progressive spring are

1) how much does the static spring rate change;

2) at what load/displacement does the spring rate increase;

3) to what spring rate does it increase;

4) is there another stage or more and at what displacements/loads does the other stage(s) occur, and what are the spring rates.