S3 rear suspension ride height.

[nmauduit]

The original are 14.7" long while the new ones are 16.2". The spring rate is the same for both springs

1.5" extra length sounds about right for a plus2 spring from what I've seen elsewhere on the forum (same wire and stifness) ... then your original ones don't appear overly rusted : why not giving them a good clean and use them, the car ?

[rgh0]

The number of coils and coil diameter and wire diameter determine the spring rate not the spacing of coils and overall length. In the photo the coil count is the same and so is coil diameter so for the same spring rate the wire diameter should be the same.

A longer unloaded spring will end up at a longer length when loaded with the cars weight hence the higher rear ride height.

The spring end details also affect the height under load depending how the final coil has been flattened or ground to match the spring supports.

cheers
Rohan

[davidj]

The original are 14.7" long while the new ones are 16.2". The spring rate is the same for both springs

1.5" extra length sounds about right for a plus2 spring from what I've seen elsewhere on the forum (same wire and stifness) ... then your original ones don't appear overly rusted : why not giving them a good clean and use them, the car ?

Yes, I did think about it and could have cleaned up the originals and they probably would have been fine. However, as all the suspension on the car is new, I could not bring myself to fit 55 year old springs!

In answer to Alan's question, the spring rates were the same but the new springs had one more coil, so I guess the wire dia must have been slightly smaller, assuming the same torsional modulus of the material. However, I did not measure it before returning the springs. Doh! However, while the Kelvedon springs were 1.5" longer the car dropped at the rear by over 2" so there must be some mechanical magnification in the suspension or weight has shifted slightly towards the rear compressing the springs more.

[davidj]

Thinking a bit more about this, as the new springs had one more coil and assuming everything else was the same, the wire diameter may have been slightly greater in diameter. However, I did my engineering degree 40 years ago, so I don't mind being corrected!

[nmauduit]

However, while the Kelvedon springs were 1.5" longer the car dropped at the rear by over 2" so there must be some mechanical magnification in the suspension or weight has shifted slightly towards the rear compressing the springs more.

there is indeed, as the springs are not mounted on the wheels but on frames, leveraging the effect by the ratio of force application points/segments lengths

as for the "old" springs, if once in place they put the car at the correct height, to me this is yet an other case where new parts may not be better than old...