sny928

Hi,

I'm in the process of replacing the shocks and springs on my manual Euro 928s -81. The front shocks are Bilsteins installed by the PO (keeping those) but the rears and all springs are the original ones.
My plan was to get "harder", 600/400 springs but when I talked to a suspension guy he said the dampers have to be matched to the springs in order to work, which sounds reasonable. So I'm just wondering if you guys who have other spring rates have had your dampers modified or not?
Is it a waste of work (and money) getting harder springs without matching the dampers?
The car is BTW not my DD but I might do some tracking in the future.

Thanks,

Sverker

Adk46

I am familiar with the basic engineering principles involved - the effect of mass, spring rate, and dampening on the response curve of a system given a transient displacement - but zero practical experience such as an expert has in setting up race cars. But I'll comment anyway.

It all comes down to what you want to feel in your butt, and what makes you smile when going around a bumpy corner, and all the other perceptions you like and don't like. Unless you have a specific road and goal in mind, it will be a complex compromise. It will take experimentation, which you probably can't do. To take advantage of others' knowledge will require knowing what they were trying to optimize for their roads and goals, and translating what they report to what your own perceptions would be.

Somewhere in this gibberish is a point... oh, yes: for me, staying with a stock setup makes sense, if only because it was something that Porsche engineers did a lot of experiments in order to optimize a system best suited to this proverbial "true GT". As far as I know, my roads are similar to theirs. However, if I lived near Detroit with it's miserable busted-up concrete highways, I might seek a more "Buick-like" ride, etc.

Mrmerlin

Curt that was a great post
One thing that sticks in my mind was a comment by Greg Brown,
he said "I replace the aftermarket springs with the stock springs on most of my customers cars,
all the springs do is shake the fasteners loose"

Unless your driving on billiard ball smooth roads or going racing making the chassis stiffer with springs will not be an enjoyable path/
I have been very happy with the Bilstiens and stock springs on my 85 Euro and my 88 S4 slow riding is quite firm but hi speed handling is great for road driven cars.

sny928

Thank you guys for your input! As I'm not going to race with the car I definitely don't want to loose my teeth everytime a small bump is hit. I think I just want a more firm ride than what I have today. On the other hand that's probably due to the +30 years old present springs. So maybe stock is the way to go then.

/Sverker

Rob Edwards

I have driven/ridden in many different combinations of Koni/Boge/Bilstein/Moton shocks on stock/clubsport/eibach/hypercoil springs, with 16/17/18/19" wheel/tire combos. The best of them all for driving in the real world is Boge Reds on stock springs and Michelin PS2's on 17" wheels.

sny928

As a set of 18" Cayman S wheels is on my wish list that may also speak in favor of staying stock.

GlenL

Too much sense in this post for the Internet!

To the original post: yes, stiffer springs need stiffer shocks to control them. I use adjustable Koni's with stiffer springs. Uncomfortable on rough roads but I track the car and accept the compromise.

Stick with the stock set-up for regular use. You may like the "Bilstein/Eibach" setup with stiffer multi-rate springs and a bit more shock control. Otherwise, the pure stock system will be good. If you're going to bigger wheels, and shorter sidewalls, it'll firm up the handling and ride. In that case I suggest sticking with the stock system, saving a few bucks there, and getting nice wheels and tires. I do really like my Cayman 18" wheels, btw.

UpFixenDerPorsche

Hi Sverker,

Your suspension guy is right.

Taking rebound first: increasing the spring rates will require more rebound damping otherwise the compressed spring force may overcome the damping force with the result that the car is bouncy.

Rebound adjustable dampers like Koni can be adjusted to suit, within their control range of course.

Next: bump rate: there is damping applied as the suspension compresses to prevent the wheel completely bouncing off whatever it hits. Hence the term 'bump' rate.

The bump force chosen is designed to make the tire flex a little as the wheel moves up so as to increase grip. Once the up force exceeds a pre-set value (U hit a big bump) the bump valve opens to release fluid so you don't get hydraulic lock and knock your teeth out.

This is what causes the hard ride that everyone complains about, and NOT harder springs. Especially if Koni Reds are used. (More on that later if you want).

My GT was set up for track work by the ppo: 1000lb fronts, 800lb rears, with Koni Reds revalved to suit: more rebound damping, but, most inportantly, lower bumps rates.

The roads here have all shapes and sizes of pot-holes and pot-humps. With say a 2" or 50mm vertical edge I know the wheel has run over something big, but there's no bone jarring crash through the car

So when ppl tell you that harder springs = bone jarring ride, they are completely WRONG, as are their suspension wrenches.

A spring will always compress, but a hydraulically locked shock WON'T. May as well have a solid bar there instead.

HTH.

UpFixen.

ROG100

In my experience stick with stock springs. They are actually quite reasonable to buy at circa $300 a pair front and rear. All springs are the same now at least 87 to 95. Instead of the matched color codes they have one part number for the front and one for the rear. I believe this is because manufacturing of springs is now a much more accurate process.
I often have customers asking for GT springs but they never existed apart from CS fronts.

sny928

Thanks UpFixen for the thorough explanation! Now I'm beginning to understand a bit more (how complex this is..). I think I'll go with stock or maybe one level harder (500/350) and take it from there.

/Sverker

Bilal928S4

If you have original shocks, they probably totally shot. Just change the shocks and see if you like it, before spending money on springs.

sny928

The front shocks were changed a few years ago by the PO but the rear shocks and all 4 springs are from -81 and +150000 miles so I figure it's a good time change everything while I'm in there replacing the lower ball joints.

jpitman2

I have M474 sport suspension from new (83). Replaced/rebuilt the shocks inside last 15k miles. The ride is a bit harsh on broken/poor pavement roads, but I can run over rail crossings comfortably (brief rumble), but at speed its like a magic carpet ride. Standard 16x7 rims, run at 38-40psi all round. I would suggest you find some cars that you can at least get a ride in, with similar suspension that you are considering, and see what floats your boat - an acceptable ride/handling compromise can be hard to achieve without experiencing first hand IMHO.
jp 83 Euro S AT 56k

Adk46

My rear shocks were clearly shot. I think these are most important since they seem to amplify any bounce of the front wheels. I changed them (Boge, OEM), and that fixed 90% of what bothered me - the bobbing motion. I'll do the front, then I think I'll be happy driving my GT on my mostly smooth, nicely twisted roads (absent frost heaves, that is).

To change them, by the way, I got the struts out without much trouble (be kind to the wires!). Then I took them to a garage to do the spring compression business, insisting they keep everything aligned (they did). Putting them back in took another set of hands and considerable nudging and fiddling, but it is a very happy moment when the Big Bolt slides back in. If you give the washers a mean stare, they will stay in place as you do this.

sny928

The car has been sitting on jack stands for 2 weeks now and I've started the disassembly process i.e. running back and forth between the laptop (Rennlist forum) and the garage . There is a first time for everything!

/Sverker