Eli K

Could someone explain or point me to a source of information about the benefits or drawbacks of doublespring suspension? I've googled it, searched this and several other boards to no avail....

Bull

Are you talking about a coil spring suspension where there are two springs of different rates, stacked together?

JackOlsen

You might mean two things.

One is adding helper springs to the (rear, or I guess front) shock towers to stiffen the overall spring rate.

The other is to add a tender spring (or in some cases an 'active' tender spring) underneath the main spring in a coil-over suspension.

The first one is pretty simple. For racing, Porsche needed more spring rate than torsion bars could provide, and using both torsion bars and springs was a good way to 'interpret' the rule book in Porsche's favor.

The second is for one of two reasons. A simple tender spring keeps the main spring from moving out of its place when the suspension is fully unloaded, like when you jack the car up and the wheels are allowed to drop all the way to the end of the suspension's travel. If the suspension's travel is longer than the spring, the spring can move unless there's something to hold it in place -- the tender spring. The 'active' tender spring provides the same function, but isn't fuly compressed when the car is at rest. As such, it's like the first stage of the car's springs, and it's softer than the main spring, which means you get an effect sort of like the progressive springs that some modern coil-over systems use. It's more gentle on the little bumps, but not liable to compress as much uner a lot of load, which would happen if you had one soft spring all by itself.

Eli K

Yep. This set up to be exact...

kurt M

The thin slinky looking springs are the tender springs Jack mentioned and keep things in place when the susp is unloaded. There is also stups where the smaller springs do some work and make the sprng rate more progressive.

Eli K

Sheesh Kurt...wish you'd dumb it down for us common folk ...
I'm trying to guage how these compare to your run of the mill H&R, eibach springs, etc. Are these more favorable to have on the track, street driving?

TD in DC

Unless I am misunderstanding their point, they are saying that the tender springs on your shock are most likely for main spring position management rather than for dual compression rates. As such, the existence of the tender springs most likely are more or less "irrelevant" with respect to whether the shocks are more or less favorable to have on the track or for street driving. The main springs are doing all the real work on the track.

JackOlsen

If they're simply tender springs, like the ones shown, there's no effect on performance at all. If they're active tender springs, you get a kind of two-stage spring rate. It won't change the car's performance capabilities, but it will make things slightly more comfortable at low speeds.

Steve Weiner-Rennsport Systems

Well said, Jack,......

Active tenders can make a significant difference in the overall spring rate, depending on the chosen rates.

For those who have driven a 993 with PSS-9's, thats a good example of this effect with its 400 lb main/685 lb tender rear configuration. Nobody can accuse that setup of being uncivilized,....

mds

I find it strange that stacking two stiff springs results in a rather soft equivalent. But that is what the math says. In particular, 400lb/685lb main/tender combo is equivalent to a 252lb spring until the tender becomes coilbound, and then reverts to 400lbs. So its not surprising that it rides softly.

Geo

OK, let's correct some terminology here....

Tender springs are high rate springs that are used to create a dual rate compound spring set-up. When the tender springs coilbind the main spring rate is the rate in effect. Until then, the compound spring rate is in effect.

Helper springs are low rate springs used to keep the main springs seated in a coilover set-up.

Never heard the term "active tender" before. It's either a helper or a tender.

http://e30m3performance.com/tech_art...ings/index.htm

I further doubt that Porsche could not get enough spring rate from torsion bars.

kurt M

Interesting link and thread. I have heard the terms used in reverse of the link and interchangeably as well. I guess it does not matter what we call them as long as all use the same term for the same thing.

Eli, Call me and i'll explain this post to you in little words

Geo

They do get used interchangeably, but that is in error. If you call Eibach and ask for about tender springs they will think the high rate springs used for compound spring rates. If you call and ask about helper springs they will think about the low rate springs that keep the main spring seated.

As anything what we call them only matters if we want to understand each other.

Geo

To asnwer your questions, the benefits are the ability to set up the suspension with relatively lower springs rates for initial travel and then switch to a higher rate. Why does someone do this? To be honest I can only speculate - probably good speculation - but I cannot pretend to say definitively.

The drawbacks? Extra cost. Extra complication. Potentially sudden abrubt changes in spring rates that could upset the suspension balance.

I'm sure there are folks on this board who have played with this sort of set-up with a real plan. Perhaps they can chime in.

For me, managing multiple spring rates like that is a little much for my monkey brain.

JCP911S

Spring rate is a factor of two things... the inherent torsion of the material used.. (e.g. the gage and material of the spring) and the length. As you increase the length of the spring the rate goes down.... Stacking springs increases the overall length