Are these progressive or linear springs?

The coils look to be all same thickness,  but does the tapered shape cause progressiveness also?  These are tt quattro rear springs. Kw v1

i found some threads claiming they are progresssive but looks linear to me  

Wow those are short! I imagine they’re linear until the bump stop, which is almost immediately, so they act progressively. ;)

Coil diameter is one of the parameters that effect spring rate so they're progressive.

In reply to APEowner :

If I compress that conical spring without binding it, the diameter of any given active coil stays the same. Since the mean diameter of the active coils is unchanged, their contribution to the spring rate remains so as well. Thus it would be a 'linear' spring.

A coil spring is simply a smartly packaged torsion bar, that has been wrapped around on itself. The mean diameter of the active coils combines with the number of active coils to provide the wire length contribution in the formula. Generally speaking, as long as the length of wire being twisted is stays the same, so too does the rate of the spring. What really makes a spring rate progressive is the shortening of the length of active wire as the amount of it in 'bind' increases.

Which is not to say that bind occurs simultaneously across all active coils of the spring still. I doubt anybody here could accurately determine that from this picture, so there certainly could be some progressive nature near the end of the travel. But it does generally appear to be predominantly linear for at least the substantial  majority of its travel. What the conical shape does do is increase the travel before said binding occurs.

OK you guys are contradicting eachother. one person says that the diameter change of the spring makes them progressive.

Another says that the "mean diameter of the active coils" means they are linear.

Which is it? 

Also I don't know if saying that a spring is "just a torsion bar wrapped on itself" is correct.
A torsion bar is a wire that is twisted. A spring is a wire that is bent. Pretty big difference.

In reply to CyberEric :

Yes. the audi tt and vw r32 rear suspension is an odd trailing arm setup.
The springs must be SUPER short if you want the car lowered properly.
And there is a crazy motion ratio (or is it spring ratio?) as well. Which makes the rear springrate not as effective.

IE you need really high springrates in the rear.  Which means you'd need even shorter springs for the same ride height.

I think like 4" springs are pretty normal for these cars.

But bumpstop, and suspension travel have never been an issue so I suppose it's no big deal.

In reply to Driven5 :

There is a range in which a conical spring is linear but they go non-linear before coil bind.  What happens is that the as the load increases and the spring compresses the force is eventually high enough that the stiffer coils are activated. Graph

Ok. Good answer.  That explains a lot.  I guess i have to go back to hunting for a standard 2.5 id race spring.

those graphs make a lot of sense.

One question pops up.

i have some softer and longer linear springs lying around.

does it do them harm to cut them down to proper length?  I would want the increased springrate.   And i could cut 5mm top. And 5mm bottom without changing the springs perching shape.

now assuming that i dont heat up the springs any while cutting.

is there any hidden downside to doing this?

Driven5 said:

In reply to APEowner :

If I compress that conical spring without binding it, the diameter of any given active coil stays the same. Since the mean diameter of the active coils is unchanged, their contribution to the spring rate remains so as well. Thus it would be a 'linear' spring.

A coil spring is simply a smartly packaged torsion bar, that has been wrapped around on itself. The mean diameter of the active coils combines with the number of active coils to provide the wire length contribution in the formula. Generally speaking, as long as the length of wire being twisted is stays the same, so too does the rate of the spring. What really makes a spring rate progressive is the shortening of the length of active wire as the amount of it in 'bind' increases.

Which is not to say that bind occurs simultaneously across all active coils of the spring still. I doubt anybody here could accurately determine that from this picture, so there certainly could be some progressive nature near the end of the travel. But it does generally appear to be predominantly linear for at least the substantial  majority of its travel. What the conical shape does do is increase the travel before said binding occurs.

You actually get a pseudo-binding effect very early on as the tighter coil starts laying flat against the mount. As this lays down, the diameter effectively increases, increasing rate. You can see how each coil can basically sit inside the next as it compresses, providing much more travel than a straight helix which would bind where the tapered spring still has un-bound coils available after the softest ones have bound. 

The tapered shape itself does not change the rate.  However, as the coil compresses and more of it sits against the seat, the effective length of the spring changes, making it stiffer.  Same idea as longer coils that have several coils very close together.  Compress enough that the top 4 coils bind, the spring is now much shorter, and higher rate.

And coil springs twist just like a torsion bar, they don't bend.

tl;dr: Progressive springs have some parts of the spring go into coil bind before others,which is how they are progressive.  If it looks like the coils can compress evenly and go into bind at the same time (or not at all before the suspension bottoms out) it's effectively a linear spring.

In reply to Tadope :

Cutting without heat will be fine. I just don't think you are going to get into active coils only cutting that little off the spring so you won't raise the rate.  A picture of the spring would help.

They're just like these

https://www.ebay.com/itm/232394872329

I suppose it would be nice to drop the ride without rate change as well. But would be could to know what is possible

Those are called square end springs. If you cut anything off the ground ends it will affect how it sits on the perch. You could cut the ground end off one end and end up with a tangential end. It might get you the 1cm shorter spring you are looking for though.

Changing the end design will change the rate a little. The amount of live coils will change and you will stiffen the spring.

In reply to jgrewe :

What If I only cut 5mm of the end?  The coils are 11mm thick.  It would still lie flat on the perch.

My only concern is that the thin tip would be super thin. Or perhaps not resting perfectly on a higher coil anymore. Right now it barely touched.  With a light grind there might be a few mm gap there.

I don't know how these springs react to that kind of thing.  Would they deform under load?

As you can see there is room to grind a few mm off both ends, and the perch is virtually the same. But the wires tip would be very thin