Coupe Springs - dimensions - rates - OE - H&R

[twoqu]

Ok, getting peed off with companies not supporting the S2 Coupe or suggesting that their "one type fits all" approach!
Seems rather short sighted of companies such as H&R only supporting the Avant/RS2 platform when at least half the components would work on the Coupe!
Likewise, nearly every other company offers springs with too great a drop (40mm+)and don't seem to appreciate that different models need different rates.
There are snippets of info on here but need to sort of condense them. So if people have fairly accurate measurements of any of the following can they post them here.
Some of the critical dimension are:
1) Wire diameter
2) Number of coils
3) Coil diameter (OD)
4) Free height.
We can determine some of the other stuff like coil mean diameter by subtracting the wire diameter from the coil diameter. That should allow us to get close to calculating the spring rate.

I know the H&R TUV documentation lists the wire diameter and number of coils for the front and rear of the Avant 29909 and the RS2 29969. But that is all the info! Bearing in mind I am looking at Avants, then the fronts are 14mm wire diameter and 7.5 coils.
Looks like the RS2 is quoted as having 305mm free length with the same wire diameter and number of coils! Ok, we are getting somewhere!

[twoqu]

The knackered ones off the front of mine spec out at the following:
1) 14mm
2) 7.5 (see below)
3) 145mm
4) 290mm

One difficulty is in counting the coils. I am not sure what effect the reduced diameter at either end is going to make or if it effects the coil count.
Going by a previous thread it almosts sounds like these could be rear as opposed to fronts! Wouldn't surprise me at all!
If you have a good shot of a front H&R or Std spring that might be useful too as we can compare them in design visually.

[mcandmar]

Just to add to the complexity the H&R springs are variable rate, the closer wound coils are actually softer than the open wound section. Attached pic shows a Coupe and Avant front spring side by side and from memory using my own weight the yellows compressed more then the blues, but visually they look the same.

I think the only way to compare like with like is with a use of a spring tester, i.e. apply a fixed weight/pressure and measure the compression.

[twoqu]

Agreed! This isn't going to be an exact scientific experiment but nonetheless at least its empirical!
Interesting about the comparsion, thanks for the pic. If the springs are identical in construction and all the variables listed than they should be the same rate. At least we can see the coil count method is ok, basically every complete coil and due to the fact the ends are at 180 degrees, thats where the half comes in!
Not sure how you would begin to assess a progressive rate spring against a linear one. Presumably its going to be something like the amount of coils in a certain distance or the relative gap between them that we would need. Guess we really need on off the vehicle spring to measure up.

[twoqu]

Thanks to Acidburn I can now add some standard data. Obviously these are used springs so the free height values might be a bit out when compared with new ones. Other than that the other values should be ok for comparsion.
Front spring non a/c (p/n 895 411 109)
Colour code orange/orange/blue/white
1. 14mm
2. 6.5
3. 140mm
4. 305mm

Rear spring (p/n 895 511 115)
Colour code silver/silver/orange
1. 14.8mm
2. 6.5
3. 145mm
4. 285mm

So it is looking like the PO put the rears on the front and they were weedy rears to boot!
Curiously then the only difference between the front springs and the H&R lie in the extra coil and their progressive nature rather than linear. Preumably most of the drop must come from the progressive part of the spring with the rest effectively being stiffer. Clever stuff!

[fozzy]

any one suggesst where i can get a set of springs for a 2.3 20v quattro coupe 1990 , i tried euro car parts , ces, cause we deal with them , and i get a plain "no dont do them for that model"

[twoqu]

Found some data for the correct S2 Coupe H&R 29939 VA/HA blue springs.

Front:
Wire diameter: 13.5mm
No of coils: 7.5

Rear:
Wire diameter: 13mm
No of coils: 6.5

Interestingly the Coupe has smaller diameter wire than the equivelent Avant front springs!

[mcandmar]

Found these links for the H&R springs...

http://www.h-r.com/bin/29909.pdf S2 Avant Yellows

http://www.h-r.com/bin/29939.pdf S2 Blues

http://www.h-r.com/bin/29969.pdf RS2 Black

[Rusty]

Interesting that the coupe and avant springs are different diameters, that will make a difference for sure! Also interesting that the Avant and RS2 items look to be the same!

[Rusty]

Coupe information taken off my OE springs

http://www.s2forum.com/forum/showthr...=front+springs

Also HnR data taken off my 29939 HA/VA springs:-

I've done a bit of measuring and have come up with the following numbers:-

Front
D = Outside Diameter = 139mm
d = Wire Diameter = 14.1mm
l = free length = ??
c = no. of coils = 6.5

Rear
D = Outside Diameter = 145mm
d = Wire Diameter = 14.7mm
l = free length = 290mm
c = no. of coils = 6.5

Now then, here's the science bit as I see it,

Assumptions:-
The OEM springs are not variable rate
The OEM springs are made from the same material
Both front and rear springs have equal numbers of coils (I've counted them and this is true).
The spring rate of a helical spring is proportional to d^4/D^3 using the assumptions above and that they have the same number of the coils. Using this formula, the rear springs have a spring constant approximately 4% stiffer than the fronts!

Here's the HnR blues that I've just measured:-

Front
D = Outside Diameter = 148mm
d = Wire Diameter = 13.7mm
l = free length = 310mm
c = no. of coils = 7.5

Rear
D = Outside Diameter = 152mm
d = Wire Diameter = 13.7mm
l = free length = 280mm
c = no. of coils = 6.75

The OE springs have 6.5 coils but the top 4 coils on the front HnRs and 3 on the rear HnRs the HnRs will lock very early, thus they will act more as though they have around 4 coils. The top and bottom coils don't do anything really so could call it 4.5 and 2.

Using the d^4/D^3 the relative relative stiffness of the helix (arbitary units, to clear things up I've removed the decimal places, the numbers are not spring rates) is:-

OE Fr 148 (23 for 6.5 coils, 37 for 4 coils)
HR Fr 109 (14.5 for 7.5 coils, 28 for 4 coils, 55 for 2 coils)

OE Rr 153 (23.5 for 6.5 coils, 38 for 4 coils)
HR Rr 100 (14.8 for 6.75 coils, 25 for 4 coils, 50 for 2 coils)

However, this is 6.5 coils vs. approx. 4 so they will be stiffer when the top coils lock. but are softer to that point using simple helix theory.

This would seem to back up your results and that when the top coils compress the springs get a lot stiffer ( your graph is starting to show that).

Also, it's worth noting that the HnR springs looks to be around 10% softer at the rear when compared to the OE setup where the springs are 4% stiffer at the rear! Just what's reaquired on a nice understeery car to begin with . Glad I've got that 26mm RARB to play with

HTH

[Rusty]

With regards to the different coil spacings, they all twist by the same amount but what happens is that the top close coils bind, thus preventing any more deflection on these coils. After this the number of coils that the load is shared over is then reduced, thus there are less springs working in series (if you imagine each whole coil as an individual spring) so the spring rate increases due to the inverse addition of springs in series.

For an individual spring with constant K:-

For 7 coils you have 1/7K

For 4 coils you have 1/4K

Thus, with the coils bound, you have a spring quite a lot stiffer.

From Lloyd's thread here, he quotes the HnR spring rates for an Avant as:-

At last, a very helpful person has confirmed actual H&R RS2 spring rates.
They are progressive and rated as follows:

FRONT 43 N/mm [16.2% stiffer than OEM, 37 N/mm]
REAR 55N/mm [5.8% stiffer than OEM, 52 N/mm]

The conversion to lb/in is x 5.5

So

Front - 43 x 5.5 = 236 lb/in
Rear - 55 x 5.5 = 302 lb/in

As spring rate is governed by d^4 / D^3 and the avant springs have a marginally larger d (14 vs. 13.5) the coupe spring rate is IIR 90% of the Avant items, thus around 200 lb/in.

The reason for the much stiffer rear springs (comapred to the coupe) is that the Avant has different suspension front and rear.

All of this point towards a pair of Avant front springs as being the better choice for a coupe IMO (for performance, not ride) and that sentiment is helped by the relative cheapness of the Avant springs compared to the Coupe.

[MGaz]

Interesting reading, don't suppose anyone knows the standard spring rates for the heavier c4 URS6?

With regards to the different coil spacings, they all twist by the same amount but what happens is that the top close coils bind, thus preventing any more deflection on these coils. After this the number of coils that the load is shared over is then reduced, thus there are less springs working in series (if you imagine each whole coil as an individual spring) so the spring rate increases due to the inverse addition of springs in series.

For an individual spring with constant K:-

For 7 coils you have 1/7K

For 4 coils you have 1/4K

Thus, with the coils bound, you have a spring quite a lot stiffer.

From Lloyd's thread here, he quotes the HnR spring rates for an Avant as:-



The conversion to lb/in is x 5.5

So

Front - 43 x 5.5 = 236 lb/in
Rear - 55 x 5.5 = 302 lb/in

As spring rate is governed by d^4 / D^3 and the avant springs have a marginally larger d (14 vs. 13.5) the coupe spring rate is IIR 90% of the Avant items, thus around 200 lb/in.

The reason for the much stiffer rear springs (comapred to the coupe) is that the Avant has different suspension front and rear.

All of this point towards a pair of Avant front springs as being the better choice for a coupe IMO (for performance, not ride) and that sentiment is helped by the relative cheapness of the Avant springs compared to the Coupe.