aftermarket spring kit spring rates

[sambb]

Hi all,

For those that are interested in what the actual spring rates are of the springs in commonly available spring kits versus stock, here is a bit of info:

Stock OEM - measured in a spring press
front - 26.6N/mm = 152lb/in (green green orange orange orange) - in eibach sydneys spring press.
rear - 25 N/mm (*5.710) = 142 lb/in (orange - white - white - orange)

H&R 30/35 drop spring kit 29333-3 (by email to germany)
front - 27 N/mm
rear - 27 N/mm

Eibach 20/15mm drop Pro Kit (by email)
front - 27 N/mm
rear - 23.5 N/mm

Weitec ( green springs) - measured in a spring press
front - 27.5 N/mm = 2.80 Kg/mm = 157 lb/in (#VW582VA/000)
rear - 22.5 N/mm = 2.29 Kg/mm = 129lb/in (#VW582HA/000)

ST by KW 20/15mm drop spring kit (#28280075) - by email to KW germany
front - 27 N/mm
rear - 22.5N/mm



anyway...food for thought for you all

sam

[sambb]

Edited the first post. A dumbass oversight that needed correcting. When I got the weitecs back off the the mechanic with the spring press, they'd put the labels on the wrong springs ... got it backwards. So when I copied the figures down I got them backwards until I realised my mistake. sorry 'bout that. I've made the correction in the first post. The fronts are in fact stiffer than the rears which makes more sense.
Now I'm doubting the figures that H&R told me (in emails to 2 seperate people at H&R though?). I've asked them to triple check.

[PoloGIT]

To add to this, I've just finished installing H&R springs into mine and the drop (after about a 5km drive), by my calculations, is 26mm at the front and 36mm at the rear. I'm hoping she'll settle some more over the coming weeks and I'll get close to another 10mm out of the front...looks great at the back.

This might be of interest as some people (or some manufacturers) think the drop is based off the standard (non GTI) car.

[sambb]

Just an update to the spring rate figures. I removed my stock Gti rear springs (orange - white - white - orange) and had them measured in the same spring press as where I had the Weitecs tested. Get this, they came back as stiffer @ 25 N/mm (142 lb/in).
I doubted this - how could a stock spring be stiffer than an aftermarket lowering spring?
To recap the weitec rear was measured @ 22.5 N/mm (128.5 lb/in). Putting them side by side I could maybe see why. The weitec wire diameter looked slightly bigger but it looked to have one extra coil and wider coils too.

So I did some measurements and some spring rate calculations.

Stock: wire di = 0.413in
# active coils = 5.750
mean di coils = 3.563 in

spring rate(calc) = 157.3 lb/in versus spring rate(measured) = 142 lb/in

weitec wire di = 0.425in
# active coils = 6.375
mean di coils = 3.787

spring rate(calc) = 132.5 lb/in versus spring rate(measured) = 128.5 lb/in

While the calculated numbers are different to the actual measured numbers, they are atleast vaguely proportionate to them. The calcs back up the measurements, confirming that the stock springs are in fact stiffer.

What can be seen is that the weitecs despite having thicker wire were softer because they had an extra 2/3rds of an active coil but more significantly (because it gets cubed in the formula) a bigger mean diameter to the coils themselves ie. although having a shorter free height they probably have over 100mm of extra wire in them. Longer lengths of spring steel are softer than shorter lengths.

This was then confirmed once I installed them in the car with a pair of koni yellows on their softest setting. The car was understeering slightly more. The konis couldn't be accused of causing this because stiffer rear dampers usually tend cars more toward oversteer on turn in. If you're into the theory stuff its more than likely because rear roll stiffness had been reduced in two ways at once - ride height had dropped (20mm) which had lowered the rear roll centre (geometric roll stiffness) slightly and also the spring rate had dropped too (elastic roll stiffness) ie basically the opposite effect of putting on a rear anti roll bar on a stock car.

Moral to the story - don't assume that those aftermarket lowered chunky looking springs are going to always be stiffer than the OEM ones. If you're happy with a stock ride height you may be on stiffer springs already!

[sambb]

heard back from H&R Spezialfedern GmbH & Co themselves and they said that the front and rear springs of the 30/35mm spring kit 29333-3 are both 27 N/mm.

To put all the above spring rates into perspective, the current model ford fiesta ST (a very similar platform to ours and universally lauded for its handling) has 30 N/mm fronts and 25N/mm rears. I'm betting our OEM rates are nearly identical. once I get the fronts measured. I was reading an article on the differences between the standard fiesta and the ST. The biggest things were that they used a 70% stiffer torsion beam (rather than adding an external anti roll bar they used higher tensile steel). They even reduced rear negative camber to only half a degree to take grip away from the rear end. At the front they gave the car different spindles. Apart from having shorter steering arms on them to give a shorter rack ratio, these spindles raise the front roll centre due to lower ball joint positions. The effect of this was that geometric roll resistance was increased compared to the standard fiesta allowing them to get away with only slightly uprated/soft 30 N/mm front springs and actually allowed them to reduced the size of the front anti roll bar compared to the standard car. This really shows the importance of having a high front roll centre.

I doubt our rear torsion beam was uprated compared to normal 9N polo's. By the looks of it then, adding a rear bar to it probably makes a lot of sense. We obviously don't have trick spindles so who knows where our scrub radius and camber gain geometry is compared to a fiesta but leaving front ride height where it is would seem to be pretty important considering that going any lower than stock (our control arms are already parallel) moves the roll centre away from the COG and achieves the opposite of what they did when they tuned the ST. So on the face of it I reckon we have a pretty good 'fast street' setup as is and just need a good re-bushes and dampers and to learn to drive FWD better. On paper I just can't really see the sense in a lot of these strut based spring kits.

[noone]

Nice thread.

I'm suprised the front and rear would be the same in H&R given the weight difference over the axles, but I'm not as educated on the matter.

Had my KW V2's installed in my S3 today, they are a progressive design which I found interesting to read up on.

IMO, stay away from upgrading the front sway bar, it seemed to be the tipping point for the Polo becoming too rigid / jiggly, but hard to say for sure as mods were not all done 1-at-a-time. Could have been just an issue with the whiteline or the geometry after lowering as well...

[sambb]

Thats good to know regarding the FARB causing alot of NVH. The stock bar is 20mm from memory but the arms are so short on it that its got to be pretty stiff in real terms, so a jump up to 22/23mm is a big one. I was/am leaning towards stock springs and adding a FARB (rather than going with stiffer springs) to hopefully keep a better ride but from what I'm reading and what you said the ride will get choppy either way. I'm thinking that with stiffer springs the car wouldn't pitch/dive as much under brakes as with stock springs/FARB but hopefully the better damping will take care of most of that. Still can't decide

[noone]

Its very hard to read an opinion online and learn much from it.
I had the H&Rs for a while, they were soft, but unless paired with suitable shocks, they kill the stock shocks (well, did for me).

The Polo NEEDS to be dropped IMO, its narrow width makes it feel tall unless lowered or pitches easily if on tight suspension. If you go too low, the steering geomerry goes to mush.

I've not ridden in a Polo with KW V2's, but they are on sale atm. Very good hardware for the S3, but it is a different car...

If you don't stiffen it enough, you'll always want more. If you go too far, you'll wish you hadn't...

Get a test drive if you can, maybe see if the guy who bought my old car will give you a passenger ride (assuming the ride is still like it was), you might think its ok.

Lots of opinions on Polo.uk as well, but the good threads are a few years old generally.

[sambb]

Just got confirmation from KW on the ST 20/15mm spring kit. Updated the 1st post to reflect this. INterestingly it has the same front:rear spring rates as what the Weitecs were measured at. I could be wrong but this kind of confirms other things I've read that ST's are just rebranded Weitecs. And if thats true, it may also confirm that the spring press readings I'd gotten are accurate.

On the face of it it looks like the Eibach, Weitec or ST kits would probably need a RARB to acheive a similar elastic roll resistance to the H&R kit because they run softer springs in ride - BUT it must be noted that they carry the rear a little higher which means they would have slightly higher geometric roll resistance. Its very possible then that the H&R method versus Eibach/Weitec/St method results in the same total rear roll resistance. Its just that H&R does it with stiffer/lower springs in ride and roll and the others do it with with softer springs in ride and more geometric roll resistance in roll. The later will have a better ride.

[sambb]

I finally fitted my front springs so that I now have the weitec springs listed above fitted all round. Thankfully they barely changed the front ride height and do feel slightly stiffer than OEM.
I took the front OEM springs down to eibach who worked out that the stock springs are 26.6N/mm (152Lb/in). Its all updated in the first post now.
I asked them why the weitecs feel stiffer than OEM when there is only about 10 lbs in it. Since the weitecs were measured in another spring press they said that it could be that the procedure was different to theirs. Eibach looked up the captive pressure the spring is under when the car sits at ride height and also the spring length at this pressure and started the test from there. They said its possible that the weitecs were tested from too early on eg spring seat half coils not taken up properly, which may have underestimated the reading. I'm ok with that though. It feels that I could now probably go back to soft on the whiteline front bar to have similar front end roll stiffness to OEM spring/hard bar setting so that's good in that I have headroom to adjust in now.

With all of them tested then, it seems that:
- H&R and OEM spring rates are very similar F:R, its just that the H&R's are significantly lower.
- The Eibach pro kit springs and ST's are more or less the same for spring rates F:R and ride height drop. They have more front bias to the spring rates compared to above so would likely be more biased towards understeer in steady state cornering provided no other changes eg bars are thrown into the mix.
- weitecs were the only ones done in an independent press. All others were either quoted from the manufacturer or done at eibach who were super professional. By the looks of it they may be the stiffest of the bunch at the front, and feel the same as OEM/H&R at the back.

If you are just starting out with all of this and just to keep things safe if you are intending on going to the track, keep in mind the effect it has when you pull weight out of the car. If you remove the spare, tools, rear seats and run a low fuel load you have 80kg less in the rear of the car. In real terms you haven't altered your suspension at all but in relative terms you have made the rear of the car much stiffer relative to the front - the rear spring rate relative to the weight it is carrying becomes much higher. The combination of your rear springs/rear bar setting that might have felt sporty on the street can suddenly be REALLY rear stiff, especially if you start adding bar at the same time too. Just something to be aware of so that you don't swap ends out there and not understand why/how. When I do the same, my car goes from feeling a tad too soft in the rear to being plenty stiff enough on the track. So for me I don't make any suspension changes between the street and track - just pulling the weight out of the rear moves the F:R bias to where I want it and beyond that tyre pressures can be fiddled.