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      08-24-2020, 12:16 PM   #1
proboner
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Stock vs H&R vs Eibach spring rates

So I just purchased and installed H&R springs this weekend and while I love the drop, I'm now immediately worried they'll prematurely wear my shocks. Curious if anyone knows the spring rates of the stock, H&R, and Eibach springs?

Although I love the drop, if the H&Rs have a significantly higher spring rate, I may have to go with Eibachs. Here are a few pics for you all as well, since I'm loving how this car looks right now!

Stock adaptive suspension:






H&R springs:




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      09-10-2020, 10:22 PM   #2
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Car looks amazing!
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      09-17-2020, 04:14 AM   #3
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Eibach pro-kit rear springs are progressive: 65.5 N/mm initial rate and 131,6 N/mm final rate (374-752lb/in). The HR looks to be stiffer and progressive as well.

I have been trying to find out what the stock spring rates are forever. I run stock springs front in combination with Eibach + spring spacers rear (to get the car back to stock ride height). My intention with this setup was and is to increase the wheel frequency rear while keeping the front stock, hoping to dial out some understeer on track. However I might have just ended up with a shorter rear spring and no significant change in wheel frequency. But I guess the shorter spring would at least mean that the increase in progressive stiffness would happen faster rear compared to the stock front as the body rolls.

If it is any help, below are the 1/3-series rates. They indicate that Eibach Pro springs are pretty close to stock, just shorter. I couldn't find any info on HR.

e90 ZSP or e92
Front: 145 lb/in
Rear: 460 lb/in

e9x Eibach Pro-kit
Front: 148 lb/in
Rear: 456 lb/in
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      01-22-2021, 11:26 AM   #4
proboner
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That's much stiffer in the rear. I'm guessing has to do with rear suspension geometry and spring placement, being placed half way on the lever arm, as opposed to being on the furthest point of leverage in the front.

Ended up with ST XA coilovers and now just have a harsh/bumpy ride. Want to find a coilover with softer spring rates but not running into a lot of spring rate variety. A majority are in the range of 390lb-450lb (7k-8k) front and 615lb-670lb (11k-12k) rear except the Ksports which are 500lb (9k) front and 335lb (6k) rear?

No clue what the current STs are, since they're progressive. Same with most of the reputable brands out there like H&R and Bilstein.

I'm really trying to figure this suspension situation out, because I've got a wheel and tire purchase coming up and this could potentially change how that all works. The questions never end. I wish this community was more active with people who've done more experimenting!
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      01-27-2021, 03:12 AM   #5
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Yeah the placement of the rear spring vs the front cuts the wheel spring rate by a large factor and also limits how high you can go at the rear. The front stiffens up much faster without hurting the comfort as much. This is especially true on the smaller engine Z4s.

I ended up running stock front spring with HR rear + a large spring spacer to avoid lowering the rear of the car. With my front sway bar in its middle setting and my custom rear sway bar in its stiffest I now finally have a setup that feels balanced almost towards oversteery on track.

On the street I would still prefer the eibach rear spring + stiffest sway bar setting front. Although such a setup causes more understeer, the stiffer front sway bar makes the car feel more direct and fun to drive on the street. The softer eibach rears also give you more grip over bumps and makes the car feel more comfortable to drive. Because the seating position is where it is, you really feel the rear springs more.

On the street a softer rear + stiffer front gives you more usable grip and a car that feels more fun and comfortable to drive. Because lets face it, on the street we don't get much enjoyment from going fast into corners. However we get enjoyment from how the car reacts to steering inputs and from how much rear grip we have when accelerating.
I mean how often do you really brake hard from 120mph to 40mph and then turn in while trail braking at the limit of grip on the street? You rarely have an opportunity to do this, its not exactly safe, and it doesnt provide much excitement either. Braking hard and turning in is not nearly as fun as accelerating hard and sliding out. However at the track, braking hard and late is all you do all the time. If you can brake later and still get the car to rotate and turn in, you win lots of lap time. Thats why a track-going car needs to be balanced towards less understeer by running a stiffer rear. To summarize:

Track/time attack preference: Soft front, stiff rear = OEM-ish front, maxed out rear = what I have done. Doesnt feel particularly fun or comfortable on the street, but it offers the most overall grip and is super fast and challenging to drive at the track. The car will loose the rear end if you do something silly like lifting off in a high speed corner.

Street preference: Stiffer front and only sligthly stiffer rear = what most coilovers + aftermarket sway bar combinations offer. You keep some comfort and rear grip by not over-doing the rear-to-front spring rate ratio, while getting the sharper response and feeling that the car can handle the extra power without squatting and diving as much. The car will understeer at the limit, which is also much safer on the street.

Personally what I do is simply adjusting the front sway bar to be stiffer on the street and softer on the track. I also adjust the dampers to be a bit softer on the street. Finally I use different tires/wheels (square setup) and brake pads on the track, but thats a different story.

Last edited by Asbjorn; 01-27-2021 at 03:18 AM..
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      01-27-2021, 11:33 AM   #6
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Appreciate the response, glad to see there's SOMEONE alive on here!

I mentioned this in my build thread a few weeks ago, but I took a ride in my buddy's 135i with Solo-werks coilovers and was absolutely blown away by how much better it rides than my ST XA sprung Z4 35i. I ended up giving Solo-werks a call to see if they ever planned to make a setup for the Z4 and long story short they're local to me and going to piece together a custom setup and potentially deliver a SKU to the community! Pretty stoked to get this all going and can hopefully understand what spring rates and shock valving they go with.
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      02-01-2021, 11:06 PM   #7
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Quote:
Originally Posted by proboner View Post
Appreciate the response, glad to see there's SOMEONE alive on here!

I mentioned this in my build thread a few weeks ago, but I took a ride in my buddy's 135i with Solo-werks coilovers and was absolutely blown away by how much better it rides than my ST XA sprung Z4 35i. I ended up giving Solo-werks a call to see if they ever planned to make a setup for the Z4 and long story short they're local to me and going to piece together a custom setup and potentially deliver a SKU to the community! Pretty stoked to get this all going and can hopefully understand what spring rates and shock valving they go with.
Thats awesome!

Do you have the solo werk specs for the 135i? Such as spring rates? Are we talking 3way damping?
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