TheBruce

I'm gearing up for my biggest DIY to date: installing Eibach lowering springs. I have a 997.1 base Carrera and the rear wheels have the much-discussed 4x4 wheel gap. The front wheel gap looks fine and I actually think if I installed just rear springs the gap on the fronts and rears would equal out.

Is this possible or would is screw up the geometry??

cosm3os

No

platinum997

Car would be unbalanced. The spring rates are different between the oem and eibachs.

If the front is already a lot lower than the rear, your front shocks or springs might be bad already. Even more reason to swap everything out.

Tj40

The simple answer is measure your ride height, from the factory the cars have a slight rake front to rear (i.e. rear is slightly higher) I think it's supposed to be 10mm, measured from the ground to the suspension mounts, someone posted a clip of the Porsche manual recently showing the factory procedure for measuring.


Pic of the car would be good - if the fronts are already much lower than the rear I would wonder why?

TheBruce

Definitive answer. Love it :-)

TheBruce

@platinum997 and tj40 - many thanks! I assumed this was not recommended but wanted to understand why. Spring rate sounds like the answer.

I will try to find the video on measuring rake and report back. I think the stock shocks are good, there is just a more pronounced rear wheel gap on base model cars. I will try to take a profile photo.

Thanks again.

TheBruce

Tj40

I have a base so understand but the front gap is still there and if I just dropped the rears I would have a smaller gap at the back. This would not be good for looks or for performance :-)


OK so this is probably more information than you want but I'm going to geek out on spring rates here..........

Base 997.1 sits on 154 front/245 Rear, (lbs/inch) rears are 159% stiffer than the front.

The 'Sports chassis' suspension from Porsche for the 997.2 sits on fixed rate springs at the front and progressive rears,
205 front/371 rear (initial) rears are 180% stiffer than the rear
205 front/542 Rear (final) rears are 265% stiffer than the rear. On the .1 the rears are slightly softer at the final rate.

If you just change out the rears the best numbers I have for Eibach springs will put you on these numbers. (Eibach are progressive).

154 front (stock)/293 Rear, (initial) so a 190% increase in stiffness at the rear
154 front (stock)/514 Rear, (final) so a 333% increase in stiffness at the rear

So you are a bit more aggressive than the sports chassis due to the softer front springs. This will give less understeer. FYI if you look at coilovers KW have a 322% increase in stiffness Front to rear, Ohlins have 200% and the PSS9 have 198% - all with overall stiffer spring rates than you have. So you are not dramatically out of line with what some people have on their cars with relation to F/R stiffness (KW V3). V3's are typically considered a more 'track' orientated setup, hence stiffer overall springs with stiffer rears to dial out understeer.

What I can't tell you is how much travel you have at the initial rate vs final on the eibachs - depends on how much is taken up with static sag and how much initial rate they 'wound' into the spring

FYI the best data I have is that Eibach rates are similar to the sports suspension from the factory (- but this is unverified data from the web) but with progressive rates front and rear. Most people with Eibachs report better handling of bumps - which might imply that they have a little of the softer initial spring rate with improved handling overall.

Simplest answer that will horrify the purists is to replace the rears (you are doing it yourself) put the car on level ground and see what it looks like - drive it and see what you think. Just make sure you either don't drive too much or check the alignment as that will affect handling and tire wear. My curiosity wonders how this would turn out :-) You can always put the fronts on the following weekend.

Unless you are of the experimental mindset you might just be happier changing them all out. If your fronts are already low vs. stock you will probably not see as much of a drop as the rears when you do the install.


I'll stop rambling now.......

Tj40

Ignore everything I just said - you posted pictures - do both otherwise your rear will be lower than the front!!

Petza914

When people (like me) install coil overs and corner balance their 997s, about 1degree rear to front rake is recommended. You can roughly check this by putting a digital level on the painted door sill and check both sides. From your pics though, you do seem to have an unusually high rear or low front. Change them all to get the proper spring rates and rake that Eibach tested the car with when designing their spring kit.

Or install a full set of coilovers instead where you can full customize the suspension settings and ride height.

TheBruce

Epic. That was exactly the answer I was looking for. Where did you come by this trove of suspension knowledge...?

I bought the car with a set of old ContiSports on it. The ride was brutal on city streets but I thought that was just the 19" wheels + non-pasm ride quality.

I recently put a set of Michelin PS4s on it and the ride has improved DRAMATICALLY around town. I guess the Contis were so old they petrified.

On the flip side the PS4s don't feel as planted in high speed corners. If I hit a bump in a corner just right I can feel the back-end float and it gives me a second or two of pause to reflect on how the story will end. My old Conti bricks never did this.

My goal is to stiffen the backend up at high speed while keeping the softer around town ride quality with the PS4s...oh, and for it to look badass.

Tj40

I live in Canada, we don't drive our P-cars much in the winter :-)


Also I'm an engineer (or was) so I love tech stuff like this, data is out there - I've published my spring rate roundup xls here on Rennlist with all the data I've found across springs and coilovers.


To your handling comments above - have you tried changing rear tire pressure? might have an impact as these cars seem sensitive to this. Now if I only had data on tires......... hmmmm there a winter project!


TJ

TheBruce

Petza914 - thx! Re rake, I tried taking a level but if gave inconsistent readings depending on which part of the door sill i placed it on. Next to the plastic scuff plate it was just under 1 degree. On the painted lip below it was 1.7 degrees.

Some additional data points:

- Rim to fender: 5" rear, 4 1/4" front
- Center wheel to fender: 15 1/4" rear, 14 3/4" front
- Tire to fender: 2 1/2" rear, 2" front

OKB

the whole car needs to be lowered, if its good for the rear, its good for the front. If it was acceptable in any circumstances they would be selling just rear springs only
keep in mind, usa has bumper height regulations, thats why euro cars are made for the rest of the world, with NORMAL springs and for usa with tall usa springs
So you need to mod the car with row springs, this is about the same as all aftermkt springs height

sm1074

I have a 997.1 C2...I want to lower car and add wheel spacers, however not sure if i should just use lowering springs with Eibachs or coilovers. I will not track the car...I drive the car for fun primarily on weekends. I'm reading how just adding lowering springs may put more stress on the factory shocks, however I don't know if spending the extra $$ on coilovers is worth it. Please let me know what you guys think. Thanks.