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Help slightly stiffening the rear suspension

 
Old 07-07-2019, 05:33 AM
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cschou
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Default Help slightly stiffening the rear suspension

Hi all,
I need some suggestion for how to slightly stiffen the rear suspension on my '89 S4.
Currently running Bilstein shocks with the original S4 springs. I also have the Ott-droplinks and set at max stiffness. I do mostly street driving on country roads and in the city, with an occasional longer trip. Once or twice a year I do a track-day. Generally, the setup is, in my opinion, really well matched for street driving, and still gives plenty of comfort. The front is spot on how I want it, great control, feels tight, but far from harsh. The rear however feels a little too soft. The Ott-droplinks helped a lot with the body roll, but going over a speed bump, the rear "sags" a little too much for my taste. Especially compared to the front, it feels soft. For what it is worth, the car is a widebody with increased track width - might that play a role?

Any suggestions on which route to take? I would prefer to keep the Bilsteins since I already paid $$$ for those.

I have been through many many threads on suspension setup here at rennlist, but not really sorted my challenge, so kindly asking for specific advice. So far, my considerations have been:
- Buy new original springs for the rear. The ones on look OK, but they are still 30 years old.
- Cut the original spring to increase spring rate. I have plenty of adjustment left to compensate for ride height.
- Buy aftermarket springs, e.g. hyper coils, eibach, Weltmeister (if available)
- Get custom springs made - I would need a lot of help on determining the rate, length etc.

Thanks!
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Old 07-07-2019, 05:45 AM
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What size tyres do you have front and rear?

The purpose of the rear link was to provide a relatively cheap and simple way to reduce the built in under steer of the stock setup such that on a track day the driver can quickly adjust the setting to the stiffer one and thus reduce the somewhat chronic natural under steer.

If you have not done so I would also recommend undoing the lower mount of the link and check to make sure there is no pre-load at the settled ride height.

if there is a need to change anything else then you have other issues that need to be resolved that have nothing to do with the use of these links.

Regards

Fred
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Old 07-07-2019, 05:52 AM
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Originally Posted by FredR View Post
What size tyres do you have front and rear?

The purpose of the rear link was to provide a relatively cheap and simple way to reduce the built in under steer of the stock setup such that on a track day the driver can quickly adjust the setting to the stiffer one and thus reduce the somewhat chronic natural under steer.

If you have not done so I would also recommend undoing the lower mount of the link and check to make sure there is no pre-load at the settled ride height.

if there is a need to change anything else then you have other issues that need to be resolved that have nothing to do with the use of these links.


I have 235/35-R19 in front and 275/30-R19 rear. Just to be sure, I should undo the droplink where it attaches to the lower control arm and check for preload on the sway-bar, right? I will check when I get to the car, but I don't think the problem lies with the droplinks, but honestly is a question of too soft springs/shocks. Thanks...

Last edited by cschou; 07-07-2019 at 05:55 AM. Reason: Added info
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Old 07-07-2019, 08:01 AM
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Originally Posted by cschou View Post

I have 235/35-R19 in front and 275/30-R19 rear. Just to be sure, I should undo the droplink where it attaches to the lower control arm and check for preload on the sway-bar, right? I will check when I get to the car, but I don't think the problem lies with the droplinks, but honestly is a question of too soft springs/shocks. Thanks...

With the car well settled [wheels on the ground] undo the nut that holds the bolt that passes through the lower end of the rear link. The captive bolt should be free/easy to move- if that is not the case then you need to undo the two lock nuts each end of the shaft and adjust the length until the lower mounting bolt is free to move.

if adjusted correctly I would not expect the link setting to make any noticeable difference to the feel of the suspension but then I run with stiffer springs compared to the stock items.

Your rubber combination will increase the car's tendency to under steer and thus negate the impact of the links to some extent- not a huge issue but a consideration nonetheless.

For what it is worth I always felt the stock springs are a bit on the soft side but then the car is suppose to be a Grand Tourer not a sports machine per se. My springs are Eibachs that are some 25% stiffer than stock and for me that is perfect but then our roads over here are excellent- other folks may well feel as though their fillings are about to come out of their teeth- it sure is a firm ride.
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Old 07-07-2019, 05:04 PM
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Originally Posted by FredR View Post
For what it is worth I always felt the stock springs are a bit on the soft side but then the car is suppose to be a Grand Tourer not a sports machine per se. My springs are Eibachs that are some 25% stiffer than stock and for me that is perfect but then our roads over here are excellent- other folks may well feel as though their fillings are about to come out of their teeth- it sure is a firm ride.
Thanks, Fred - I will check the links as per your instructions.
Regarding the springs - I was considering the Eibachs, as I can still find them up for sale some places (e.g. Ebay), but I've read several complaining they are harsh when paired with the Bilstein shocks. Which shocks do you run? I fully agree that the car is and should remain a GT car, I just hope for slightly more stiffness. It is fine balance, which is why I am asking here. The roads here in DK around where I live are also excellent compared to many other places.
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Old 07-07-2019, 06:16 PM
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Originally Posted by cschou View Post
Thanks, Fred - I will check the links as per your instructions.
Regarding the springs - I was considering the Eibachs, as I can still find them up for sale some places (e.g. Ebay), but I've read several complaining they are harsh when paired with the Bilstein shocks. Which shocks do you run? I fully agree that the car is and should remain a GT car, I just hope for slightly more stiffness. It is fine balance, which is why I am asking here. The roads here in DK around where I live are also excellent compared to many other places.
Originally I had Bilstein gas shocks all round. I lost a front shock when I wrote off my S4 and replaced the fronts with Koni internally adjustables that I set to the mid setting of the three options and have never adjusted them since. Fit stiffer springs and the ride is bound to be harsher just as it is when running 18 inch wheels instead of smaller diameter items- no free lunches as it were. Ride over small speed bumps and the rear feels as though it is solid!

The rear links are OK but what is really needed is something like my Devek front roll bar that can be progressively tuned. That is why I now run the rear links on the softer setting that as I understand mimics the stock rear link stiffness.
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Old 07-07-2019, 06:37 PM
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The car feels soft, because it's a GT car, not a race car.
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Old 07-07-2019, 06:40 PM
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The effective spring rates are affected some by the wheel offsets. With your widebody I'm guessing you have spacers plus wheels with more offset so the wheels fill the wider wheelwells. You'll be fine with the stiffer Eibach springs except they are a progressive-wound coil, so the initial feel is still relatively soft. Next option is hypercoils, which are available from 928 Motorsports. Talk to Carl about your exact geometry and wheel/tire/spacer specs, and you can get springs closer to your desired performance. The hypercoil sizes are common, so you can "tune" the ride and handling with a relatively inexpensive spring change once you have the mounting pieces in place on the dampers.
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Old 07-07-2019, 06:49 PM
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Originally Posted by dr bob View Post
The effective spring rates are affected some by the wheel offsets. With your widebody I'm guessing you have spacers plus wheels with more offset so the wheels fill the wider wheelwells. You'll be fine with the stiffer Eibach springs except they are a progressive-wound coil, so the initial feel is still relatively soft. Next option is hypercoils, which are available from 928 Motorsports. Talk to Carl about your exact geometry and wheel/tire/spacer specs, and you can get springs closer to your desired performance. The hypercoil sizes are common, so you can "tune" the ride and handling with a relatively inexpensive spring change once you have the mounting pieces in place on the dampers.
IMHO, avoid progressive springs..the shocks will have a single damper setting which will never really match the springs constantly shifting compression and rebound rate(s)..which are..ever changing.

You'll "bounce" alot..comparatively.

A custom setup with a stiffer linear spring is bees knees, and on the track, wont try to whip the car into oversteer in quick transitions.
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Old 07-08-2019, 02:09 AM
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Originally Posted by Speedtoys View Post
IMHO, avoid progressive springs..the shocks will have a single damper setting which will never really match the springs constantly shifting compression and rebound rate(s)..which are..ever changing.

You'll "bounce" alot..comparatively.

A custom setup with a stiffer linear spring is bees knees, and on the track, wont try to whip the car into oversteer in quick transitions.
Interesting observation and I have seen folks write similar observations- no idea if you have ever driven the Eibachs but my reaction was nearly the opposite of what you perceive to be the case when compared to the stock springs. On the stock items I found the suspension ponderous when entering bends and weight started to transfer- it was only transient thing until the stock springs took up load. That weird sensation disappeared with the Eibachs fitted. I was under the impression that the Bilstein gas shocks were designed to cope with such thus why they have no adjustment capability.

The recommendation to use such came from my long time dear friend and 928 guru Mr Kim Crumb- figured he might just know what he was talking about given I understand he developed these springs specifically for the 928 in conjunction with Eibach- he also taught me how to set the car up -it worked and then some.

As a street car intended for track excursions I find it hard to imagine a better compromise. I dare say for a dedicated track machine there may be better options but the notion that the Eibachs are a bit like riding a pogo stick i find rather amusing but then I am no Lewis Hamilton either. On the other hand at a Porsche track day experience at Dubai Autodrome some 15 years ago a factory instructor in a 996 twin turbo invited me to go as fast as I wanted round the prepared short circuit section of the track. Overtaking was not permitted as he set the racing line etc and the intent was that he would go as fast as was needed to keep me "happy". At the end of the hot lap he got out of his car in utter astonishment- in the bends I was coming along side him because he was not going fast enough for me! The car's performance cooked his goose completely- he could not in his Teutonic mindset understand how an old automatic car that to his mindset did not handle anywhere near as good as the car he was driving could possibly do to him what I did- happy days! I sure got some respect after that.
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Old 07-08-2019, 04:27 AM
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Various street cars, last one being a mk3 supra. Tried eibach progressives once..three laps at thunderhill laughing my **** off, and I went back to my ST linear springs.

If you don't have a consistent spring rate, you -never- have a complimentary shock rate. If those two are in different time zones..its a hot mess.

A high speed turn on a mostly even road..will wobble in and out of the loaded spring 'set' on the outside of the car, as it hunts in and out of a stabilized compression for body roll and accel/decel corner weight transfer.

Transitions..you load up one side, shift quickly to the other...and you have taken a high rate spring on the loaded side (with too soft a shock rebound) and now you're tossing car weight to a very soft (in comparison, and too stiff of a shock compression rate) unloaded side of the car with a much lower spring rate. It's not good for even controlled weight transfer. Which is why we have linear rate springs.


It's just an unhappy car performance wise in aftermarket use. Many cars use progressive rates OEM, but with a narrow band of rate. Not so much aftermarket. Theyre trying to be soft for an inch or so, and then get hard as heck to keep the oilpan off the concrete, because the use case is almost always some kid trying to slam the car.

It's an attempt to feel sporty while being 'soft'...but like I noted, just 'existing' while driving all 4 corners dipping in and out of soft-hard-soft spring rates with anything less than a really good dual rate shocks to deal with a generally softer compression and at the same time a stiffer/stronger rebound...it's weird, it's attempting a compromise.

Ill always take a linear option, even at a higher cost, than aftermarket progressive.
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Old 07-08-2019, 06:48 AM
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Originally Posted by dr bob View Post
The effective spring rates are affected some by the wheel offsets. With your widebody I'm guessing you have spacers plus wheels with more offset so the wheels fill the wider wheelwells. You'll be fine with the stiffer Eibach springs except they are a progressive-wound coil, so the initial feel is still relatively soft. Next option is hypercoils, which are available from 928 Motorsports. Talk to Carl about your exact geometry and wheel/tire/spacer specs, and you can get springs closer to your desired performance. The hypercoil sizes are common, so you can "tune" the ride and handling with a relatively inexpensive spring change once you have the mounting pieces in place on the dampers.
Yes, I have wider track width with both spacers and lower offset rims, and it might explain why the rear feels a bit softer than the front. I have been looking at the hyper coil kit from Carl, but due to import taxes, customs etc, I have been hesitating on pulling the trigger on them - but maybe they will be my best option...Thanks you!

Originally Posted by Speedtoys View Post
IMHO, avoid progressive springs..the shocks will have a single damper setting which will never really match the springs constantly shifting compression and rebound rate(s)..which are..ever changing.

You'll "bounce" alot..comparatively.

A custom setup with a stiffer linear spring is bees knees, and on the track, wont try to whip the car into oversteer in quick transitions.
I would also prefer to avoid progressive springs. The hyper coils are linear, right? Thanks...
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Old 07-08-2019, 10:23 AM
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I can't stand the progressive springs on Bilistiens. Way too bouncy for me. Get with Lizard931 (colin) and he can tell you what to buy for off the shelf parts to make your own hyper coil set up cheaply.
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Old 07-08-2019, 10:25 AM
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Yes, the Hypercoils are linear. Recommend the Koni's with them, not the Bilsteins unless using spring rates in the lower/softer ranges. The Bilstein valves can be overpowered at higher spring rates, while the Koni's still can dampen springs that are much stronger. They are less expensive, too.
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Old 07-08-2019, 10:30 AM
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I can't stand the progressive springs on Bilistiens. Way too bouncy for me. Get with Lizard931 (colin) and he can tell you what to buy for off the shelf parts to make your own hyper coil set up cheaply.
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