ptuomov

Gents, and the occasional lady, here's what I've been thinking on the suspension. I am venturing far outside my comfort zone with this handling stuff.

First, as a background, the car is '87 S4 5-speed with above-stock power. The car is not lightened, it's about the original weight which includes a fairly comprehensive list options. This is mostly for street rac... driving, but my tastes are such that I usually drive a 997 C2SC with the PSM in the sport setting and I think it's still pretty cushy.

How does the following plan sound?

Front:
- 928 Motorsports solid spring perches (http://www.928motorsports.com/parts/...ring_perch.php)
- 600 lb/inch rate 10" Hypercoils (10B0600)
- Bilstein shocks revalved to the "Carl Leguia" spec of 40% more low speed compression dampening and 60% more rebound dampening
- J9/18/et68 rims, brand/weight/cost undecided
- Yokohama ADVAN Neova AD08 255/35/18

Rear:
- 928 Motorsports spring perch adapters
- 400 lb/inch rate 12" Hypercoils (12B0400)
- Bilstein shocks revalved to the "Carl Leguia" spec of 40% more low speed compression dampening and 60% more rebound dampening
- J10.5/18/et48, brand/weight/cost undecided
- Yokohama ADVAN Neova AD08 295/30/18
- Rolled fenders

Here are my concerns.
- How much NVH from solid spring perch in the front?
- Solid perch in the front, but stock perch with the rubber mount in the back? Is this going to drive fine or is it going to feel weird?
- Ride height adjustment? Do the Bilsteins have the threads on them? How does the adjustment actually work
- Ride height in the rear. I want to put the minimum stress on the drive shafts and whatnot. What's the safest ride height?
- The inevitable offset question. "Do these offsets make my wheels look fat?" Will it rub and how much I have to Rrroll the rear fenders?

Ispeed

Nice! Another convert!
The Bilstein shock body is threaded, adjustment is easy with the right wrench. I put a mix of anti-seize/waterproof grease on the threads so that they move easy and don't corrode.

BC

I think your spring rates are too low.

ptuomov

Why?

IcemanG17

That sounds like a nice balance for a street car.......Sean has bilstein (unknown valving) with 600F 400R with BIG tires..265 front 305 rear and it handles very well for a full weight street car....

BC

Because when people have discussed the 928 and its suspension ratio, it has been stated that the normal 600/400 is simply too low. I understand that everyone equates a higher number with a harsh suspension, but the 928, imo, requires much more spring to be a modern feeling vehicle.

ptuomov

The stock started as low as F 215 R 120.
Eichbachs are progressive F 399-559 R 285-372

Here are 928 Motorsport's recommendations:
Front Rear
OEM 215 120
Street 300 200
Street+ 400 250
Street/Sport 500 350
Sport/Race 600 400
Race Mod 850 450
Race Pro 1000 500

I drove Carl's F 600 R 400 and it felt like a sports car.

I guess I am not understanding you correctly.

BC

I understand that the spring rates that people sell right now are at 300 and 400 or a bit higher. We don't have adaptive ride control on the 928, so some may want more squish. But the facts that Mike Simard gave us on the real suspension geometry ratio are that we should be talking about 800s or above for the front. Add in the fact that with your turbos you are adding probably around 100 pounds or more to the front of the car....

But I know nothing. Move along without a second thought.

ptuomov

The extra weight in the front is a good point, the total weight gain so far is 70 lbs and most of it is in the front.

Ispeed

I respect your opinion, but have you logged many miles on 600/400's? They are as stiff as I would ever go in a 928 for the street. At times it can be terribly harsh. If I were to do another 928, I might go less extreme and have more of a comfortable cruiser than extreme sports car.
After 12,000 street miles on 600/400 I can see how softer may be acceptable depending what a person is looking for.

Mike Simard

That's the damping you feel as harsh.

What B' is talking about is the motion ratio unique to the 928 that has a spring giving a much lower wheel rate than the spring's rate. The wheel rate with the 600s is something under 200. It's a motion ratio way beyond other cars with arms and another world from strut cars and their 1:1 spring to wheel rate.

Your combo may work well for you. There seem to be different schools of thought and if you like the way your car feels than that's a good setup.
I like to set up a race car to have no more damping than it takes to keep from porpoising and I do know that 600s are way too soft and will have silly body roll.
There seems to be a broad range that the spring rates can be and still feel fine, at least in a racer. I've run mine with various springs from 400-2000 (front) and it always felt fast but with problems like bottoming and leaning alot in turns that I didn't realise until seeing pictures. With the soft springs it did feel like there was too much movement but it wasn't like an undriveable pos.

hans14914

I am personally still skeptical of using a solid shock mount on an non-caged street car. I have to think there will be some pivoting forces in there somewhere, which is why I would think a jointed top-mount would be a better solution in this application, such as:

http://www.racersedge-inc.com/racers...open&id=11.4.1

Certainly not as affordable as the standard solid mounts, but may be an better strategy for the stock chassis.

ptuomov

Very interesting info.

[EDIT: Hans posted the below info while I was writing this message]

(My understanding of suspension issues has been elevated from the level of my 1.5 year old to the level of my 4 year old in the process!)

Here's a question. Someone suggested to me these solid spring perches:

http://www.racersedge-inc.com/racers...open&id=11.4.1



They have joint in the shock mount, which was highlighted to me by the said person. The 928 Motorsports one doesn't:



What's the practical signifcance of the difference? Please discuss.

ptuomov

Just got the same suggestion from another source some minutes ago. Thanks! Keep this stuff coming. I have no idea about this suspension stuff, so all of this is very helpful in helping me learn a bit.

ptuomov

Mike --

Do you happen to have the info at hand on what's the leverage ratio for the front and rear? Does the rear suspension also have a similar leverage ratio?

Also, if I just look at the geometry of the setup, is it enough to assume that the tire midpoint is the contact point and then just compute the leverage ratios from the angles and the part lengths?

Best, Tuomo