hacker-pschorr

One of my customers has a shock dyno and he's been designing / modifying shocks for various racing teams for decades. When I told him about the 928 and various suspension options he said it would be fun to test out the various shocks and get some data.

I just need to get my hands on the various shocks to test them. Preferably all at the same time.

GregBBRD

That's exactly what I did.
I sent over two complete brand new sets of every commonly available shock over and had them all done, at the same time.
I also had a set of custom JRZ's done....to see what is possible.
Wasn't cheap, but I sure learned a bunch.

I'll post a picture of a couple of the Sach dyno tests....hilarious stuff.

My own shock dyno is on my Christmas list.
Turns out that no only shocks, from the same manufacturer be like fingerprints, but the same applies to springs.

hacker-pschorr

This will probably cost me a case of beer.

GregBBRD

That's great!
One of my very long time friends and suspension expert (one of my mentors) did mine. (Glenn Yee)
He really didn't want me to pay for the work....it took me almost a year of asking, before I finally bypassed him and asked his wife for a bill for the dyno work.
Even she would not bill me for the dozens of hours of explaining what it all meant, how important it is to know the exact weight of each part of the suspension, wheels, and tires, what the weight of each corner it is, to determine what spring rate to start with and what shock valving is actually needed to control all of that stuff. (Glenn actually teaches suspension development at the collegiate level.)
The cost of the dyno tesing was chump change, compared to all of the education.

For what it is worth, Glenn came by one day, when I was working out the final assembly of pieces for the custom "street/race" suspension for the "Ultimate GTS" car.
He stood around, looking at the pieces, while I explained what they all did and why I made them, like I did.
Didn't say much...just got what he needed and wandered on.

A couple weeks later, he stopped by to take another look, after thinking about what I was doing.
His comment was: "I've been thinking about this, for two weeks, even comes up in my dreams. You are a fricking genius! That's some of the most innovative stuff I've seen in years, from anyone."

I guess the proof is in the pudding. The car drives around, on the street, like it has a well tuned stock style suspension (with main spring rates WAY over 1000lbs), yet corners on the track at G forces that embarras many.

Thermal, where the car lives, is filled with multiples of every modern 911 "GTWHATEVER", equipped with "modern turn on hero suspension".

The nickname of this 928, at Thermal, is "The Beast".

When asked what the he!! is going on, by the 911 guys, I simply respond "It's magic."

Darklands

(Our next step, for our official test street car, is to put a version of this suspension (with much lower spring rates) and go attack the canyons above LA.)

Thanks for the opprotunity to have a "civil" exchange of thoughts!

This is something I would like to do to one of my cars!
Was the iris blue S one not your testbed? Or is this another attempt with Boge red?

PK68

This is occupying my mind as well as I am preparing for a suspension refreshening at some point next year.
I am thinking on just replacing parts with new ones and keep it as stock as possible (same dimension tyres as well) but the big unknown for me is how well do the “old” suspension settings and components interact with new rubber which is obviously way better than the original P7s the car was designed for. Do I have to make adjustments or replace parts in order for the suspension to cope with modern tyres level of grip?

Panagiotis
928S, 1986 Euro

GregBBRD

Yes.
The suspension changes we make, throughout the various "levels", are all designed around modern tires, which have far superior grip. This is actually why we undertook a development program. We realized that really nothing had changed in 928 suspension development in 30+ years. (I think that 928 Intrrnational's springs were the last development, other than some sway bar drop links and associated pieces.

Truthfully, I have not figured out (or spent any time to find out) what the specific issue is, with the new P7. My initial impression was that it had very soft sidewalls, which makes the car feel like it is cornering on tires made of "Jello".
We had stock inflation pressures in these tires. Perhaps some additional pressure (say 10 mote pounds) would have made a difference?

GregBBRD

I sent you another email.
It would be great if you could give me an idea of how low your front ride height is and where you eventually want it to sit.
Your request is actually fairly simple, until we get into the ride height question. (I'm going to assume that you are going to set-up the car at ~150mm.)
Bilsteins, modified so that the car can be lowered down to 150m, with no issue. Stock replacement springs. All new rubber parts, with some poly bushings in a couple of places. Replacement upper and lower front control arms, modified. Some modified pieces to give you increased travel to the front bump stops. A fairly sticky tire. If you have or can obtain a '78 front hollow sway bar (I've got a few), that would reduce the inherent understeer. The other option would be to buy one of Hans' rear sway bar stiffening kits, with drop links and do the needed modifications to make this safe and functional. New wheels bearings, while you are there, if mileage/age warrants this.

Simple and done.

streetsnake

Hi Greg. I didn’t see your other email, sorry about that. Unfortunately my car is not home yet so I can’t get an accurate measurement. Below is a pic of its state 2 years ago, when I just got it. Stock height or slightly below is fine. This will just be a driver, not a track car, and I’m trying to restore it as best I can, to that. Fun on curves but not teeth-rattling on bad roads. Lol However with that said, I’ve seen some that seem to ride high up front and looks a bit off, but that is just preference I think. Yes, straightforward. I’ll have to educate myself on the front sway bar (sounds intriguing), I’m not familiar. And don’t want to modify the rear very much, if at all. Yes, I was going to replace the front upper/lower control arms, bushings and all rubber pieces all-around, new shocks/springs all-around, bump stops, bearings, ball joints, etc, etc. Same with the rear. The more that is bolt-in ready the better. I even have a spare set of front hubs that I can send to have the bearings installed. Lol. I can turn a wrench, clean, plate and get things to WSM specs but don’t have access to any “specialized” equipment and don’t have anyone local.

You mentioned stock replacement springs, I didn’t think those were available, or did you mean using the 84s? Everyone on this thread is much more knowledgeable than I am. I just try to glean information as I can. You all have been very patient as I’ve gone through this process and greatly appreciate it. The suspension restoration is just the newest project im planning on undertaking.

The Forgotten On

^^ For adjustable front suspension like the Bilsteins and later 928s, just use 928 S4 springs.

They will always fit.

PS, the reason so many 928s sit high in the front is that the springs they are using are for non adjustable front suspension and they installed them on adjustable shocks.

The adjustable ones are a good bit shorter in length.

streetsnake

Okay. Can someone explain why a hollow front sway bar from a ‘78 helps prevent understearing? My build was actually in ‘78. I wonder if I them rollbar on mine. I’ve looked around for the specie topic but can’t seem to find anything.

Darklands

A nice looking car, worthy a new suspension!
I would say the car is on the lower side in the front, especially for an earlier car!

Speedtoys

A softer bar on one end, allows more side to side weight transfer at that end, which also allows the opposite end to rotate 'easier'.

So instead of "off throttle push" you can steer the car with the throttle easier..the front end it biting quicker (outside tire gets more weight and quicker) and the rear end is not kept as flat by a tight front end...it will then kick up (weight transfer moving forward), get light, and 'come around' as you wish easier.

GregBBRD

Early front hollow front sway bar is softer, which reduces understeer.
You can use later model (used) springs, but they can have incredibly different spring rates, since they are 40 years old. The computer wound new springs (yes, '84 type) are far more consistent and softer than new later model springs. (Softer reduces understeer.)

I am one of those people who agree that stock ride height 928's look bad and a bit lower is good. 150mm is a good look...not slammed, but not the "Paris to Dakar" look. The trick is to get the bump stop "away", so the front suspension isn't "using it" through every single corner, which increases understeer at a prodigious rate.
It seems like very few of the "suspension experts", here, have done much (if any) of even the simple measurements of the 928 suspension, to help them understand the basic dynamics of what they are doing!

The front "wheel rate" is 2 to 1, which means that in the front, every millimeter the wheel moves, the shock moves .5 millimeters.
Simple math tells us that if the suspension starts at 170mm ride height and the bump stop becomes active at 140mm, that's 30mm of shock travel, which equates to 60mm of wheel travel. That takes a fast, big corner (on the street) to get that much "roll"....and if that occurs, the beginning stages of the bump stop induce a "warm/fuzzy" amount of understeer.
​​​
Go through that same corner, starting at 150mm ride height (or lower), and that bump stop quickly becomes a significant part0 of the front spring rate....inducing high levels of understeer....right from the very start....which increases exponentially.


Throughout this thread, my points are very consistent::
People need to look at the "entire picture" before doing any modification. "My car looks absurd at stock ride height 170-180mm and I want it lowered", one needs to look at what Porsche designed, for the car to function at that ride height, before randomly lowering the car to 140mm.
"I need new shocks, I'm going to use XXX brand." That decision is completely dependent upon ones' goals and what else has been done. (Absolutely, I can send someone a Koni that is adjusted to close to the performance characteristics of a stock, original Boge, by keeping the rebound adjustment very soft. One can also "ruin" that shock/handling by cranking up that rebound too high...but the car will ride and feel like what that person thinks a "track car" should feel like....a foir wheel drive pickup truck.

You can buy basic suspension refresening pieces from a multitude of places. A "parts guy" will sell you whatever he has in stock, or whatever his "feedback loop" says "feels" the best.

A actual "developer" is going to sell you pieces and technology that work together, along with improvements in both ride quality and handling, for modern tires.....5 to 15 years ahead of everyone else.

Yeah, we joke back and forth about "magical" changes. However, when things all come together and work in unison, it really does seem like magic.

Speedtoys

GB: How do you manage the challenging geometry in your new gear, when lowered and your LCA/UCA angles are already working against you with additional available travel?