Gus

Working on creating damper curves , but I don’t like what I am seeing on the wheel rates.

The basic issue is rear motion ratio is about 0.88, so the wheel rate is the square of this number or 0.77. For every 100 lb the rear spring or damper exert, the wheel only sees 77 lb.
Front suspension on the other hand is much closer to a 1.04 motion ratio, resulting in a wheel rate of 1.08. This means that for every 100 lb the front spring or damper exert, the wheel sees an even larger 108 lb.

As a result, the rear stiffness does not match well with the front stiffness. The natural frequency of the rear suspension is about 23% less than the front, when the rear should actually be a few percent higher.

The front natural frequency calculates at 2.58, which is a little high, but OK for a serious track car, but the rear natural frequency is only 2.10, which would be fine for a nice comfortable “sporty car”. This suspension setting would create lots of under steer unless using a relatively stiff rear sway bar.

Has anyone calculated the motion ratio on their 964 car ( race or otherwise)? Can you tell me if your motion ratio was about the same, what numbers did you come up with ? What spring rates did you elect to run based on the results of your calculations ?? and what settings do you run for your shocks as - compression and rebound??
Thanks

KaiB

Gus, I love you man, but....(and my degrees are in in Math and Chemistry)...

I don't worry about what I consider to be trivialities, and leave the thinking to others (like you and Bill). These cars have been out and raced for decades now, and those who build and drive them know what works....without re-inventing the wheel.

What works for me may not work for you, and only track time at the limit, data and a stopwatch will yield the best setup for any given individual. For instance, I like a somewhat softer car, a bit loose at all phases of the corner...you may be faster in something entirely different.

Numbers are FUN, but when it comes to my car, I try not to think about it.

FWIW, I run softer springs than many and my damper settings vary from track to track and from tire to tire.

Gus

LOL - understand - at least that is some feedback. I understand that everyone likes a different flavor. And choices are not limited to vanilla and chocolate -
Me real reason for the post was to try and get confirmation that the motion ratios I came up with are in the ballpark/valid - so I can move on
Thanks for your input. Greatly appreciate, numbers provide a starting point after that ( to me ) it is drivers preference to what feels right.

Bill Verburg

964 Cup used 600/800 springs
964 M030 was 169/263

Seems like a good place to start

more front spring wrt to the rear increases understeer
more rear spring wrt the front decreases understeer

Here are the shock rates rebound/compression
To give you some idea of shock rate differences, here are the Bilsteins:

964 to 1990, Front: 332/104 Rear: 274/122 (note HD shock)
964 1991 on, Front: 332/104 Rear: 277/124 (note HD shock)
964 RS, Front: 259/84 Rear: 415/237
964 Cup, Front: 220/67 Rear: 379/172
964 3.8 RSR, Front: 237/96 Rear: 402/216

KaiB

FWIW Gus, I run 600/800 and very rarely feel the need for more.

I've been able to balance the car with damper canister pressures to this point.

I believe in the open wheel (say FF, FM, FA) world that the tendency on the West coast is less spring. For our cars, I'm told that new damper technology is also allowing less spring.

It's my honest belief that many in the club race world are oversprung. This is certainly the case for many of the guys in the GTC classes (6 and 7Cups), who are forced to run factory spring rates - engineered for high level Supercup races on European tracks.

Gus

Thanks Bill and KaiB for the feedback - Gives me a starting point - I am trying to work up a balance combination between DE and AX events. The WRT works to advantage in AX, while not so much for track Right now I have 300 frt and 450 rear - from what I am reading working on I need to increase rear for a better balance - 600 -. I am running MCS shocks with external Comp Can's at 125 psi. But have just put on and have only run 1 DE with lot of playing - so am working to try and get setting and spring rates closer over the cold season. I was trying to work up my Motion Ratio to plot some Damper Curves to see what the picture was, but my first calculations showed that my current spring weight is not where it should be to even begin - hence wanted to see if I was off on the Motion Ratio.
Your input is most greatly appreciated, thank you.

KaiB

Let's ask what the car is doing that you don't want it to do...or that you can't drive around...or that may be slowing you down.

THIS is really where one should start.

Gus

Valid point - front of the car seems to track very well, goes were I point it - track or AX -
On AX course the rear is loose (oversteer) to the point of breaking free under power -
On Track front is stable, rear is loose and drfts out in "S"'s or fast turns, will hold on under power but is on the boarder of breaking loose, uncomfortable feeling, spend time getting car stable. all which tell me I am driving a 911 - understeer - but would like to get a little more neutral. Car has more body roll than I like with in long fast turns. Some of this is being worked out slowly - which is what started this post.
Side note - I am on R888 which work well on the track - Springs are 300 frt and 450 rear. Current shock settings on track are 1/4 of the way on rebound and 1/3 of the way on compression on the front. 1/3 the way on rebound and 1/2 the way on compression on the rear. That is saying I have 16 clicks on rebound and compression. I am working my way into different settings and trying different combinations to see what seems to work.
In AX the setting are back off considerable to 1/8 or 2 clicks on all -

Gus

Oh! MCS shocks with remote cans at 125 PSI

KaiB

Let's be VERY careful here - in an effort to find out exactly what you want the car to do. (The internet can be funny this way...)


All food for thought....

ToSi

How did you measure the front MR? I come up w/ ~ 0.75, granted on a 993 but the parts are basically the same. Interestingly, the rear matches yours. Working out the ride frequencies, end up higher in the rear by 15 to 30% depending on model.

alexjc4

Interesting subject in lots of ways. I'm at a similar stage with my track car and having spoken to some very open and generous damper builders have kind of reach Kai's conclusion tbh. But I'd love to hear more about the process you are working through Gus.

Re the op question: Geoffery had MR info for his race car, he posted a few times on the subject. But he only ever publicly stated "front <1 and rear >1" which is out of whack with what you have. Make of that what you will.

Gus

Originally Posted by Gus Questions from KiaB -View Post
Valid point - front of the car seems to track very well, goes were I point it - track or AX -
This is a good thing, yes? YES

On AX course the rear is loose (oversteer) to the point of breaking free under power -
If it's actually at the "point of breaking free", is this not good?
YES and NO - as in AX it breaks free very fast when it does decide to go
On Track front is stable, rear is loose and drfts out in "S"'s or fast turns, will hold on under power but is on the boarder of breaking loose, uncomfortable feeling, spend time getting car stable.
If you can plant the rear end with the throttle, then all seems to be good here. Yes, but I am looking to get a little more grip/control from the rear of the car and a more neutral feel.
all which tell me I am driving a 911 - OVERSTEER ( my fopar ) -

But above, you're discussing oversteer conditions, all of which sound rather controllable and actually tend to be desirable. Yes, to a degree

but would like to get a little more neutral.

Again, you say you have a bit of looseness, but then indicate OVERSTEER - my mistake???

Car has more body roll than I like with in long fast turns. Some of this is being worked out slowly - which is what started this post.

Is this a hindrance (does it slow you)...is it a problem elsewhere? Yes to me, It is a matter of comfort with the way the car feels. I know that there is more to be had with the adjustments to the suspension and settings. It is just going to take time to test and set.

Side note - I am on R888 which work well on the track.

It's my understanding that these are pressure sensitive. Have you been over to the racing forum to search what others are using for hot pressures? (Quite a bit of R888 info there) Good idea - will see what I can find

Springs are 300 frt and 450 rear. Going to 600 rear

Keep in mind the fact that if you go up in spring rate, you will have to insure that you're more "on top" of the car, as it will be less forgiving and ask that your inputs and reactions are quite a bit quicker (and SMOOTHER). Understood, have been bit before

Current shock settings on track are 1/4 of the way on rebound and 1/3 of the way on compression on the front. 1/3 the way on rebound and 1/2 the way on compression on the rear. That is saying I have 16 clicks on rebound and compression. I am working my way into different settings and trying different combinations to see what seems to work.

Have you spent a day working settings, i.e....bump until the track JUST smooths out (while rebound is zero, i.e. done first) and then rebound until roll becomes acceptable, and then back one click?
NO - need to get track time to set up a test day to do -

In AX the setting are back off considerable to 1/8 or 2 clicks on all -

Thanks for the questions and feedback greatly appreciated -

Gus

ToSi -
How did you measure the front MR? I come up w/ ~ 0.75, granted on a 993 but the parts are basically the same. Interestingly, the rear matches yours. Working out the ride frequencies, end up higher in the rear by 15 to 30% depending on model.
What I did was measure the change in the shock height to the rise of the wheel or hub - You need 5 or 6 measurements -
First measure the distance of the wheel at static position normal ride height. I measured from fender lip to rime - 2 7/8 inch - this gave me a static know position.
I then raised the car up and measured the distance again 4 3/8. At this point I removed the wheel and measured the distance on the shock from two fixed points one top and one bottom. You will have to back the springs all the way off to be able to raise the wheel without raising the car when you do. I then raised the raised the wheel in 1/2 inch increments pulling a measurement at the shock each time.
I pulled 6 measurements and took the difference between each - the first to the last.,
Measure hub lift change Shock change
0 start 10.25
.5 9.843 diff .40625
1.0 9.375 diff .46875
1.5 8.906 diff .46875
2.0 8.437 diff .46875
2.5 7.875 diff .5625

Add all together and take average 2.375/5 = .475 = The Motion Ratoio is .475/.5 or .95

You can use this info to determine your wheel weight front and rear and then determine the force front and rear to decide what spring rate will give you a balanced spring ratio front and rear -
In my case a friend of mine helped me determine that 300 lbs front and 600 rear which gives me a natural frequency in the front of 2.36 and 2.43 which is only 3 % higher than the front - which is a good thing

Bill Verburg

The front is similar but not the same
the shock top is the same
the A arm is the same
the longitudinals that the A arm bolt to are different in that on a 993 the A-arm pivot is 12.5mm further outboard than on a 964, this will alter the motion ratio.