Type 964 - Detailed suspension questions: The Redux

roadsleeper · 02-06-2009, 09:45 PM

Hi all,

This thread is sort of a follow-on to my previous thread: Type 964 - Detailed suspension questions

Long story short: my project car is into its 10th month and the bare shell is currently sitting on a rotisserie cradle in the paintshop after having had the sunroof taken out and welded shut, and the entire shell seem welded. They need to finish scraping off the remenants of the underbody soundproof coating and then it'll be resprayed.

Anyway, prior to embarking on the project of turning my Carrera 4 into a lightweight track monster, we weighed it and the weight was [i]exactly[i/] as it stated on the plate in the trunk (I think it was 1,475 kg, but the plate is buried somewhere; that would be 3,252 lbs).

What will be going back into the bare shell will be lighter parts like a G50/21 (i.e., 2 wheel drive), 993 GT2 Evo uprights, lightweight carpet on only the rear section and transmission tunnel (i.e., no insulation or soundproofing), plexiglass rear and 3/4 windows, carbon fiber hood, fibreglass RS America wing (i.e., no electronics parts), pared down wiring harness, manual windows, RS door panels, no radio, etc... Basically, I expect the end result weight to be in the area of 1,100 kg (~2,400 lbs), if not lower.

My question is: what suspension I should use for a car like this?

In my previous thread, we had discussed things like KW Variant 3s, KW Clubsports, Bilstein PSS9s, etc. which encompasses the realm of generic suspension (i.e., that which can be bought off the shelf), as opposed to custom suspension like Motons. My concern is that with the significant weight loss, things like the spring and rebound rates should (logically speaking) be wrong, right? If so, how would I go about determining what the correct rates are?

Thanks in advance for any insight you all can share.

Can I just say... I MISS DRIVING MY AIRCOOLED!!! Thanks.

Don Plumley · Don Plumley's Garage

Waiting for Tom Weber and Bill V. to chime in...

Tom runs JIC Cross on his lightweight 964 chassis. Search is your friend: spring rate, more, more

Bill Verburg · Bill Verburg's Garage

Obviously when a car is significantly lightened, the existing effective spring rate goes up. If you loose 1000# it's like gaining 1000# of effective spring rate. So you'll have to decide what you want in that department

Factory spring rates for Cup and RSR type cars is ~600/800# for a ~2500# car

obviously the shock needs to be matched to the spring and chassis, you can get custom valved Bilstein HD's w/ the spring of your choice or use used adjustable like JIC, Bilstein PSS10 etc.

Best, if you know what you are doing, is the fully adjustable like Moton. Again w/ the springs of your choice.

Cost is a pretty good indicator of desireability.

To use the 993RS/Evo uprights you will want 993 front struts.

to get the full benefit of all the rest of the suspension pieces you will want full monoball w/ camber plates.

38D · 02-07-2009, 08:01 AM

Quote:

Originally Posted by Bill Verburg

Factory spring rates for Cup and RSR type cars is ~600/800# for a ~2500# car

As a driver of a 964 cup, I can tell you that the cup springs are not enough for smooth tracks like Watkins Glen or Road America (at really bumpy tracks like Sebring, its not bad). If I were allowed to change the springs, I would go with something in the 800-1000/1000-1200 range.

Tom W · Tom W's Garage

I think my springs are about 600/800 with the car at about 2150 lbs (without driver) and I think I had about 750/900 when the car was about 2400 lb (if you search you should find it in a post from 1-2 years ago). I still have the JIC-Cross suspension I had put in 4 or so years ago. It may be swapped for a KW double/triple adjustable this year. I've not wanted to go to double or triple adjustables in the past for the reason Bill states - you better know what you are doing and be prepared to play around with them to get the full benefit.

It's going to be tough to get to 2400 lbs if you put all the stuff you say you will back in. If you look at my posts for the Turtle Mk 2, you'll see what I did to get to that weight.

roadsleeper · 02-12-2009, 11:04 PM

Guys, thanks for the input all very helpful!

Bill - I've got the mechanic building the car for me sorting out suspension setup, and he has suggested Motons in the past. I do have the monoballs all around, the camber plates and the adjustable spring plates for the rear as you and Tom W previously suggested.

38D - Thanks, I'll bear this in mind.

Tom W - Noted. I'll look up your Turtle 2 thread as I'm interested to see what else can be done to lose weight.

38D · 02-13-2009, 03:36 AM

Quote:

Originally Posted by Tom W

It's going to be tough to get to 2400 lbs if you put all the stuff you say you will back in. If you look at my posts for the Turtle Mk 2, you'll see what I did to get to that weight.

I would agree 2400 will be very tough. My cup weighs ~2475 running on fumes.

One thing to bear in mind on spring rate is the final weight of the car will matter. You may want to select a middle of the road rate, and then weight the car to confirm you are in the 2500lb range.

Bill Verburg · Bill Verburg's Garage

If you do use the 993RS wheel carriers(uprights) it makes it very easy to update the brakes to 964RS/Cup spec. Now is the time to think about that as you will also want 993 struts to use the RS uprights.

Porsche_monkey · 02-13-2009, 07:25 AM

Quote:

Originally Posted by Bill Verburg

Cost is a pretty good indicator of desirability.

Unfortunately this is true....

Tom W · Tom W's Garage

I checked my data and for the current car at 2150 lbs (without driver) I'm using 650/800. In reviewing the MoTeC data from last year, it has become clear that I get too much "squat" under acceleration and I'm going to be increasing my spring rates to reduce it.

IIRC, when I was at 2450 or so pounds I was running 800/1000.

Just don't put anything back into the car that's not essential for maintaining your race class and you'll do the best you can on weight. Most of us struggle to remove it because the time and effort to strip it back to the tub doesn't seem worth it. In my case, the target weight was 2100 lbs ('cause class rules say 2300 off the track at the end of the race). I only got to 2450ish the first round where we did not strip it to the tub. For the Turtle Mark 3, it was stripped to the tub and every little bit of metal not required was removed. To the extent that hood hinges (3-4 lbs) or the fender fill gas bits (3-4 lbs) and everything similar were removed. We still didn't get to 2100 lbs but very close. The 300 lb difference in the car is huge on the track.

roadsleeper · 02-15-2009, 07:06 PM

Bill - I've got the 993 GT2 Evo uprights that you suggested in a past thread, and will couple those with 993 front struts. The brakes are Big Reds from the 993RS (can't remember the disc diameter off-hand).

Tom - I took a look at some of the threads on your Turtle II and it looks intense! Unfortunately, the problem I face is that because of registration rules on cars in Singapore, I have to keep the car somewhat road driveable, and that means I can't build it as a dedicated track car.