Bill Verburg

Only links #24 and #29 in this diagram are specific to the RS and will have the harder rubber RS sport bushings. And for #24(A-arm) only the rear arm has the harder rubber sport bushing, the leading arm has the same mono-ball like end as all the others.


When you add a mono-ball link like the Tarret or Rennline to replace the stock toe link(#25) you stabilize the rear end immensely, on the street it's slightly harsh and abrupt at first but you get used to it, for the track it provides a great deal of control over what the rear is doing, I hated what the stock suspension was doing on track it always seemed to be doing it's own thing and only reluctantly following what I was asking it to do.

As a compromise for mostly street use you could certainly use Elephant sport rubber bushes in the stock arm, I'm sure it would help some, but the other issue is the tendency of the eccentrics on the toe arm to be less than positive in controlling the alignment settings

Bill Verburg

I know that both Chris and Colin are fans of the tilt kits, and they are probably correct as to the improvement over even the flat solid sides, I objected more to the altered front cross link mount when using them than any other reason.
the cross link is #2 here


here is what the altered connections looks like w/ the tilt kit


and the flat solid kits work great too w/o the above alteration.

V

Thanks Bill. In the package from geos source I've got 8 of these rear bushings (all of which are identical) per rear end per car. That means 4 per side. I suppose those are meant for the arms with numbers 12,29,25 and 24.

I might stick to the RS rubber bushings on the toe arm, since my car sees mixed use with more street than track. I also read in another post that longetivy is compromised with the Rennline links and that on bad roads they deteriorate the ride quality quite a lot. I should also add that I have solid (aluminum) subframe bushings. I assume that the rubber insert on the rear toe arm is the same size as the other inserts? Or does ERP and others use a different size rubber insert for the toe arms? If yes, then I need to buy that because as per above, all the ones I have are the same size..

As for top mounts and camber plates, I will decide on that when I buy the shocks themselves. Or are you guys using 993RS parts from Porsche?

OP: When talking about track suspension setups, don't forget the brakes. A good brake upgrade is the 993RS rear brakes, it will shift the brake bias rearward with the big reds up front.

Bill: On a 993TT, would you say that replacing the rear calipers with 993RS items (I've done this) creates a well balanced brake setup? I have retained the standard discs and pads, just changed the fluid to Castrol SRF. Also, I have not done anything to the MC. I was thinking that if this setup shifts brake bias too much rearward, I could replace the front calipers with Brembo 355mm brakes or similar. But as far as I've heard, the big reds are up for the task, even for the powerful and somewhat heavier 993TT. There are a few pros with the newer Brembo GT 355 and 380 brakes in that they reduce unsprung weight and that they last longer. Also, the brake discs and pads are not as expensive as for the big reds. At least, they don't have to be replaced as often. Thoughts?

Bill Verburg

That's correct, though #24 is the A arm w/ a front component and a rear component, the rear bush is what should be replaced w/ sport rubber. I have found that the leading mono-ball like connection is often very loose which means the whole arm needs to be replaced, do check that, it should be quite firm and stiff.






That is also correct, all the bushes at the inner end of the rear arms are the same size, for mostly street use the sport rubber bushes all around is a very good choice, and they last a long time, just be sure to index the bushes to the ride height that you will be using.





I like the design that Porsche used on the RS shock mounts, Cupcar is using all factory RS/RSR parts on his and it's a great way to go but also a little expensive, Mode has a nice front setup that is a copy of the factory parts but w/o seals, Rennline has a very nice rear mount w/ factory lower seal but not the upper seal. Elephant has a nice sealed setup too. I dislike the sliding front camber plates because of a bad experience I had w/ them, both Elephant and Rennline front camber plates are of the sliding design




To fully utilize the RS rear brake potential the car needs to be low, stiff and have an effective lsd, the tt m/c is perfect, I wish that I could get one that size in my car. Most of the hot tt guys over here have gone to either 355 or 380 fronts, I suppose there must be a good reason for that but for a n/a car I don't think that's necessary. It would be nice to be able to use lighter brake components and the wide Big Red pad is not the best solution in the world either.

ToSi

See dia 17.15b:


Or, to make it more obvious:

^^ above image depicts a 4-link SRA but the instant center location / effects are identical for an SLA/IRS (as installed in 993/996/997/991) in side view.

Tilting the front of the subframe down *reduces* anti-squat, among other things. For ideas that might explain why Porsche would do this, think about what happens if anti-squat exceeds 100% (as it would with significantly lowered CG height in diagram 17.15b).

Don't mean to be stickler re: semantics but the differences are important, depending on the desired outcome..

Bill Verburg

what the tilt kit does vs what the flat kits do depends on the planes of the arms and how they change, it is not clear to me what the planes were to start w/ nor what the planes are as a result.

It is clear that the flat kit, moves the whole suspension up into the chassis, whilr maintaining the preexisting planes, this will restore the geometry to reduce bump steer.

The anti-squat aspect is purely antectodal in that many proponents have claimed that result, I have never previously heard anyone claim that the tilt kit reduces bump steer more than the flat kits.

I don't know the truth of the matter and w/o access to modeling software can only conjecture and compare our various experiences.

From my own experience the flat kit does everything I had hoped for it and then some, if the tilt kit is better, great, I would like to understand why, and then install one. At this point the bulk of the anecdotal comments seem to indicate that for high power cars like turbos the tilt kit is better but for lower hp cars not so much.

The only factory info on this subject can be seen here, where it appears that the planes of the upper and lower arms are both tilted up at the leading edge

pvdw

Although you lost me on the detailed technical formula's, I got a huge amount of info thanks to the sharing of the experienced posters.
Thanks a bunch for that !

To sum up my suspension overhaul:
KW Clubsports
Techart 8.5 & 10.5 18's on Michelin semi slicks
RS top mounts front (any idea it oem's will work with KW spring diameter?)
RS wheel centers and Evo uprights
Rear kinematic toe arm from rennline or tarret
Solid side mounts rear
RS sways
Rear mode top mounts
RS drop links
Rs or walrod bushings (still undecided on that one, thinking walrod)
RS or rennline engine mount

Did I miss something ?
Except for the obvious driving instructions ;-)

camlob

Your thread is a complete guide to upgrading a 993 suspension

I would go for the monoball's in your toe control arm. When I installed mine, even on stock suspension(at that time), it was night and day. You wont feel the rear end wandering like a stock car would. My ride quality is not bad. You can feel it feels more solid bec it is!

vanguard

so getting rennline or tarret toe links would straighten the rear? meaning the rear wouldnt squirrel around and wag as much under hard breaking. i notice under extremely hard breaking, the rear would wiggle around. anything short of straight line braking would cause this to happen on my car. i would imagine i would have to match this up with stiffer bushing as well?

bobt993

^^^^^ There are several factors that can lead to stability issues under braking. You definitely want the factory toe links upgraded to better lock in the critical toe setting. I run a total of 1/8" toe in the back with all solid bushings. Car is 100% track/race. I don't think my car has seen a stop sign in 7 years so I would the last person to give you an opinion on how "streetable" the car will be. You need to consider a trailer once you start down this path. A buddy of mine would often put a dealer plate on his Grand Am prepped 993 3.8 to get two cars to the track. He had multiple failures of bushings and bearing likely due to the added bruising the suspension was taking.

Bill Verburg

9M appears to be using stock RS shock tops w/ Kw's, you might want to contact him, since Smart Racing closed up shop sourcing some of these parts has become somewhat of an issue, you can always contact Mode, ERP, Elephant, Tarret or Rennline by phone or e-mail

You want Rs wheel carriers aka uprights w/ RS outer tierod ends and mono-ball inners, Gert used to have them I'm sure FVD or Cargraphic probably do to.

KT arm leave stock as long as the bushes are ok, even the RS doesn't use stiff bushing here, You want a mon-ball toe link which is the bottom rearmost arm

Rennline has one of the nicer and more complete solid side kits, both flat and tilt

Rennline rear shock mount or RS gets my vote, Front RS, Mode, ERP, Elephant, Rennline in order of preference, I'm sure that they all work fine just small detail differences

If you really want better control than sport rubber provides spend the $ for mono-*****, you can buy the factory RSR parts or Elephant has sealed mono-ball kits

jdistefa

Vanguard, late to the show here, but a few points:

I'm running the same setup as Paddy. As Bob said, keep in mind your spring rates depend on driving style/preference/skill... but also on environment. If your home track surfaces are variable in terms of grip and bump then consider erring on the side of 'softer' springs and sway settings. One of the things that slows people down here at Mosport is running the car too stiff (loss of warp, compliance, contact, grip).

I run a wee bit less rear toe than Bob but in the same ballpark. One of the advantages of reducing rubber in the rear end, especially the toe links, is that you can run less rear toe which reduces scrub. Agree with Bob that you need to run low kinematic numbers and as has been mentioned many times on this forum, it's important to buy a gauge if your shop doesn't have one - or change to a shop that does.

Lastly, if your car is wiggling its *** under hard braking... in addition to alignment settings, rake, weight transfer, tires, it's always possible that your LSD is toast or doesn't have a high enough decel lockup ratio to be effective.

jdistefa

Oh, forgot. I run double locking collars on the rear drop link mounts, two above and two below. I found with the one above/below setup I was still getting some movement. Didn't want any surprises so took the whole thing apart again, PITA, and with the uber-locking approach it's been rock solid since.

vanguard

Thx for the feedback jdistefa. My current springs rates are just way too stiff, especially if its to see some normal street use. I'll swap my front 950lb springs to the rear and buy new 650/700 for the front (is there an ideal ratio of front to rear and how does the weight of the car come to play?). Are 6" long coils ideal to get enough travel plus allow the car to ride at RS height? I need to get another collar to snug the rear drop link mounts. I'll check with my alignment shop. I believe its set at 1/8 degree for toe and the sways are in softest of the three positions.

Bill Verburg

f/r spring ratios are all over the place street apps tend to be on the lower side '5 to .7 or so track oriented are on the higher side .75 to .9