Bill Verburg

The only rear horizontal arms that are specific to the RS(stiffer) are the KT and
rear of the A-arm bushes om #4 & #5

For track use a full mono-ball setup w/ locks would be used on the toe link, #2 but that wouldn't be necessary for street use

the joint on #1 is already a mono-ball and is very stiff(when new)

the KT effect comes from the stock soft joint on #5 and further exacerbated by play in #4, if there are mon-***** on #2 and #5(like yours) there is no KT. If the soft joints are retained on #2 & #5 KT is adjusted by changing the caster at #4, the more caster the more the toe will change w/ vertical wheel motion, this is one of the reasons I dislike a tilt kit.

I've had the pleasure of swapping between mine w/ all RS/Elephant and Tarrett mono-ball#2 and a full mono-ball 993, there is little difference so I have put full mono-***** on the back burner.

chaoscreature

1) For a non-track car I don't think you need mono-***** anywhere, and unless you drive on glass like roads all the time I would avoid using "track springs" as well. The PSS10 system is pretty fantastic for a dual purpose car, I have ridden in a few cars with them and they handle great on/off the track.
2) I am not sure what the panel bushings are... but my rear suspension is all stock. The rear bushings still felt tight so I left them alone. The front control arm bushings have the highest incidence for wear and Walrod replacements are a great value for those.

Without driving others cars that have gone a similiar route I would take this project slowly. I went too stiff with my springs the first time out (still driving on them). I don't mind the harsh ride or the NVH, it's the fact that I LOSE significant traction on rough roads because my car hops around that I don't like. Now get me on a nice smooth road and I am all grins

Cupcar

My wonder is if the replacement at links 2 and 4 with thicker rubber annulus RS designs may make the car better than old worn out rubber, but not as good as if the links were replaced with new links of the correct thickness rubber annulus with less range of motion.

My interpretation of the explanation of the suspensions kinematic geometry is that the main point of deflection is at link 5 for the toe change and deflection at link 4 changes caster and thus the force vector driving the toe in compression of link 5.

In a corner the force makes the outside tire toe in and the inside toe out producing a steering force to counteract oversteer.

I think the reason an adjustment lock out is required when the toe link is turned to a monoball is because the main force is directed at link 5 in cornering and if this link is flexible and the toe link is solid then the force is taken by the toe link where it was not designed to be taken and the adjustment can't resist the force. (this sentence was really screwed up when I hit the post button and I edited it afterward)

I think that perhaps the best combination of street and track is to replace link 5 with a monoball (essentially what Porsche has done on the GT3 RS 4.0) use an RS bushing at link 4 a new camber arm and a Tarrett or new link 2. Then replace the 4 rubber suspension mounts with solid or RS rubber units.

My experience has been that ride is more affected by shock valving than spring rates. My 964 USA Cup (964 RS suspension) which is known to be over damped from the factory, actually seemed to ride better with a change of spring rate from 250 to 450 in the front and 450 to 675 in the rear but with less aggressive shock valving than stock. The car also worked better over rough pavement.

Bill Verburg

I see what you are saying and you are likely correct, I never experimented w/ that, I just bought all new links, RS for #4 and the A-arm#s 1&5

That may be, changing those 2 certainly changed the rear steer of the car

Perhaps, the first& second year I tracked the car was miserable w/ just new stock toe links#2, the third year I had the all new suspension arms including the same stock toe links#2, but handling was still not the best, 1/2 way through the season I switched to the Tarrett mono-ball toe links and that did the trick of nailing the rear end down.

excpt for RS A-arm that is what I have and I agree it's great.

I agree, again that's what I have both the street ride and track ride are vastly improved from the adjustable shocks I had even though the springs are far stiffer. Friends w/ similarly equipped cars but w/ Motons report the same. To me properly valved Cup shocks are the way to go. I know that you will be very pleased when you get yours on the road.

Cupcar

One thing Bill alluded to was the wear in the stock forward monoball of link 1 (link 1 is actually formed as one piece with link 5)

I was amazed at how worn and sloppy mine were after 107,000 street miles and thus replaced the lower A arm. So, if one has this issue as well and wants RS rubber, just buy a new RS lower arm from Porsche rather than just a new replacement rubber bushing.

Bill Verburg

Ditto and at about the same milage

IXLR8

Bill, I read lots of posts discussing replacement of the bushings in the rear suspension links. In this thread, there are cars with over 100K miles on them and they talk about the slop in them.

I find it hard to believe that the bushings are done and the ball joints at the other end of the link are still fine. Are the ball joints that durable?

Cupcar

I know this was for Bill but my ball joints seemed fine front and rear at 107,000 miles, but as I said the monoball on the lower control arm was sloppy. If you look at my picture below, the new bushing on the left looks crisp, the other two with 107,000 are cracked and a bit crumbly.

One does get a new rear ball joint with a new lower control arm though.

ToSi

Yep, on the other hand there is so little rubber in the toe & camber link that, unless the original has failed, there isn't much point in replacing these.

Interestingly, the mono-***** in my a-arms & all ball joints still feel good w/ ~100k. The rear a-arm bushings look OK but the k-arm bushings have crept a fair bit, not torn though. The camber arm bushings have seen better days.

To the OP > leave the rear alone unless something is broken.

Not sure what you mean by 'tighten up' but stiffer bushings will tend to transmit more impact harshness over sharp bumps while stiffer shocks will make it feel like the body of the car is following the road more closely..

Cupcar

Here is how I think the system works:

Below is a scan of Porsche's explanation of the 993 Kinematics. I have added numbers, text and arrows to the illustration with number 188 at lower right of it.

I labeled the link numbers as seen from above that correlate to illustration with 190 at the lower left.

Note that there is a squiggle line at 5 to indicate this is a relatively "squishy" link. Link 1 rotates around a monoball at the forward pivot in the box next to the 1 and link 5 moves either in or out depending on what side of car it is on during cornering, this motion is resisted by link 2 and the wheel turns.

The vertically canted line in illustration 190 is the virtual axis of rotation of the wheel.

Note the distance between the large force arrow tip and the curly arrow in illustration 190, it is important because its length is proportional to the force acting on link 5. This distance is controlled by the caster change allowed by link 4 and is set during alignment.

The squishy bushing in link 5 and the bushing in link 4 are the ones changed in the RS to harder material to limit the motion in these bushings and thus the kinematic steering effect during cornering.

Monoballs at links 4 & 5 would eliminate the design kinematic effect, but now there can be minor kinematic affects in toe and camber due to compression of the remaining bushings during cornering.

Also, this is why I think the monoballs wear out relatively quickly if installed in link 2 because then the load tends to all go link 2 rather than shared with link 5.

What do you guys think?

Bill Verburg

The blue line is the axis about which the wheel pivots, It is the summation of camber, caster and toe settings and the pivot is caused by the forces applied dynamically

I don't understand the reference to castor force

Cupcar

Yes, I called it the "canted vertical line" but that blue makes it clear.

The wheels both rotate inward with braking and that force is proportional to the camber force line dimension.

Bill Verburg

again replace the soft #5 bush w/ a harder RS(to greatly limit) or monoball(to eliminate) and you have a 996/997 rear suspension which does not have KT, You can adjust castor though(this is fixed on a 996/997) which affects the camber and toe patterns

ToSi

^^ the later cars don't have the provision to adjust the angle (castor) of the instant center (blue line) but still have some amount of rear toe-in due to lateral loading. This is tuned-in via different relative bushing stiffness. Presumably, a longer wheelbase & electronics in the newer cars allows a reduction in 'compliance steer' while still being sufficiently spin-resistant. The terminology is somewhat misleading since Porsche refers to alignment changes due to suspension motion as 'kinematics' in the later tech manuals.

For the same reason, replacing bushing #5 with a monoball would be bad idea without also installing a monoball in the toe link (bushing #2) - the softer rear bushing would result in toe out.

I think the loads through the various arms are determined by their relative locations, meaning that the toe links share the majority of cornering loads w/ the a-arms in a 993. This in unfortunate as any slippage back there results in toe-out. The other downside is that compressing bushing #5 draws the bottom of the wheel inward, reducing camber. This is reduced somewhat by using the same, softer bushing in position #4 but not fully.

The 991 was cleverly redesigned to move the toe link above & ahead of the axle - this still allows for tuning of toe in due to cornering loads, but reduces the influence on camber.

Cupcar

Interesting on the new 996/997/991 suspension details.

Except the GT3 engine, I have not even looked at one of the new water cars yet in detail.

Tosi- Do you have access to the Porsche company intranet? You seem to have interesting information.