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Discussion Starter #1
Lowering a car appreciably in any manner generally requires stiffer shocks to prevent bottoming out in compression. It can also create bump steer issues.

Will I have bottoming out and bump steer issues if I move from a 20 inch summer performance tire to a 19 inch DOT race tire?

Stated another way, is the shock and valving and the bump steer setting the same on a stock 981 suspension with 20” wheels as it is on a stock 981 suspension with 19” wheels?
 

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No you will not have bottoming out issues or bump steer issues. Both of these issues are caused by changing the relationship of the Lower Control Arm (LCA) to the chassis. Changing rim size these relationships do not change.

Lowering spring Example.
Using the chassis as the vertical reference line when you use lowering springs without changing struts the angle of the LCA is reduced, now the strut has some travel used even when sitting still. Add additional movement and it can bottom out.
Same example for bump steer the LCA is now at a lesser angle than before because some travel of the strut used even sitting still, now the arc the LCA makes in it's upper range of movement is now outside of the steering connections to ability to handle and bump steer occurs.

Changing wheel diameter example.
This produces no change in the relationship of the LCA to the chassis vertical reference it simply changes the ride height of the car if smaller diameter tires are used. If the same diameter tire is used then then is no change in LCA angle or ride height.
 

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No you will not have bottoming out issues or bump steer issues. Both of these issues are caused by changing the relationship of the Lower Control Arm (LCA) to the chassis and LCA arc to steering connections. Changing rim size these relationships do not change.

Lowering spring Example.
Using the chassis as the vertical reference line when you use lowering springs without changing struts the angle of the LCA is reduced, now the strut has some travel used even when sitting still. Add additional movement and it can bottom out.
Same example for bump steer the LCA is now at a lesser angle than before because some travel of the strut used even sitting still, now the arc the LCA makes in it's upper range of movement is now outside of the steering connections to ability to handle and bump steer occurs.

Changing wheel diameter example.
This produces no change in the relationship of the LCA to the chassis vertical reference it simply changes the ride height of the car if smaller diameter tires are used. If the same diameter tire is used then then is no change in LCA angle or ride height.
 

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Discussion Starter #4 (Edited)
Thank you fsae99.

We all find our own approach to track day use. I am still evaluating mine, but it is premised on “first do no harm”—no permanent major alterations such as X-73—to the excellent job Porsche did in making the 981 S in stock form a superb road car. With the reversible Softronic ECU flash and the reversible exhaust upgrades, it is remarkable for its combination of performance and comfortable road use.

The premise here is to preserve that while improving on-track performance.

Consistant with that premise, temporarily lowering cg by use of track day 18” wheels and race tires would aid greatly in minimizing weight transfer during cornering. This is a huge factor in maximizing mechanical grip. Couple that with upgraded pads and brake fluid, adjustable upgraded sway bars front and rear for balancing the car dynamically, and simple suspension tweaks to easily adjust camber and toe from track to street and VV, and you go a long way towards a relatively high level of track day viability which is simple and which observes the major premise.

I understand the role adjustable shocks play in optimizing race performance. I may elect to do that at some point, although I question its cost-effectiveness for my level of use. I just didn’t want to HAVE to do that to avoid bottoming out or bump steer issues caused by the use of smaller wheels.

There is nothing particularly comment-worthy about this post. I put it out as an example of one approach to car development so we can pool ideas about how to approach this and perhaps discuss. I have found it useful to read similar posts from others.
 

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While I was participating in a PCA tour event, one fellow member, who just installed Tarett LCA's and had his car fine tuned for camber and caster at a specialist shop, told me that he had to lower his rack and pinion steering assembly with shims to prevent bump steer in the front with a 30 mm drop from coilovers. I am not sure my Ohlin coilover install shop did that work. My car ended up 30 mm lower than stock due to corner balance issue with 200 lbs in the drivers seat. So, do I need to inspect my steering assembly for these shims. I have not felt any bump steering issues so far. I mentioned Tarett new Cup LCA's, which I intend to have installed by this same shop, with the idea that these control arm's thrust puck, allow better management of caster than other arms like GT3's. At this point of the control arms install, I certainly could have the steering shims installed. What caster number is best for a -2.7 camber f and -2.4 r with just a hint of toe out in front, toe in in the rear. Caster to allow for accurate straight ahead driving without fear of side wondering, with the toe out, going across truck groves in the highway at say 85 mph.
 

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I like 9.5 or so caster at -2.5 front camber on a stock suspension 981CS. Great grip, heavier steering, helps with dynamic camber in corners. It doesn't have any adverse side effects even without messing with the rack on my car, but YMMV depending on suspension height and the like.
 

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Just wondering what your secret is getting -2.5 front camber on a stock suspension 981cs. I have been working hard with Cantrell Motor Sports to get -1.6 camber in the front without resorting to either camber plates or lower control arms.
 

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Just wondering what your secret is getting -2.5 front camber on a stock suspension 981cs. I have been working hard with Cantrell Motor Sports to get -1.6 camber in the front without resorting to either camber plates or lower control arms.
GT3 LCA with adjustable thrust pucks and monoballs. So the struts, springs, and ride height are stock but not the LCA.
 

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Thanks, that makes sense, I am upgrading to Tarett Cup lower control arms and Porsche Cup brake ducts F and 997 Turbo ducts in the rear in a couple of months to get to -2.6 front and -2.4 rear. The caster is a head scratcher, as too much adds a dead feel to the steering wheel, and not enough could be deadly with a little toe out (4-6 minutes) for quick response.
 

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Thanks, that makes sense, I am upgrading to Tarett Cup lower control arms and GT2 brake ducts in a couple of months to get to -2.7 front and -2.4 rear. The caster is a head scratcher, as too much adds a dead feel to the steering wheel, and not enough could be deadly with a little toe out for quick response.
Stock caster is somewhere around 8, which would be fine. More is better up until you have bump steer issues, which I have not encountered at 9.5. 9.5 feels better steering, not worse. I did have a tick of fender liner wear, a mm or two, but it is all good now.

You didn't ask for advice on camber and toe, but toe out on a street driven car is a no-no in my opinion, since it vastly exacerbates wear and causes the car to wander on the highway. Car setup is personal, but I also like more of a gap between F/R camber. I run -2.5 and -1.8 right now, but plan on going -3 and -2 F/R at some point, with zero toe up front in all cases. It's a balancing act.
 

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The toe out is for autocrossing with RE71R's 255/35 F and 275/35 R. I am trying to get the car to rotate rapidly, but need the caster to prevent wondering like you said on the freeways especially where trucks have formed a grove in the road. I have my track out as far as possible to minamize weight transfer, but that causes slower turn in, so the need for just a couple of minutes of toe out in front.
 

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Stock Porsche GT3 LCA with 10 mm shim and you get the max spec on the caster of around 9 degrees. Perfect for me. The thrust bushing does not need to be adjusted and comes with some higher the stock durometer rubber in there for a little bit better NVH. I don't notice any bump steer with my set up and with the shimmed LCAs the car is about 25-26 mm lower than stock with the X73 springs. I have been using 3-6 minutes of toe out in front. Very subtle. The car bites a little better into a corner and I don't feel it wandering on the road. As for wear, can't say much because the tires get worn out on the track in less than a year before road wear complications can be seen.
 

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Stock Porsche GT3 LCA with 10 mm shim and you get the max spec on the caster of around 9 degrees. Perfect for me. The thrust bushing does not need to be adjusted and comes with some higher the stock durometer rubber in there for a little bit better NVH. I don't notice any bump steer with my set up and with the shimmed LCAs the car is about 25-26 mm lower than stock with the X73 springs. I have been using 3-6 minutes of toe out in front. Very subtle. The car bites a little better into a corner and I don't feel it wandering on the road. As for wear, can't say much because the tires get worn out on the track in less than a year before road wear complications can be seen.
So the GT3 LCAs lower the car 5-6mm? Would not have expected that. You have X73? LCAs front and rear?
 

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Just wondering if the PASM shock spring perch is slightly longer than stock, so the addition of the X73 spring, which maybe shorter than the PASM spring, lowers the car slightly. The more we dig into your suspension set up, it seems the better it becomes, especially if the car came with PASM.

So, I am thinking the use of Tarett Cup LCA, but to get the Porsche Cup brake cooling ducts to work we will need to use straps holding the ducts in place but Cantrell says they have done this procedure all the time when installing LCA's. The project also includes cutting open the rotor backing plate so the Cup duct ends with no interference close to the inner brake rotor.
 

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When you take a GT3 LCA and add 13mm of shim, that pushes the front wheel bearing assembly out by that amount. At the same time the shock tower is not getting any longer. How does it reach an extra 13mm to attach to the LCA? The LCA assembly must move up a proportionate amount to be able to attach to the shock tower. Get your measuring tape out, measure the suspension parts triangle, plug in the before and after numbers and you get almost a 6mm rise in the outer end of the LCA which is actually lowering the car by that much. Add 6mm to the rear LCA assembly with an eccentric monoball and you have about 3mm of lowering in the rear. You get a wider track, more negative camber, and this also comes with the added bonus of lowering your car while not affecting the ride compliance with shorter and stiffer springs. A trifecta of benefits with one modification.

The shock spring perches are the same I believe. The ride lowering comes from the geometry changes that I just explained above. Not cost effective for Porsche to make three different perches for the three different stock suspensions. The GT4, however, is another whole animal in its own.

Ride Heights:
0mm Standard
-10mm PASM
-20mm X73
-24.5(avg) X73 modified
-30mm GT4
 

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To review, my original PASM suspension now consists of the following parts and is essentially a modern day SPASM with DSC controller. The car drives amazingly well at both the track and on the road. It feels better than stock and drives just like an OEM car and not something cobbled together just to lower the car. It is, after all, mostly Porsche OEM parts. These guys kinda know what they are doing, so I went down this road. Adding the DSC sport PASM control box adds another dimension to what Porsche already put into the PASM suspension and builds on their success and makes a good thing even better.

1. X73 springs

2. X73 sway bars front and rear

3. PASM shocks

4. DSC control Box

5. GT3 front LCAs with thrust bushings rotated 90 degrees

6. 12 mm of shim and 1mm litronic bracket for a total of 13mm (increases track by 26mm for more front end bite)

7. Tarret or Elephant rear eccentric Monoballs with 6 mm of offset (increases rear track by 12 mm)

8. Tarret rear toe links (necessary to get full available rear negative camber)

9. Sport rear LCA thrust bushings (25% stiffer rubber comparable to GT3 part)

What might I do differently? I am happy with everything the way it is, but would likely do rear LCAs instead of the bushings the next time around. Another option to this would be to add a Tarret GT3 front sway bar and a matching rear if you are looking for a racier car. Beware though that the links are more prone to making noise than the stock parts. Camber plates are another option but my mechanic says that he has taken out almost as many of these as he has installed over 30 years due to eventual noise and ride deterioration. You also don't get quite the ride height lowering and the increased track as you do with LCAs. What is the first thing that they do with a race car suspension? Wider, lower, and stiffer.


 

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So, from your build list and the outcome, my take away is adding Tarett Cup lower control arms to increase negative camber from the max OEM of -1.6 f and -2.1 to say -2.6 front and -2.4 rear. Pushing out the lower control arm like you did using the 10 mm shim with a 1 mm litronic bracket, would be close to -2.6 camber and lower the car another 5mm, plus push the wheels out another 11 mm. I can get rid of my 10 mm front spacers and 18 mm rear ones as well. Not sure what the Tarett Cup control arms will do to the corner balance that was setup last year for the Ohlins.
The new Cup arms:
increase track by 22 mm, (2) 10mm shims plus (2) 1mm litronic brackets
Lower the car by 5 mm, (car really does not need this as it sits about 30mm lower than OEM and will requires a shimming of the steering rack to prevent bump steer),
give negative camber of -2.0 to according to Tarett, plus base adjuster,
Have a solid thrust puck for greater control, wheelbase and caster adjustment,
Have a more robust (larger compared to GT3)) outboard monoball bushing with a teflon inner bushing coating that is replaceable.
So, overall this upgrade excites me,
Down side is the Porsche Cup brake ducts needs a non elegant install by being strapped in place rather than clipped onto the ridges of the GT3 arms.
 

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That pretty well sums it up. I would use 10mm of shim in the front to start with and not the 12 like I did. This will put your caster right at the upper limit without having to add a solid thrust bushing in front. There are two holes in the GT3 LCA to accommodate for the shimming so as to maintain stock caster. I am right on the limit and just over on one side with the 12+1=13 shim setup on the front. You can probably get close to -3.0 in the front as I have -2.7 here with room to go for more as you can see in the photo. The rear maxes out at -2.8 with the eccentric monoballs, both rear eccentric alignment bolts maxed out, and the subframe centered and adjusted to use all of the available negative camber and apportion an equal amount to each side. Rear LCAs will get you to -3.0 but are more expensive.

 
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