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Randy Pobst "Marvels" at the TPC Racing GT4

Old 06-21-2016, 05:05 PM
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Default Randy Pobst "Marvels" at the TPC Racing GT4


On May 17th, TPC Racing had the pleasure of hosting acclaimed driver Randy Pobst of Motortrend; independent journalist Matt Farah of The Smoking Tire; and automotive editor of Ars Technica, Jonathan Gitlin, at Summit Point Motorsport Park for a private track day.

Before dawn, the TPC Racing crew loaded up a fleet of DSC Sport-equipped cars; including a Porsche Cayman GT4, Dodge Viper TA, and a Corvette C7 Z06; and shipped out to Summit Point. By the time the crew had pulled into the paddock, the rain was falling and puddles were accumulating on the track. It was looking like the day was going to be a total bust.


As we hopefully waited out the weather, Pobst and TPC Racing owner Mike Levitas shared stories of their racing days together in Grand-Am. After that, Mike gave tutorials on how to tune an active chassis using the DSC Sport tuning software. Finally, at about 1pm the sky began to clear up. By 1:30pm, the track was hot and the real fun began.

Randy Pobst began (and spent most of) his day in the GT4. He and Mike had exchanged notes relentlessly on their experiences in the GT4 leading up to this track day, and this was Randy’s first time in the driver seat of a TPC Racing GT4 fitted with DSC Sport. Mike’s knowledge for chassis tuning paired with Randy’s ability to drive the wheels off anything that has four, made it appear as though Randy had been driving this car his whole life.



Even as an extremely well-sorted 700hp Viper TA violently ripped around the track, cooking a set of Hoosier A7s, somehow the GT4 on street tires, piloted by Randy Pobst, looked like the fastest car on the track. Of course, in reality it wasn’t (the Viper was). However, the amount of grip and speed he carried out of turn 10 onto the main straight was enough to make your head spin. Based on unofficial hand timing, Randy was consistently pulling 1:20s (with a passenger!) on the main course; an impressive feat for a nearly stock GT4 on half-worn OEM tires.



TPC Racing Cayman GT4



Of course, the TPC Racing Cayman GT4 wasn't totally stock. It was very lightly modified with some suspension goodies in order to correct some of the shortcomings from the factory. Let's start by identifying said issues before we discuss TPC Racing's solution for them.

It's been well documented, by Randy Pobst and others, that the GT4 suffers from a case of understeer from the factory (see here, here, or here). Pobst also mentions that, on track, the Cayman GT4 feels "slightly underdamped," particularly over bumps. Additionally, many experienced drivers (many of whom have expressed this here on Rennlist) have noted issues with corner entry oversteer, tire wear issues on track, and excessive nose dive/bodyroll. So how did Michael Levitas of TPC Racing and DSC Sport go about correcting these characteristics on his personal GT4?

Understeer

Reducing the understeer to a reasonable amount is a fairly easy task. In fact, Randy already shared his trackside quick fix in his second Motortrend review of the GT4:

I got so frustrated that I crawled under its gorgeous body and adjusted the anti-roll bars myself: full soft front, full hard rear, textbook to reduce understeer. It certainly helped, but the overriding feel remained a numbing push, hurting the response through the middle of the corners.

As his experience suggests, adjusting the sway bars doesn't fully solve the issue with understeer the car has coming out of the factory. The other inexpensive - and vital - step in reducing the GT4's understeer is a proper alignment, which Randy also addresses in the same article. It's something the TPC Racing team does very, very well.

Tire Wear/Alignment

As most of us here already know, a proper alignment is everything for a performance car. As we took our car in for an alignment, we used shims to widen the rear track for increased lateral grip. As the majority of our customers use their cars both on street and on track, tire wear was also a factor in determining our alignment specs. However, as we aligned again and again, we found ourselves walking a very fine line between understeer and corner entry oversteer. It seemed as though the more we attempted to dial out the understeer, the more trouble we were running into with oversteer. So we turned to the damping.

Damping

We knew that a lot of that oversteer was a result of the geometry of the car when under heavy load. As the suspension travel increased, we were experiencing more and more oversteer. We decided to see what we could do with the damping. Whereas years ago we might have used stiffer springs to address this issue, we now turned to the DSC Sport tuning software. We dialed in more damper into the rear of the car to mask the oversteer. It helped, quite a lot in fact, but we still knew we were just masking the problem, rather than solving it. So we turned back to the rear geometry.

On a similar note, we were also able to greatly reduce nose dive and body roll, allowing us to threshold brake deeper into the braking zone, by improving the damping via the DSC Sport controller. We feel on such a high tech modern sports car, such as the GT4, the DSC controller and tuning software matches the sophistication of the car.

Rear Geometry

After multiple outings at the track, we determined that the rear toe curve was the major culprit, and we were masking it with stiffer damping via DSC mapping. More specifically, the OEM rear toe link - as well as any other link we've been able to find on the market - produce a greater amount of toe deviation as the suspension travels. After multiple progressions of track days, street testing, and parts development, our engineer developed an improved toe link design specifically for the rear of the GT4 that reduces the amount of toe deviation throughout the suspension travel range; during compression and rebound.

TPC Racing GT4 Toe Link Kit

The TPC Racing GT4 Toe Link Kit eliminates entry oversteer by solving the geometry problem at its root, rather than simply masking the issue with stiffer damping or springs. As a result, we were able to create a custom DSC calibration map for a GT4 fitted with the package that allows us to soften the rear dampers for even more grip coming off the corner.

While TPC Racing already offers a toe link package for the 981 that fits the GT4, it does not feature TPC's proprietary offset bushing design, intended specifically for the GT4 upright. The TPC Racing GT4 Toe Link Kit, paired with the proprietary TPC Racing setup specs, allows for an improved toe curve on the rear Macpherson strut design. What this means is even at the highest of g loads, the toe settings are still optimal for grip. Pair this package with the DSC Controller, and the GT4 becomes an incredibly well-balanced car to drive.



As Randy Pobst himself later reflected, "I marveled at the balance and control of the TPC-tuned Porsche GT4 I drove at Summit Point. TPC racing makes this great car better to drive."

While the TPC Racing GT4 Toe Link Kit and the DSC Sport V2 controller are available separately, we refer to the combination of these two products as our stage 2 suspension package for the 981 GT4, which also includes the full setup support that has become synonymous with TPC Racing products.


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Old 06-21-2016, 07:18 PM
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Nice article. I had a few questions about it:

1) Could you post some more data on the engineering design of the new offset toe link? Something as simple as a plot of the toe vs. suspension compression for stock vs. the new toe link could help make the case for a real improvement, vs. just qualitative feel.

2) Do you have any thoughts on why Porsche doesn't program the OEM controller to have similar tuning to that of the DSC Sport, at least in "Sport" mode (assuming there is some trade-off in street driving)? Certainly it's possible, so I'm just curious why you think they leave performance on the table. Is there a trade-off even on track? If not, have you considered approaching Porsche to sell them your tuning or tuning approach?
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Old 06-21-2016, 07:43 PM
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How is your toe links different (or better?) than the offerings available from RSS or Tarett that many of us already invested in?
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Old 06-21-2016, 07:56 PM
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Originally Posted by myBailey07 View Post
How is your toe links different (or better?) than the offerings available from RSS or Tarett that many of us already invested in?
Yeah, hoping to see some data on exactly what changed... looks like a ~1/2" offset on the bushing.
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Old 06-21-2016, 10:13 PM
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Originally Posted by Mech33 View Post

1) Could you post some more data on the engineering design of the new offset toe link? Something as simple as a plot of the toe vs. suspension compression for stock vs. the new toe link could help make the case for a real improvement, vs. just qualitative feel.
Yeah, the curve would be awesome!
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Old 06-21-2016, 11:27 PM
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Originally Posted by Mech33 View Post
Nice article.
Thank you. We have put a lot into the development and the event.


Originally Posted by Mech33 View Post
1) Could you post some more data on the engineering design of the new offset toe link? Something as simple as a plot of the toe vs. suspension compression for stock vs. the new toe link could help make the case for a real improvement, vs. just qualitative feel.
As you would expect any front running race team would do, we certainly bump steer tested our GT4 multiple times. Initially, our highly experienced test/race drivers felt there was a hint of unwanted rear toe steer. Which is what prompted us to bump steer test this car. We definitively concluded we were masking a rear geometry issue with stiffer DSC mapping once we have bump steer tested the car(which is okay because we consider DSC to be a "stage 1" product). A number of prototype offset bushings were machined and tested in order to produce most ideal dynamic curve throughout the rear suspension's travel range. I am afraid we are not wiling to share the bump steer numbers, these numbers are a race team's "secret sauce". I don't know you and you don't know me, but those who know us know that we are serious about race/track setups to the point of obsessed. With all due respect we do have to protect some of our knowledge. It is our livelihood here at TPC Racing. It is more accurate to say that we sell what we would use on our own car than to say we use what we sell.


Originally Posted by Mech33 View Post
2) Do you have any thoughts on why Porsche doesn't program the OEM controller to have similar tuning to that of the DSC Sport, at least in "Sport" mode (assuming there is some trade-off in street driving)? Certainly it's possible, so I'm just curious why you think they leave performance on the table. Is there a trade-off even on track?
We don't know for sure why. We can only speculate. Our speculations are 1) could be their head decision maker thinks it works good enough for the price point of the car, or 2) they planned on incrementally making the PASM system more dynamic over a period of x number of years. In which case we are offering your the future now with DSC. The factory PASM mapping on the 991 GT3 is better than on the GT4, and the GT4 is better than Cayman S, and so on. Currently the widest dynamic range factory controller that we've seen is on the BMW M4. In any case, I doubt that OEM will offer free custom tuning software + support for their suspension controllers.


Originally Posted by Mech33 View Post
Is there a trade-off even on track?
No. From our racing experience swapping springs and even conventional mechanically-adjusted dampers(via adjustment *****) is a compromise to make the car do something specific at certain part(s) of the track. Not that there's anything wrong with that when DSC technology isn't applicable or available. Tuning the dampers near real time to the amount of g-force, direction of g-force, and rate of range of the CAN-bus inputs either eliminates or minimizes the compromises. The factory PASM does all this but at this time the factory system just doesn't have nearly enough dynamic control range that DSC has. And of course, PASM has no tuning software and support to the end users.

Originally Posted by Mech33 View Post
If not, have you considered approaching Porsche to sell them your tuning or tuning approach?
Funny you asked. Believe it or not, TPC Racing has done pretty well in GT class racing over the years for have a relatively small budget. Perhaps its because the key personnel here have amazing synergy or the environment allows more creativity to think outside of the box compared to giant companies that have to go through committees and departments to get things done...who knows. Anyway, in the past we have set up quicker cars than factory-supported race teams with much bigger budget and resources. "They" don't like it when we out think them. I think there's no chance that they will admit that by buying this tech from a small American race team. Haha. But like I said above, its quite possible its already in their plans for future cars.
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Old 06-22-2016, 12:03 AM
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Originally Posted by myBailey07 View Post
How is your toe links different (or better?) than the offerings available from RSS or Tarett that many of us already invested in?
The more common design has symmetrical bushings for the inner spherical rod end(shown on top of the image), which was the design we were using earlier. The new TPC Racing GT4-specific version have offset bushing(show on bottom of image).

On earlier cars such as the 987/997 the outter spherical rod end can be shimmed to tune the toe curve but this is not applicable on 981/GT4 so we have to tune the toe curve at the inner(same as on 997.2 Cup and RSR) instead of the outter.

The symmetrical bushing design has been around for almost 20 years. It fits and works well on a number of cars from 986,996, 987,997, to 981. It works well in that it offer adjustability for "static" rear toe and it is a requirement on a GT4 with increased rear negative camber. But we can't possible expect a nearly 20 year old design to be optimized specifically for the GT4 so this is where the newest TPC Racing version comes in.

Other features are TPC Racing adjustable rear toe links have hex turnbuckle that are longer than others on the market to increase thread engagement into the inner and outter portions, the longer length offer option to widen rear track width, and have fine thread pitch for more precise toe setting while having more thread surface contact area into the mating part.


Premium Aurora rod ends are standard. All machining is done by USA machine shop with military contracts using USA source metals. Stainless steel bushings(not plated plain steel!). Hand assembled by yours truly...when I'm not responding to posts at 10pm.

Having improved the rear dynamic toe curve allows us to run a softer DSC mapping without experiencing unwanted corner entry toe steer(entry oversteer). The softer rear damper mapping produces more exit grip. This pairing subsequently makes the front end of the car even better because the rear damping have less influence on the front!

I know its only a matter of time til the offset design is copied by others. For what its worth, I just wanted to say that it so much harder to be the innovator than the copier. We bought a GT4 to develop, we rented race tracks, made prototypes, tested and tested until we had a setup that we are proud of. We truly put our hearts and souls into developing this formula and illustrated the issues and fixes systematically to the readers. I hope users will enjoy the fruits of our labor.
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Old 06-22-2016, 12:16 AM
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Originally Posted by Tom-TPC Racing View Post

A number of prototype offset bushings were machined and tested in order to produce most ideal dynamic curve throughout the rear suspension's travel range. I am afraid we are not wiling to share the bump steer numbers, these numbers are a race team's "secret sauce". I don't know you and you don't know me, but those who know us know that we are serious about race/track setups to the point of obsessed. With all due respect we do have to protect some of our knowledge. It is our livelihood here at TPC Racing. It is more accurate to say that we sell what we would use on our own car than to say we use what we sell.
Sounds like you got the absolute best dynamic toe curves, I mean other people have some okay dynamic toe curves, but they all pale in comparison to yours. Let's make Driving great again.

Seriously though. You're first to market, your competitors need only measure your device to replicate it. Why not provide the data so I can see quantitatively what you did, how much of a difference it made and make sure there isn't a compromise? Then I'll congratulate you for doing it first and purchase your product. The board has a long memory and will buy your stuff because they'll know you engineered it. Again, competitors could replicate it whether you tell your customers or not.

I would understand you refusing to release the source code for your DSC modules. There's a lot of secret stuff there, probably, like what damping is best for an off camber decreasing radius turn. That's complicated. However, I personally have greater respect for companies that know they are so far ahead of the game with their products, they they are open to share the simple stuff. An example would be: release the toe curves for your toe links, give some basic quantitative data on your damping coefficients for your DSC module, but kept all the curves of exactly how the shocks stiffen up with braking to yourselves.

Conversely, if a company refuses to release the most simple information, then I will tend to assume they don't have any more complicated intellectual property that is actually really innovative, because if they did, they could tell the difference.

(Just to keep beating the dead horse, here's an example of an MX-5 tuner who gives tons of information, proves he's the best through information, not trust, and his stuff sells: http://forum.miata.net/vb/showthread.php?t=584935)

Just my 2 cents.
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Old 06-22-2016, 12:31 AM
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Originally Posted by Tom-TPC Racing View Post
I am afraid we are not wiling to share the bump steer numbers, these numbers are a race team's "secret sauce". I don't know you and you don't know me, but those who know us know that we are serious about race/track setups to the point of obsessed. With all due respect we do have to protect some of our knowledge.
Conceptually, I certainly respect keeping Trade Secret knowledge secret. But usually that is reserved for things that can't be so readily reverse engineered. In general, I'm a fan of being more transparent with information that can be easily measured or analyzed by a competitor that actually has the desire to copy or reverse engineer.

In this case, quantitatively measuring the toe vs. suspension compression for stock configuration vs. the TPC configuration wouldn't take too much effort to do for someone in the business. However, potential customers will never do it, and without such information many may remain skeptical (and unnecessarily so).

On the other hand, the data may not help if it is not particularly convincing. For example, perhaps the difference in toe vs. compression doesn't actually make intuitive sense for why it would be better, yet the racers are convinced it is actually better. Then there's a case where you might not want to share the data after all...
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Old 06-22-2016, 12:49 AM
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Originally Posted by Mech33 View Post
On the other hand, the data may not help if it is not particularly convincing. For example, perhaps the difference in toe vs. compression doesn't actually make intuitive sense for why it would be better, yet the racers are convinced it is actually better. Then there's a case where you might not want to share the data after all...
This.
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Old 06-22-2016, 01:51 AM
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Originally Posted by Tom-TPC Racing View Post
The more common design has symmetrical bushings for the inner spherical rod end(shown on top of the image), which was the design we were using earlier. The new TPC Racing GT4-specific version have offset bushing(show on bottom of image)....

Hand assembled by yours truly...when I'm not responding to posts at 10pm.
Great informative answer, even for 10pm. Thx for taking the time.
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Old 06-22-2016, 02:40 AM
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I'll throw in my 2 cents here:

TPC asked me to test their DSCsport unit. I was sent a unit and did a back to back experiment stock vs DSC at Thunderhill on 6/3. The car didn't feel any better to me. It felt mostly the same. It somewhat felt like the car was diving and rolling more than stock. Lap times were essentially the same. I relayed my experience to Josh at DSC, and he was understandably surprised that I didn't noticed a difference. At this point in time, I saw no reason to run the unit and switched back to the stock box, intending the send the unit back.

HOWEVER, it was discovered my DSC unit was part of a faulty batch of units. I was notified of this immediately and was instructed to go back to stock for the time being. A new set of units were made and programmed, but I was about to head back to the track again. TPC shipped the unit to the track for me so I could do another back to back test of the unit on 6/17 at Thunderhill again. I ran stock in the morning, and then switched to the DSC unit after lunch. This time, I actually felt a difference with the unit. The car dove less under braking, and leaned less during corner entry. The car felt more flat under braking, more stable entering turns, and I noticed that corner entry oversteer disappeared. My lap times were much more consistent and I was able to knock an extra second off of my best time on a ~3 minute course. Definitive proof? Maybe not, but my senses were sharp enough to know that the first defective box didn't do anything and the 2nd functioning box did. At this point, I would choose to keep the DSC unit in my car over the stock unit.

My front tires are wearing faster than the rear, and the outer edges of my front tires are wearing excessively still even with -2.8 camber up front (-2.4 rear). One solution is the add more negative camber, another is to run stiffer springs. I would like to see if the DSC unit alone can solve this uneven tire wear problem without having to add camber or stiffer springs. I'm mounting a fresh set of tires on the car and will run the DSC unit for the next 2 track days and see what happens.

In regards to the toe links, many of us have already purchased aftermarket toe links because we couldn't dial in a proper alignment or run 19" wheels without them. I think there will naturally be some resistance to switching over since many have already invested aftermarket toe links already. I'm curious to hear your thoughts on a post that John Tecce at BGB made several months ago regarding toe deflection:

https://rennlist.com/forums/gt4/9205...l#post13072793

Originally Posted by BGB Motorsports View Post
I wanted to resume this discussion and share what I have found. You have rubber inner control arm bushings front and rear; you have a rubber lined caster puck in the front and an aluminum one in the rear. When we first started engineering these cars for Pro racing we struggled. I had ZERO support and while parts or money would have been great, even information would have helped us save in testing costs. One night while in my Cayman laboratory the light bulb went off that we were experiencing toe deflection. I had some of the best sports car drivers in the country struggling with this car and I stared at the 911 and then the Cayman and then the 911 and BOOM, divine inspiration occurred. The series gave us monoball rear toe links but we had to use rubber rear control arm bushings. In the back, a Cayman has 3 links vs the 911's having 5 and it finally occurred to me that a hard mounted toe link needs a spherical monoball control arm. Without it, the rubber inner control arm bushing gives up when it's loaded. The 911 has the upper dog bones to rely on for this. I made a rules request, got the inner monoballs approved and the radio chatter went silent. Suddenly "that's how this thing should have been handling all along."

I was concerned last week that by selling you all aftermarket toe links I was recreating the problem but it already exists from the factory given that the front and rear OE toe links are spherical in nature and the control arms are not. I was sort of shocked to learn this last week as I studied my car on the lift. A lot of folks don't like monoball inners because of the road noise but I think that as you all outfit your suspension wish list, you need to consider upgrading the inners. The car has such an abundance of rear tire that it isn't the ill handling car that our GS race cars were running 245/275 tires with a soft sidewall but if you are thinking about going to a more track oriented suspension package, it's worth considering. My $.02.
Is this toe deflection issue that John is describing above the same issue that the TPC toe links are designed to address? Does this mean that there are 2 ways around the problem? Or are we talking about 2 different issues?

Also, when will the TPC toe links become available?
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Old 06-22-2016, 03:42 AM
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Joe, great commentary. So... the DSC units don't get functionally tested prior to sending them to customers? Especially customers that they intend to leverage for reviews to drive sales? Or... did TPC just dial yours up a notch since you didn't feel much of a difference initially?

And if it was a failed part, would a less advanced driver have noticed? Maybe not...

I'd be interested in understanding how the units were faulty in a way that didn't cause problems with the car. Wrong firmware? Wrong calibration stored? Just curious really.
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Old 06-22-2016, 04:24 AM
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Originally Posted by orthojoe View Post
Is this toe deflection issue that John is describing above the same issue that the TPC toe links are designed to address? Does this mean that there are 2 ways around the problem? Or are we talking about 2 different issues?
While we wait for an expert's opinion, I'll guess.

As I understand it, the figure of merit, the toe curve, should be plotted against suspension compression, I'll guess without a lateral load. I don't know if the toe curve is "non-ideal" on the GT4 because I haven't seen the data, but the hypothesis is that mid-corner bumps, or weight transfer under braking simply extends or compresses the rear and then the toe changes, making the car less stable. Again, I don't think we have definitive evidence for this and I think this assumption just comes from the fact that the rear is a strut design.

The issue John Tecce raised is deflection of the inner control arm bushing. It seems this could happen under a lateral load, but even in the absence of suspension compression/extension. [A lateral load pushes on all the suspension links, however the stock toe link has solid bushings and doesn't deflect. The inner control arm bushing deflects and this unevenness (control arm bushing deflecting, toe link not) induces a change in toe, maybe]

Therefore it seems both, either one, or neither of these could be issues (toe curve or bushing deflection). However, the explanation from John Tecce seems very convincing that the control arm bushing is an issue. (I hope to be corrected by the more knowledgable if I misinterpreted something)
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Old 06-22-2016, 11:47 AM
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Originally Posted by Mech33
Joe, great commentary. So... the DSC units don't get functionally tested prior to sending them to customers? Especially customers that they intend to leverage for reviews to drive sales? Or... did TPC just dial yours up a notch since you didn't feel much of a difference initially?

And if it was a failed part, would a less advanced driver have noticed? Maybe not...

I'd be interested in understanding how the units were faulty in a way that didn't cause problems with the car. Wrong firmware? Wrong calibration stored? Just curious really.
I think TPC is gonna make you come out to my open track day and blind swap the oem and DSC units on me now. Lol.
I'm also happy to let you plug in my unit for a session to try it out as well. It's easy to do. Good to get more opinions and data points, especially from critical thinkers like us.

TPC can answer this for sure, but my feeling was that the pasm system just wasn't functional at that point and locked in 'default mode' which would explain why it felt like it was leaning more when I first got into the car. I adapt pretty easily so it didn't bother me or notice it anymore after I drove it multiple sessions. However, I will use the hot weather and nonfunctional pasm as an excuse for not running sub 2:00 that day. lol

Originally Posted by Yargk
While we wait for an expert's opinion, I'll guess.

As I understand it, the figure of merit, the toe curve, should be plotted against suspension compression, I'll guess without a lateral load. I don't know if the toe curve is "non-ideal" on the GT4 because I haven't seen the data, but the hypothesis is that mid-corner bumps, or weight transfer under braking simply extends or compresses the rear and then the toe changes, making the car less stable. Again, I don't think we have definitive evidence for this and I think this assumption just comes from the fact that the rear is a strut design.

The issue John Tecce raised is deflection of the inner control arm bushing. It seems this could happen under a lateral load, but even in the absence of suspension compression/extension. [A lateral load pushes on all the suspension links, however the stock toe link has solid bushings and doesn't deflect. The inner control arm bushing deflects and this unevenness (control arm bushing deflecting, toe link not) induces a change in toe, maybe]

Therefore it seems both, either one, or neither of these could be issues (toe curve or bushing deflection). However, the explanation from John Tecce seems very convincing that the control arm bushing is an issue. (I hope to be corrected by the more knowledgable if I misinterpreted something)
I know nothing about suspension tuning, so that was a great post. Interested to hear more from the pros.

Last edited by orthojoe; 06-22-2016 at 12:14 PM.
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