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[ANSWERED] How do the Various PTM Modes Change Parameters When in Track Mode

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[ANSWERED] How do the Various PTM Modes Change Parameters When in Track Mode

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Old 08-19-2015, 05:01 PM
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jvp
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Default [ANSWERED] How do the Various PTM Modes Change Parameters When in Track Mode

Original question is here.

descartesfool asked:
There are 5 modes which can be selected on any C7 equipped with MRC when it is put into Track mode. The owner's manual and a few other documents already released provide some very brief descriptions of the changes to the car's various systems, but it would be very informative to learn more about how the different systems are affected for each mode, and how all the various systems interact. A more detailed explanation of each PTM mode would be very informative for people who drive on track. As well it would be good to know when all the nannies are turned off, which ones are still operating if any. And finally which mode should be used by which type of driver, and is it possible to have a dash display showing which PTM mode the car is in?
Tadge answered:
Although we have talked publicly many times and published quite a bit of documentation about PTM (and other chassis controls), there continues to be additional questions. Assuming this section of the forum attracts a very tech-savvy audience, we are going to give a more technical description than our usual public statements. At the risk of helping our competitors, I have asked Alex Macdonald, our lead Corvette development engineer and expert in chassis controls to provide the definitive treatise on PTM. So, thanks for askingÖ.

Performance Traction Management was first brought to the market on C6 with the 2010 ZR1 and was available on C6 Z06ís with MR from 2011-2013. It has evolved with each new model of Corvette since then but still retains the same functions and philosophy as when it was introduced. All of this discussion applies to any Corvette with PTM, not just the C7. For C7 any Z51 with MR dampers and all Z06ís are equipped with PTM.

The first important part of PTM is the traction control function. This is the core of PTM. Later I will describe the other adjustments it makes to the chassis but the fundamental change in each mode is the logic and calibration of the traction control.

It will help to understand how the standard TCS system works when not in PTM. See Fig 1 for an example of a C7 Z06 turning left at 25mph and approximately 0.8G lateral acceleration (a spirited left turn). The driver steps to 100% throttle (point #1) which causes the rear wheels to start spinning. Prior to spinning up the tires, the TCS system doesnít know much about the road surface conditions. It could be wet, dry, gravel, coarse concrete, smooth asphalt, etc. To determine the surface condition the engine torque is allowed to increase until the wheels start slipping a certain amount (point #2). They would continue to slip at this torque level so torque is quickly reduced to stop the slipping (point #3). The initial engine torque that caused the wheels to slip along with how much torque needed to be removed to reduce the wheel spin are used by the TCS to calculate the type of surface the car is on. Once the surface grip is known the torque is added back in to maintain an amount of slip thatís appropriate for that surface (point #5).



The drawback to this method is that the wheels have to over-slip past the ideal target at the start. This disturbs the cornering of the vehicle and requires the driver to adjust the steering angle. Further, bringing the wheels back down from the initial over-slip with a torque reduction dramatically reduces acceleration in order to regain stability.

The use of the PTM switch tells the car that it is definitely on dry asphalt (or wet asphalt in in mode 1) and that the driver is prepared for some amount of wheel slip. Since the surface is known the PTM system can make a good approximation of the maximum possible engine torque that will not over-slip the tires based on how hard the car is cornering and how fast itís going. This torque estimate is used as a starting point (since no new information about the surface is needed in PTM) and then depending on the amount of slip that results more fine adjustments can be made. As the driver unwinds the steering wheel the tires gain grip and torque will be fed back in due to the lower slip. See Fig2 for the same maneuver described above but using PTM mode 2 this time.



As you look at these two examples one thing to note is how much more gradually steering and lateral acceleration can be reduced on corner exit in PTM vs normal TCS (Red and Tan lines). The initial over-slip in normal TCS is what prompts the driver to make this abrupt steering adjustment.

Each of the Traction Control modes in PTM are differentiated in two ways. First, the target slip levels are lower in the lower modes as you would expect. Second, the estimate we make for the starting torque is lower. In DRY we err on the low side with our estimation and may have to increase torque to reach the target slip. In RACE we err on the high side and the driver may have to use more steering correction to manage the extra slip in the rear until the target slip can be recovered.

Along with the traction control, the other chassis subsystems are modified for the conditions expected in each mode. The following is a chart of the status of each subsystem for each PTM mode:



Mode uses:
WET: This mode is intended for any driver who is on a wet track. The track should be wet enough to be glossy, not just damp. A damp or drying track will require the driver to decide between WET and DRY modes to get the best performance. Standing water is not recommended as hydroplaning is possible and no TCS system can accurately control a tire that is hydroplaning.

The slip targets are very low, lower than in standard TCS. The MR is in tour mode to maximize mechanical grip since the lower cornering forces donít require the extra control offered in Sport or Track. ESC is on and uses the normal calibration, not the competitive calibration.

DRY: This mode is for a novice driver on any track or an experienced driver learning a brand new track. I also use it to warm up the tires or run the mold release off of new tires, especially if itís cold out.

Slip targets here are very similar to normal TCS but will feel very different due to the entry prediction. MR is in Sport to compliment the usage we expect in this mode. ESC is on but uses the competitive calibration.

SPORT 1: This mode is for any driver who is ready to run very competitive lap times while still having stability control on in the background. I use this mode whenever I have a passenger, possible distractions such as data collection, or as I am continuing to become more familiar with a new track.

Slip targets are higher here than in normal TCS. MR is in track mode and ESC is on using the competitive calibration.

SPORT 2: This mode uses the exact same traction control settings as SPORT 1 but turns off ESC completely. This mode is designed as a way to turn off ESC while maintaining a fairly stable TCS calibration. I use it very often. Basically any time where tenths of a second are not critical to my testing or when the tires are getting too hot or wearing out. After 10 to 15 consecutive laps it is likely that your fastest times will be achieved in SPORT 2 rather than RACE.

Slip targets here are the same as SPORT 1. MR is still in track mode and ESC is turned OFF.

RACE: This one is pretty self-explanatory, itís as fast as we know how to make the car go. This mode is for a well prepared car on relatively new tires with an advanced driver that is completely familiar with the track. The track should be somewhat warm and the tires should be up to temperature.

Slip targets are 100% optimized for forward acceleration, any small variances in the track or tires can result in momentary overslip that will have to be managed by the driver.

A few notes about RACE mode: I approach this mode as a tool to go faster meaning that I think specifically about it when I go to WOT and I try to change or optimize my throttle application point and steering to help the computer do its job. It can do the job better than me but only if I give it good information. A specific example is how I unwind the steering wheel mid corner and as the corner opens up. When learning to drive on track it is good practice to automatically unwind the steering wheel as you apply throttle regardless of the vehicle response (a string tied from throttle toe to steering wheel is the analogy used by some instructors). The problem in RACE PTM is that by doing that you are telling the car you are ready to go straight. It will add power and drive you to a wider line. The best results are found by holding the wheel as steady as you can and pointing the car to corner exit only when itís time. Obviously you have to be ready to correct for overslip but as you gain trust you will find this is required less often than it feels at first. It is very useful to practice using PTM in the lower modes where you can focus on letting the car go where you point it and not correcting your steering too early.

We find that the calibration settings that achieve the fastest lap times often result in expert drivers feeling like they are being held back slightly on corner exit. Most of the time that extra little bit of slip that an expert driver wants is over the traction peak of the tire and may feel good but is wasting forward acceleration. The restrictive feeling is not there as a safety net, rather, itís as close as we can operate to the peak capability of the tire in a robust way.

Other notes: The eLSD ďPTM modeĒ is only a slight alteration of its normal track mode. This alteration is required since it is likely to see engine torque values that would not occur without PTM. However, the philosophy of what itís trying to do and how itís controlling vehicle dynamics does not change between PTM and TCS/ESC OFF mode.

As for the nannies, that term is a big gray area. I would personally call TCS, PTM, ESC and active rev match nannies but would not call the eLSD or MR dampers nannies. ABS is very gray. ABS, TCS, PTM, ESC and active rev match are trying to do a better job of something you normally do as a driver, steering, throttle, brake or shifting. However, no driver ever controls a differential directly or a shock absorber directly. They are part of the base chassis tuning and can be thought of as a calibration component like a spring or anti roll bar.
ABS is more of a gray area but it is not allowed to be shut off because the capability of the vehicle is so limited without it. You may need 200 bar of brake pressure to get max decel from a wheel thatís on the outside of a corner but the inside wheel would be totally locked at 50 bar. Unless you want a lot of flat spotted tires you need ABS in that situation. Also, somewhat like eLSD and MR, individual wheel brake pressure is something even the best driver canít physically control without the electronics.

Thatís a long way of saying you can shut off TCS, PTM, ESC and active rev match but you canít shut off MR, eLSD or ABS.
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Old 08-19-2015, 07:21 PM
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I've been waiting for this mostly complete description for a long time. It's been nearly a year since I personally talked to Alex about PTM at the NCM anniversary. Someone also needs to tell the guys at Spring Mountain that yes the MRC does change with PTM mode (I got into an argument with one of the instructors about that).

The only thing that is missing is that although Alex indicates that ABS is never disabled, it's calibration does change with the mode; it's going to kick in earlier and more agressively in Wet as compared to Race.

Alex also told me that PTM on the C7 is very similar to its implementation on the Camaro ZL1. See http://youtu.be/jHeCT8k2LrA (tried to add as a link on my original edit but it crashed on my phone!)

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Old 08-19-2015, 09:26 PM
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As an engineer, its threads like this that remind me of how amazing corvette forum truly is.

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Old 08-19-2015, 09:41 PM
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Originally Posted by LrgZ51 View Post
I've been waiting for this mostly complete description for a long time. It's been nearly a year since I personally talked to Alex about PTM at the NCM anniversary. Someone also needs to tell the guys at Spring Mountain that yes the MRC does change with PTM mode (I got into an argument with one of the instructors about that).

The only thing that is missing is that although Alex indicates that ABS is never disabled, it's calibration does change with the mode; it's going to kick in earlier and more agressively in Wet as compared to Race.

Alex also told me that PTM on the C7 is very similar to its implementation on the Camaro ZL1. See http://youtu.be/jHeCT8k2LrA (tried to add as a link on my original edit but it crashed on my phone!)
Here you go.



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Old 08-20-2015, 04:56 AM
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Fantastic reply! Great to get such detailed technical info.
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Old 08-20-2015, 08:04 AM
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this was an excellent read I enjoyed going through it
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Old 08-20-2015, 08:47 AM
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Very cool!!! What is Launch Control - Drag Strip??? Higher RPMs for prepped surface?
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Old 08-20-2015, 09:09 AM
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Thank you. I am truly impressed and appreciative of the through response.

I too would like a bit more explanation of the launch control differences.
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Old 08-20-2015, 04:24 PM
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Jvp - I know I hit you offline on this but is there any option for a follow-up or did I misunderstand something?

The quote below seems to indicate that PTM assumes a certain grip from the tire and pavement. That would also seem to imply that in cases where you run a stickier tire (i.e. Hoosiers) it might be withholding power needlessly. Conversely if you are on a track with poor grip (such as Autobahn that uses sealant on purpose in places) or well worn tires then the driver will need to be extra mindful as PTM will assume more grip than is present.

"The use of the PTM switch tells the car that it is definitely on dry asphalt (or wet asphalt in in mode 1) and that the driver is prepared for some amount of wheel slip. Since the surface is known the PTM system can make a good approximation of the maximum possible engine torque that will not over-slip the tires based on how hard the car is cornering and how fast it’s going."

Finally, is the PTM tuned differently for the different models and assumed traction (i.e. FE6 with MPSS tires, FE7 with MPSC2, Z51 with MPSS). Seems there would be value in some user adjustability of this expected grip parameter.
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Old 08-20-2015, 04:58 PM
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It's interesting you bring that up Poor-sha, I was debating the same idea earlier. I kind of quashed it in my brain though, as the Z06 comes with multiple tire compounds from the factory, as you know.

What I took away was, there is some variation, and you need to somewhat "dial in" your slippage, based on the compound and conditions, but I also understand your question is more directed at the "torque estimation" that is applied. I think, though this was given as a "technical" explanation, there is some proprietary information possibly, on how this works exactly. Of course, I'm am assuming and giving benefit of the doubt I suppose.

I'm no traction control system engineer but this all still sounds like just an advanced fuzzy logic controller, taking various parameter inputs, coupling them with pre-programmed values, then outputting a negotiated command. So, while there are some programmed torque output value assumptions (determined by the ptm mode), it'd combine wheel slippage, lateral acceleration values and steering angles with those values. So, essentially it'd always make the "best assumption" based on the grip calculations it's making. This is essentially the same as the engine control system, where there are hardened volumetric efficiency assumptions made, but that then negotiate via incoming sensor information to make variation adjustments.

Like I said...that's what I'd assume.

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Old 08-20-2015, 08:20 PM
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They could have tuned the PTM different for FE6 and FE7 cars since the standard tires are different as are the suspensions. In theory in that case the PTM in a Z07 would allow more torque to be put down by the motor since it is tuned for more grip. Of course, I'm just guessing and inferring so it's likely nonsense.
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Old 08-20-2015, 10:12 PM
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Originally Posted by Poor-sha View Post
They could have tuned the PTM different for FE6 and FE7 cars since the standard tires are different as are the suspensions. In theory in that case the PTM in a Z07 would allow more torque to be put down by the motor since it is tuned for more grip. Of course, I'm just guessing and inferring so it's likely nonsense.
Put simply, if so, the Z07 calibration would be faster tire-to-tire compared to a Z06. Tell me again why I didn't buy a Z07?
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Old 08-20-2015, 11:29 PM
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It's certainly possible the base parameters could be different, but keep in mind that the system could easily calculate actual grip levels by looking at steering angle, lateral loads and wheel slippage. I really think the system has the ability to compensate, but I'm a nobody so.... They'd have to get more in depth with the explanation.

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Old 08-21-2015, 05:21 PM
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If the rear tires are spinning faster than the front tires, the rear tires are slipping. It doesn't matter what the type of tire it is. System would just allow a certain % of extra rotational speed for the rear tires than the front tires before throwing in some traction control. The % might change slightly depending on tire type, but basically if you are running Hoosiers, they will have more g's before the rears start spinning, so traction control will not intervene until they start to slip by the given % relative to the front. That is for the simplest of traction control systems. Then they can add a bunch more stuff into the equation, like the eLSD setting to determine how much one of the rear tires can slip more than the other without any intervention. It's all a very complex algorithm I am sure, but it seems to work quite well on track with R comps if you are in Sport 2 or Race mode.
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Old 08-21-2015, 08:29 PM
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^ You're missing the point of the conversation sir. Go back and reinterpret the data. No one is disputing the system responds to traction loss and that response will wait for wheel slip....that's obvious.
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Old 08-22-2015, 09:42 AM
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Originally Posted by Poor-sha View Post
The quote below seems to indicate that PTM assumes a certain grip from the tire and pavement.

Finally, is the PTM tuned differently for the different models
The basic problem is the system cannot measure the available grip until the rear tires spin (for longitudinal traction) and/or yaw error (ie, slip for lateral traction) is detected.

The different grip assumptions used in the various PTM modes are simply the calibration system's initial point until the actual grip can be measured. This point, in combination with other measureable parameters such as actual ambient temp, tire temp estimate, throttle and steering angle and rates of change, etc, allow the system to more quickly react when the measureable grip data is available, as shown in the charts presented.

And yes I'm sure the PTM tuning and calibration is different for the different models (Z51, Z06, CTS-V, ZL1), but not so 'tight' as to be that sensitive to differences in tires for a given model (such as base Z06 vs. Z07), because again, all we're talking about is that initial grip estimate. And as Alex mentioned, when the driver knows the tires are cold or worn, he should 'dail back' the PTM mode so the system has a better initial estimation of the grip to start with.

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Old 08-22-2015, 08:04 PM
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See that...no one listens to me...lol.
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Old 08-24-2015, 07:34 AM
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Thanks for responding. I appreciate it.
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Old 08-24-2015, 08:02 PM
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Great info.
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Old 08-24-2015, 08:45 PM
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Thanks JVP and tadge.

This is why I come on here every day. For information like this straight from the guys whom designed it.
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