A little inside knowledge on the Corvette's TC/PTM modes

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Most of you know this information (not me). For the ones that don't here is the inside scoop on Chevy's PTM.

Originally Posted by jvp

Original question is here.

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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 asking….

Performance Traction Management was first brought to the market on C6 with the 2010 ZR1 and was available on C6 Z06’s 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 Z06’s 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 doesn’t 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 that’s appropriate for that surface (point #5).

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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 it’s 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.

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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:

Click the image to open in full size.

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 don’t 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 it’s 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, it’s 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 it’s 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, it’s as close as we can operate to the peak capability of the tire in a robust way.

Other notes: The eLSD “PTM mode” 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 it’s trying to do and how it’s 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 that’s 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 can’t physically control without the electronics.

That’s a long way of saying you can shut off TCS, PTM, ESC and active rev match but you can’t shut off MR, eLSD or ABS.

[M6-Coupe]

Thanks for sharing

[Used2be]

Originally Posted by URBAN LEGEND

Tadge answered:

Thanks for sharing, that's a lot to think about. Do you have a link to the figures Alex MacDonald is referencing?

[Vic55]

You had me at "urban legend"

[SocalBM]

As much as I love the Vette (had 2 Z06's, not sure I want this C7Z) I really don't trust Tadge, or anything he says. The company has put so much spin into their PR crap and the community had debunked a whole hell of a lot of it.

Not to mention, the worst part is their refusal to admit there are serious problems: Power steering overheats, transmission overheats, differentials grind and then pop, engines have blown far too often - press cars for fuck's sake - manuals popping OUT of gear while on the move, and the notorious bad panel alignment.

Some of those things are "meh" and some are fucking serious as hell. Nothing was fixed after a year of the cars being on the market.

No thanks, my hunt for a GT3 Unicorn continues.

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http://http://www.corvetteforum.com/...rack-mode.html

Originally Posted by Used2be

Thanks for sharing, that's a lot to think about. Do you have a link to the figures Alex MacDonald is referencing?

Click here. It's the 1st reply.

http://www.corvetteforum.com/forums/...rack-mode.html

[turbo8765]

Originally Posted by SocalBM

As much as I love the Vette (had 2 Z06's, not sure I want this C7Z) I really don't trust Tadge, or anything he says. The company has put so much spin into their PR crap and the community had debunked a whole hell of a lot of it.

Not to mention, the worst part is their refusal to admit there are serious problems: Power steering overheats, transmission overheats, differentials grind and then pop, engines have blown far too often - press cars for fuck's sake - manuals popping OUT of gear while on the move, and the notorious bad panel alignment.

Some of those things are "meh" and some are fucking serious as hell. Nothing was fixed after a year of the cars being on the market.

No thanks, my hunt for a GT3 Unicorn continues.

Just like blown f8x engines due to oil cooler placement, chassis malfunctions on track etc..... These issues are highly overblown. That's the nature of a car forum.

[froggy47]

A lot more info than I could find on Corvette Forum, thanks.

Specific question:

In SCCA autox, as you may know, it is required to run your competition laps (at least the first & maybe second) on "cold tires".

So what specific tire temp is required by PTM to get it "working".

If I turn ALL OFF does that cancel the requirement for tires to be at a certain XXX temp?

Would it be smart to run the first lap or two ALL OFF, then when the tires have some heat in them, use the PTM higher levels (lesser intrusion)?

Thanks if anyone knows, please don't guess, little point in that.