I believe the Bosch Motorsports ABS control algorithm doesn't bother with rough road surface detection. i.e. basically when the wheel with the slowest rotation and the wheel with fastest rotation change places during a single iteration of the main control loop. They have dozens of patents going back 30+ years that attempt to resolve this but don't seem to have managed for all use-cases.

 

997-Rich - thanks. My ratios seem to look good. My 997S (not a 996GT3. That was an older car) :

OEM (PS2s)
R - 295/30 ZR19 - Circ = 81.584 (inches)
F - 235/35 ZR19 - Circ = 80.035
F to R ratio = 0.981

New Wheels/tires (Cup2s)
R - 305/30 ZR19 - Circ = 82.325
F - 245/35 ZR19 - Circ = 80.902
F to R ratio = 0.982

So...I am guessing it is a combo of aggressive pads (PFC08) and cup tires. I may switch to pagid yellows as I had no problem with them on the 996GT3 and cups.

 

It is also in part the way you use the brakes...

Ray

 

Rbhar- I have done DEs for 14 years. I have carted, raced open wheel renaults, and have had 4 different porsches including Carreras, gt3s and boxsters all of which I have tracked. I agree braking technique is important but the fact that this set-up is the first time in 14 years that has resulted in "ice mode" would lead me to believe it is something other than technique in this case.

 

ECS: No offense meant, I would suggest that you look at this thread:
https://rennlist.com/forums/996-gt2-...-ice-mode.html

about ice mode. In addition there is a very good video which is a technical discussion about brakes and 'ice mode' that I would recommend.

There are two significant components at play here - HOW you use your brakes, and how effective the brakes (pads, rotors, tires) are, anyway that article and the video has good information...

Ray

 

Thanks rbahr. Good video. I am actually going to look in to the suggestion of a more aggressive rear pad to take advantage of the initial higher rear bias on the 997. I have seen a number of guys do that.

 

Glad you liked it... I have (successfully) done that in the past - I use PF pads, so I might use 01 / 08 mix or something like that. With the GT3, I am currently using the 08's which seems to work OK...

Ray

 

Hello Everyone,

I've been hesitant to chime in on this thread, but the speculation and misinformation has reached a point that I feel I must respond.

First my background: I am a senior chassis control engineer at Bosch with 17 years of experience calibrating ABS, TCS and VDC (stability control). I started tuning ABS in 2000, and worked extensively with ABS calibration on the system that is installed in the Porsche 997 variants. I also know many of the Bosch engineers in Germany who work on the Porsche projects.

I recently saw this thread after purchasing a 997.2 GT3 and joining Rennlist. In fact, there is a reference earlier in the thread to an ABS article I wrote for the E30 M3 community back around 2003 (Post 118 by Stuttgarter, who was a former colleague of mine). Unfortunately the link no longer works, but I will see if I can find it and repost.

Here are the main items I want to clarify:

Ice mode: Whenever I hear this term, it’s usually in reference to poor deceleration upon initial braking. There are three main causes for this:

1. Component changes (tires, pads, rotors, suspension parts)
   
2. Tuning strategy to meet customer specifications on ice and snow
   
3. Driving style (timing and rate of apply)

Erratic ABS behavior such as bump sensitivity or wheel lock: mainly caused by component changes that are incompatible with the production calibration (see #1 above)

Component Changes: When tuning projects for series production, every component on the vehicle contributes to the performance of the entire system. It’s not uncommon for manufacturers that have very high performance expectations (like Porsche) to request an ABS or stability control retune for a small design revision from the tire supplier. This could be something like a slight change to the compound composition or cord structure ON THE SAME design/size OEM tire. Think about that when you are installing oversize Hoosiers on your car.

Same thing with pad/rotor changes – the controller is no longer tuned for optimum performance with a pad/rotor interface that has different friction coefficient and apply/release characteristics.

Also think about suspension changes like dampers, springs, and even bushings. The ABS, TCS and stability control are no longer optimized for a vehicle that now has different weight transfer and suspension articulation characteristics from the OEM spec vehicle.

To summarize, the OEM ABS, TCS and stability control tune becomes invalid when installing aftermarket tires, pads and suspension. Even slightly different tires and pads could be enough to reduce robustness of the system performance. It’s a testament to the robustness of the Bosch system that it works as well as it does under these circumstances.

Tuning strategy to meet customer specifications on ice and snow: All manufacturers have requirements for ice and snow performance for road cars. So the ABS, TCS and stability control have to provide optimal performance for ALL surfaces. The purpose of the logic development and tuning process is to maximize performance on each surface, with the least amount of compromise for other surfaces. Obviously this is very difficult, requiring fine tuning to achieve all of the OEM performance goals. This is the reason why component changes can have large effects to the robustness of the system.

Driving style, especially with modifications: as mentioned above, many of the modifications made for track use result in much different wheel dynamics that the controller is not optimized for:

• Sticky tires: Mue/slip curve is much more “peaky”, meaning the tire tends toward lock very quickly when exceeding the linear range of the curve.
  
• Track pads/rotors: higher coefficient of friction and “bite” result in faster wheel lock at lower pressures.
  
• Stiffer suspension/dampers/bushings: slows load transfer and build-up of normal force, resulting in earlier wheel lock during quick brake applies.

All of these mods cause the tires to slip faster during braking than the controller was tuned to expect for dry pavement. Now add in an aggressive driver who spikes the brakes at initial apply and the tires will tend toward lock very quickly before the vehicle builds much deceleration. The controller will release pressure to recover the tires (prevent deep slip/wheel lock) and the driver feels the lack of deceleration as “ice mode”.

So if you want to avoid ice mode on a vehicle with heavy modifications, the best recommendation is to slow the initial apply and then quickly ramp in pressure after the initial weight transfers occurs. This is proper braking technique to prevent wheel lock for non-ABS cars as well.

There are additional situations that can contribute to ice mode, such as bumpy surfaces, and aggressive turn-in during braking. Remember the primary goal of ABS is to prevent wheel lock so the driver can steer. The only way to prevent wheel lock when the tires are skipping across a bumpy surface or the inside tires have no load is to RELEASE PRESSURE. The pressure loss will be proportional to the time the unloaded tires have no grip. ABS braking performance in these situations is usually not a problem with an OEM spec vehicle, but track modifications will affect robustness.

The Motorsport ABS controller works so much better for track driving because it is optimized for race tires/suspension and has a selector switch for different surface conditions such as rain, cold tires/cold pavement, hot tires/hot pavement. Ice/snow performance is not even considered. Obviously this is a much more robust solution for track only driving in a modified car if you have $10k to spend. Just don’t try driving this same car on a snowy mountain pass…

Additional Warning: Under NO circumstances should you try to defeat the ABS system by pulling a fuse, unless you install mechanical rear brake proportioning. You will lose EBD control, and lock rear the rear wheels before the fronts in almost every situation. Very dangerous.

Steve

 

The previous post was the best general explanation of ice mode which I've seen. I can tell you if you take a Lotus Elise and add stiff track suspension, Hoosiers, and race pads, then nail the brakes late at the bumpy end of a long straight (Summit Point), you will get absolutely terrifying ice mode, beside which GT3 ice mode is a walk in the park.

 

Thanks Steve,

First, congrats on the car!

Back to the care and feeding of ABS systems... I appreciate the more technical explanations...

One thing stood out, It sounds like we are pushing this system way beyond its design envelope by just bolting good go-fast/go-slow parts on. You very clearly support the driving style that the video I posted suggests, BUT it also sounds like trail braking is a major no-no Another question: Why does Bosch charge so much for a proper track system? It would seem to be mostly software - which I know is not cheap, but we are talking safety here...

Thanks

Ray

 

Excellent explanation Steve. Thank you for chiming in. I am running the 996Cup Motorsport ABS and it is an absolutely phenomenal set up compared to the street unit. It doesn't mater how bumpy the braking surface it, how quickly I apply the brakes, or how hard I brake at large slip angles. The ABS is simply unflappable works with 100% reliability even with large wheel diameter differences. I have a couple of quick question. I have driven the car on the street in dry weather but have not driven it in ice and show or on street tires. Would the Cup ABS controller provide worse performance under those conditions compared to the stock controller?

Also, the Cup ABS unit is physically the same as a simple 986 Boxster unit / 996GT3 street unit. Heck, the Boxster ABS is available on ebay all day long at $150. Used Cup ABS is easily $5K plus if you can find one and they usually sell the same day. I have a couple of spare 996Cup ABS units and one of them actually has a 986 part number on them with the Motosport part number stamped over it by PMNA. Obviously the only difference is in the programming of the "brain". Would it be possible to reflash the stock Boxster ABS with the Cup ABS programming? The Cup ABS communicates with the car over K-line while the street units over CAN bus. Because of this I'm not sure if there is more to it than simply the programming. Thank you...

 

Thanks for taking the time Steve, I will try to remember the above.

 

Thanks Steve, your input has put some clarity to all this, however it would also be good to try and understand how, other than braking technique, modifications can be managed/chosen so as to have minimal effect on the ABS robustness. I'm sure that people will not stop modifying their cars, so understand around what, say stickier tyres or wheel diameter ratio changes etc.... have on the ABS system would be invaluable.

Ie. I have a Cayman R and recently upgraded the front calipers and rotors to the larger 997.1 GT3 350mm rotors, 6 pot calipers and master cylinder. Rears are OEM.

I have noticed in my first trackday since the upgrade that I can induce varying levels of 'ice mode' which I never had before with OEM brakes.

I'm still glad I did the upgrade, as the brake performance (even with this 'ice mode') is far better than before. However it would be good to start ubderstanding what effect this has had on the ABS system so I can potentially improve these negative effects with further upgrades to say, the rear brakes, with either rotor size, pad type or even caliper size...

Is it as simple as trying to return every factor to as close to OEM as possible? And if so what are the main factors to consider ie. tyre size ratio between front rear, brake bias etc...

Some input from a ABS programmer would be invaluable here..

Cheers

 

Good post, summarizes well my 'end user' understanding of the issue and the remediation options. Good to know the intuition is the right way to handle the situation. :-)

 

Thanks for the positive feedback everyone. I will try to answer your questions as effectively as possible.


Ray:
To clarify my statement, braking hard while in ABS then turning in quickly can cause deep slip on the inside wheels and a release of brake pressure. If you are trail braking properly, you should be slowly releasing brake pressure upon turn-in all the way to zero brake pressure at the apex. So you should not even be in ABS during trail braking if you are doing it right. It's the driver who stabs the brake pedal hard and stays in ABS all the way to the apex that will have an issue with ice mode on a modified vehicle.


Powderhound:
Absolutely the performance on ice and snow will be nowhere near as safe as the stock controller. As you mentioned, the Motorsport controller is tuned to allow much more wheel slip before releasing pressure. I would expect your car to lock wheels fairly easily on snow and ice even with winter tires. It may not be to the level that the car is unsafe to drive, just be careful if you decide to try it! I can't comment on the possibility of mixing ECU/hydraulic units or reflashing the Motorsport tune in another ECU as I do not know the details of the hardware you mentioned. But my guess is that it's not possible or someone would have done it by now.


I don't have any experience with the Bosch Motorsport ABS system, but I know some of the guys who tune it. My guess is that it cost so much due to economies of scale: They probably sell a few hundred of these systems per year vs. millions of street car systems.

Moochier: I will be in your neck of the woods at the end of July at SHPG (Snow Farm) on the South Island for winter testing!
In my initial post I explained the negative effects of many common modifications. In your case, you installed different calipers, rotors, and pads which combine for a HUGE change to the brake system characteristics. Think about it: You have changed the apply/release characteristics (pads), friction characteristics (pads/rotors) and fluid consumption characteristics (calipers) of the system. Other than returning the vehicle to stock configuration, there is no "guideline" about how to improve the system with further modifications. As a general rule though, the more modifications you make to your car, the larger the effects will be on ABS system robustness with the stock controller and calibration.


Best Regards,


Steve