Fweasel

iTrader: (6)

I got the ABS system to engage for the first time since owning the car over the weekend. Someone merged in front of me on the highway, then hit the brakes hard. I hit my bake pedal pretty hard in response, not surprised ABS kicked in, but how the car reacted was very surprising. The nose of the car dove pretty severely (stock suspension's fault mostly) but the pedal continued to travel further than I pushed it, as if there's a panic brake assist to the stabilitrak system, which resulted in significantly more braking force applied than I intended, or then the situation even called for. Has anyone else experienced this?

I've read about people disabling ABS systems on other platforms for various personal reasons, but I never felt that any ABS system was a hindrance to my driving, until now. The conditions were pretty ideal in my case, dry, flat, new pavement, straight line, Michelin PSS tires, proper inflation pressures... hardly any need for ABS in my opinion. The additional brake assist applied could have created a problem for me in terms of being rear ended. I'm wondering if Competitive Driving Mode would prevent this?

Andringa

I'm assuming you have a V1, if you have a V2 my comments might not apply.

From my experience, the V1 does not have any sort of advanced ABS or brake assist.

I've driven my 2004 V through 6 winters in WI, so I'm pretty familiar with what it does, and it has always been simple and predictive.

Additionally, in the summer, I autocross it and instruct at a Teen Car Control Class, during which we do panic stops (55mph-0) on dry concrete and a wet skid pad. I've never had anything but normal ABS behavior.

If it is a V1, my best guess would be that the ABS control module had air in it which caused odd behavior when it cycled. When bleeding the brake system, you can use a Tech II to cycle the ABS module to bleed out the trapped air.

If it is a V2, I'd be interest in hearing from other owners, as I wasn't aware that it had a brake assist feature either.

Naf

iTrader: (1)

I have driven cars with and without abs..

You just have to teach yourself how to use the abs

No abs hard braking leads to tire lock. you will just travel in a straight line regardless of steering input

ABS typical situation
-Brake pedal drops to the floor and there is no feelin of braking, however steering input is available to get around stopped object.
-If there is no way around stopped object impact will happen

ABS learnt technique
-Once ABS is engaged, i lift my foot from the pedal and reapply, this way whatever event triggered the ABS in the first place has passed and i am starting fresh, I also down shift to 2nd/3rd and use engine to assist in the braking.

Its the same for a manual and automatic

barrok69

I'm not sure you would be able to avoid that situation even in comp mode. Panic assist comes on due to the sudden rate of change of applying your foot on the gas pedal to the brake pedal. You can work around the system generally by waiting a brief moment to push the brakes.

Unloading the rear tires probably didn't help your situation with the stock suspension therefore causing the abs to put more pressure to the fronts.

The system is also not calibrated for the grip level of your tires so you could be experiencing more decel than the system was actually calibrated for because all it knows are pressures from the abs module and level of slip generated at the wheel speed sensors.

my 0.02c

Fweasel

iTrader: (6)

Yes, 07 V1, sorry, I forget not everyone can see signatures. I DD this car in the winter in Chicago and get plenty of ABS then. Funny, I said it was the first time for ABS last weekend, but actually, ABS happens in the winter often enought. After two winters, the braking response in cold and slop was normal and predictable and never even blipped my radar as an issue. The brakes were just bled, twice, at the dealership with the TechII after they did the brake line recall. I suppose they could have air still in the pump, but the pedal has been firm before that service, after, and after the ABS event this weekend.

I've got a respectable amount of seat time in my previous cars at Autobahn in Joliet. I never had an ABS engagement so severe and overpowering in two other cars there, or on the street. I really think this is less about my abilities and more about a difference in software intended function.

So there is a panic assist on the V1 for sure? That would make the most sense as this was a throttle to immediate brakes, there was no time for a pause. I might test that out next time I'm on the back roads; throttle then sudden stop vs. coasting then sudden stop.

DMM

Yes. The V1 does have this integrated into the stability control system. It's only functional on dry pavement and shortens stopping distances by about 20% (my guestimation, anyhow). I've never been able to invoke this feature intentionally in any of my vehicles. I don't know what the programming logic is, although it seems to be able to confidently determine an emergency stopping situation from an intentional application.

barrok69

I don't know the exact time on our cars but to test it out do the following (Make sure nobody is behind you!!!):
Go from the gas to the brake pedal as fast as you can (easily doable if you two foot).

You will get into this mode and it will be apparent. I'm not sure the exact calibration details but it could vary from speed so at higher speeds it will apply more Brake pressure than at lower speeds.
Your pedal will go down all the way as the ABS pump is taking over and it will come to a stop very abruptly.

Andringa

Ok, now I'm really curious, I'm going to try and engage this mode.

Naf

iTrader: (1)

Guys where are all you?

There is no software that helps your car stop in a shorter distance. Abs increases the distance, stabikitrak only keeps your car from spinnin out.

Your experience and wisdom whilst pressin the brake pedal is the only means to get shorter brakin distances...

Volvo has the automatic brake system but its not shorter than an abrupt stop

Fweasel

iTrader: (6)

I'm not saying it did. I reported that the ABS system chose to input more braking force than my brain and foot did during a panic stop. Never have I driven a car before with ABS where there was anymore feedback than pulsations through the brake pedal and a buzzing noise from the pump during an ABS assisted stop.

barrok69

If you think you can out brake ABS then this conversation is over. Maybe if you are talking first gen ABS from the 70's, but modern ABS, aka anything newer than 1995, will do a much better job than the human foot ever can. ABS will not increase stopping distances. The brake hardware is designed in such a way that it's actually really hard to brake without ABS these days. You'll flat spot your tires real quick without ABS.

Most of the newer cars I've driven have a form of panic stop configured through the ABS module and it will make you stop faster because it puts maximum line pressure into your brakes in a very short time, therefore reducing stopping distance.

Naf

iTrader: (1)

I dare you to a wet street contest...

Find an isolated street with zero traffic but asphalted. Place cones for when to stop and where you stopped.

A controlled test would be to have the street dry get the shortest distance then wet the street and try with the abs goin crazy...

Abs is a pulsin system that helps give you steerin response at the cost of stoppin distance.

Gen 1 or gen 3 its all the same, but the pulses get quicker and the stoppin distance slightly less but still a dry hard brake with great tires cannot be beat or even matched by the abs

barrok69

I help develop these systems on numerous vehicles and you are completely wrong in every case.

The ABS car will win every time vs. the same car with ABS disabled.

Common misconception with ABS systems from "old school" people are that the stopping distance is longer...yes that is true for the Original ABS developed in 1971. But it is not true in any modern ABS system.

A skidding tire has less traction than a tire that is not skidding. End of story. ABS prevents the skid.

I win the race. Where is my prize?

Naf

iTrader: (1)

But that wasnt the challenge...

If you read my earlier post, a skidding tire has no steering, ABS has a shorter stopping distance than skiddin, but longer than no abs active.

The ABS helps keeps our car in control where it gives you the opportunity to steer around the object. Skiddin will let you slam into it.

My story, which hasnt changed, is when your abs activates your braking distance is increased compared to when it doesnt activate.

The reason being is it pulses the brake pressure allowing you to steer and not lock up the front tires.

Most of the time when you upgrade your brake system you have to also upgrade your tire/wheels or else the abs activates under hard dry driving. Case and point I use to have a 2003 tahoe where i had the stock wheel and tire combo but i had the baer 6 piston calipers up front with 15" rotors and eradispeed +1 rotors in the rear. I was drivin a little quicker than i should have and had to come to an abrupt stop, well, lets just say i had to press the E-brake pedal as well to help me come to a screeching stop...

The rear tires today have stabilitrak where if it feels you starting to spin out it will correct it by calculated braking at certain points of the car. Which keeps you in your turning curve.

barrok69

You really can't compare stopping distance between different friction surfaces ie dry, wet, snow. Stopping distance will always increase when you go from dry, to wet, to snow, and ice being the worst.

If you are comparing dry ABS braking to wet ABS braking then of course the distances will get longer to stop on wet than dry.

The "race" you proposed, wet course. If I drive an ABS equipped car, you drive the identical non-abs car. I win, everytime. End of story.

What I'm saying is that ABS vs. no ABS on any surface, the ABS vehicle will stop shorter than the non ABS vehicle when comparing equivalent road surfaces.

If you still cannot grasp this concept I challenge you to try it for yourself. If you really want to ruin a set of tires then pull your ABS pump fuse and do a stopping distance test by pushing the brake pedal as hard as you can from 60mph to zero. Record the length and see which is better. The no-ABS stop will, skid uncontrollably, it will flat spot your tires making them useless, so even if you do achieve a shorter distance, now you need a new set of tires, and you'll probably end up in a sand ditch. Lose, Lose.

Comparing truck ABS/stability to a Car is a non-starter. The roll over mitigation is extremely aggressive on suv's and bigger vehicles whereas cars do not need this due to their lower roll centers. Stabilitrack plays a huge roll in this as it mitigates slip much more aggressively and does not allow the same amount of yaw that a car would be able to handle and recover from.

With regards to your Tahoe mods,
Stopping distance is generally tire limited. Increasing the size of a brake system generally decreases the time to lock the tires up. If it doesn't the pad selection you are using for your application is inadequate or the sizing of the master cylinder might be wrong for the additional fluid you need to push. You have to realize that the whole ABS/Stabilitrack is calibrated for each vehicle configuration and based off a pressure model. If you start changing the components, putting bigger calipers that require more volume to move the pistons the same or more distance, then that affects alot of things like not being able to even get to tire lock/ABS. The pressure model is no longer valid and you will get sub-par performance out of the entire system.

If I had to guess what the problem with your truck is that with your oversized front calipers they require more fluid to push the extra 4 pistons causing pedal to travel longer and get less decel than previous system and you probably ran out of fluid to push.

There are so many other great features with ABS/Stability control, I don't have enough time to type out the books worth.

For what it's worth, a McLaren P1 doesn't use a limited slip diff. It uses "electronic brake differential" a feature in the ABS module

Lbar

OP so you think the pedal somehow separated from your foot and applied the brakes even harder? I've had cars get pedal feedback from the ABS that might create this feeling. I believe weak brake hoses exaggerate the feeling of ABS pedal pump. I've also driven GM vehicles that have a wheel speed sensor issue which causes the ABS to kick on with much less pedal input. I don't know if that issue affects the ABS style on the V in the same way as the late 90's GM's I've experienced that with.

Fweasel

iTrader: (6)

Not severe enough to cause separation between foot and pedal, but the pressure against my foot definitely and noticeably decreased as pedal travel continued on its own. All I've ever experienced in the past in other cars was a pulsation felt through the pedal. I felt that here again, but there was no doubt additional travel in the pedal not caused by me, and the resulting braking force was more than I input myself.

Fweasel

iTrader: (6)

Just a follow-up on this. I recently put on my winter wheels and tires which don't have the same wet traction as my summer tires. I was driving to work in the morning on wet pavement and just as I was pressing the brake pedal while approaching a red light, I hit a manhole cover that wasn't flush with the road surface. The bump was just enough to kick in the ABS, which was barely noticeable both in vibration through the pedal and audibly inside the cabin. My point, there's certainly varying degrees of ABS engagement on this car, and I'm now certain that my previous experience was an emergency brake assist scenario and not the traditional lack of traction variety.