fatbillybob

Geerookie or others,

Hows does the ABS work in out C5 cars. Do we have true 4 wheel abs? If your abs senses slip in the rear under braking and you go into felt ABS mode does the ABS bleed pressure to prevent slip only at the 1 wheel or does it bleed off pressure to the front axle too so that stopping distances are actually increased?

sothpaw2

Pretty sure it's 4 wheel and it would not cause the front to slip due to a rear slip. That would be a very inefficient system.

Bill Dearborn

It is a 4 channel ABS and just bleeds pressure at the wheel that is turning too slowly. It can also increase pressure to keep the wheel speed at the proper slip level for max traction. Based on the wheel speed sensor data the EBCM can detect which wheel is in incipient lock up and control that brake. Traction control uses the same function to control the brakes at the rear and AH uses it to control a single brake to change yaw.

One thing to think about with ABS is the wheel that is being controlled cannot contribute full braking capability if it is on a surface with less traction than the other wheels have. To counteract that you might need to increase pedal pressure to get more braking from the other wheels. I see this all the time in the winter on my other cars when driving on ice or snow. When I get into the ABS it is almost always better to increase pedal pressure to decrease stopping distance. In winter you can have a situation where one side of the car is running on almost dry pavement and the other is riding in a pile of snow. The wheels on the snow side will ABS like mad while the dry side wheels provide most of the braking.

By the way the Corvette has never had one of those cheap ABS systems. When introduced on the 86 model it was a 3 channel ABS where the two rear wheels worked together but the fronts were independent of each other and the rear.

Bill

fatbillybob

Bill,

Excellent! I know exactly what you mean about 2 wheels on snow and 2 wheels off snow. The wonky long stopping distances in those conditions I just chalked up to 3 channel ABS in my truck which bleed off pedal pressure to the entire system once sensing the lock-up on the rear axle.

Very interesting on the increasing pedal pressure even if ABS is on. I thought once ABS was on you got max G deceleration from the tires. But I guess you are saying that one wheel could be causing the ABS I feel at the pedal and with more pedal effort I can get more decel G's out of the brakes until all 4 wheels are into ABS mode. Am I getting that right?

New question...Can we really change the brake bias by differing pad compounds on front axle vs. rear axle? I think the answer is yes up until threshold braking but no for ultimate stopping distance. If ABS is the ultimate ruler of the brakes then as long as a brake pad can lock a wheel the ABS will take over and it does not matter what the compound is. But why does a high friction carbotech pad feel so good vs. OEM pads? Is it all inital bite and function up until ABS that we like so much from a high friction pad? Iknow the high friction race pads have higher thermal capacity and that is reason alone to use them.

Bill Dearborn

Yes. You got what I meant about pushing harder on the pedal. I have had to do this when driving in traffic and the RF wheel started to ABS. To get the vehicle to slow with traffic I had to apply more brake and it slowed quicker even though the right front wheel was in full ABS. It turned out there was something spilled on the pavement which the front wheel was running over while the other tires had good traction and were no where near lockup.

Since 2001 the Vette has had Dynamic Rear Proportioning which is done by the EBCM. No more brake bias spring. Within limits the system will give you as much rear brake as the tires can stand.

The purpose of the brakes is to turn momentum into heat. A pad like a Carbotech can do that quicker/longer and will make the car feel like it is stopping faster because it is. However, the tire has a lot to say about how quick lockup can occur. The Carbotech race pads can lock up a street tire under hard braking while they might not lock up an R compound tire under the same conditions. The stock pads can lock up a street tire but will probably not be able to lock up an R compound tire (built in ABS).


Bill

geerookie

Bill's explained it well. On the '01 and later Vettes GM uses Delco/Bosch 5.3 ABS. On earlier C5's they used the Delco/Bosch 5.0 system which didn't have the DRP. This was done through a mechanical spring in the distribution block right under the master cylinder.
Here's GM engineering's explanation of operation but remember, this applies to 01' and later C5 Corvettes. I think the C6 is similar but there are some other things integrated into the system.

When wheel slip is detected during a brake application, the ABS enters antilock mode. During antilock braking, hydraulic pressure in the individual wheel circuits is controlled to prevent any wheel from slipping. A separate hydraulic line and specific solenoid valves are provided for each wheel. The ABS can decrease, hold, or increase hydraulic pressure to each wheel brake. The ABS cannot, however, increase hydraulic pressure above the amount which is transmitted by the master cylinder during braking.

During antilock braking, a series of rapid pulsations is felt in the brake pedal. These pulsations are caused by the rapid changes in position of the individual solenoid valves as the EBCM responds to wheel speed sensor inputs and attempts to prevent wheel slip. These pedal pulsations are present only during antilock braking and stop when normal braking is resumed or when the vehicle comes to a stop. A ticking or popping noise may also be heard as the solenoid valves cycle rapidly. During antilock braking on dry pavement, intermittent chirping noises may be heard as the tires approach slipping. These noises and pedal pulsations are considered normal during antilock operation.

Vehicles equipped with ABS may be stopped by applying normal force to the brake pedal. Brake pedal operation during normal braking is no different than that of previous non-ABS systems. Maintaining a constant force on the brake pedal provides the shortest stopping distance while maintaining vehicle stability.

The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the EBCM. The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure.

Traction Control System (TCS)
When drive wheel slip is noted while the brake is not applied, the EBCM will enter traction control mode.

First, the EBCM requests the PCM to reduce the amount of torque to the drive wheels via the requested torque signal circuit. The PCM reduces torque to the drive wheels by retarding spark timing and turning off fuel injectors. The PCM reports the amount torque delivered to the drive wheels via the delivered torque signal circuit.

If the engine torque reduction does not eliminate drive wheel slip, the EBCM will actively apply the drive wheel brakes. During traction control braking, hydraulic pressure in each drive wheel circuit is controlled to prevent the drive wheels from slipping. The master cylinder isolation valve closes in order to isolate the master cylinder from the rest of the hydraulic system. The prime valve then opens in order to allow the pump to accumulate brake fluid in order to build hydraulic pressure for braking. The drive wheel inlet and outlet solenoid valves then open and close in order to perform the following functions: Pressure hold, Pressure increase, Pressure decrease

The vehicle stability enhancement system (VSES) includes an additional level of vehicle control to the EBCM. The VSES is activated by the EBCM calculating the desired yaw rate and comparing it to the actual yaw rate input. The desired yaw rate is calculated from measured steering wheel position, vehicle speed, and lateral acceleration. The difference between the desired yaw rate and actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. If the yaw rate error becomes too large, the EBCM will attempt to correct the vehicle's yaw motion by applying differential braking to the left or right front wheel.

The amount of differential braking applied to the left or right front wheel is based on both the yaw rate error and side slip rate error. The side slip rate error is a function of the lateral acceleration minus the product of the yaw rate and vehicle speed. The yaw rate error and side slip rate error are combined to produce the total delta velocity error. When the delta velocity error becomes too large and the VSES system activates, the driver's steering inputs combined with the differential braking will attempt to bring the delta velocity error toward zero.

The EBCM also uses the input from the brake fluid pressure sensor for more accurate braking control during VSES.

The VSES activations generally occur during aggressive driving, in the turns or bumpy roads without much use of the accelerator pedal. When braking during VSES activation, the brake pedal will feel different than the ABS pedal pulsation. The brake pedal pulsates at a higher frequency during VSES activation.

Rear Stability Control
When the vehicle performs a high speed turn or curve, the EBCM will enter rear stability control mode. The vehicle speed is greater than 48 km/h (30 mph) and the vehicle lateral acceleration is greater than 0.6 g. The vehicle will exit rear stability control when the vehicle speed is less than 40 km/h (25 mph) or the vehicle lateral acceleration is less than 0.4 g.

During a rear stability control event, the EBCM performs a pressure increase on the outside rear brake and a pressure hold on the inside rear brake. The driver may hear the pump motor run and may feel a vibration in the brake pedal.

geerookie

Too high of initial bite can cause the rears to go into ABS quicker than the fronts, resulting in a significant loss of stopping distance. This is a good reason to have a rear pad with less initial bite but be careful that you don't give up coeffcient of friction to get less bite.
This is part of the reason why I run the same pads front and rear. Same friction levels and then bias truly adjusts pressure for maximum stopping power front and rear.

I think the reason most people would say a competition pad "feels" better is because of compressibility. Assuming a pad can get the wheels to lock up then the difference is going to be in how firm and linear the pedal feels. These are controlled by the torque curve (coefficient of friction vs. temperature) of the pad compound and the compressibility of the compound.

trackboss

I'd say the abs as described above does not actually work so well. In my '99, earlier this year, I picked up a screw (as best I can tell) prior to entering a corner on the race track and the tire went flat. I had no brakes. The pedal was hard and I pressed it on/off three times before it actually became effective. I would have thought that abs would have pulsed the flat tire corner and allowed the other 3 tires to slow the car down, but in my case I ended up in the tire wall. The brakes did nothing until I was off the pavement. I have never experienced anything like that before and it has put some fear into me as far as driving my car on the track.
All the race cars I've ever driven have never had any abs and I was always confident. With my street car I have doubts about diving into corners now. I honestly would rather not have any factory abs after my experience.
I've heard some describe something like what happened to me as being "ice mode", but I can't see why that would have happened. Prior to the incident the brakes where phenomenal. Just one corner before I was telling myself in the car that it has never slowed down so well.

Bill Dearborn

My White Student at the event we both attended this weekend had a very real problem with having too little bite in the rear. He had a GT500 Mustang with DTC70s in the front and HPS pads in the rear. During the first session he had significant brake issues and called it quits before the session was over. We talked to Jay Andrew (the Hawk dealer at the event) and he said the HPS pads caused most of the braking to happen at the front of the car (they also glazed over which made them even less effective). This caused more heat in the front which boiled the new fluid. My student bought a set of rear DTC60s from Jay and flushed his brake fluid and went the rest of the weekend without any brake issues. He also was stopping faster and the car was more stable under braking.

Bill

fatbillybob

If Bill is correct and my understanding is correct of the 4 channel ABS then your first sentence can't be right. The rears going into ABS mode early would mean that if pedel pressure was increased and continued the front axle would be doing its work and stopping distance should not be effected.

But I hear what you are saying because I think I have experienced rear abs first then incresing pedal does nothing because the 4 channel is bleeding off pressure to the fronts and stopping distance increases.


So we have a fundamental qestion of how the ABS really works your way or Bill's way. Bill's way meaning full 4 wheel so that abs in one wheel does not effect what is happening at the other wheels as long as there is not slip.



To your second sentence "...bias truly adjusts pressure for maximum stopping power front and rear..." you mean the ABS will act more in concert front and rear and the designed bias will do its job. But what I do not understand is that many C5/C6 guys are using pads front one step more friction than the rear pads like Xp12 and Xp10. The reason is bias but some say the reason is because this set-up keeps the pads in the right operating heat range. For whatever the reason if you use the same front pad on the rear which is higher friction than the step down rears don't you increase the inital bite and then throw the rears into ABS sooner and make the problem worse? I guess I don't really understand the dynamics of what is happening. Thanks to you and Bill trying to help me understand all this.

fatbillybob

You story here also implies that GEEROOKIE's concept of abs activation in the rear first can cause increase stpping distance and decrease work by the front axle.

PaintballaXX

With sticky tires more weight will be transferred to the front before lockup. This means you need to move the bias forward. Apparently, the abs in 2001+ models can do some self biasing, but that would only occur once the abs has been activated (if I read the above posts correctly). Up until that point, the rear might feel a bit twitchy during turn in. From what I hear most people try to stay out of the abs to begin with, increasing the need for the bias to be balanced pre-abs activation. Setting up the bias right would also increase how hard one can stop before the abs comes on.

geerookie

As stated in the above description of operation, it applies to '01 and newer cars since that is what the OP has.

I also have a '99 and the ABS, AH, front and rear bias are completely different....Also the ABS/AH are not as good/refined.

geerookie

This is starting to make my head hurt! You're making me have to think I tried to address (in red) each item after you made the statement or asked the question.

geerookie

Sticky tires have no effect on how much weight is transfered to the front of the car. They only have a higher coeffiecent of friction which can result in a greater rate of deceleration (higher G forces, so to speak). Unless of course you slow down so fast that something that isn't bolted down well enough in your car comes loose and slides to the front of your car

The Dynamic bias doesn't require the ABS to be active. "The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operation software in the EBCM. The DRP uses active control with existing ABS in order to regulate the vehicle's rear brake pressure."

It works in conjuction with ABS and other systems to provide better control.

Olitho

I usually think your posts and opinions are spot on, but with this one I think I must disagree.

The car always weighs the same amount, but with faster slowing more torque will be applied to the chassis causing weight to transfer to the front. After years of motorcycle racing I can attest to the fact that with good traction you can slowly transfer all the weight of the motorcycle to the front wheel to the point that the rear wheel is suspended in the air.

The same can be said for swift acceleration with good traction. The front of the motorcycle rises and lifts.

At that point all the weight of the entire machine and rider is sitting on one patch of tire. The same holds true for our cars.

Conversely, if you cannot hook-up and get good traction very little weight transfer occurs.

geerookie

Thank you for the kind words.

Maybe I misunderstood what was being said or I read too much into it. I'm not trying to say that weight doesn't transfer to the front of the car under braking, just that sticky tires won't cause more weight to transfer. Unless of course you can slow the car down so fast that you get the rear tires off the ground, therefore increasing the height of the CG a significant amount.

OK, I'm getting tired and probably should go to bed but I'll take a swing at this, if you do the actual math on this the difference in weight transfer wouldn't be that much but it would be higher by a few pounds assuming a 3200 lb car and a 50% - 50% weight distribution and a 6" - 8" CG..
What I'm trying to say is that sticky tires don't work better because they cause greater weight transfer, they work better because they have a higher coeffecient of friction and this would have no impact on the performance of the ABS and little if any difference for the Dynamic Brake Bias.

trackboss

FWIW, my car does not have active handling. Only ABS and traction control, which I always turn off.
Great information, thanks for all the input.

davidfarmer

COG in a Corvette is closer to 20", not 6-8". The floorboard alone is 6-8" off of the ground.

Big problem with ABS in snow, or other extreme conditions, is that the modules can only release a finite amount of pressure/volume. Once a solenoid is maxed out, the system basically gives up and the brakes become unreliable. I've seen this trying to stop downhill in the snow, and I've seen it entering VERY bumpy brake zones at an angle (darting across the track after making a late pass, for instance). I honestly don't know that the "maxed out" description is mechanically correct, but it works for me.

fatbillybob

GEEROOKIE...thanks for trying to enlighten my denseness. I think I get it.


To trackboss and DFarmer you guys are bringing up an issue that is important. What some have called "icemode" Dave is calling "maxed out" I think of as ABS confusion. The result is the same. I have experienced this with no ill effect. But one of our C5 T1 guys at LBGP support race lost his car due to this confusion and luckily he did not get hurt. So we are hitting a braking zone at 100+mph that is only a couple seconds long and the ABS gets confused...

What conditions can we all agree this is most likely to happen? Bumpy brake zone? Maybe made worse by wheel sensor upset from bad hubs?

What is the result? No brakes? A spin?

In the split second you realize you have no brakes what can we do to reduce your impact with the tire wall?
Pump brakes? Stand on them with 2 feet?

Does anyone have an idea as to why the ABS gets confused?