Threshold Braking

[twiget]

I was out cruising around some of the back roads off HW30 in Oregon today, giving the Z-51 suspension and the 315's on the rear some much needed exercise (ok, I needed it more than the car). In my travel's I had several opprotunities to exercise the brakes too. Is it normal for a C4 to shudder under heavy braking? Is this what all the racing books I've read term "Threshold Braking"?

Jason - The Wanna-Be racer

 

[FX GS]

You mentioned that you were doing some heavy braking. Did you activate your ABS? When this happens, you will get "pulsations" from your brake pedal, which is normal when the ABS activates. The ABS
lights in the message center should also light up when this happens. Hopefully, this is what you experienced. :cool If not, the possibility exists that your rotors are warped, especially if you feel the shudder under lighter braking:eek

 

[Viper 10]

Jason:

The shuddering that you are experiencing is probably the ABS kicking in. The problem for most is that they aren't used to the shudddering and the end up letting off of the brake. Do not do this. It's difficult to develop threshold braking techniques with ABS brakes. Many people on the track, turn off (or unplug?) the ABS so they have a more predictible feel for the brakes.

The definition of threshold braking is to brake hard enough that will take you just short of lock up situation (allowing you to brake deeper into a turn). With ABS the computer takes this out of your hands. One of the problems with ABS on the track is that the computer kicks in at a different time as you enter the same turn. This makes it very difficult to develop any rhythm and braking technique. What you end up doing is braking earlier than needed, which results in slower lap times.

The same complaints are said about traction control on the track. Although it is wonderful to have on the street (especially in adverse weather), it makes for a change in driving technique for the track. The reason is because there are many situations where you want the tires to break loose (i.e. to bring your rearend around in a very tight turn). This cannot be done with traction control turned on. The same goes for sliding a car in an oversteer posture, where a driver can use a technique called throttle steering. This cannot be done with traction control on.

For the average driver with limited track experience, these tools can make for a fun day. But if you are truly trying to develop and improve track skills, ABS and traction control can hamper your development.

Sorry if I gave you a little more feedback than you wanted...

Brad

 

[Tom Bryant]

ABS

Jason,
It sure sounds like ABS activating to me. It pulses so rapidly that it feels like a shudder. Riding with my wife in our '99 Tahoe I have become very used to the feel of ABS. :s

She got her driving habits from her mom who was also a Corvette owner. They both feel that the gas pedal has 2 positions, up or down. They prefer down.

Tom

 

[twiget]

Thanks for the replys guys. ABS was my first thought too, but the light never came on on the DIC and I felt the shudder through the steering wheel more than through the brake peddle. I've never intentionally tried to lock the brakes on the GS, but I have had the oprotunity to experience ABS on my S-10 and '99 Camaro. When ABS kicked in on these cars I could feel the peddle pulsing under my foot.

I've tried to re-create the shudder a few times today and last night but no luck. I've never experienced this shudder at normal crusing speeds either, under light or heavy breaking. I was doing some farily heavy engine braking durring that cruise but not durring my little test runs. Could it just be the engine at high rev's close to red line?

Brad,

When it comes to racing, there is no such thing as TMI in my book.

Jason

 

[Ken]

Braking...

Normally, I'm very, very easy on brakes. They last a long time for me (on the street, that is) because I can usually see far enough ahead to eliminate the need for braking. I'm constantly trying to out-guess the other driver (read: "Drive Defensively") :L

However, seeing as to how I live in Los Angeles, I sometimes am forced into some really heavy braking. When this has occured, I never once lost braking traction, never had a warning indicator light that my ABS failed, nor ever felt the pedal pulsating one bit. Of course, I was pushing as hard as I could on the pedal at the time. So hard, as a matter of fact, that when I came to a stop, I found that I was revving the engine because the side of my foot was touching the gas pedal. :eek Luckily the clutch was disengaged at the time.

The front end sure did "nosedive" though. :L

 

[Jack]

warp

Purely a guess...but could it be rotor warp...or mis-alignment between rotor-to-hub...or loose / worn wheel bearings?
Jack

 

[Viper 10]

Jason:

It could be a warped rotor if you feel the pulsing under very light to moderate braking. I would go out and test the brakes since your aggressive run and see if the brakes still do it. Corvettes are notorious for warped rotors..

Is the car pulling in one direction or another? If so, you might have a caliper hanging up on you.

Let us know how it turns out? Good luck.

Brad

Ps: What is TMI? It's been bugging me all night.

 

[GreyGhost]

Chat Lingo

Viper:

TMI usually means Too Much Information.
__________
:cool
Al

 

[Viper 10]

Thanks Grey Ghost. I learn something new everyday.

Take care,

Brad

 

[twiget]

I've tried to re-create the shudder several times since my little romp but no luck. As for the car pulling to one side, I've tried light braking from about 20mph to a stop with my hands off the wheel and I've yet to notice the car pulling. I have not gotten up the guts to try this technique with heavy braking though.

Jack mentioned loose/worn wheel bearings. I dont think that is a problem. She was up on the lift about 2 months ago when I had to replace a rear tire. I didnt notice any excesive play in the wheels then. I'll re-check it this comming week though. My baby's going to get an intensive detaling session and the the wheels are going to come off and get cleaned good.

How does one check to see if the rotors are warped? My first guess would be to put a straight edge against the rotor and look for day light between the edge and the rotor. Am I close?

Brad, sorry about the TMI thing, I thought everyone knew that.:s

Jason

 

[Jack]

straight edge / dial indicator

Jason: More TMI (LOL). You're close on the straight edge but you'll get far more useful/accurate results by using a dial indicator...one with a magnetic base might be easily attached to a nearby steel part. The rotor areas just outside and inside of the swept path are probably higher/thicker and will let light pass across swept path. Put dial indicator's spring-loaded tip onto & perpendicular to swept path and rotate rotor while watching dial for "runout." If one side of rotor is good...check other side. Your manual should give runout tolerance(s) in thousandth's of inch or mm's. By inducing a lateral stress to rotor, you can also check wheel bearings & hub-to-rotor alignment in similar manner. An inexpensive dial indicator w/ mag base can be had for about $30...the best go for nearly $300. Not something you're likely to use often...but a cheap one's real handy. You can get either at www.nolansupply.com or maybe a machinist-type buddy might loan you one.
Jack

 

[Tom Bryant]

runout

Jason,
A quick and easy way to check for runout is to set a jack stand or other solid base next to your rotor. Lay a pencil on top of it at a 90 degree angle to the rotor surface, with the tip just barely not touching the rotor. Turn the rotor slowly by hand and observe any variation in the clearance between the rotor and pencil.

Not too scientific but if it is out enough to shake violently you should be able to see it.Tom

 

[BubbleHead]

I had the EXACT same feeling in my car the other night. I was making some high speed runs on the highway (2am Monday morning, no traffic). I would hit 135 and back down. When I got on the brakes to slow down quickly, I had the same shudder feeling. It wasn't the ABS because the light never came on. I had later tried a panic stop from 65 to engage the ABS, totally different feeling.

I was thinking that the rotors were heating up very quickly and maybe warping. After a few miles of driving to let them cool down, I tried the brakes again and the feeling was smooth braking.

I want the spoiler with brake cooling ducts.

 

[Viper 10]

This definitely sounds like ABS brakes. The reason why you don't feel the same sensation when trying to rereat the shudder is probably because your pads have warmed up significantly. Your first stops (and probably the most shuddering one) were on colder pads. As the brakes pads heat up, they are much easier to stop (with the less pressure from the ABS than before).

I remember driving an AWD Turbo Quattro Coupe with Bosch ABS on it. When slamming the brakes on, it was almost a violent kind of shudder that came on (as well a correcting itself mildly from side to side). This is the nature of ABS. I would be willing to bet that if you unplugged or turned off the ABS in you cars that you would not get the same shuddering and shaking. I don't know if traction control would contribute to this (even though it is initiated through the cars fuel injection system)?

A warped set of rotors will shudder at low speeds and even light pressure (versus high speeds and heavy pressure). On the track, C4's and C5's are notorious for warping stock rotors. If you were on and off of the brakes long enough and maybe even left your foot on the brake pedal at a stop, you probably not only warped the rotor, but you probably melted the part of the pad onto the rotor. After doing a bunch of laps or driving in the mountains aggressively, we stop and set the cars in gear (or use the emergency brake), but never stand on the brakes at a stop light or inthe pits.

 

[BubbleHead]

But the ABS light never came on and it was a different feel from the "panic" stop I did to test the ABS (light came on). I'm thinking it's a heat issue. Dropping from 135 to 66 mph that quick does create some serious heat.

I know it's not warpped rotors for the exact reason you gave. Once it's cooled down, the brakes are smooth.

 

[Viper 10]

One stop from that speed on cold pads might give you these results. Heat will cause the rotor to distort, but should also cause a serious amount of fade.

Sorry, but you've now reached the limits to my technical knowledge. I would have a brake person take a look at your brakes if this persists (or you can find a way to duplicate the problem on a regular basis). I wonder if your suspension is shifting under heavy brake load?

Good luck.

 

[Jack]

burping ?

I've seen the rotors with slots / grooves / holes both advertized and on cars in pits...I've been told this helps permit gas to escape. Beyond this, I'm also on limit of technical savvy. So, uninformed as I am...I ask: could it be that during brutal braking, gas builds, but quickly burps out in a rapid-fire fashion...inducing a shudder?
JACK

 

[Viper 10]

For most performance applications slotted is the preferred choice. Slotting helps wipe
away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage.

Out gassing that occurs from the slotting or cross drilling reduces heat build up and glazing of your pads. I don't think that the build up of gasses with cause any shuddering, but like I said before it will cause the brakes to fade and the fluid to overheat.

Brad

 

[BubbleHead]

This has been a good topic. Not sure what the cause is, but I feel it's heat induced. I don't know about the suspension shifting, but it's always a possibilty.

Now that I think about it, I do have some minor vibes at higher(100+) speeds. Maybe the tires are not balanced as good as they should be and this shuddering shows up more while braking from those speeds. Hmmmm... I may take it in and have the balance checked. Now If I can just find some time.