I have a 1998 Corvette coupe, and the brakes pulsate when braking, possibly indicating that a rotor is warped.
How can I diagnose which rotor is warped? I dont want to buy any more parts than I need.
thetitan

 

Pulsating brakes could mean there are high spots on the pad surface area of the rotors. It is normally good practice to replace them in at least front and rear sets. Not such a good idea to replace just one. As far as telling which one is causing the issue, sometimes you can see the higher contact area where the surface will have a different color on the rotor then the surrounding surface area. You can use a micrometer to measure width at a number of locations on the rotor. You might have some luck by pulling them and having them turned. This is the cheapest fix though in my experience this is a short term fix and the pulsation will eventually return. In my opinion, I'd just start with the front set and replace them with good units.

 

If youre driving at 50-60mph and press on the brake pedal and you feel a vibration in the steering wheel its going to be the front rotors. If you feel it more in your seat its probably going to be the rears. The front braked do most of the stopping and create most of the heat in the brake system so its most likely the front rotors that are you problem. You might also want to check you pads to make sure they aren't getting to low. Low brake pads create more heat which will cause rotors to warp. You will probably be fine getting your rotors resurfaced as long as they are still thick enough, that totally depends on how many time you have had them cut already.

 

First things first. Find a safe place to run the car up to 55-60 and then stop as hard as you can. The anti-lock should be working HARD!!! Do that a couple of times. If there are deposits on the rotor or if the pad is dirty, that will clean them off.

If you still have pulsations after that, remove the tire and put the lug nuts back on the studs and tighten them down to normal torque. One the rotor is torqued, use a dial indicator and check for rotor run-out. You will have to clamp it to the suspension or magnetically mount it to the frame.

BC

 

Best way to do it and be 100% sure. Of course, you need a dial indicator and mount.

 

Agreed-but chances are it's the front ones. Have them mike'd and turned if there's enough meat on them.

 

That's pretty much the industry standard method for diagnosing if a problem is in front or rear.

Don't bother having the rotors turned. Turned rotors never last long - before they start pulsating again, that is. It's not much more money to just buy new rotors.

Just my $.02

-

 

I flat wore out the rotors on my 02 ZO6. They had cracks an inch long in them. I got the NAPA Premium rotors for $60 each front and $50 each for the rears.

You get a 7% discount if your an AAA menber or Military.

Cant beat thet with a stick!

BC

 

Here's what I did to permanently solve the dreaded "pulsating brake pedal" syndrome.

http://forums.corvetteforum.com/c5-g...ulsations.html

 

It doesn't cost anything to try and may solve your problem. When you have the brakes real hot, don't keep the brakes clamped on before the rotors/pads have cooled down. Pad material transfers to the rotor when you stop with the hot brakes applied and sit there. That transfer of material is what causes the pulsation.

RonJ ...

 

Many times it's just build up on the rotors that can be removed by perrforming the method described. This procedure worked on my wife's mini-van as well as the vette

 

Unfortunately, the buildup returns rather quickly - hence my permanent solution of getting slotted rotors. It's been many years and almost 20,000 miles and I have not had one incidence of pulsating brakes since those rotors were installed.

 

Do as Bill says and run the brakes hard to see if you can scrub off the deposits, although don't push them hard enough to get into the ABS. Check out the Stop Tech web page to read about the warped rotor myth http://www.stoptech.com/tech_info/wp...rakedisk.shtml .

Bill

 

I just wire wheel'd the rotor to hub mounting surfaces and then machined the rotors. It's been about 15k miles and the brakes are still perfect. I'm pretty sure I just stuck the pads back on because they were almost new from the safety inspection. I might have taken a belt sander to them though and flattened them out a little, I can't remember.

I've been running NAPA ceramic pads. They work OK for street driving but I can tell they don't have the bite of normal pads. It's OK because the car has enough brake that it still stops better than most anything on the road.

I've never been overly convinced of the "runout" theory where a little rotor runout causes the rotor thickness to vary. A floating caliper should follow the rotor keeping the wear thickness the same. I still did what I could to keep the runout of the newly machined rotors as little as possible. I can't tell you if it helped but it doesn't hurt.

From what I've read, I'd check any cheap new rotor for runout and uniform thickness before installing it. I've read and been told in person a number of times about machining new rotors so the brakes work smoothly and last and don't come back.

Peter

 

Great link! That's where I got my information from when I thought I had warped front rotors. It didn't make sense because the runout was within spec. After reading the link, I did a few non-ABS stops from about 60 with some improvement, but not all the brake pulsing went away. Encouraged from the results, I ran over 100 mph and applied the brakes medium hard to assure that the car remained straight. After realizing that nothing exciting was going to happen at that speed, I did 3 full ABS stops from 100-10 mph in quick succession. The key was heating the heck out of the rotors, and then making sure I kept the car rolling long enough for the rotors to fully cool BEFORE coming to a complete stop. If you stop a car with hot rotors, the pads sitting in one location on the hot rotor can transfer material from the pads to the rotors. This can make the pad grip more on that one part of the rotor, inducing a surging effect that you can feel through the brake pedal. There's actually quite a bit of info about this phenomenon if you Google.

As a side note, I hadn't done a full ABS stop at high speed until then. Let me tell you, these cars will haul down in a hurry, and Id encourage anyone to experiment with the ABS system if they haven't done so yet. Just make sure there is plenty of straight road in from of you and no traffic. Also ascertain that the car will remain straight at low speeds before attempting any high speed stops.

Another thing to check and recheck a few times- is to make sure your lug nuts are all tightened to 100 ft/lbs. Use a torque wrench. They are inexpensive. (don’t go by "oh this seems tight enough" while using a lug wrench or ratchet) I was surprised that after a few heat cycles, they can require retorquing a few times if the wheels have been off the car. This also impacts rotor stability. If you aren't sure about the lug torque, break em lose and retorque them. Then run the car as normal and retorque again before "testing your brakes".

Just my .02 cents...

Good luck!

 

This is exactly what I did too. Replaced the rotors with slotted ones and in the last 30k miles I have not had a single issue. I hated the stock brakes, they always ended up with buildup that caused pulsing.

 

Before resorting to buying new rotors, I tried the "slamming on the brakes" method, which only delayed a repeat of the pulsations. I guess if you race every weekend, you won't have a problem, but why is it that the bedding or slamming-of-brakes process is not needed with other cars? My guess is that the best pad compound for braking is also a good candidate for allowing deposits to stick to a hot, blank rotor.

The new Cadillac CTS-V comes from the factory with slotted-only rotors, as does the Viper ACR and amazing Caparo supercar. I think they now know what we know - slotted rotors work well with sticky pads!

 

Well it looks like this a common issue I am having. Thanks for the info, I am going to try the braking sequence tonite and see what results. Thanks a ton!!!

 

I had the same issue as well. I tried everything on here short of replacing the rotors. I went and had my brakes inspected, and my rotors were so thin they could not be machined, so I got new rotors, and I had brake pads already so I had them install those as well. Now, my brakes feel better than ever, and the problem is gone. A costly fix for what could be a small problem, but you can't beat the results.