vibration amplified when braking

[nagemong]

Hello everyone.
When driving at 60 mph, or more, there is a slight pull to the right, and slight vibration that seems to come from the rear. When the brakes are applied, even lightly, there is a major vibration. You can feel the vibration the most in your seat, not very much at all in the steering wheel or the brake pedal. It begs for a diagnosis of "warped rotors" because it is only noticable at high speeds and mostly during braking. However, I have nearly ruled out that possibility.

Here are the things that I have done or had done recently:
- new brake pads
- turned all 4 rotors
- new Koni's in the front
- new tires ( Kumho Escta )
- alignment

The vibration only started recently. This is the order of events:
Day 1 - brake pads and rotors turned
Day 30 - new tires ( didn't get the shocks or the alignment here because the shop didn't know how to do the shocks, but they went ahead and did the tires )
Day 31 - Vibration starts
Day 32 - Koni's
Day 33 - Alignment
Day 40 - Re turned the right side rotors to see if it made a difference but the vibration is still there.

Would anyone say that it has nothing to do with the brakes, and that I should look else where?

Thanks!

[nize]

wheel out of balance?

and btw, turning your rotors (ie; grinding them down) is really unnecessary and only shortens the life of your rotors.

[Lawrence Coppari]

I had a problem once with a rear vibration. It took me a while to find it. One rear rotor always ran hotter than the other and the big axle nut would loosen. The stub axle was slightly bent. I finally figured it out and replaced it and the bearing.

Turning rotors is a waste of time. Replace them.

[nagemong]

Why would an out of balance wheel be so much more noticable during braking?

[Lawrence Coppari]

Good quiestion. I've noticed it too. Only thing I can think of is that when a wheel is out of balance, there is slight deflection of the axle at the frequency of the imbalance. Perhaps the axle can slop around a little in the wheel bearing without imparting all the movement into the entire wheel assembly. All bearings must have a little play, however minute. But when you apply the brakes, the pads grip the rotor providing another path with no slop for the imbalance motion/forces to get into the body of the car.

Just conjecture on my part.

[nagemong]

That sounds reasonable. I will go back to the place I just had them installed an balanced and see if they will recheck the balance on them.

I'll let you know. Thanks.

[Lawrence Coppari]

I have a sedan that does exactly that. The slightest wheel imbalance at the front and bingo, it gets magnified when you brake.

[1.2gees]

Wheel imbalance should not cause vibration under braking. Vibration under braking is caused by warped rotors or uneven friction surface along the rotor/pads. No friction surface is perfect, loose components in the suspension magnify vibration under braking (sometimes to the point of causing noticable vibration, when otherwise there wouldn't be any).

The symptoms you describe sound like warped or partially glazed rotors (can happen with some of the cheaper street pads if the car's parked soon after applying the brakes very hard), or a wheel bearing with play in it. Either case, from the rear. I would start by checking the right rear wheel bearing.
Ahmet

[nagemong]

I have a feeling it has something to do with the right control arm. I'm not very familiar with the specifics of these things so I took a couple pictures to see if anyone could see anything.

Here are two shots of the right side. There are two things that makes me wonder.
1 - the rubber is cracked around the washer
2 - that little metal piece, or collar, is pointer down and the left one isn't. What is that used for?





Here is the left side for comparision:


Thanks.

[1.2gees]

The collar is there for caster adjustment. It's not an issue that they don't look the same. If you don't feel the vibration coming from the steering wheel --and sure those bushings could use being replaced but it's not absolutely necessary-- I would be looking at the rear...
Ahmet

[nagemong]

Well, who ever said rotor and / or tire balance was right. There were 2 problems at work causing the vibration. The tires were not balanced properly, so that caused the vibration at high speeds. And the rotor was messed up so bad that turning it didn't even help for a moment. Once I got new rotors and a proper tire balance, all the vibration disappeared.

I will say that I don't hold PepBoys in very high regards, and I don't yet have anything bad to say about Big-O. I never thought that wheel balancing would be so hard and that it could be done so wrong.

There is still a slight pull, but even though I just got it aligned as well, I am going to go to another place to try again. Has any one else had a problem with finding competent shops to do standard work on these cars?

Thanks for all your help!

[nize]

the problem is you're probably going to places that are not familiar with porsches. try and find shops that deal exclusively or mostly with porsches and you should have much better results, though you'll pay a bit more.

also, when you get your wheels balanced, make sure you ask them to use the tape weights instead of the hammer-in clips. the clips mark and damage the wheels.

[nagemong]

PepBoys used the tape weights, but they put them across the tire instead of around it. It would seem like they didn't know if they should go on the inside or middle or what, so they just spread it evenly across the whole thing.

But your right, I'm used to doing all the work myself and figured balancing a porsche tire was just like any other. So I went to any old tire shop and learned my lesson. Same with alignment, it is now at the best place with a guy that has done alignments on a 944 before. I learned that not everybody knows how to work on these cars, even though they think they might.

[nize]

i've learned that everybody WANTS to work on these cars, because it's actually fun with the right tools and equipment. so every joe-wrench wants to have a go at it if you'll let him.

[nagemong]

Over the holidays I took my baby on a long trip. I loaded the back up with serval hundred pounds of clothes and presents and immediated noticed a new problem. The symptoms are as follows:

- In first and second gear I would hear a bad cracking/popping sound from the right rear. Only once or twice per gear.
- On the freeway there was a small vibration from the right rear only when the accelerator was despressed. It was directly dependent on having both weight and acceleration. When the freeway was a little "wavy", the vibration would be there on the down sides, and go away when I went over the top and some of the weight would come off the back. Similarly, removing my foot from the accelerator would cause the vibration to immediately go away.

Any ideas on what I could be? Could it of been caused by the violent braking vibration that I just fixed?

Thanks.

[nize]

hmm, cv joints?

it also sounds like this car's got some other issues. has it been in an accident? is the body straight? did you pull the carfax on it?

[Evan70]

Originally posted by nagemong
But your right, I'm used to doing all the work myself and figured balancing a porsche tire was just like any other.

Balancing a tire on a Porsche IS the same as any other car, the shop you went to was just incompetent. I had the same small town Mom-n-Pop tire shop balance two sets of tires on my 86 951 and two sets on my 95 Formula. I've also had probably a dozen sets (street and race) for my 87 Firehawk, 01 Z06 Vette, and 02 RSX-S balanced at another small town shop using the same procedures and never had a problem. Personally, I feel you usually get better results with a smaller shop where you can get to know the mechanics and watch them do the work. There really isn't anything to it as the machine tells you what to do.

One catch that I can think of off-hand is that sometimes even though a tire/wheel will show balanced on the machine, it will still not be perfectly balanced, or may be out of round. The thing to try with that is take it back and have them dismount the tire and rotate it 90 degrees, rebalance, and try it on the road again. If you have new tires, the best thing to do is balance the wheels seperately, mark the heavy/light points on the wheel, and then match the tires to this (there is usually a colored dot on the tire from the manufacturer showing which point is heavy/light (I can't remember the color code off the top of my head)). Of course, if the tire is out of round, then you are just screwed and need to get a new tire (this usually occurs on cheaper tires, but I have had it happen on expensive ones too). Another thing to check too, is the wheels. If they aren't "straight", then that could conceivably cause a balance issue as well. Even if it holds air and appears straight to the eye, it may not be. Of course, this can usually be seen if you balance a wheel by itself with a tire.

Anyway, a good practice to follow is the first you get new tires for a car, get the wheels balanced seperately and mark the heavy/low spots on the wheel. Then you can check them in the future if you ever have a balance problem to see if something has happened to the wheel.

[nagemong]

I'm not to sure that the machine at PepBoys told them if it was to go in the inside, middle or outside. However, at Big0 the did say that a wheel was bent, they were able to balanced it, and I think it went back on the right rear.
Also, I wanted to add that when it was loaded down with weight and going down a steap hill, I could feel the brake pedal pulse in and out slightly when braking.

[1.2gees]

The dots on the new tires indicate the lighter point of the tire. This is intended to line up with the hole on the rim for the valve.
Ahmet

[Evan70]

Originally posted by 1.2gees
The dots on the new tires indicate the lighter point of the tire. This is intended to line up with the hole on the rim for the valve.

That's not nescessarily true. Most shops line it up with the valve stem because they haven't bothered to balance the wheel seperately, and since the valve stem adds weight they just figure that will balance out. As I said above, if you really want it balanced right, balance the wheels seperately and mark the heavy spot, then match the dot on the tires to this.