Knocking Sound When Spinning Front Rotors Back and Forth
 

I am trying to track down some steering vibration I get when driving 75-80 mph. When checking the front suspension, I noticed a knocking sound when I spin the rotors backward and forwards by hand. I sounds like it could be coming from the lower control arm (coffin arm) ball bushing or the spindle in the hub. There is no play or looseness when pushing or pulling the wheels/rotors at 12 and 6 or 3 and 9. So I don’t think it’s the wheel bearing.

It does it on both sides. Is this normal?

Out of curiosity, are the brake pad wear sensors installed?

Yes, and the pads and rotors are fairly new. They were replaced before I got the car. Interestingly the right side stuck a bit before the brakes were bled.

Following along on this one as I have an almost identical situation...I've also balanced all 4 wheels and done an alignment without success.

A few months ago I had the front tires road force balanced and it helped a little. While the wheels were off the car this time I had all four unmounted, had the rims checked for any bending and remounted. The tech said there was some wheel hop before remounting them. I’ll let you know how that turns out.

Unfortunately the vibration is even worse now :mad: it still could be the tires, but I think it maybe the inner tie rod on the right. And if I do that I may as well do the lower control arms…. And then get an alignment done

I’d suggest that all the balancing in the world won’t solve the issues with truly put of round wheels or tires.

Bent wheels can be straightened, just do be selective in who you have do the work.

This is interesting and might be related to your car vibration issue:

https://www.youtube.com/watch?v=XA4LExidcmY&t=334s

Check whether the calipers are properly mounted / torqued down.

I just took my front left caliper off and it went was difficult to remove the bolt. Once apart (with some alu particles coming out..) I can see marks of the bolt thread in the mounting hole of the caliper. Obviously they haven't been torqued down properly and the braking load became a shear load on the bolts....

That could be the problem. It is important to carefully clean the contact surface of the brake disc on the hub (and then apply assembly paste) to ensure balanced running, which should ideally be checked with a dial gauge. I recommend doing just that, you just have to take the wheel off and put the dial gauge on.

First of all, thank you for all the great ideas. I really appreciate the interest and responses. Here is a run down of the cars history with the issue and my thoughts on next steps.

- bought the car in 2023
- the car had the steering shake when I bought it (in the 75ish mph zone).
- had a post purchase inspection done by a well regarded indy Porsche mechanic.
- they noticed the front right brake caliper sticking and bled the brakes to improve it.
- they also replaced one of the rear bearings, others seem okay.
- They also noted that the stock shocks were shot from bouncing around on lowering springs. However, the various control arms seemed okay.
- so I installed bilstein PSS9s, new strut mounts and bearings, and drop links all around. I also installed new caliper bolts after removing them (torqued to spec, but I'll check again)
- Then I had the same indy shop corner balance and align it for me.
- The shake was still there and they recommended I get the wheels balanced.
- I found a place that did road force balancing and had them do the fronts. There was some improvement (maybe 50%) but not gone.
- since the car ran over something, possibly a curb (bumper tabs torn off and front part of the fender liners missing. I went to a scrap yard and picked up some fender liners and 996 bumper (in matching black color)
- Since the car ran over something big enough to tear up some of the under body panels (front and rear right, all replaced now). I thought maybe some of the rims were bent.
- I took all four tires to a local shop known for fixing bent rims, including on P cars.
- They unmounted all the tires, checked for bends, and remounted/balanced them.
- They said there were no bends, but the tires were a bit old and one side looked like the toe angle was off and the wear looked like the tire pointed inward. They noticed some wheel hop when balancing them and tried to change the mounting orientation to minimize it.
- the tires are 5 years old and I don't know if that wear was before or after I had the car corner balanced and aligned.
- I put them back on the car and it's way worse than before (back to 100% or more of the old shake).
- While the car was still up with the wheels off, I banged around on the front suspension arms.
- On the right side near the inner tie rod, I can get a metallic click and a little wiggle hitting it with my hand or a rubber mallet. It's not a lot, but there is no sound at all on the left side.

So here is what I am thinking for next steps:
1. new tires - Since the problem is better or worse after changing the tire balance this may solve the whole issue.
2. check the brake calipers and rotors (as suggested below) I'll have to figure out how to trouble shoot this one.
3. New inner and outer tie rods - If i go this route I will also do control arms since I'll have to get an alignment anyway. I'm a little worried its the steering rack, but the fact that there is no play on the left side is encouraging.

With all that information what does everyone think? With my luck the issue will be a combination of all of three and I'll have to sacrifice a few thousand to the Porsche gods :rolleyes:

I would start with the obvious, eliminate the wheel hop, tires and the clicking tie rod. The 986 is very sensitive to alignment, so if you are thinking control arms, then do both sides and also replace the "tuning fork" arms. If the car had a suspension impact, then it might be wise to check the subframe, and also the shock towers. I recently replaced my suspension, brakes, bearings and struts, and found 2 of the 4 wheel bearings shot, even though they didn't wiggle during the usual tests. 2 of them were noisy with the brakes removed, and in the rear it couldn't be detected until the drive axle was out. I replaced all the wheel bearings, of course. I am amazed how well the car handles now, but I am equally impressed how well it still handled with the old suspension and questionable components. The vibrations in the steering are now gone, and the car feels like I imagine it would have when new. Keep on it and you will find the issue.

Update, on just the right front: the clicking/looseness is the rack and not the inner tie rod ☹️. So I need to order a remanufactured one. The wheel bearing is tight and a little grindy (halfway done replacing it). The knock I was hearing from spinning the wheel back and forth is almost certainly the lower control arm ball joints. I’ll know for sure later today. Swaybar bushings looks okay, but replacing anyway

Before you replace the steering rack, you may want to check the inner tie rod.

Update: the right side is back together with the exception of the tie rods because I am waiting on the new rack. Before putting the calipers back on everything is tight and smooth. HOWEVER, as soon as I installed the calipers, the rattle is back when I spin the wheel back and worth. It sounds like maybe a piston is loose and rattling inside? Or possibly the pads sliding backs forth? Has anyone encountered something like that?

Did you or are you able to check what I described in #10?

Not yet, I’ll have to pick up a dial gauge and some assembly paste.