Is it rotors or bearings or what?

alan_florsheim · 07-05-2009, 02:33 PM

I'm tracking down a rotational vibration in the of front end of my 1980 911 SC

There is a bit of play when I pull the wheel (with my hands) back and forth.

There aren't any grinding noises that I normally associate with a bad bearing - the ones that change frequency as you rotate the wheels.

There was a VERY minor vibration when I stepped on the brakes, so I took the rotors to a machine shop and had them resurfaced.

Here's the funny part. I put it all back together and when I spin the rotors sans wheels there is still some binding as they go around. It does always happen in the same place, so it feels like it's a warped rotor, but when I drive the car, I don't feel any vibration through the brake pedal.

I do, however, feel the same very distinct thump thump thumb through the steering wheel, especially over 20mph or so.

Rather than just throw money at the problem (new rotors, bearings), is there something I'm missing so that I can only replace what I need to?

Thanks so much for your help!

Steve@Rennsport · 07-05-2009, 04:34 PM

Alan,

First, grasp each front wheel at the 12 and 6 o'clock positions (off the ground) to feel any play in the bearings & races. There are several places that exhibit wear such as bearings, races, and the spindle itself. Sometimes it takes some practice to determine precisely where any excessive movement is coming from.

Second, if you are hearing noises as they rotate, remove the brake pads to see if a caliper piston or two may be dragging. If your rubber brake lines have never been replaced, I strongly encourage you to do so as thats a major reason for dragging brake pistons.

Make doubly sure that the brake rotors and wheels are both correctly torqued to the hubs.

alan_florsheim · 07-05-2009, 05:01 PM

thanks Steve.

There's definite play SOMEWHERE in the system. I can move the wheels in and out a bit when I do grab at 12 and 6. Is this the bearing. As I said earlier, it doesn't seem to make noise while driving.

and would this cause the rotor to grab as it rotates around.

Is the bearing causing the hub to be slightly off?

Sorry for all of the questions - I tend to dive into these things and learn as I go (while reading the book - which I have

)

sfoster13 · 07-05-2009, 05:01 PM

thrown wheel weight? try balancing again.

Steve@Rennsport · 07-05-2009, 05:48 PM

Quote:

Originally Posted by alan_florsheim

thanks Steve.

There's definite play SOMEWHERE in the system. I can move the wheels in and out a bit when I do grab at 12 and 6. Is this the bearing. As I said earlier, it doesn't seem to make noise while driving.

and would this cause the rotor to grab as it rotates around.

Is the bearing causing the hub to be slightly off?

Sorry for all of the questions - I tend to dive into these things and learn as I go (while reading the book - which I have

)

When you say "in and out",...can you be more specific? You should feel a VERY VERY slight amount of movement when you rock the tire at the 12 & 6. Its a finesse thing,...

Are your wheel bearings adjusted precisely to factory specs?

If the bearings are not seated and out of adjustment, the rotors move around in the caliper and that generates pad noise & drag.

docrodg · 07-06-2009, 04:24 AM

Measure the play with a dial indicator, that will take out the guess work on the WB.

Also, the periodic grabbing of the rotor could be runout, do a check on that too with the indicator. When you have rotors surfaced you should measure the runout and mark the high spot and measurement. Take it to the shop and have them mount it on the machine with the same measurement. That way you get 0 runout when you reinstall.

The thumping could be many things, thrown wheel weight, abnormal tire wear, hard spots in tire to name a few. Inspect for balance and if ok then try swapping front to rear and if it changes then you know it is tires.

rpo420 · 07-06-2009, 05:38 AM

hey, had a similar problem. if you have any float in your steering wheel, any at all, that can cause the vibration too. I have a 78 SC, I had a vibration that would come and go, usually around 55mph and then go away. I had a 1/4 to 1/2 inch of play in my steering wheel. I rebuilt the steering box with new bearings. There is a great tech article on pelican for that procedure.. At any rate, now the steering is knife like and more precise than I can believe, and all vibrations are done. If your steering wheel has some slack in it, I would bet its the steering rack..

bkreigsr · 07-06-2009, 05:52 AM

Quote:

Originally Posted by rpo420

.....I rebuilt the steering box with new bearings. There is a great tech article on pelican for that procedure..

RPO -
please post a link to this tech article.
I could not find it
thanks
Bill K

Oh Haha · 07-06-2009, 06:07 AM

On my "to do" list.
SC Steering Rack DIY Bearing Replace

KTL · 07-06-2009, 10:36 AM

It is very important when replacing rotors to check the cleanliness of the hub mounting surface on both the hub and the rotor. Like Steve said, you need to make sure your rotors are properly attached to the wheel hubs.

If the contact surfaces are not very flat and true, you can get a vibration. I had this problem on a friend's '86 Carrera. We installed new quality rotors from Porsche and he got a lot of runout from one wheel. We kept "clocking" the rotor on the hub (unbolt rotor from hub, change relative position of rotor and hub by rotating them, reassemble) until we could reduce the runout as much as possible. Still had more runout than allowed per the rotor specs, but vibration was not detectable.

alan_florsheim · 07-13-2009, 03:42 PM

No luck so far.
The rotors are resurfaced (no pulse when brakes applied
The bearings and races have all been replaced.
The surface between the hubs and rotors were cleaned. torqued down to spec

It is exactly how it was before. I start getting the slight clunk, clunk, clunk rotational thing around 20 mph on upward, getting more rapid as the car speeds up (though I'm not sure it's 1:1 with wheel rotation.

To my untrained eye there is just not that much up there to go wrong! Are there other areas that can cause this? Spindle? struts? something that is loose?

Please help!

Oh Haha · 07-13-2009, 03:52 PM

can you hear anything when the wheels are spun by hand with the car in the air?

I wonder if you have a caliper hanging up causing the brake pads to touch the rotor.

If not, then I would check the tires for a possible broken belt if there are no obvious flat spots.

try a friends tires on the front.

sfoster13 · 07-13-2009, 04:09 PM

just for fun switch the front wheels from right to left and see what happens

alan_florsheim · 07-13-2009, 04:15 PM

one other thing I forgot to mention. There is definitely a little play in and out when you grab the tire at 9 and 3. I've read in the forums that a little play is normal, but I don't know what a "little" really is. Also remember that the bearings have just been replaced and packed etc...

KTL · 07-14-2009, 05:31 AM

Did you check the runout at the rotor face and OD to determine if the problem lies within the hub-bearings-rotor assembly?

tharbert · tharbert's Garage

Quote:

Originally Posted by sfoster13

just for fun switch the front wheels from right to left and see what happens

+1 for moving/swapping tires. Have you done a tire rotation recently? I had a steel belt slip once causing a decernable "thump" in the ride.

alan_florsheim · 11-22-2009, 07:14 PM

The folks who suggested swapping tires are the winners. I borrowed a set of tires from friend and wala!no more bump. Turns out left front tire was out of round. required new tires, but at least it's fixed.

RWebb · RWebb's Garage

huh - surprised that showed up as "clunk, clunk, clunk"

thump, yes; clunk no...