Arcturus

Hello,

I'm getting some vibration in the steering wheel when I brake. It is noticeable at highway speeds. It only seems to be at the start of when I press the pedal. The harder I press the more it goes away. I did a brake job recently and replaced the front rotors and pads and thought it might be that one of the rotors was warped.

I checked the lateral runout on both sides today with a dial test indicator and it read 0.002" or less on each side, which is within the OEM specification. Just to be clear, I checked the lateral runout at the extremity (outer diameter) of the rotor on both inside and outside surfaces of the rotor on the driver's side and passenger sides rotor.

Has anyone had a similar experience?

Thanks

rkw77080

The disc runout measurement should be taken at 10mm (0.39") from the edge of the disc. This measurement is the combination runout of the disc and the hub. Maximum on-vehicle combination disc/hub runout is 0.05mm (0.002"). For a new rotor, you should try to find a mounting position that yields the lowest "combined" runout measurement. This is done by taking measurement for all 5 disc/hub mating combination, and select the lowest one.

Arcturus

This was my setup. I used a dial test indicator with the rotor on the car. If the rotor is spinning around the central hub and there is an out-of-parallelism condition, the combined lateral runout should be the greatest at the rotor outer diameter.

As I reinstalled the rotors previously without a dial gauge, I did not attempt to minimize the runout. However, 0.002" and less to me is pretty good and in conformance with the service manual. Are you saying I should attempt to try and minimize this further?

I will have to take the calipers off for this and I know these bolts are aluminum. Although it says to use a new set each time, can I re-use them?

Also, the manual is interesting where it shows placement of the magnetic base. The knuckle is aluminum and hence non-magnetic. I had to put it on the sway bar instead.

Arcturus

You know what. Looking at those pictures those rotors sure look shiny. Are those glazed? They had a zinc coating on them and I wonder if it glazed the pads. Opinions are appreciated.

rkw77080

The runout measurement is for the braking surface and it should be taken from a position 10mm from the edge of the rotor. I cannot tell from your pictures if you're measuring the out-of-round (from the edge) of the rotor, or are you measuring the run-out (rotor surface wobbling). Please take a look at this video I found on Youtube (it is not mine!)... You will probably need a c-clamp to attach the base to the aluminum knuckle. Important: Install at least 3 lug nuts to ensure that the rotor is firmly attached to the hub.

Arcturus

Thanks for the help. Here's another photo showing a closeup. I am in fact measuring the run-out of the rotor on the face of brake disk. This is the displacement in the direction parallel to the axis of the wheel and wheel hub rotation. Although I am using a dial test indicator (lever type), it is equivalent to the dial indicator measurement (plunger type). In fact I confirmed the same value of (0.0015"-0.002") with the plunger indicator but found that the dial test was more convenient to use and had finer graduations (0.0005" resolution).

I measured at the edge of the disc as this would normally be the area of maximum out-of-parallelism. However, I can repeat the measurement at the 10mm from edge as you suggest to be safe.

Tom57

I have (carefully) re-used those brake caliper bolts. BUT be absolutely sure you have clean threads, do NOT add any lubricant to the threads, and be sure to carefully torque them.

Arcturus

Points are well taken - I used the same method above when assembling them.

By the way, have you ever heard of someone snapping off those bolts? They look pretty massive to me!

rkw77080

I also noticed on the pictures that your rotor is not held in place with lug nuts - this can give you bad measurements.

Tom57

Those bolts won't break, but the threads on them and/or the steering knuckle will strip out.

Tom57

Excellent observation!

Arcturus

I'll recheck the measurements with the lug studs tightened. In my opinion no lug studs would make the measurement worse not better, but I'll recheck as you suggested to be sure. It could be that the back of the disc is not flat.

Tom57

You should use a wire brush to clean off any residue on the brake rotor and bearing.

Arcturus

Alright gents. I checked with the lug nuts tightened. Didn't seem to make a big difference, but I wired brushed the hub on the drivers side which had some rust, then indexed the rotors until I found the minimum lateral runout on each rotor.

I started with about 0.002" per side and managed to get down to around 0.0007"-0.0008" per side. This is right near the limits of my dial test indicator, which is about as good as it gets. Starting to get near aerospace tolerances. Actually kind of surprised the rotors and hub are machined that tight!

Then I put the car back together tightening the lug nuts with the torque wrench to the correct torque in a star pattern.

If this doesn't work I think I can safely rule out the rotors. I'll give it a try tomorrow.

sushiboiii

iTrader: (1)

have you check the rear rotors? i've worked on a few ls's and the rear rotors will also give you pulsations if they are warped