I've read many posts regarding rotor runout here and other forums. And watch several videos. Only one instruction that the rear hubs/rotors should be raised to approximately riding height before measuring runout. Is this a requirement? If yes, does this mean the front should also be raised? 69 BB.
Thanks
Mike

 

The rotors are secured to the hubs which are constrained by the spindles (front) and bearing housing (rear). As long as the rotor is evenly bolted to the hubs you'll get the same readings no matter if set to ride height.

The only difficulty you could run into is if the rears are hanging so that the u-joints are binding a bit and not allowing easy rotation of the rotors. In this case, place the rear on jack stands under the trailing arm to simulate ride height (straighter axle to hub angle on the u-joints).

 

Thanks for the quick reply Owen. One concern to check off the list.

 

Glad you asked as I never really even thought about that even being a concern.

 

Ride height makes no difference
I setup run out on the bench

 

On the car it matters for binding of the rear u joints......if the rear half shafts are disconnected, then no, it would not matter. If you replace the lip seals with orings then you dont have to worry about run out much at all....atleast not as much as those stupid lip seals

 

Sorry to be so dumb, but what lip seals are you talking about? I’d rather not have to buy the tools and set run out if it can be avoided by replacing those seals.

 

Sorry to be so dumb, but what lip seals are you talking about? I’d rather not have to buy the tools and set run out if it can be avoided by replacing those seals.
Also, another dumb question, but say I replace those seals, and do not check or set run out. How would I know if I am having a run out induced issue? Would I feel it in the brake pedal?

 

Original calipers used lip seals on the caliper pistons. They work but if the car sits a long time tend to leak. O ring kits require new pistons and use o-rings instead of the lip seals. Some have had issues with them as well.

 

If you are setting up the parking brake at the same time, I think the shop manual calls for disconnecting the half-shafts, to allow the rotor/hub to run free. This would eliminate any need to raise the trailing arm, as there would be no binding in the U-joints, and no drag from the differential.

This would then be equivalent to doing it on the bench, as @GTR1999 does.

 

GTR1999 answered your lip seals vs. o-rings question. The run-out would affect the "pumping" action of the pistons exactly the same and I think you'd feel excessive run-out in the pedal no matter what seal you use. Best practice would be to eliminate as much run-out as possible for best operation.

The "pumping" action of excessive run-out pulls air and moisture into the piston bores. The original cast iron bores rusted and pitted, compromising seal contact with the walls. On stainless lined bores this moisture isn't a great concern since they don't rust like the original walls, however, the DOT 3, 4, 5.1 (non-silicone) brake fluids will readily absorb any moisture that makes its way to the fluid side of the lip/o-ring seals. Any moisture in the fluid has the potential of turning to steam under braking heat -- that's no bueno! The moisture also has the potential to migrate up the non-stainless lines and rust them out.

This is why many of us old-timers use DOT 5 silicone fluid - we are from the lip seal era when one of the protections of moisture absorption into the fluid was eliminated with the use of silicone fluid. Silicone behaves oppositely of non-silicone fluids, it repels water, so the moisture isn't aided in being pulled into the system. But, to echo GTR: some have had issues with it as well.

 

No its not a requirement but the rear half shafts u joints will bind during rotation messing up your measurements.

 

Thanks for the clarification/answers guys!