Why is my car vibrating when I brake between 60-80km/hr?

[RKD89]

I have a Chevorlet Lumina 2001.
It has got Disc brakes in front as well as in rear wheels and its automatic transmission.
Recently I had replaced the Brakes shoes and had to resurface the Discs by Grinding ,for the front wheels.

The real problem is the cars start vibrating every time I brake only in the speed region of 60-80km/hr
I hv replaced d shoes n resized d discs twice now.
THe problem repeats after few days.
I believe there is a problem in resizing/surfacing of the disc brakes.
But Y does d prob only occur between 60-80
I hv checked different parts like suspensions , engines seats etc...everything is fine

please any help will. do...I just want to find out Y d problem occurs only between 60 n 80
I m currently studying Mech Engineering

 

[ponjavic]

3 ways to change the natural frequency or response. Mass, stiffness or introduce damping. As an engineer I would introduce damping but unfortunately I'm not a mechanic so I do not know what the correct automotive solution is :D

 

[brewnog]

If the wheel balance is good, I'd suggest the discs have been skimmed incorrectly leaving a surface which isn't perpendicular to the axis of rotation. Don't be tight, buy some new discs.

 

[mugaliens]

I had the same high-speed braking shmmy problem with my Ford Explorer, and was positive it was the rotors. When I put on my winter wheels and tires, the problem flat out disappeared. I'm now thinking I might have a bent wheel.

I had a Honda civic that developed the same problem, but that was a bent knuckle, and never did get it fixed.

 

[marcusl]

Check/replace the rear brakes and rotors.

 

[Ranger Mike]

good advise sop far from al..and i hope you had the rotors turned on a brake lathe by competent operator. ifin you got brake shimmy thru the brake pedal its the brakes and most likely rotors. ifin yo ugot shimmy thru the steering wheel , all the tine, it is the wheel balance.

 

[tyroman]

If the rotors have been resurfaced too many times, their thickness may have been reduced below their design minimum. The problem this can cause is that the rotor will warp when it gets hot (under breaking at speed) and the pad contact will be intermittant which causes vibration. Have someone mike the rotors and replace them if under spec.
.

 

[tyroman]

After my post, I googled "rotor warp" (just to see if my suggestion was supported) and found this;

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

According to the ref above it, appears I am the victim of an urban myth and my earlier post reflects that.

However, the link describes other possible causes for the symptoms you have... one of them is improper break-in procedure.

Elsewhere, I read that improper lug nut torque can cause rotor warp.
.