My ls is having a bad vibration while breaking at a speed above 50mph.
The shake is usually worst at higher speed and harder breaking.
I dont think its my rotor and doubt that wheel and tire has anything to do with the shimmy. I know alot of the OG ls owner will say lca , but how will I know for sure. The ls gets pricey when you start to replace random stuff, its got roughly 160k miles on it.
Any advice would be appreciated.

 

Generally, shaking/vibrations coming ONLY when the brakes are applied lead one to assume that the brakes are at fault. When i first purchased my car, it was quite the same problem. Braking at speeds above 40 MPH caused a vibration. I suggest taking the car in for a proper brake evaluation but my 2 cents: R&R Front Rotors/Pads.

Goodluck!

 

You have a warped rotor - no doubt

 

pull your front rotors off, take them to a parts house and have them checked/resurfaced.
usually the rear are ok.

 

the only reason I doubt it was my rotor is because I had similar incident with my little Bmw convertible. I started out replacing my rotor and it ran the same, so I replaced lca with new bushing and ball joint still ran the same. Anyways I thought it might be the tie rod, but theres no relation between them.

 

There actually is. Check the tie-rods to see if there is movement in them.

 

Yeah, the first place you always look is the rotors but bad bushings or ball joints could theoretically cause something like that. If you notice vague, sloppy steering, maybe it's the tie rods / ball joints. Poor tracking (drifting, vagueness) at highway speed, uneven tire wear, it could be ball joints or control arm bushings. You said you felt a shimmy when braking; do you also feel a pulsing feedback through the pedal?

 

My car is actually doing the same thing. It vibrates under braking only, but it really doesn't feel like a warped rotor.

 

It may be ball joints... but if you feel "shimming" through your hands while braking... I'm almost certain the "Strut Bar Rubber Cushions" are torn or damaged. You'll also may have experienced irregular/premature tire wear even after getting an alignment. The "Strut Bars" are connected from the lower suspension arm and mount forward just behind the front bumper.

I've attached a front suspension diagram... the Rubber Cushin is actually pressed into the Strut Arm and and isn't available separately.

Choritsu-shi

 

I think it's one of the strut rod bushings in the front of the car. In the lower left corner of the diagram it shows the strut bar. I had a 91 LS and had this same problem.

 

Update : got rotor checked and turned being it was warp so now it is ok but maybe the back is in need to be turn also. I can still feel a little under the seat. My brake pedal travel too far before the car even slow down.

 

If you feel it under your seat... your transmission mount could be going bad. When brake rotors are warped, you should only feel pulsing through your foot, while on the brake pedal.

In most cases, brake rotors warp because wheel lug nuts aren't torqued down evenly, or missing one. Particularily with aluminum wheels... always tighten the lugs nuts as best you can while the wheel is OFF the ground... torque lug nuts after bringing the car down.

If you have a lot of pedal travel, your master cylinder may be leaking (internally). Try bleeding your brakes and see if the pedal travel doesn't improve...

Choritsu-shi