Vibration when braking or hard cornering

[jangell]

I've been trying to figure this one out for over a year now:

After the car has been driven for a bit, braking, especially at higher speeds, has a strong vibration. I have also seen it when hard cornering, notably while spiraling to the let down an off ramp while going a little faster than I probably should. The vibration can be most strongly felt at the steering wheel, but you can feel it through the body of the car as well. The vibration appears to be coming from the front left, and it seems to pull in that direction a bit as well. It's not so bad that I won't drive the car, but it's annoying and I'd like to get it fixed.

A secondary issue that may be related is a kind of non-linear braking at slow speeds (just a couple of MPH). Normally, if you hold the brake at a constant pressure, the car slows down at a constant rate. What I'm seeing is that the car slows, then speeds up very briefly and slightly (like I took my foot off the brake a little), and repeats this loop until the car stops. This also seems more common after the car has warmed up.

What I've done:
- Replaced the steering rack and tie rod ends (and steering bushing, while I was in there).
- Bled the brakes multiple times.
- Swapped the front wheels (so, rotors and bearings) between the passenger side and the driver's side, thinking it might be a wheel bearing or rotor issue. I didn't think to swap the tires, though; I should try that just to rule it out.
- Checked the swap bar; Helicoiled one of the driver side bracket bolt holes to make sure item sway bar was secure
- Minor welded repairs to the front frame extension.

The only real modification to this part of the car is the Big Brakes upgraded. It sort of correlates to that kit being installed, but that was when I did the 3.0L EFI conversion and the car had been off the road for 5.5 years, so it's not necessarily that. The swap bar is about 15 years old, and wheel bearings and tires are about 2 years old. The shocks are probably 10 or 15 years old. Everything else in that area is original, if I remember correctly.

I'm running out of things to test.

Anyone have any thoughts on what this might be?

Thanks!

-- Joe

[Michael]

My first thought was rotors, a dragging caliper, broke pad, etc. but now I'm wondering if it's not a LCA bushing. Maybe even a bent or damaged LCA(especially if you are still running the OE type).

Those things flex enough with stock brakes, I can only imagine the stress is amplified with better brakes.

[Helirich]

I suspect the lower control arm. It is a known defect. If you disconnect the sway bar, you will find it is easy to move the control arm forward and back by hand. You can imagine the forces brakes can apply to it. The swaybar is the only thing that locates it forward and back. If your sway bar bushings are in good condition, it should be alright. I've done some tests with a ratchet strap and I could barely move it. (Like a 1/16") I doubt you could feel that driving.

I would suggest testing yours with a ratchet strap and see how far you can move it in relation to a fixed point on your fender or frame. If it moves very far, I would change all the swaybar bushings. Maybe you could see where the play is. The best solution is a lower control arm brace. I installed one from Delorean Industry and I'm very happy with it, but there are others and I'm sure they are good also.

[David T]

There is something loose, worn, bent, or broken in your front suspension. I would also inspect the rear suspension, some problems seem like they are in the front but are actually in the rear. One critical thing to check in the rear are the TAB's. Make sure you have the proper tire pressures and the lug nuts are torqued in sequence. Have the wheels checked to see if they are bent and have the balance checked. I would not ignore the problem, the car is trying to tell you something is wrong. Get it fixed before it gets worse. If you get into a panic situation the car could get out of control if you had to panic stop in a turn. Once you find and fix the problem have the alignment checked. You said you did minor welding repair to the crumple zone. There may be other damage that needs repair that you have not fixed. That section is what holds the front suspension and if it is not 100% you would have a problem like this. As mentioned, that sway bar not only transfers loads side-to-side, it also absorbs the braking loads and transfers them into the frame. If that sway bar is not securely mounted, the wheels will move fore and aft and that can cause severe steering problems. The crumple zone (aka frame extension) is what holds that sway bar.

[jangell]

Welp, the LCA bushings are toast. That explains a lot. The other stuff I replaced, like the steering rack, needed to be replaced anyway, so I don't feel too bad about trying to fix this problem with that. The LCAs themselves seem OK.

The TABs are in good shape, and straight and clean. That broken TAB mounting bolt I had a few weeks ago forced me to inspect the TABs themselves, and they look good.

The minor welds to the front frame extension are froward of the suspension components, although I realize that it could still cause flexing if the box around where the LCAs are mounted is compromised.

I had my sway bar snap at the LCA about 15 years ago. The car was basically underivable, with the wheel seemingly randomly changing direction on me.

I'm going to pull the LCAs out in a week or two. A friend is going to clean them up, weld a reinforcement plate in, and press in new bushings for me. We'll do some front shocks while we're in there; they're over a decade old now too.

I'll report back when I get stuck. Thanks!

-- Joe