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Higgs Boson-like Phantom Brake Shimmy/Shudder

Okay, this is one of the oddest automotive ailments a car of mine has ever had…Last year I bought a 1999 Subaru Legacy GT (2.5L, 5spd, AWD), and for several months now, there is an odd shimmying/shuddering that happens from time to time that appears to be related to lateral forces on the front end brakes or drive line.

It comes and goes, but I find the easiest way to make it occur is to make a hard left turn while accelerating. After doing so, there is a shaking/shimmying/shuddering that is at wheel speed, coming form the front end that sticks around for as little as a few miles or as long as a few days of regular driving. At freeway speeds you can see and feel it in the steering wheel. Stepping on the brakes when the shimmying is occurring will make the brakes pulse, squeak and clatter. Eventually the symptoms just kind of disappear, potentially after backing up or with just enough mixed driving.

Even though they did not look overly worn, I replaced the front brake pads and rotors, which seemed to temporarily limit how often the problem occurs. However, after just a few thousand miles of driving the problem seems to be back, and with a vengeance…enough to make emergency braking a very noisy and scary experience (had to slam on them today).

The only things I can think of is that one or both calipers may be intermittently dragging, or (God forbid), there is some weird lateral run-out problem with the front axles or differential, causing the brakes to rub, generating brake friction and heat.

Before I go and replace the calipers, does anyone have any ideas or advice? I’m pretty technically inclined, but this baffles me.



Higgs-Boson, huh? Don’t worry then. The feeling is only theoretical. You can always trade it in for a Higgsless model.

Seriously, your symptoms would not be unknown for a worn CV joint, psrticularly an inner one. A worn ball joint, tie rod end, or any other articulating joint that’s worn to the point of exhibiting physical displacement when lateral (to the car) forces effect it could also cause your symptoms.

Based on the symptoms you posted I don’t see this as a brake problem. I think you have some axial movement of a half shaft or some lateral movement of a joint.

Can you get the car up on a lift? Or the front end SAFELY up on stands? You’ll need to get under it to find the problem.

I’m with mountainbike - I think that the brakes are the wrong tree.

The only thing I’d add as a suggestion is to also give the motor & trans mounts a careful once over. If there does happen to be a problem there it doesn’t sound bad enough to be easy to detect - so you need more than a visual inspection.

Thank you, gentlemen! I’ve already eyeballed all of the suspension components and engine mounts. Short of a mushrooming sway bar bushing or too, all looks good. I also spun all four wheels while the car was on jack stands to see if anything leaped out at me but found nothing.

I’ll see if I can do some sort of comparison between the two axles (4 joints) to determine if one can be be hyper-extended more than the other. No clicking on corners so if it is a CV joint it’s likely an inner (as you suggested). The good news is that replacing both axles on this car would still be cheaper than one caliper.

I’ll post the outcome.

Again, thanks much!

You may not be able to check the CV joints when parked, but have the car checked by an alignment shop. They will know if the ball joints are bad. Sometimes you have to do this stuff every day to be good at it.

Sure sounds like a suspension problem or just maybe a tire/wheel problem. I’d check the wheel bearings, ball joints, and swap the front tires right for left. Also check the strut mounts.

With the car safely supported, try pulling the front wheels back and forth, in and out, etc.

Next time it happens and you can do so safely, stop the car using the engine and parking brake, not the foot brake. If you are lucky, the front brakes will be cool enough to get your fingers near. If one is decidedly hotter than the other, you might have a sticking caliper although that doesn’t really seem to match the problem very well.

Eyeballing won’t do it. You need to grab the pieces with your fist and try moving them. I’d recommend making very extremely doubly sure the car is secure before doing this, with backup supports, and you may even want to have someone there just in case. I couldn’t live with being the cause of the loss of a poster.

Thanks all, for the sage advice. Unfortunately it looks like the dark side has prevailed…(see image).

These nasty little bits came out when I drained the gear oil. I will be opening up the transmission case to determine which sub-assembly (transmission, diff or transfer case) ate itself and needs to be rebuilt/replaced. Prior to seeing the damage I’m assuming its the differential, since lateral forces turning are what bring on the symptoms. Wish me luck, because my wallet will need it!

I was thinking diff. related right when I read that it shudders turning hard and accelerating