Brake pulsation

[Dr. Overkill]

Gang,
I've had some intermittent brake pulsation lately in my '97 hoe. I notice it mostly when I'm creeping along around 3 MPH at a metered on-ramp to the freeway by my work. Kinda feels like I'm riding a slow moving horse, if you can imagine it.

Anyway, when I do feel it at faster speeds, I don't feel it through the steering wheel, so my first guess was that it was the rear brakes. So to test this, I found a safe area near my house, and cruised along slowly while pressing the e-brake down by small increments until I knew it was dragging. I didn't feel the least bit of pulsation, even though not long before that I had felt it while driving normally.

I also don't feel any pulsation whatsoever in the pedal--and I have SS lines, so I feel *everything*.

Is it possible for the front rotors to be warped and cause the whole vehicle to pulsate yet *not* feeling it through the steering wheel? That's what I find so weird about this, because every other time I've needed the rotors turned or had sticky caliper sliders, it had a pretty obvious vibration in the steering wheel.

Other thoughts as to what this could be? I'd like to narrow down my hunt a bit if possible.

Thanks!

 

[ravingmadman]

Good job troubleshooting with the p-brake. I would also try removing the ABS relay in the relay panel under the hood, if you have ABS, to rule that out. Rotors can warp without it jerking the wheel around, especially at that low speed with wide low pro tires.

Any noise coming from anywhere? Sometimes you can hear a warped rotor grind when the warped section hits as you push the pedal down.

This might not be brakes, this might be something in the driveline, though I wouldn't like to wager on what. Jack the front end up, and spin the tires, see if they go freely all the way around. Push/pull on the steering components, wheels, and all joints. See if you can find something out of whack there? Do the same to rule out the rear end (much less to do, just spin the wheels, maybe have a buddy gently press the brake pedal...) Report back?

 

[haks310]

Worn or damaged wheel bearings?


Front brake pulsation test:

From 20 mph with transmission in neutral, LIGHTLY apply the brakes to stop the car. If there is pulsation, turn the vehicle 180 degrees to change the phase of the rotors and repeat the stop. If the pulsation is still present it is only one rotor, if the pulsation is reduced or gone, it is both rotors. The thickness variation on each rotor will go in and out of phase with each other after turning and reduce and/or cancel the effects on pulsation.

 

[Dr. Overkill]

The ABS and 180 degree turn suggestions were new to me--interesting. Thanks.

I'm 99% sure this has to be brake related, because it only happens when I brake. And because it's intermittent, I think that rules out bearings or driveline, as those should provide full-time problems (although stranger things have happened).

Oh, I completely forgot to mention, I had a rear drum that did NOT want to come off the other day. Never had that happen before. It was definitely hanging up on the brake shoe and not the studs or hub. Took the drum off again a week or two later with no problem. I wonder if the brake cylinder is sticking intermittently?

 

[ravingmadman]

Oh, I completely forgot to mention, I had a rear drum that did NOT want to come off the other day. Never had that happen before. It was definitely hanging up on the brake shoe and not the studs or hub. Took the drum off again a week or two later with no problem. I wonder if the brake cylinder is sticking intermittently?

There's a little toothed wheel that advances the brake shoes as they wear, whenever you really mash on the brakes. If you're feeling froggy, you can take the shoes off, and hose everything down with brake cleaner, and scrub the springs with a brush, and then put everything back together. But that's a lot of work for not knowing what's wrong yet. I'm with haks on checking the wheel bearings, too.

And not to discredit, but from your picture it looks like you've got some pretty nice low pro's on your rig. The bigger/stiffer your footprint, the less torsional feedback you'll get from a single wheel pulling. If you have big squishy tires, you can tell when they're a couple pounds underinflated, because they pull really hard. High-speed tires don't pull nearly as much. So don't write off your rotors just because you aren't feeling pull. My $0.02.

---------- Post added at 03:48 PM ---------- Previous post was at 03:46 PM ----------

Oh, and your rear brake drums getting hung up on your shoes is really common, don't worry about it.

 

[Dr. Overkill]

I may be wrong, but I think the brake adjuster star only works when you are backing up and braking.

Whatever the case, it doesn't work for shiznet, and I (and many others I'm aware of) have to adjust them by hand every few months

I do get into the rear brakes every so often to not only adjust the shoes but also clean them up. The axle seals are weeping a bit and once a tiny bit of that gear oil gets on the shoes, they glaze and make a nice howl sound when I'm at a near stop. Bleah. I clean everything up with brake spray and take some emery cloth to the shoes to scuff the top layer, and it quiets them down for a couple days, tops. Really annoying, but I don't want to deal with replacing those seals until it gets bad.

---------- Post added at 02:05 PM ---------- Previous post was at 02:02 PM ----------

And not to discredit, but from your picture it looks like you've got some pretty nice low pro's on your rig. The bigger/stiffer your footprint, the less torsional feedback you'll get from a single wheel pulling. If you have big squishy tires, you can tell when they're a couple pounds underinflated, because they pull really hard. High-speed tires don't pull nearly as much. So don't write off your rotors just because you aren't feeling pull. My $0.02.

Oh yeah, my tires are pretty middle-of-the-road as far as aspect ratio goes. They're just 50 series (255x50-17 up front), so I do have a fair amount of meat compared to some of the 35 series rubber bands some are running. I know what you mean about feeling when they're down a couple PSI, as it tends to track the road variations a lot more--and I know it needs air