low speed vibration in brakes
 

Howdy folks. I searched around a bunch and didn't find anyone having this exact problem, so here we go.....

When slowing or coming to a stop, from about 30mph down, under very light braking there's a really faint vibration/rumble in the truck. Almost like I'm driving over really light rumble strips. I can "turn on" and "turn off" the vibration by pressing/releasing the brake pedal.

My powerslot rotors have about 30k miles on them now. Does anyone running these ever "feel the slots" in the pedal? That's almost what it seems like.

It was really bad a few weeks back and I replaced my rear brakes which were in bad shape. This got rid of my bad brake vibrations, but the really slight vibes are still there.

I've checked all the usual suspects -- no play in the front/rear wheel bearings, all u-joints are good, carrier bearing seems fine, ESOF hubs are unlocked, lug nuts are tight, ball joints are good, etc. I greased the slide pins about 4k miles ago.

If you're still with me this far into this post, thanks for hanging tough!

Thanks in advance!

rotors may be warped, check them for runout with a dial indicator.

Ditto on the warpped rotors. Milage doesn't matter, it can happen even when they are new.

To me, it feels different than warped rotors. All the warped rotors I've had I could feel pulsating in the pedal but this time its just a vibration (like rumble strips) you can feel through the whole truck. Not really in the pedal so much.

I'll put the indicator on them next chance I get though.

Thanks!

You guys need to learn the difference between high runout of a rotor that you describe as warp and thickness variation that can be caused by installing rotors with a high runout in the first place.

Jason, you might be having an issue with an ABS sensor rather then the mechanical side if you only feel this in the brake pedal. If a sensor / tone ring are dirty or damaged at lower speeds the signal gets erratic and the ABS controller believes the tire is starting to skid and will start to pulse the hydraulics on that wheel. The signal is not erratic enough at higher speeds to fool the ABS controller.

Any vibration through the pedal or steering wheel?

I think your pads may be slightly glazed over. If your back brakes were in bad shape that just puts more strain on the front brakes. My mom is very good at glazing pads over and it feels like your describing with no shakes in the steering like with warped rotors. It feels kinda like the pads grip, slip, grip, slip and etc. Don't know what she does but she glazes pads.

My dad has Power Slots on his truck and they worked much better after he had them turned. The brakes squealed too much with under 10k miles of driving so he got the brakes done. They turned the power slots and I bedded them in for him and they work better now than when new.

This is what I was thinking as I was reading over the last few posts. My truck will do this same thing but not all the time. I've had to replace the pig tail to the vss to take care of mine, the cable was starting to rot off up by the rail.

Thanks for the info fellas! I'll look into the ABS and VSS stuff.

I took them apart and cleaned/inspected them. Runout was 0.002" on the driver's side and 0.003" on the pass side. That's on the high side of spec so maybe that could explain it since its really light vibrations. It was hard to measure though with the slots.

Another thing I've noticed since I installed the "upgraded" slide pins is that the one with the rubber on it is always crazy tight after a while. When I first grease it up and install it its not bad. But its only been ~4k miles since I last did that and it was really hard to remove. Anybody else have this? I don't think it explains the vibes but it seems wrong to me.

Maybe I'll try getting my rotors turned...?

I'm having the same issue on my 2002. An occassional rumble strip feel at low speed while lightly braking. I feel it through the floor board beneath my feet. The first few times I was turning into a driveway approach but not anywhere near full lock. It felt like the corner of the tire tread rubbing the leaf spring. I don't see anything that's been rubbing and was wondering about ABS due to the frequency of the vibration at such low speeds. I'll be looking at sensor wires and such.

Did you check rotor runout when you installed the rotors? That may be the best runout you are going to get with aftermarket rotors. It would be better to index them and see how low of a runout you can get.

Runout usually doesn't develop and pulsation in the brakes until you are at high speeds and the runout is about 0.005-0.006", when at that point the acceleration of the calipers slapping side to side start to generate hydraulic pulsations.

However if incompatible grease was used on the slide pins the lower pin's rubber bushing will swell and cause more sliding resistance, then that could also be contribution to the pulse even with 0.002-0.003" runout. If they have that much resistance, just remove the bushings. They were not used on the early model vehicles.

Runout in excess of the 0.0015" spec will start to cause the high runout points of the rotor to wear away so you end up with disc thickness variation, which is what you really feel when the steering and brake pedal pulse.

It can be problematic for some shops to turn rotors with slots, and if you are going to have them turned on a bench lathe you might as well throw them away. If you are going to have them turned mounted to the hubs with an on-car lathe, then it would be worth doing. But I would be indexing the rotors first to see if they can get closer to spec.

Well, kudos to those who predicted it was the front rotors :-X22 I replaced my powerslots with a set of OEM rotors (and new pads) and the vibes are completely gone.

The runout was so small on them (0.003" max) I guess that's why the vibrations were so light. It was just starting. For the record, they only had around 30k miles on them.

I'm going to look into getting my powerslots turned if I can find someone who'll do it. Maybe give them another try once these wear out.

Thanks again everyone!

A runout of 0.003" is not light, it exceeds the service manual limit by a factor of two. Runout would not cause the pulsations. As I said earlier, excessive runout will wear a thinner section on the the rotor and the thickness variation will cause the pulsation.

So if those much heralded Powerslot rotors came from the factory with 0.0030" they were doomed from the start. So are these new rotors unless the installed runout was within spec.

I didn't measure the runout on the powerslots when they were new. But this vibration just started within the last 1000 miles. They were smooth as glass the first 29k miles. I did expect them to last longer though.

I measured the new ones and they're less than 0.001"

Thanks for closing the loop. Sure didn't sound like a runout problem to me so I'm glad to know that I actually do know the differance between the two.

Then you should be good with these. One thing I would recommend is that after 500-1000 miles loosen the lug nuts and re-tighten them in steps. Clockwise every third nut, first 80lbft until all are tight, then 120lbft in rotation, then 165lbft as a final. If there were any residual thermal stresses in the rotor that were released during the early running, this will take care of that. Just a precaution for that 1 in 10,000 chance.

A runout like 0.003" will not pulsate initially. Again, it is the off-brake wear of the rotors high runout point that is the problem. As miles go on this high point wears against the pad and you slowly develop a thin spot*. Drivers typically notice pulsation when the thickness variation exceeds 0.0008". It might take 20k, 30k, 50k miles before this happens. It depends on the abrasiveness of the brake pads and the driving conditions.

If that rotor had initially installed at 0.006" runout you might have had the pulsation at 15k miles.

* When the rotor transitions between a thick and thin areas of the rubbing discs there are two forces involved. One is that as the brake pads fall into this thinner area the hydraulic pressure drops as the caliper piston move to this thinner rotor condition and the brake pedal drops slightly. Then as the rotor transitions into the thicker region the pressure goes up, there is a slight hydraulic push-back to the brake pedal. With this the driver feels the vibration in the brake pedal.

The second force is the frictional change, again going between low hydraulic pressure and high hydraulic pressure. This results in torque steer, especially on a vehicle like the SD that has a high scrub radius. With the minute changes in torque steer the driver feels the steering wheel move side to side.

This is similar but opposite of what happens when a hard spot forms in the rotor. The hard spot is wear resistant so you form an area that is proud or thicker then the rest of the rotor. But the situation ends up being the same with a transition between higher and lower pressures, and friction changes.