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Discussion Starter #1 (Edited)
2016 R/T, 11,000 miles, 5.7 auto.

When braking from approx 55 mph or higher going downhill, the car sometimes shakes but steering wheel does not. If I brake from any speed on a level surface, no shaking. Warped rotors come to mind, but the randomness and downhill aspect are throwing me off. Dealer couldn't repeat the issue. Thoughts?
 

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Most likely a warped rotor. My first set lasted about 12k miles. The dealership replaced that set for free and fixed it for about 20k more miles. The stock rotors are really bad from Dodge - maybe not on the SRTs but both my Challenger and my Ram had warping problems way too early. I've had a set of Powerstop slotted & cross-drilled rotors on as well as their Z26 hi-perf ceramic pads. Stops much better than it ever did with stock, plus hardly any brake dust.
 

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"braking" not "breaking"
 

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2016 R/T, 11,000 miles, 5.7 auto.

When braking from approx 55 mph or higher going downhill, the car sometimes shakes but steering wheel does not. If I brake from any speed on a level surface, no shaking. Warped rotors come to mind, but the randomness and downhill aspect are throwing me off. Dealer couldn't repeat the issue. Thoughts?

Warped rotors are rare. More likely -- if rotors are the cause of your car's behavior -- one or more rotors experienced a braking event that resulted in uneven pad to rotor material deposition.


Had this happen to a car of mine years ago: VW Golf TDi. Washed the car and forgot to drive it afterwards to dry the brakes. A day or two later used the car. Didn't have any cause to use the brakes until I was up to some speed and a car pulled out in front of mine. Slammed on the brakes and brought the car to a complete stop. Left the pedal applied while I looked daggers at the butthead that ran the stop sign.


Afterwards under light braking the brakes pulsed. Tried bedding in the brakes (again: I bedded them in properly right after I bought the car) but it didn't help. Fortunately since the pulsing only occurred during very light braking and my braking technique is to avoid light braking when possible so I managed to mostly avoid the pulsing. But when I went to sell the car at nearly 150K miles the buyer a woman was of course a very light braker and noticed the pulsing right away. She bought the car but I gave her a slight price break.


My advice is to on a smooth stretch of road/highway get the car up to speed and lightly apply the brakes and see if the brakes pulse or shudder. Try this using different degrees of braking: light, medium, and heavy. This is not a bedding in process so try to arrange to drive the car long enough after each braking event to let the brakes cool down.


You want to brake over the same stretch of roadway starting from the same speed to eliminate road surface from the equation.


If you detect some pulsing/shuddering that's probably rotors that have experienced the uneven pad material deposition event. Regardless if warped or having uneven pad material deposition nothing you can do other than have the rotors resurfaced or replaced.


If you don't get the pulsing or shuddering then the problem is less likely a purely braking hardware problem. Suspension/steering comes in for suspicion: something loose/worn or bent (what do the the tires look like?), or possibly the wheel is not mounted properly on the hub.
 

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Discussion Starter #7
I was thinking of taking a dial indicator to the rotors and find the high and low spots. Then apply a thin steel .00x” shim over a stud between the rotor and hub where needed to reduce the indicator runout.

Would doing this be bad ju ju?
 

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I was thinking of taking a dial indicator to the rotors and find the high and low spots. Then apply a thin steel .00x” shim over a stud between the rotor and hub where needed to reduce the indicator runout.

Would doing this be bad ju ju?
You are certainly welcome to check for rotor run out.

If you find run out I would not attempt to use a shim. If the rotor has no serious run out -- I don't have the tolerances/limits handy -- then it is not run out that is at the root of the car's behavior but something else. Uneven material deposition is one but something with the steering/suspension can also explain the behavior.

But if you find too much run out, you need to figure out why the rotor has run out.

One possible reason is the hub face to which the rotor bolts is not flat. A piece of rust/dirt/paint/burr is between the hub and rotor. Before mounting a rotor to a hub I'd use a small round knife sharpening stone (around 1/2" thick and maybe 3" in diameter) to smooth over the hub face and rotor face to remove any burrs or anything that would prevent a good fit of these two surfaces.

While the rotor and hub now mate together with nothing between them the rotor may still not be true running. The rotor could be warped. As an (ex) journeyman machinist with plenty of experience dealing with machining cast iron (and uncounted other materials) parts it is very unlikely the rotor is warped but I can't guarantee is it not warped and you want the brakes working right.

(It is typical a foundry after it casts up in this case brake rotors either runs them through an oven to heat them up then let them cool to relieve any stresses or in other cases just puts the raw castings outside and leave them sit for a while -- weeks or even months. The exposure to the natural changes in temperature over time will let any stress leave the casting. During machining I'd take a 1st cut to get under the casting skin and get the part to within say 0.016" of an inch of its finished size. Then I'd take the finish cut. The 1st cut gives the part a chance to release stresses that might have remained in the part. The 2nd cut brings the part to its finished size with of course any run out due to warp removed as well.)

If there is still run out you have to determine if it is something amiss with the hub face, the rotor face, or the rotor itself is bad. Without proper equipment this is hard to do. I'd be tempted -- more than tempted -- to just replace the suspected bad rotor and its counterpart on the other side.

Or the rotor is not warped but because it was not mounted to the hub properly has developed uneven wear: the rotor faces are no longer parallel; and there is nothing you can do about this other than try a resurfacing or opt for replacement.

Back in the day resurfacing was often done but nowadays replacement is the preferred option. Mainly this is because there are issues possible with resurfacing, issues I won't cover here. As a result most shops prefer to rely upon the odds a new part will be ok and if not is warrantied by the manufacturer.

Of course you still have to make sure whatever course of action you take the now straight/parallel rotor bolts up with no run out issues.
 
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