"Engineers"
 

So my Hellcat has a couple relatively minor but annoying issues I wanted the dealer to resolve. Since I have installed a new aluminum driveshaft, I was a bit fearful of going to the wrong dealer and getting flagged. At the recommendation of a fellow HC owner, I contacted a dealer in North NJ to see if they would play ball. I spoke to the service writer and he seemed to be speaking the right language. He even went as far to mention they have 305mm MT DR's in stock. :)

The 1st issue I've been chasing for a bit is vibration at highway speed around 55-65mph. I get a slight shimmy in the wheel and you can feel it in the car a touch. The car did this when new and I ended up having all 4 Pirelli's replaced under warranty. The process was annoying but we got it done. The shimmy was gone but after a few thousand miles, it's back.

The 2nd is a whine from the rear differential at very light throttle between 50-65mph. Other HC owners report this issue as well and while no one has reported breaking one for just a whine, I wanted to get it addressed.



The tech looked like a competent guy and I was surprised to hear he owned a HC as well. No f***s were given regarding the DS so that was nice too. I drove the car and re-created the symptoms. They spoke to the SRT "Engineers" and here are the solutions.

Rear - Replace the diff fluid. OK but likely a band aid. I assume the lash is off a touch and the new fluid will mask it until it comes back.

Tires - The SRT engineers recommend.....unbolting all 4 wheels, rotating them 180 degrees and bolting them back on the car..... I made a confused face when the service writer mentioned this to me and he agreed it sounded odd. I did mention how can a 5 lug wheel be rotated 180 degrees. I cannot fathom what this will do but I will play along for now.

Don't get me wrong, I love the car and will see how this plays out. They didn't have the rear fluid in stock so I will need to make another appt to get all of the "work" done.

:rofl:

I find this interesting...

I've heard of matching tires on the wheels for excessive roadforce, but never unbolting wheels/bolting back on.

Also, the rear fluid exchange... our V's (V2/V3) with a rear noise, a fluid change fixes it for a little while. Ultimately it needs a new diff. we don't rebuild the V diff while the escalade/SRX/everything else gets rebuilt in-house.

Yea I've heard of matching them as well so perhaps there was a misunderstanding. Regarding the fluid, I see it being a bandaid and ultimately they will replace it.

Everyone nowadays is an "engineer", it's a joke in the US. Even the guy who drives nj transit train is an "engineer" (despite the fact he/she merely opens the doors..). But that's a topic for another day lol

If in fact the tech actually meant unbolt the wheels, spin them 180* and mount them back up, it makes no sense and it's a waste of time. When you go around a corner, the outside wheels will cover more distance and no longer be flipped 180* and eventually they will all line up to the way they were before you flipped them (even if for only a short distance).

As you stated, changing the diff fluid is a band-aid. If it's whining then it already has an issue unless they have an additive that will help.

In for mid-July update. :D

She will have a sticky tire on it before then ;)

I think you have conductor and engineer mixed up on that one.

Yeppers. The guy who drives the train is the engineer. The guy who opens the doors is the conductor.

Could the 180 mean rotating the wheels side to side? Left on right, right on left?

Just a thought.

Should have told him you only operate in radians and 180* is not a valid number.

I always tell people I drive trains when you ask what I do. Its fun

I'm not in the car business but a business that requires us to call the factory tech guys for problems and/or help. Tech is no longer a guy that was in the field and had hands on experience. Now, it's a person that asks questions and inputs them into a computer for an answer. I have had some success but also get " I have no idea" answer a lot.

By rotating the wheels 180 degrees on the hub they are trying to eliminate the possibility of run-out on the hub surface. GM had a similar procedure for brake rotors. We even had a "tool kit" that had wedge shaped shims to install between the rotor and hub. The 180 degree part is obviously not possibly on a 5 bolt wheel but it gets the general idea across.

We had a Cadillac engineer come out for a vehicle with a noise and his recommendation was to fill the frame rails with expanding insulation foam. In my experience the engineers really aren't anything special. It's mostly just to try and make the customer feel better.

Assuming they haven't done it yet I would look into have the tires road force balanced. Had a car about 10 years ago with a shimmy issue and no shops around had one of those machines. We ended up driving about 45 min away just to have the tire balanced.

The tires were originally replaced under warranty by Pirelli. 3 of the 4 tires would not produce an acceptable balance when road force balanced. The thought from some of the HC guys is the tires may be spinning on the rim. I don't think there is enough traction with this car to warrant this claim but I suppose its possible given the weight of the vehicle and power. If we road force again, I will mark the wheel/tire to rule that out.

"tires may be spinning on the rim"

Yeah, I can't picture that under normal driving conditions. GM had that issue with the Z28 during severe track usage, and that car likely ran stickier tires. then again the HC weighs a lot, so maybe.

I'm skeptical as well. It is happening for some guys running drag radials but street tires...ehhh.

Maybe check the balance now? See exactly which tires are off and how much. Spinning on the rim, I would imagine you'd only have issue with the rears. I don't think you can spin them cornering, I don't have a fancy title either though :lol:

Ya. I will delve into all of this next time I'm at the dealer. Just figured it would be a fun discussion topic. I'm thinking it's the right rear.

Tub it already...

lol at the 180 degrees

305s are ready for wheels. Just waiting on another shipment as one got nicely waffled by a lazy shipping company.

Yeah, this is pretty common with brakes and aftermarket wheels. We used to rotate either the rotors, if brake related and the tires if tire related. I'd also be interested in rotating the tire in relation to the wheel or if it was mounted correctly by putting the "heavy spot" in the right spot.

I was getting a pretty nasty shimmy on my Saab that turned out to be built up gunk that accumulated on the hub. I ended up taking a wire cup to the hub to smooth it out and problem solved.

I know your car is much newer, but its worth a shot.

I don't have an answer just questions that may help you.

I hope they are torquing lug nuts to spec.

What air pressure are you keeping the tires at?

Could it be a bushing, body mount, engine or transmission mount that needs to be replaced? I had to replace a poly transmission mount because as it aged the vibration got worse.

Could the vibration be coming from the body due to stiffness, I have heard of newer GM trucks do this because the body's are so stiff.

Could it still be a tire problem and worth a shot asking if you could switch to a different brand? And or take your wheels completly off the car and install wheels from another challenger on the lot and take for a test drive?

Lug nuts are torque'd to the proper spec by me verified with another tool as well. Shake will happen at the speeds specified, not per a certain RPM, gear, etc. I will admit I'm pretty picky with this stuff so perhaps I'm just overly sensitive.

I could switch to another brand but I really doubt they would cover any of that cost. I think I will just wear these out and then try another brand if they don't come up with anything reasonable.

As for the last option of using a known set of good wheels/tires, I again doubt they would go through all that hassle.