Hi Guys,

Over the last few months either I have become extremely sensitive to the ride of my car or something is wrong - the car/suspension is just riding much 'harder' lately.

I am getting a vibration coming up into the car which feels like suspension but is really hard to nail down. I don't think it's coming from the front wheels as I'm not getting any particular shudder through the steering column.

I foolishly took it to Mossy Nissan in Oceanside to see if they could nail down the problem. They kept the car for a day and came back saying it was because my back tires were worn and 'cupping' then offered to replace them for $520 as I am out of warranty. I decided to get the tires done elsewhere so paid my $96 'assessment fee' and left. Spent $350 getting 2 new Khumo Exsta's on the rear and sure enough nothing has changed - if anything it may be a little worse.

Any ideas? I'll try and give any further information I can if people need it to help diagnose probable causes.

Many thanks

Joel

 

Bent wheel? Worn out dampers?

 

I doubt it's a bent wheel (unless my wife clubbed one and hasn't told me) especially as they had the rears off at least on the weekend for the new tires and didn't mention any problems balancing them.

I was concerend that it might be worn out dampers as it does seem to have come on over time, the car 40k on it now. How can I check/verify if this is the case without dropping more money on 'expert technicians' to find the problem?

 

Hi Joel, can you be more specific as to where the vibration feels like it might be coming from? My 350 is an '04, I've spent a lot of $$ on fixing up it's issues with vibration etc from that area, including others. I'll help if I can.

Is the ride quality, harder, or harsher?? IE, slamming into small bumps or road surface changes??

 

Hi there,

It feels like the vibration is coming from the rear right of the car although it's very hard to isolate. It felt like it has gotten slightly worse since installing new Khumo Ecsta's on the rear, although it isn't a constant vibration that you would get from an unbalanced or bent wheel.

Bumps in the road tend to increase the intensity of the vibration when you hit them and then it dies down a little until the next bump. The ride quality seems harder, I remember when I first got the car thinking how smooth the ride was compared to my s2000 but these days it just feels 'hard'.

At this point I'm speculating that it may be bad/worn shocks.

Hope this info helps

 

Measure the frequency and spectrum of the vibration and associated that with rotating components.
Everything that rotates creates a specific frequency and harmonics of that fundamental as it rotational speed varies.

For example a tire/wheel/rotor has x number of rotations per second at a road speed same with engine, tranny, drive shafts, half shafts, etc. Depending on how many imbalances this can create a vibration at primary and multiples of primarry 2,3,4th harmonics.

http://mb-soft.com/public/autovibr.html
http://madgetech.com/pdf_files/data_.../svr101_ds.pdf

Hopefully you will see how difficult it is without knowing frequency, and how easy it is with frequency.

Obviously a vibration being felt is below the range of hearing so under 60 Hz. usally the first one is ~~10-13 Hz.

Take two defective tires each with a 13Hz [single point of imbalance] at 60 mph. The points will move as they relocate as the tires move in and out of phase timewise so the vibration intensity comes and goes as they add and subtract from each other over a few seconds.

Free spectrum analyser software but you need a vibration microphone that can feel vibrations.
http://dl4yhf.ssl7.com/speclab/install_speclab.zip

 

One normally isolates the vibration by test wheels to oem specs without tires, then test tires on perfect oem wheels with a Hunter 9700 radial force balancer.

We use a set of Michelins and perfect oem wheels to diagnose on Q and Lexus.

Many times it is a warped rotor interreacting with out of balance wheels and tires.

Premium Michelins have the lowest radial force [irregular sidewall stiffness] in the industry so they are used as the reference to compare other tire brands.

If the vibration changes at a steady speed as the vehicle warms up it points to tires and brake rotors.

 

Wow, if that's as complicated as you just made it sound then I am pretty much screwed as there's no way I will be able to perform those kind of test...I mean come on...I'm just a guy with a car.

The only other symptom that I did notice for a while wa a 'knock' that would come from what seemed to be the rear right of the car as it went up or down my rather steep driveway - as if the weight shift with the angle caused something to move forward or back depending on whether I was going up or down the slope. That seems to have gone away but it could be related.

 

Well, a relatively cheap fix if it is damper related, is to swap them out. You can pick up a set of Tokico Blues for roughly $60 each at the rear. Given the description, and the year and milage of the car I would say that for that little of money invested it wouldn't hurt to replace the OEM dampers. I swaped mine out at 30k miles which made a world of difference handling/braking wise. The Z rides firm as is, so it's hard to really say dampers are shot but I could tell that road irregularities and handling were becoming compromised.

 

350z?

 

You could always try swapping the front and rear wheels (rotating) just to run around your block to see if the vibration changes at all. It may be your front tires, which, from what I understand, you didn't have changed.

The best thing to do is eliminate the easiest causes of the problem first before you go and rent an oscilloscope and open MRI unit like the other guy recommended.

 

Is it definitely a vibration and not a humming?

 

Yep definately vibration without any audible noise. I feel it coming up through the chassis in the seat more than the steering column or anywhere else.

You might say I can feel it with my butt

 

So many possibilities that it could be, without test driving and hearing for myself.

I replaced my right hand axle assembly because of "unusual axial looseness". Basically the the axle pops in and out a tiny bit more than it should banging on the housing. Causing harmonic vibration and makes quite a distinctive knocking noise. Quite audible, and nothing like axle clicking. Usually most obvious going over uneven surface, or road surface, or angled driveways were another one for my 350. I've also replaced many other major components. Sigh...................

 

If you feel it in the butt it should be coming from the rear of the car. Make sure the wheels have been "road force" balanced. Makes a difference.

 

What specifically is road force balanced?

 

It is a type of balancing machine. Seems to give a better wheel & tire balance.

 

http://www.gsp9700.com/
http://www.gsp9700.com/pub/features/how-1.cfm

Named Product of the year in 1998, so no excuse in not knowing.

Road force vibration is caused when a tire is constructed or changes. there is a softer or harder [than average] stiffness in the sidewall as the circle overlaps.

Tires are like springs except they are 8-12 x stiffer than coil springs. Tires being in series with body springs reduce the sum total stiffness by 5-10%.
This soft / hard spot starts the body undulating and creates the vibration.
Shocks are slow reacting devices [less than 2 HZ] and as such they just pass these >10Hz vibrations thru as if they were not there!

A guy will a new car ['04 Silver/Black Touring Roadster, 7" Touchscreen Car Computer. Real cars go fast around corner] should at least be a college student and have had some High School/College Physics under his belt.

"no way I will be able to perform those kind of test...I mean come on...I'm just a guy with a car."

And whose fault is that? Then you get to find experts [with expert test equipment] somewhere in US and pay them to find and repair problems.

Warning dealerships may not have experts in the technology of vibration control.

 

Well yep that's why I took it to the dealer, and yes it seems they do not have experts in the technology for vibration control.

Thanks for your over technical and arrogant replies though.

 

Hey, be thankful that people have replied, and tried to help.

My comment from experience, take your Z to a suspension expert. You're more likely to get somewhere.