Severe High Speed Vibration

[1.2gees]

Not having a garage sucks. Since moving to an apartment I've had to pay for diagnosis/repair on my car...

Recently I've developed a severe high speed vibration. It starts around 95mph and gets worse up to 140, I haven't taken it past that, as I still have my snow tires on the car (a set of that flashy new yokohama AVS is going on this weekend). There's absulutely no vibration up to about 95 though. Also, something feels very loose upfront. I have parked the car tonight, after a long drive until I figure this problem out. BTW that marks about 3k on the engine rebuild this is good Now if I could fix the power steering leak, the turbo oil leak, and that fuel leak...

Anyway, the car's been to two shops now, one of which guessed it was a wheel bearing (replaced), and the other a tie rod (also replaced). It has not gotten any better. Which pisses me off btw, because I'm paying them to fix something and it seems they just throw parts at it! Any ideas on what this problem could be?
Ahmet

[SoCal Driver]

Snow tires at 140 MPH.......

Interesting.

Put the new tires on and get back to after a few run ups.

There is really nothing trick about the front and rear suspension on these cars -- steel or aluminum. Things that move and rotate do need to be kept tight though.

One thing comes to mind is that the newest replacement tie rods have a rubber dampener built in. It's just outboard the inner tie rod end under the billows on each end of the steering rack. You don't have these on yours do you?

[adrian jaye]

Gee's

Get your car properly aligned/tracked and wheels balanced at an OPC.

Estimate on 4 hours work.

It will work wonders, and any problems the will detect, saving you money in diagnosis

(talking from experience)

Adrian

[Blackfoot]

so ahmet, when they take away your license, how long will it be for this time? will there be a big fine along with the suspension?

my guess is the simplest of causes...wheel deformity, either being out of round to start with, or the tires don't like 95+. cary law enforcement doesn't either, i'd hazzard to guess. be safe & smart.

[1.2gees]

Blackfoot, I went for a drive in a convoy of cars this time. BTW, have not been pulled over in a little over a year now (if memory serves me right)last ticket at least 4 months before that. However, if I get another ticket license will be suspended for 3 years, no fine. If If my front tires/rims were out of round I hope they would've told me, I'll double check that. They also should've been properly balanced.

SoCal, is the new style tie rod worse? I haven't looked at tie rods for 944s since 99? I don't know what type of tie rods were installed, might want to check that. BTW snow tires are in the rear, fronts are older dunlops (z rated). No obvious parts have play, I've completely redone the suspension more than once on my old 944, this time around I don't have the cash but this thing gets downright scary at speed. What puzzles me is the amount of play in the steering @ all speeds but no detectable play w/the car on a lift, and the fact that there's no significant vibration before I approach 100mph.

Adrian, I'm planning to align the car but I'd like to do that after figuring out this problem. Don't want to pay for an alignment twice.
Ahmet

[divezic]

Harmonic resonance. Every rotating mechanical device has a resonant frequency and harmonics of that frequency. Generally, the "looser" and "heavier" a system is, the lower the resonant freq will be. For example a steel ball on a string will have a low swing frequency. Tight and light (a guitar tuning fork) will have a higher freq. Harmonics are integer multiples of the base resonance. In other words if a base freq is 100Hz then harmonics can (but not always do) exist at 200Hz, 300Hz etc. Engineers design systems so that resonances and harmonics occur outside of the normal operating range of that system. Take a Cessna 152. When you start the engine, there is distinct shudder at around 600-700 rpm which is not there at idle. The resonance is at this point. Thats why the idle is around 1000 rpm. The propeller is only rated for I think 2900 rpm. Over that rpm the system (engine, driveshaft, propeller) may hit a resonance and blow itself apart.

On the front suspensions of our 944's, there generally seems to be a resonance around 70mph. Your problem, Gees, is happening at a greater speed but is the same problem. Anything in the suspension may be a contributing factor, but it is the entire system together that causes the resonance. As SoCal mentioned about the rubber dampeners in the tie rods, the engineers are trying to dampen out the resonance (shift the frequency or eliminate it). I found that balancing, alignment, repacking the bearings, and rotating the tires did not work. I still get the 70mph vibe. New tie rods, ball joints, a-arms, bushings, and tires are coming. If this doesn't work then I'm going to install a steering dampener, kind of like they have on raised trucks but smaller.

The trick with Gees is that the higher the speed and resonance, the quicker your system will fail (break apart). At 100+mph you have little chance of unscrewing the situation when a tie rod breaks. Your problem is not a ticket or license. It is killing yourself and anybody you take out. From an engineering standpoint, your system will fail eventually. Statistically speaking, you will die and you will kill others around you.

Fix the problem, buy a helmet, join SCCA or PCA and do the 140 on the track after some instruction.

[SoCal Driver]

Guess if one is so inclined they could measure everything and throw it into a 3D model for FEA (finite element analysis).

I think it's the wheels (tire and rim combo's) and inadequate balancing. These cars are the same design that travels the autobahn at plus 120 mph speeds. There is no reason the suspension design can't do that here.

A couple of local high end shops around here cater to the Ferrari crowd and I think they use on-the-car high speed balancing to tweak the off-the-car machine balance.

In fact there is a Ferrari dealer just a half mile down the road. I'll have to stop by and ask on my way to Starbucks just across the street. Ooops! With all the big names around there I think they changed it to an Avenue or Boulevard.

[eligunn]

Harmonics are multiples of 2. It goes 100hz-200hz-400hz

[1.2gees]

divezic, I said:

Originally posted by 1.2gees
I have parked the car tonight, after a long drive until I figure this problem out.

you respond with:

Originally posted by divezic

The trick with Gees is that the higher the speed and resonance, the quicker your system will fail (break apart). At 100+mph you have little chance of unscrewing the situation when a tie rod breaks. Your problem is not a ticket or license. It is killing yourself and anybody you take out. From an engineering standpoint, your system will fail eventually. Statistically speaking, you will die and you will kill others around you.

you also said:

Originally posted by divezic
Fix the problem, buy a helmet, join SCCA or PCA and do the 140 on the track after some instruction.

and to that I will respond with this:


I'm trying to fix the car FOR a track event...

Other that though, I do appreciate your response thanks again.

SoCal, I doubt I'd go as far as analyzing the whole system But I will look over the wheels and tires. Thing is usually that type of thing appears at much lower speeds 55-70mph.

eligunn, so are you saying that my problem couldn't be a resonance since it's through a broad speed range?
Ahmet

[Bob's944]

I had a very bad vibration/shutter at 70 mph when I bought the car which had just ok older tires. New tires balanced properly along with 2 sway bar bushings completely cured the problem. I bet it's just your tires.

I started thinking about the harmonic vibration problem and kinda thought of a way to explain it that is not to complicated to be understood.

Everything along the drive line that has weight and is rotating, has a sine wave like variation to the balance of the rotation. The heavier components and furthest away from the center of rotation having the greatest effect . Tires have the most effect because of their weight and distance from the center of rotation. As the engine RPM increases, it increases the cycles per second (hertz, Hz) of the component along the drive line changes. Some may have peaks at say 15 mph, 30 mph.. 45 mph (the speeds are directly related to the Hz) that are barely noticable. Your brake discs, flywheel, crankshaft, cam, wheels (2nd most important because of weight) are all involved.

At 70 mph there must be a number of waves that peak all at the same time , one being tires. The severe shudder that I was getting was the tire jumping up and down off the road as a way to help disperse the energy. It fades off as the sine wave retreats, but when it returns, it should be much worse because the doubling of the rotational speed.

I never got close to finding out what that 2nd tire peak was like, I had the new tires on the car the day I bought it, I bet it's brutal.

I bet the new tires will drive perfect.