Bad Vibration In High Speed Turns

[nelsonian101]

The vibration is most noticeable at turn 1 at Sydney Motorsport Park.

The vibration also occurs at any speed over 150kmh and even in a straight line though much worse in corners. Fast Left handers are the worst.

Car (FGXR6T sedan) has done 185,000kms but has been maintained meticulously.

The whole car shakes which ruins any confidence for a decent lap time.

There arent many stock parts remaining on the chassis/car. Shocks springs bushes etc Front camber kit with 3.5deg neg.

Most obvious causes are wheel balance with the used Michelin slicks I am using but these tyres are continually flipped rebalanced and rotated.

I though it may be the diff but it was recently replaced with a Truetrac which did not effect the vibration.

I have narrowed it down to 2 possible causes:

1. Rear Wheel bearings/hub. I had the rear bearings only replaced not long before the vibration started. The mechanics found it very difficult getting the new bearings into the existing rear hub housing and I am thinking would this slight misalignment be fine at normal road speeds but be terrible at the circuit?

Would turn 1 at Eastern Creek be possibly loading the rear RHS bearing causing the bad vibration?

2. Rear diffuser was fitted around the same time the vibration started too but would a rear diffuser vibration be worse in corners than straight line over certain speeds? Once again, nothing is noticable on the street.

I am getting desperate to diagnose and any help is appreciated.

The frustrating thing is, I cant test anything on the road as the car drives perfectly on the street at road speeds.

Any ideas?

[ScatterBrain]

Sure it's not your tyres sliding? What hot pressures do you run?

[nelsonian101]

Hot pressures for the Michelin Porsche Cup slicks I have been using are around 30 to 32psi.

First session they went to 35psi hot recently and were dropped back down.

Vibration was a bit worse at lower pressures.

I dont have a lot of experience with slicks (have done about 5 track days with different full slicks) but thinking about it, their slide characteritics seem to be slip/grab, slip/grab.

Far less predictable than a R spec.

It is quite a violent vibration though, and surely has to be more to it?

[ScatterBrain]

Whens you next track day? Maybe see if you can do a run in the track with your road tyres and if it still vibrates than it's not the tyres. I doubt it would be to do with wheel bearing or the hub. Also do you have any in car footage of the car when you get the vibration?

[nelsonian101]

Next track day will be at Wakefield late January sometime.

Unfortunately Wakefield doesn'e seem to be as bad with the vibration as SMP.

I will be experimenting with some tyre combos there though. Thanks for the the idea.

The camera was mounted very securely but you can see the vibration through the car.

It is demonstrated quite well at the 2min35 mark through turn 1.

I haven't also ruled out the slicks slipping and grabbing causing the vibration and I need to experiment with a few tyre combos next time at the track.

[ScatterBrain]

Yeah that's a bad vibration alright. How low is the rear of the car? maybe your maxing out the CV's travel.

[nelsonian101]

  On 21/12/2012 at 7:58 AM, ScatterBrain said:

Yeah that's a bad vibration alright. How low is the rear of the car? maybe your maxing out the CV's travel.

Rear of the car is very low on Lovells and revalved Konis, and has been for years. (Though after spring sag I am onto my 2nd set of Lovell springs at the back which raised it up about 10mm again, but still far lower than stock)

I am hoping it isn't CV's as they are expensive but the car has been low at the back now for over 100,000kms (and has done a dozen circuit days). Would my lighter track wheels/slicks and higher cornering loads highlight this shagged CV symptom more and would this cause the vibration both on and off throttle?

[ScatterBrain]

The more g-force on the car during high speed corners could compress the bushes in the rear control arms causing the CV to move in an potentially maxing out their travel. If It Is the CV the amount of throttle applied would have little to no affect on the vibration, It would all be related to cornering speed and grip. Just on a side note if the cars too low your roll center goes through the floor and your handling will get worse and you may end up with heat issues.

[BRK]

Assume all engine and trans mounts are okay. Does the vibration only happen at that speed ( I.e. a Harmonic) or does it get progressively worse past 100KPH? Guesswork here but it might be the driveshaft. You have eliminated tyres and bent rims. I had a 280E merc a long way back that used to get the tremors at speed and that was the CV nearest the gear box.

I was told the driveshafts on these cars are balanced on the car - that was/is according to a dealer.