Vibration

[Woz]

Hello, I have a 68 911T (SWB). A couple of months ago I start to experience a severe vibration throughout the car when travelling at speeds between 70 and 95 MPH. My local classic Porsche specialist believes it's caused by degradation of rear trailing arm bushings. For this he wants USD1800 to replace them and the spring plate covers. This sounds like a heck of a lot of money especially as the parts are relatively low cost items, so I asked my local approved Porsche dealership to take a look (as they have a lower man-hour cost). Unfortunately they came back with a different diagnosis, that it's due to the incorrect gearbox that has been installed on my car (it has had some kind of adaptive connector welded on).

So, my question is, who is most likely to be right? I have not always experienced the problem and as it doesn't matter what gear I am in I would not expect it to be the gearbox. The clutch was replaced less than 3000 miles ago. If it's trailing arm, how many man-hours would it take for a decent mechanic to change the trailing arm bushes and make any necessary balancing adjustments? For 1800 USD I could probably find the right gearbox but could be a wrong diagnosis.

Thanks.

[Oh Haha]

How old are your tires? Does the car sit for long periods of time?

Have you had the tires balanced recently?

What "incorrect" gearbox do you have?


Explain a bit more about your setup, please.

[Zeke]

Borrow some mounted tires and try them . I just had this and it was a tire separation below the tread. Out of round tires by other reasons do this too. No balance job will fix it.

[docrodg]

Vibration at speed that gets worse at certain speeds is caused by an out of balance rotational component (Including seperated tire treads and abnormally worn tires, as these throw off the balance). Easiest thing to do is first check the for something that is amiss (missing bolt or something stuck around the axle) as well as missing wheel weights. Check the tread for abnormal wear If that fails take it in for a wheel balancing.

Post a picture of the "adapter"... could be throwing off the balance.

[Woz]

Thanks for your replies so far. The car is used most weekends. The tyres are not worn. Local Porsche dealer checked wheel balance but this is also OK.

[Woz]

According to my Porsche dealer, the installed gearbox is a 911.01 (5 speed for a 1970 car) when should be a 905.01 (for a 68 T). Previous owner has welded some kind of adapter to enable it to mate on my car.

[Zeke]

Can you tell us more? Is the vibration definitely related to road speed?

AS I said, a balance job won't always expose the underlying problem. Modern computer balancers only rotate the tire at about the equivalent of 60 MPH. Tires will come out of round at any speeds, but often at higher speeds.

If you can't put some other tires on temporarily, you won't cover this aspect. I understand that is not always convenient, but how else are you going to check? Maybe others have an idea.

I don't think the tranny is the problem at this point. What I do think is the 2 shops are guessing big time. I would NOT spend any money on this until you check the simple stuff.

How about taking the car down to the tire shop and having them spin up the tire while on the car using the type of balancer that sits on the ground in front of the car with the little wheel. Those machines will spin the tire a lot faster and are used to pin point problems in rotors and the rest of the spinning mass. If the tire has a heavy spot in it (they all do to a certain degree), it will show up at the higher speeds that you say are when you get the vibration.

What do they call those? I've used one many times and feel stupid describing it like that.

I'm pretty certain about this and would just about bet money.

[Woz]

Yes, vibration starts at approx 75MPH and stops at around 95MPH. It doesn't feel like it's just through the steering wheel although it's a little hard to tell. I will take your advice and take it to tire specialist to see what they can test. Thanks for your help.